GAS-COOLED REACTOR INFORMATION MEETING - UNT ...

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Proceedings ?f

CONF- 700401

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GAS-COOLED REACTOR INFORMATION MEETING

at the OAK RIDGE NATIONAL LABORATORY

April 27-30, 1970 Oak Ridge Playhouse Oak Ridge, Tennessee

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.-- . United States Atomic Energy Commission.

. 4 /' Division of Technical Information -

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DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

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L E G ’ A L N O T I C E This report was prepared a s an account of Government sponsored work. Neither the United States, nor the,Commission. nor any person acting on behalf of the Commission:

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Printed in USA. P r i c e $10.00. Available f rom the Clearing- house for Federal Scientific and Technical Information, Na- tional Bureau of Standards, U. S. Department of Commerce, Springfield, Virginia 22151.

- --- CONF-700401

REACTOR TECHNOLOGY (TID -45 00)

.-

GAS-COOLED REACTOR INFORMATION MEETING

Proceedings of Meeting at the

OAK RIDGE PLAYHOUSE

Oak Ridge, Tennessee

April 27 - April 30, 1970

Sponsored by

Oak Ridge National Laboratory

Oak Ridge, Tennessee

L E G A L NOTICE This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com- pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

-. .* .

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General Chairman:

Assoc ia te Chairman:

MEETING OFFICERS

D. B. Trauger, Oak Ridge Nat iona l Laboratory

J . H. Coobs, Oak Ridge Nat iona l Laboratory

Coordinator f o r Proceedings: E. R. Taylor , Oak Ridge Nat iona l Laboratory

Adminis tr a t i v e S e c r e t a r y : June L. Zachary, Oak Ridge Nat iona l Laboratory

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EDITORIAL NOTE

The papers and d i scuss ions incorpora ted i n t h e proceedings a r e

e d i t e d only t o t h e e x t e n t considered necessary f o r t h e r e a d e r ' s a s s i s t a n c e .

The views expressed and t h e gene ra l s t y l e - a d o p t e d remain, however, t h e

r e s p o n s i b i l i t y of t h e named au tho r s or p a r t i c i p a n t s .

The a f f i l i a t i o n s of au thors and p a r t i c i p a n t s a r e those given a t

t h e t i m e of t h e meeting.

The mention of s p e c i f i c companies or of t h e i r products o r brand-

names does no t imply any endorsement o r recommendation on t h e p a r t of

t h e Oak Ridge Nat iona l Laboratory.

vii

FOREWORD

Hel ium-cooled n u c l e a r r e a c t o r s o f f e r p o t e n t i a l f o r meet ing t h e major long- te rm requ i rements f o r e l e c t r i c power genera t i on . i n t o two c a t e g o r i e s : f i r s t , t h e High-Temperature Gas-Cooled Conver te r Reactor (HTGR o r HTR) t h a t uses g r a p h i t e f o r t h e f u e l s t r u c t u r e and moderator ; and second, t h e Gas-Cooled Fas t Breeder Reactor ( G C B R o r GCFBR) which has a c o r e composed o f m e t a l l i c - c l a d p i n s o f mixed uran ium-p lu ton ium o x i d e f u e l . o r a modern steam c y c l e f o r energy recove ry t o d r i v e t h e e l e c t r i c genera to r .

The r e a c t o r s f a l l

Both r e a c t o r s can be used w i t h e i t h e r a d i r e c t t u r b i n e c y c l e

Ma jor i n c e n t i v e s f o r a p p l i c a t i o n o f t h e HTGR a r e t h e p rospec ts f o r economica l l y c o m p e t i t i v e power p r o d u c t i o n w h i l e u t i l i z i n g more e f f e c t i v e l y t h e uranium o r e reserves. formance make p o s s i b l e h i g h burnup th rough which low f u e l - c y c l e c o s t s can be o b t a i n e d and ma in ta ined as uranium o r e c o s t s r i s e . tempera ture c a p a b i l i t y o f t h e g r a p h i t e co re s t r u c t u r e f a c i l i t a t e s o p e r a t i o n a t h i g h thermodynamic e f f i c i e n c i e s , and thus requ i rements f o r heat d i s s i p a t i o n t o t h e env i ronment a r e l o w . E x c e l l e n t r e t e n t i o n o f f i s s i o n p roduc ts and o t h e r r a d i o a c t i v e m a t e r i a l s appears t o be a c h i e v a b l e by t h e c o o l a n t c i r c u i t s housed i n c o n c r e t e p ressu re vesse ls ,

The e x c e l l e n t neu t ron economy and f u e l pe r -

The h i g h -

The he l ium-coo led GCBR o f f e r s a h i g h b reed ing r a t i o w i t h a f u e l d o u b l i n g t ime o f l e s s than 10 years , a c o r e convers ion r a t i o o f a p p r o x i - ma te l y 1.0, a smal l r e a c t i v i t y change f rom l o s s o f c o o l a n t , a p o t e n t i a l l y n o n r a d i o a c t i v e c i r c u i t , and h i g h thermal e f f i c i e n c y . The p r i n c i p a l ques t i ons concern ing t h e concept c e n t e r around t h e h igh -p ressu re c o o l a n t , p a r t i c u l a r l y f o r a f t e r h e a t removal f o l l o w i n g l o s s - o f - c o o l i n g a c c i d e n t s , and t h e performance o f m e t a l - c l a d ceramic f u e l e lements f o r r e l a t i v e l y h i g h tempera ture o p e r a t i o n , app rox ima te l y 700°C.

Development of gas-cooled r e a c t o r s for e l e c t r i c g e n e r a t i n g power s t a t i o n s has been under way i n t h e U n i t e d S ta tes s i n c e 1956 and f o r a l onger p e r i o d i n seve ra l European c o u n t r i e s . Four he l ium-coo led r e a c t o r s have opera ted - Peach Bottom, i n Pennsylvania, t h e AVR a t J U l i c h , Germany, t h e Dragon Reactor i n t h e South o f England, and UHTREX a t Los Alamos. Three o f t h e r e a c t o r s rep resen t e a r l y concepts f o r he l ium-coo led nuc lea r e l e c t r i c power s t a t i o n s . The f o u r t h , UHTREX, was modeled for d i r e c t use o f heat . The o p e r a t i n g exper ience w i t h these r e a c t o r s p r o v i d e s a broad b a s i s f o r new des igns and c o n t r i b u t e s g r e a t l y t o t h e p r o j e c t i o n o f HTGR pe r f o rma nce .

The Oak Ridge N a t i o n a l L a b o r a t o r y ho lds p e r i o d i c meet ings f o r t h e r e p o r t i n g o f i t s work i n gas-cooled r e a c t o r development. T h i s meet ing c o n s t i t u t e d an e x t e n s i o n t o p r o v i d e g r e a t l y inc reased p a r t i c i p a t i o n by o t h e r s . I n e f f e c t , t h e meet ing was conducted as an i n t e r n a t i o n a l symposium a t which comprehensive papers were presented on a l l aspec ts of gas-cooled r e a c t o r technology. The papers and d i s c u s s i o n a t t h e meet ing p r o v i d e d an o p p o r t u n i t y t o rev iew the t o t a l development program, t h e des ign concepts, and t h e o p e r a t i n g exper ience f rom gas-cooled r e a c t o r s w i t h emphasis on t h e HTGR which i s now e n t e r i n g t h e commercial f i e l d , An @

viii

important session at the meeting, however, was that on the gas-cooled fast breeder reactor held in conjunction with the Molten-Salt Breeder Reactor meeting. Here considerable advances in the study and analysis of safety problems for the GCBR indicated more clearly its feasibility and practicality. as having longer-term potential for reducing capital costs.

Direct helium turbine cycle systems were described

In the presentation o f these proceedings the Oak Ridge National Laboratory wishes to thank all o f those who participated in the meeting. Particular appreciation is expressed to the authors, the session chair- men, and panelists who were m o s t cooperative in providing abstracts, texts of papers, and discussion comments so that the timely preparation o f these proceedings would be possible.

D. B. Trauger

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CON TENTS

In t roductory Remarks . . . . . . . . . H. G . MacPherson, Deputy D i r e c t o r Oak Ridge Nat iona l Laboratory

SESSION I : WACTORS I N OPERATION

Chairman - J. S. Kemper, PE Co-Chairman - S. I . Kaplan, ORNL

. . . . . . . . . . . . . . . . Dragon Operating Experience (1/137) Is

Discussion 32

(2/103) 34

B. G . Chapman, Dragon P r o j e c t . . . . . . . . . . . . . . . . . . . . . . . . . . . Operating Experience with t h e AVR Experimental Power S t a t i o n

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . H. J . Hantke, BBK; G. Ivens, AVR; E . A . Nephew, ORNL Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Objec t ives and Plans f o r Fuel T e s t i n g i n t h e Peach Bottom HTGR (3/138) . . . . . . . . . . . . . . . . . . . . . . . . . . 60 K. P. Steward, GGA Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

SESSION I1 : PLANT CONSTRUCTION EXPERIENCE

Chairman - G . E . Locket t , Dragon P r o j e c t Co-Chairman - M. Bender, ORNL

F o r t S t . Vrain Construct ion Progress (1/105) 93 H. N . Wellhouser, GGA Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

( 2 / 1 2 0 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

. . . . . . . . . . . .

The Rela t ionship of t h e HTR with E a r l i e r Gas-Cooled Reactors

B. N . Furber, TNPG; C. S. Lowthian, TNPG

SESSION 111: H E R NEW AND ADVANCED DESIGNS

Chairman - J . D. Thorn, R i s l e y Co-Chairman - G. D. Whitman, ORNL

The High Temperature Reactor Development i n Germany (Present S i t u a t i o n , Program and Future Role) (1/101) . . . . . . . . . . . 145

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 H. K r ' a m e r , KFA

Design Fea tures and Engineer ing S t a t u s of t h e THTR 300 Mwe Prototype Power S t a t i o n (2/113) . . . . . . . . . . . . . . . . . 161 H. Oehme, BBK; J . Schgning, BBK Discuss ion . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

X

The Layout of t h e Core and Fuel Elements of t h e TYTK (3/112) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Large HTGR Design S t a t u s (Ul.21) . . . . . . . . . . . . . . . . . . K. Ehlers , BBK; H . Harder, BBK; E. Schrb'der, BBK Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . A . J. Goodjohn, GGA Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature Reactor and Helium Turbine (5/115) . . . . . . . . . -4. Yodzic, BBK; K. W . Marx W. TYvardziok, GHH; S. Fb'rster, KFA Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . .

Conceptual Design f o r a 600 Mwe Xuclear Power P lan t with High

GHH; W . StrGmer, BBK;

Development Work f o r Large-Scale Helium Turbine P l a n t s with High Temperature Reactors .(6/117) . . . . . . . . . . . . . . . . . . E. Bbhm, GHH; W . Twardziok, GHH; H. Oehme, BBK; H. Weiskopf, BBC Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . .

The Program of Plate-out I n v e s t i g a t i o n s i n t h e Gas Turbine P r o j e c t (7/127) . . . . . . . . . . . . . . . . . . . . . . . . . C. B. von d e r Decken, KFA Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . .

The HTR Direct Cycle: Engineer ing P o s s i b i l i t i e s and Mater ia l Requirements (8/102) . . . . . . . . . . . . . . . . . . . . . . G. E. Locket t , Dragon P r o j e c t ; R . A. U. Huddle, Dragon P r o j e c t ; Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . .

180

192 195

21 2

214

228

231

SESSION I V : PRESSURE VESSEL TECHNOLOGY AND SAFETY TOPICS

Chairman - T. A. Jaeger , BFM Co-Chairman - H. J . deNordwall, ORNL

S a f e t y Aspects of High Temperature Reactors (1/125) . . . . . . . . 293

of a Large HTR S t a t i o n (2/118) . . . . . . . . . . . . . . . . . 31-1

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 F i s s i o n Product Transport i n HTGR Systems - A Summary (3/104) . . . 361

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 384

Vessels (4/111) . . . . . . . . . . . . . . . . . . . . . . . . 387

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 402

F. P. 0. Ashworth, Dragon P r o j e c t The Rela t ion of F i s s i o n Product Release Limi ta t ions t o t h e Design

R . H. Flowers, AERE, Harwell

F. E . Vanslager, GGA; W. E . B e l l , GGA; 0. Sisman, ORNL; M. T. Morgan, ORNL

Shear F a i l u r e s i n End S l a b s of P r e s t r e s s e d Concrete Pressure

M. A . Sozen, U. of Ill.; W . C . Schnobrich, U. of 111.; S. L. Paul, U. of I l l .

I n v e s t i g a t i o n s of Time-Dependent Behavior of P r e s t r e s s e d Concrete Pressure Vessels (5/108) . . . . . . . . . . . . . . . . . . . . 403 J . P. Cal lahan, ORNL; J . M. Corum, ORNL; G. D. Whitman, ORNL Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 431

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By T i t l e - Concrete Pressure Vessels - 1969 Assessment (6/-) . . . . . . . . . . . . . . . . . . . . . . . . H. Benzler, Euratom; J. Terps t ra , Euratom Oral s ta tement by D. D. Tytgat , Euratom

SESSION V : H E R FUEL ELFIvENT DESIGN, PERFORMANCE AND MANUFACTURE

Chairman - R. F. Turner, GGA Co-Chairman - , J. H. Coobs, ORNL

HTGR Fuel I r r a d i a t i o n Performance and Impl ica t ions on Fuel Design (1/131) . . . . . . . . . . . . . . . . . . . . . . . W . V . Goeddel, GGA; E. 0. Winkler, GGA; C. 2;. Luby, GGA Discussion . . . . . . . . . . . . . . . . . . . . . . . . .

Development of Bonded Beds of Coated P a r t i c l e s for HTGR Fuel Elements (2/107) . . . . . . . . . . . . . . . . . . . . . . J . L. Scott,,ORNL; J . A. Conlin, ORNL; J . H.. Coobs, ORNL; D. M. H e w e t t e 11, ORNL; J. M. Robbins, ORNL; R. L. Senn, ORNL D i s cus s i on . . . . . . . . . . . . . . . . . . . . . . . . . (3/132) . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Smith, Dragon P r o j e c t Discussion . . . . . . . . . . . . . . . . . . . . . . . . .

The Development and Performance of HTR Core M a t e r i a l s (4/106) . L. W . Graham, Dragon P r o j e c t Discussion . . . . . . . . . . . . . . . . . . . . . . . . .

Choice of Fuel Design f o r Homogeneous Low Enriched HTR (5/126) . D. J . Merrett, TNFG; M. Gaube, Belgonucleaire D i scu s s i on

H. B a i r i o t , Belgonucleaire; L. Aerts, Belgonucleaire; R. A . Skinner, TNFG; J . Vangeel, CEN Discussion ; . . . . . . . . . . . . . . . . . . . . . . . .

HTR Fuel and M a t e r i a l s Development i n Germany, Present S i t u a t i o n and Program (7/128) . . . . . . . . . . . . . . . . . . . . . B. Liebmann, KFA; J . Bugl, GHH; K . Ehlers , BBK; K . G. Hackstein, NUKEM Discussion . . . . . . . . . . . . . . . . . . . . . . . . .

Behavior of Coated P a r t i c l e Fuel i n DFR I r r a d i a t i o n Experiments (8/133) . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Baier , KFA; W. i n d e r Schmitten, NUKEM; P. Walger, BBK Discussion . . . . . . . . . . . . . . . . . . . . . . . . .

Experiments with t h e Production of AVR-Fuel Elements (9/116) . . K. G. Hackstein, NUKEM Discussion . . . . . . . . . . . . . . . . . . . . . . . . .

The Design of Pr i smat ic High Temperature Reactor Fuel Elements

. . . . . . . . . . . . . . . . . . . . . . . . . Fuel Development for a Low Enriched HTR (6/124) . . . . . . . .

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437

4 39

455

456

472

474

492 494

51 7 51 8

54 5 547

50 5

586

59 7

598

607 61 0

617

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SESSION V I : ECONOMICS OF HTGR FUEL RECYCLE AND POWER GENERATION

Chairman - H . B. S tewar t , GGA Co-Chairman - R . S . Carlsmith, ORNL

Thorium-Fuel-Reprocessing Research and Development work a t t h e KFA J z l i c h (1/136) . . . . . . . . . . . . . . . . . . . . . . . 62 1 J . Bohnenstingl, KFA; B. G. Brodda, KFA; 0 . Coenegracht, KFA; E . F i scher , KFA; G. K a i s e r , KFA; H . Kirchner, KFA; M. L a s e r , KFA; W . Litzow, KFA; E . Merz, KFA; H. J. Riedel, KFA; D. Thie le , KFA; U. Wenzel, KFA; H . W i e m e r , KFA; H. W i t t e , KFA; E . Z i m m e r , KFA

D. E . Ferguson, ORNL By T i t l e - Review of USAEC-ORNL HTGR Fuel Recycle Program (2/-) . . 647

Designs (3/135), P a r t I, P a r t I1 67 1 Some Fea tu res of t h e Low Enriched HTR f o r Various Fuel Element . . . . . . . . . . . . . . . .

G. Graz ian i , CCR Euratom - I s p r a ; C . R i n a l d i n i , CCR Euratom- I s p r a ; C. Zanantoni, CCR Euratom - I s p r a ; H . B a i r i o t , Belgonucleaire; P. Haubert, Belgonucleaire; G. G h i l a r d o t t i , AGIP Nucleare; M. Baur, GHH . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion 69 2

Comparison of HTGR Fuel Cycles f o r Large Reactors (4/130) . . . . . 694 R. C . Dahlberg, GGA D i s c u s s i o n 708

Po in t of V i e w (5/129) . . . . . . . . . . . . . . . . . . . . . 710

Discussion 72 3

Elements (6/134) . . . . . . . . . . . . . . . . . . . . . . . . 726

. . . . . . . . . . . . . . . . . . . . . . . . . . . Economics of Recycle i n LWR's and HTGR's i n t h e U.S.--A Consul tants

W . V . Macnabb, NUS; J . C. Scarborough, NUS . . . . . . . . . . . . . . . . . . . . . . . . . . . Parametr ic Survey on Fuel Cycles and To ta l Generating C o s t s f o r

HTR's w i th H o l l o w Rod, Te led ia l , and Tubular I n t e r a c t i n g Fuel

H . Gutmann, Dragon P r o j e c t ; J . Daub, Dragon P r o j e c t ; H . Schnober, Dragon P r o j e c t ; C. R ina ld in i , Euratom-Ispra; G. Graz ian i , Euratom-Ispra; J . J o u r n e t , CEA; J . Malherbe, CEA Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 52

. . . . . . . . . . . . . Evening Program - HTGR's and Greenhouses 7 55 S . E . B e a l l , OFDL

. . . . . . . . Panel Discussion - Prospects f o r t h e Next Ten Years 765

F loor Discussion . . . . . . . . . . . . . . . . . . . . . . . . 780

J. A. Lane, ORNL; C. L. Rickard, GGA; R . D. Vaughan, TNPG; K. Wirtz, KFK

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SESSION V I I : GAS-COOLED FAST REACTOR DESIGN, SAFETY STUDIES AND FUEL DEVELOPMENT

Chairman - C. A . Rennie, P. Co-Chairman - P. P a t r i a r c a , ORNL

The GCFR Demonstration P l a n t Design (1/119) . . . . . . . . . . . . G a s Turbine Power Conversion Systems f o r Helium Cooled Breeder

P. Fortescue, GGA; W . I . Thompson, GGA

Reactors (2/114) . . . . . . . . . . . . . . . . . . . . . . . . L. A . Lys, EIR; G. Ciszewski, SGI; H . F ru t sch i , BST

(3/139) . . . . . . . . . . . . . . . . . . . . . . . . . . . . H. B a i r i o t , Belgonucleaire ; J . M. Tiromson, Belgonucleaire; L. A e r t s , Be3gonucleaire D i s c u s s i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . (4/123) . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. D a l l e Donne, Gfk; E . Eisemann, GfK; K. Wirtz, Gfk

Development ‘of Coated P a r t i c l e Fuel f o r a Gas E%reeder Reactor

La te s t Ca lcu la t ions f o r a GCFR wi th a Vanadium Clad Pin C o r e

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas-Cooled Fas t Reactor Fuel-Element Development (5/122) . . . . .

R . B. F i t t s , OWL; J . R . Lindgren, GGA; E . I,. Long, ORNL; D. R . Cuneo, ORNL Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . J . M. Waage, GGA; J . A . Larrimore, GGA

S a f e t y S tud ie s f o r t h e Gas-Cooled Fas t Reactor (6/109) . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . .

S a f e t y And Af terhea t Removal S tud ie s of t h e Gas-Cooled Fas t Reactor (7/110) . . . . . . . . . . . . . . . . . . . . . . . . C. S . Walker, ORNL D i s c u s s i o n . . . . . . . . . . . . . . . . . . . . ; . . . . . .

Aspects of Gas-Cooled Fas t Reactor Development a t ENEA . . . . . . W. F. Haussermann

79 5

81 2

833

852

8 54

861 864

878 879

890

90 3 904

. . . . . . . . . 907 Fas t Breeder Reactors, N o t LMFBR o r GCFR--but FBR Compton A . Rennie, P

. . . . . . . . . . . . . . . . . . . . . . . . . . 91 7 Closing Remarks D. B. Trauger, ORNL

. . . . . . . . . . . . . . . . . . . . . . . 919 L i s t of P a r t i c i p a n t s

. . . . . . . . . . . . . . . . . . . 931 P a r t i c i p a t i n g Organizat ions

. . . . . . . . . . . . . . . . . . . . . . . . . . . 935 Author Index

xv

In t roduc to ry Remarks

H . G. MacPherson

Welcome t o Tennessee and t o t h e Oak Ridge Nat ional Laboratory.

a r e deeply honored and pleased t o p l ay h o s t t o such a d i s t ingu i shed

ga the r ing of expe r t s on h igh temperature gas-cooled r e a c t o r s . If t h e r e

i s anything we can do t o make your s t a y he re more p l easan t , p l ease l e t us know.

We

Since t h i s i n t e r n a t i o n a l meeting on gas-cooled r e a c t o r s i s be ing

he ld i n Oak Ridge, I t h i n k it i s appropr i a t e t o pay t r i b u t e t o t h e e a r l y

e f f o r t s of Fa r r ing ton Daniels . P ro fes so r Daniels had a l l t h e r i g h t s e n t i -

ments and motives t o win approval of t h i s audience - t h e only t r o u b l e was

t h a t he was a c t i v e about 10 years t oo e a r l y .

t h e Meta l lu rg ica l Laboratory, now t h e Argonne Nati-onal Laboratory, he

proposed a pebble bed r e a c t o r t o use a mixture of g ranh i t e and uranium

ca rb ide pebbles or b a l l s .

t o p re sen t day s tandards . H e suggested such th ings a s ope ra t ion a t

2000 OC, a r e a c t o r diameter of 35 f e e t t o ope ra t e on n a t u r a l uranium,

cool ing with e i t h e r H e or b o i l i n g bismuth, and d i s t i l l i n g Pu and f i s s i o n

products for t h e r e a c t o r dur ing ope ra t ion .

I n 1944, i n what was then

This was a r a t h e r advanced proposa l according

These ideas l e d t o t h e a u t h o r i z a t i o n of t h e Power P i l e P r o j e c t i n

1946. T h i s was a helium-cooled BeO-moderated r e a c t o r , c a l l e d the Daniels

P i l e , which was t o be b u i l t i n Oak Ridge. Although t h i s r e a c t o r p r o j e c t

was canceled be fo re it was two yea r s o ld , t h e group of engineers t h a t was

assembled f o r t h i s purpose went on t o b u i l d t h e U. S. submarine r e a c t o r .

Fa r r ing ton Daniels d id not g ive up, d e s p i t e t h e o f f i c i a l discourage-

ment. H e modified h i s ideas , and made a s p e c i f i c proposa l of a more

simple graphite-moderated gas-cooled r e a c t o r i n 1950. This r e a c t o r pro-

posa l was very s p e c i f i c f o r t h e ope ra t ion of a gas t u r b i n e using N2 a s a

coolan t , and he even went t o t h e t r o u b l e of l o c a t i n g a v a i l a b l e gas t u r -

b ines and g e t t i n g quota t ions on t h e i r d e l i v e r y . This proposal f e l l on

deaf e a r s , b u t back a t t h e Un ive r s i ty of Wisconsin Daniels continued h i s

r e sea rch a c t i v i t i e s .

xvi

I n 1955 w e had some correspondence a t a t ime when he was seeking a

method of s e a l i n g uranium f u e l t i g h t l y i n smal l p ieces of g raph i t e .

was then wi th t h e Carbon Products Div is ion of Union Carbide Corporation,

and we provided him wi th our t hen b e s t e f f o r t a t a small sealed g raph i t e

capsule .

i n 1956 proposing a helium-cooled r e a c t o r based on t h i s f u e l .

h i s major i n t e r e s t tu rned t o s o l a r energy a s a h e a t source f o r backward

coun t r i e s .

I

This was no t q u i t e good enough, b u t Daniels d id publ i sh a paper

Af t e r t h a t

It i s too bad t h a t Danie ls ' enthusiasm and e f f o r t had t o come 10

yea r s t o o soon, both from a p o l i t i c a l and a t e c h n i c a l p o i n t of view.

The world was no t ready f o r commercial nuc lea r power when he s t a r t e d , and

he was not ab le 60 p e r s i s t long enough t o be rescued by t h e development

of p y r o l y t i c a l l y carbon-coated f u e l p a r t i c l e s , a s t h e r e s t of you have

been.

u c t r e t e n t i o n by p y r o l y t i c coa t ings i s t h e most important s i n g l e advance

i n gas-cooled r e a c t o r technology s i n c e Fa r r ing ton Daniels s t a r t e d i n 1944, i n t h a t it makes gas-cooled r e a c t o r s c r e d i b l e from a commercial p o i n t of

view.

It i s my per sona l op in ion t h a t t h e d iscovery of t r u e f i s s i o n prod-

1

mACTORS I N OPERATION

(Sess ion I)

2

Chairman : J . S. Kemper, ’

Phi l ade lph ia Electr ic Company

Co-Chairman: S . I . Kaplan, Oak Ridge National Laboratory

Paper 1/137 2' 0

DRAGON OPERATING EXPERIENCE .ur

--ly

B. G. Chapman I -'1 0 j' ;? 0 OECD Dragon P r o j e c t

' I

ABSTRACT

The u s e of Dragon as a f u e l i r r a d i a t i o n f a c i l i t y has generated a g r e a t d e a l of experience i n ope ra t iona l areas svch as helium u t i l i s a - t i o n and engineer ing , maintenance of helium c i r c u i t s , a c t i v i t y behaviour i n HTR c i r c u i t s , and t h e d e t e c t i o n and clean-up of chemical impur i t i e s . yea r s a t Dragon.

The paper desc r ibes t h i s experience over t h e l a s t f i v e

It i s f r equen t ly d i f f i c u l t t o p r e d i c t t h e ope ra t iona l performance of l a r g e p l a n t s from t h e drawing board and t h e ch ie f va lue of exper ience i s t o e s t a b l i s h t h e p r a c t i c a l l i m i t s which can be achieved. I n t h i s r e s p e c t Dragon has demonstrated t h e e s s e n t i a l v i a b i l i t y of t h e HTR concept and has enabled r e a l i s t i c design c r i te r ia t o be l a i d down.

3

4

A

INTRODUCTION

Dragon was b u i l t t o prove an i d e a and t h e h e a r t of t h a t i d e a was a

r e a c t o r core cons t ruc ted e n t i r e l y of g r a p h i t e and ceramic nuclear

f u e l s , and cooled with helium.

of t h e r e a c t o r experiment w e r e secondary t o t h a t of c r e a t i n g t h e

c o r r e c t environment f o r t h e ope ra t ion of such a core.

many des ign f e a t u r e s w e r e int roduced which are not t y p i c a l of a

commercial HTR plant . Nevertheless , i n a d d i t i o n t o f u e l t e s t i n g , a

g r e a t d e a l of experience and informat ion has accrued from t h e P r o j e c t

t o guide des igne r s of f u t u r e systems.

A l l o t h e r cons ide ra t ions i n t h e design

For t h i s reason

Since t h e r e a c t o r f i r s t achieved f u l l power i n A p r i l , 1966, f o u r

y e a r s of development have gone i n t o t h e ope ra t ion and maintenance

techniques app l i cab le t o the HTR.

15 d i f f e r e n t core l ayou t s have been i r r a d i a t e d and some major

d i f f i c u l t i e s overcome. I n p a r t i c u l a r , s i n c e t h e first b o i l e r tube

f a i l u r e i n February, 1967, t h e r e a c t o r has been dogged by water-side

cor ros ion t r o u b l e s - of almost no re levance t o t h e concept of t he HTR.

Dozens of t ubes have been plugged and 13 b o i l e r changes c a r r i e d out .

This has had a s e r i o u s e f f e c t on u t i l i s a t i o n . During Charge I1

u t i l i s a t i o n w a s 52.5% (about 60% of t h e t a r g e t set by cons ide ra t ions

of f u e l changing). For Charge I11 t he corresponding figures are 50%

and 68% (Fig. 1) ” *. There can be l i t t l e doubt t h a t , wi thout t h e

b o i l e r problem, over 90% of t h e des i r ed i r r a d i a t i o n would have been

achieved. The t i m e t h a t has been l o s t i n i r r a d i a t i o n , however, has

provided va luable experience i n maintenance and ope ra t ing techniques

and it i s p r imar i ly with t h i s a spec t t h a t t h i s paper i s concerned.

F ive main areas w i l l be considered.

Three f u e l charges cons is t ing of

The Performance of Large Sca le H e l i u m C i r c u i t s and Associated Components

The use of an expensive coolan t such as helium p laces c i r c u i t

leakage uppermost i n t h e des igne r s ’ mind. The t a r g e t leakage ra te of

O.l%/day, t o which Dragon w a s designed, has become almost t r a d i t i o n a l - y e t it i s impossible t o r e l a t e t o des ign d a t a f o r components.

5

The d i f f u s i o n of helium through metals can be below t h e l i m i t s of

d e t e c t i o n provided t h a t c a r e f u l s e l e c t i o n and c o n t r o l a r e exerc ised .

For i n s t a n c e , some leakage may be expected along a r o l l e d b a r and

small diameter cover p l a t e s must be made from r o l l e d p l a t e ( i n which

imperfec t ions are l a t e r a l ) and no t from b a r ( i n which imperfec t ions are

l o n g t i t u d i n a l ) . S imi la r ly , welds of good s tandard may be expected t o

show n e g l i g i b l e leakage.

f o r imperfec t ions r a t h e r than conformity with a des ign predic t ion .

Since, i n t h e absence of imperfec t ions , t h e leakage should be

vanish ingly small, t h e tes t s p e c i f i c a t i o n becomes r e l a t e d t o t h e

s e n s i t i v i t y of t h e test procedure.

developed by Wade

e i t h e r i n t h e vacuum mode (component evacuated i n a helium atmosphere)

o r a s " s n i f f e r s " (component p re s su r i sed wi th helium o r hel ium/ni t rogen

mixture). S e n s i t i v i t y can be increased and t h e o v e r a l l t e s t i n g of

l a r g e components s impl i f i ed by "bagging" i n polythene and al lowing

helium build-up i n t h e bag f o r a s p e c i f i e d period.

leakage rates a t least two o r d e r s of magnitude less than O.l%/day can

be measured.

Thus most components are t e s t e d pr imar i ly

The Dragon l eak t e s t i n g r o u t i n e s 3 are based upon s p e c i f i e d mass-spectrometers used

By these means,

Turning t o j o i n t s and seals, it i s poss ib l e t o l a y down average

performance d a t a from l abora to ry tests.

used on Dragon have a t y p i c a l leak ra te of 1 x 10 a ta cm /s/ft a t

25 a t m . ra te of 3 x 10-7%/day4' 5.

an e x c e l l e n t poss ib l e performance b u t are mostly very s e n s i t i v e t o

f l e x i n g and thermal cycl ing.

leakage from each coupl ing may be very s m a l l , they are used i n l a r g e

numbers widely d i s t r i b u t e d i n space.

The s i l v e r gaske t j o i n t s -8 3

There are 600 f t of such gaske t s i n Dragon leading t o a leak

Small p ipe coupl ings and "Conoseals" have

The problem he re i s t h a t a l though t h e

From t h i s i t can be seen t h a t a t a r g e t figure of O.l%/day makes a

It a l s o sets a problem f o r generous allowance f o r t h e f r a i l t y of man. o p e r a t o r s i n t h a t l eak rates of t h i s o rde r can be very d i f f i c u l t t o

d e t e c t , l e t a lone measure.

i s t o carry ou t r e g u l a r coolan t balances.

v e s s e l s , p ipes , etc., be provided with p re s su re and temperature

The usua l r o u t i n e i n gas cooled r e a c t o r s

This r e q u i r e s t h a t a l l

6

monitoring so t h a t gas con ten t s can be ca l cu la t ed .

ba lances are c a r r i e d out weekly and al though g r e a t care i s taken with

instrument c a l i b r a t i o n and Van de Waals d e v i a t i o n s from t h e p e r f e c t gas

l a w s are taken i n t o account, experience shows t h a t an accuracy of 23%

i s typ ica l .

temperatures ranging from 150 C t o 830 C, i s d i f f i c u l t t o assess. Thus

helium ba lances w i l l no t show a O.l%/day l e a k ra te i n under f o u r weeks.

A t Dragon such

I n p a r t i c u l a r t h e con ten t of t h e primary c i r c u i t , wi th 0 0

It has been poss ib l e , a t Dragon, t o measure t h e t o t a l unaccountable

leakage by use of t h e containment bui ld ing .

cont inuously v e n t i l a t e d by an open c i r c u i t system with a complete a i r

in te rchange every 22 hours.

by a mass-spectrometer which has been c a l i b r a t e d by making measured r e l e a s e s of helium i n t o t h e bui ld ing . By t h i s means w e f i n d t h a t a

l eak ra te of 0.045%/day g ives 100 vpm of helium i n air. Fig. 2 shows

a t y p i c a l h i s t o r y from which it can be seen that normal leak rates l i e

between 0.025 and 0.05%/day. The assessment of human f r a i l t y seems

s u r p r i s i n g l y co r rec t .

Fig. 3 shows t h e t o t a l usage of helium over t h e same period,

This bu i ld ing i s

The helium i n a i r concent ra t ion i s measured

computed from helium balances.

l o s s e s are h igh compared with those c a l c u l a t e d from Fig. 2.

are three reasons f o r t h i s :

It i s apparent t h a t t h e unaccountable

There

( a ) Losses occur dur ing shutdown when t h e mass-spectrometer i s no t

used because t h e containment bu i ld ing doors are open.

( b ) Large l e a k s (shown by peaks on Fig. 2 ) are usua l ly co r rec t ed

be fo re t h e helium-in-air measurement h a s equ i l ib ra t ed .

(c ) Usage of helium f o r chemical a n a l y s i s i s not metered accu ra t e ly

and a no t iona l figure i s used i n t h e balances. It would not be

economically j u s t i f i e d t o improve t h i s s i t u a t i o n .

The most important conclusion from our experience i s t h a t t h e

t o t a l usage of helium f a r outweighs t h e loss by leakage. I n a

commercial HTR t h i s w i l l be even more t rue . Usage i n purging on-load

r e f u e l l i n g machines and mul t ip l e s tand pipe c l o s u r e s w i l l be h igher

than t h a t r equ i r ed f o r t h e Dragon machinery and leakage should be less

A

A

7

(expressed as a d a i l y percentage) .

complex areas such as instrument and a n a l y s i s small bore l i n e s and no t

from t h e l a r g e vesse ls . Since Dragon i s very h ighly instrumented,

leakage experience i s probably pess imis t i c .

W e f i n d most leaks arise from

What l e s sons can des igne r s l e a r n from t h i s ?

A l eak r a t e of O.l%/day should be comfortably achievable.

Helium-in-air monitoring i s by f a r t h e quickes t and most s e n s i t i v e

technique f o r c o n t r o l l i n g leakage. If poss ib l e , r e a c t o r b u i l d i n g s

and v e n t i l a t i o n p l a n t should be designed t o f a c i l i t a t e t h i s .

Wherever poss ib l e , components should be s p e c i f i e d on a Itno

d e t e c t a b l e leak" basis. Although t h i s cannot be defended i n

terms of t o t a l leakage it g r e a t l y f a c i l i t a t e s t h e maintenance of

l o w backgrounds and s i m p l i f i e s l eak d e t e c t i o n and loca t ion . If

t h e component manufacturing i n d u s t r i e s are encouraged t o th ink i n

t h e s e terms, t h e economic penal ty need not be s i g n i f i c a n t .

A l o w p re s su re c o l l e c t i o n system and recovery compressors are

essent ia l . A s much a s poss ib l e of t h e p l a n t should be designed f o r evacuat ion

t o m i n i m i s e purging l o s s e s p r i o r t o major maintenance.

A computing data logger capable of producing r o u t i n e helium

balances would h e l p i n l o c a t i n g major leaks.

Ear ly r ecogn i t ion of p o s s i b l e l u b r i c a t i o n problems i n oxygen-free

helium l e d t o ex tens ive t e s t i n g i n t h i s area. The design and construc-

t i o n of Dragon however, preceded much of t h i s work and t h e fact t h a t no

bear ing s e i z u r e has occurred i n t h e primary circuit must be a t t r i b u t e d

t o des ign r a t h e r t han any s t a r t l i n g new materials. Wherever poss ib l e ,

bear ings (always r o l l i n g surfaces) are pos i t ioned i n low temperatures

and f l u x e s , mechanisms are designed t o accept h igh c learances , speeds

and t o t a l d u t i e s are low, and grease packing i s used. Experience has

shown t h a t sea led greased bea r ings do no t produce s i g n i f i c a n t

contamination. Greases are se l ec t ed f o r m i n i m u m tendency t o genera te

ttgummytl r e s i d u e s and later developments i nc lude oxygen r e l e a s i n g

8

add i t ives . Where condi t ions preclude t h e use of grease , s tandard

bear ings ( 3 dot-pressed cage) a r e used a f t e r N i t a l e tch ing and MoS

treatment .

d e t a i l s can be found i n . A u s e f u l summary of design

d e s i d e r a t a i s contained i n .

2 Bearings have been designed f o r more arduous duty and

6, 7, 8, 9 , 10

11

A des ign of p a r t i c u l a r i n t e r e s t i s t h e use of o i l ba th l u b r i c a t i o n

These boxes are loca ted i n cool r eg ions i n the c o n t r o l rod gear-boxes.

and are connected t o t h e main v e s s e l by p res su re tubes which ca r ry t h e

d r i v e sha f t s . Normal nuc lear grade o i l s , a f t e r degassing, perform

w e l l , and no foaming has been encountered on depressur i sa t ion . This

technique would appear i d e a l l y s u i t e d f o r PCRV app l i ca t ion .

The bear ing types r e f e r r e d t o are s u i t a b l e only €or l i g h t loads ,

moderate speeds and occas iona l use. The main c i r c u l a t o r s c l e a r l y

r e q u i r e a more r a d i c a l approach and gas bea r ings were se l ec t ed f o r

Dragon, t hus avoiding t h e r o t a t i n g s h a f t seal problem. During t h e

100,000 t o t a l machine hours t o d a t e , no machine has f a i l e d i n normal

se rv ice . Four f a i l u r e s have occurred, a l l a t m i n i m u m i d l e speed

(1,500 rpm) i n n i t rogen a t 1 a ta . Two of t h e s e w e r e considered t o be

due t o d e f i c i e n t jacking gas f low a t s t a r t -up , one t o l o s s of dynamic

balance due t o a loose nose f a i r i n g and t h e o the r t o water i n g r e s s

fol lowing a h e a t exchanger tube f a i l u r e . I n a l l cases i n t e r m i t t e n t

bear ing temperature rises preceded t h e f a i l u r e by seve ra l hours and

t h e machines r a n normally u n t i l they w e r e d e l i b e r a t e l y stopped. They

then re fused t o restart. I n a d d i t i o n t o t h e above f o u r machines, two

have been s t r i p p e d f o r r o u t i n e examination a f t e r about 16,000 hours

and found t o be i n e x c e l l e n t condi t ion. The only se rv ic ing requi red

has been minor reba lanc ing t o the s p e c i f i c a t i o n of 0.5 g cm.

C i r c u l a t o r r e l i a b i l i t y i s usua l ly a central po in t i n t h e sa fe ty

O u r experience would i n d i c a t e t h a t a n a l y s i s of gas cooled r e a c t o r s .

gas bea r ings have p a r t i c u l a r l y favourable c h a r a c t e r i s t i c s i n t h i s

respec t .

speed, t h e machine experiences i t s most d i f f i c u l t tes t a t s t a r t -up and

i d l e speed opera t ion . Thus, i f a machine starts s a t i s f a c t o r i l y , we

Since t h e bear ing sepa ra t ing f o r c e s r ise so r a p i d l y with

9

0 have confidence i n i t s i n t e g r i t y f o r normal high speed duty. To tes t

the machine's a b i l i t y t o withstand a primary c i r c u i t f a i l u r e , one

c i r c u l a t o r w a s depressur i sed a t 50 p s i / s , on a test r i g . N o bear ing

touch occurred, indeed, t h e bear ing capac i ty measurement ind ica t ed a

n e g l i g i b l e change i n c learance.

F i n a l l y , some mention must be made of diaphragm compressors.

These machines have been t h e cause of cons iderable d i f f i c u l t y on a l l

c u r r e n t HTR p lan ts . Fig. 4 shows t h e genera l improvement i n diaphragm

l i f e from about 100 hours , 5 y e a r s ago, t o i n excess of 1 ,000 hours

cu r ren t ly . This improvement has been achieved p a r t l y by modi f ica t ions

and p a r t l y by improved ope ra t iona l technique. A s suppl ied , t he

machines had t r i p l e diaphragms, two mild steel on t h e o i l s i d e and one

s t a i n l e s s s t e e l on t h e gas s ide . These were assembled dry. It has

been found p re fe rab le t o f i t t he s t a i n l e s s diaphragm between the mild

s t e e l ones and t o smear them with o i l be fo re assembly. Changes have

been made t o t h e i n t e r s t a g e pressure c o n t r o l l e r , t h e o i l and water

coo le r s , and an o i l catch-pot with o i l d e t e c t o r f i t t e d t o t h e d ischarge

l i n e . A sump o i l l e v e l gauge t o d e t e c t o i l loss has a l s o been

i n s t a l l e d . Operat ional r o u t i n e s have been devised t o avoid excess ive

hammering and s t r e s s i n g of t h e diaphragms. These inc lude unloaded

s t a r t i n g , minimised running a g a i n s t low d ischarge p res su res , p ressure

relief on shutdown and the avoidance of sub-atmospheric pressures at

i n l e t . As a r e s u l t of t h e s e changes, t h e performance of t h e

compressors i s acceptab le although w e cont inue t o be i n t e r e s t e d i n

more r e c e n t developments such as the "roll-sock" machines.

Summarising, our experience with t h e Dragon helium c i r cu i t s , I

consider t h a t , a s t he r e s u l t of conserva t ive design, c a r e f u l

maintenance and monitoring and t h e s teady improvement t o components

and opera t ing r o u t i n e s , w e have now reached t h e s t a g e of a f f i rming

t h a t helium cool ing i s e n t i r e l y p rac t i cab le . I am convinced t h a t t h e

m e r i t s of helium cool ing can be r e a l i s e d without excess ive economic o r

ope ra t iona l demands.

10

The Behaviour of F i s s i o n Products ii? the Primary C i r c u i t

T h e primary c i r c u i t a c t i v i t y l e v e l s i n Dragon are low by aln?osk

any power r e a c t o r s tandards. OY course, experimental and explora tory

f u e l s a r e d e l i b e r a t e l y p ro tec t ed by t h e purge system b u t i n r e c e n t

cores the f r a c t i o n of f u e l purged i s small (Charge I1 -35% Charge 111

Core 8 -24%)).

indicat . ion of cond i t ions app l i cdb le t o power r e a c t o r design.

Nevertheless , r e l a t i v e l y small d i f f e r e n c e s i n f u e l p a r t i c l e design,

f u e l p i n geometry and opera t ing cond i t ions can a f f e c t a c t i v i t y r e l e a s e

s t rong ly - p a r t i c u l a r l y i n r e s p e c t of those i s o t o p e s of low v o l a t i l i t y

and high tendency t o p la te -out on g raph i t e o r metal sur faces . Thus i n

Dragon P r o j e c t work, p r e d i c t i o n s of a c t i v i t y l e v e l s i n power H T R ' s a r e

be ing based on two foundat ions.

i s being assessed us ing t h e o r e t i c a l models.

c a l c u l a t i o n s i s no t being taken from d i rec t measurements of Dragon

primary c i r c u i t a c t i v i t i e s . This would be too crude and l i a b l e t o

e r r o r . In s t ead , the d e t a i l e d informat ion coming from purge gas

measurements and s t u d i e s of f i s s i o n product d i s t r i b u t i o n s i n the

p a r t i c l e s , compacts and f u e l tubes of i r r a d i a t e d elements, i s used a s

t h e b a s i c data . The need f o r t h i s i s f u r t h e r r e in fo rced by the s h o r t

i r r a d i a t i o n t i m e s i n Dragon as compared with t h e power r e a c t o r .

Thus the primary c i r c u i t a c t i v i t i e s g ive a v a l i d

Release from power r e a c t o r f u e l p ins

The d a t a f o r these

The o t h e r foundat ion i s t h e use of t h e Dragon Reactor t o study t h e

behaviour of r e l eased f i s s i o n products a s they move around t h e primary

c i r c u i t . It i s t h i s a spec t which w i l l be discussed here.

It i s p o s s i b l e t o a f f i r m now t h a t gas-born a c t i v i t y per se i n a

HTR w i l l no t c o n s t i t u t e a problem. That i s t o say, d i r e c t r a d i a t i o n

from coolant duc t s and p ipes w i l l be small. Equally, l eaks from t h e

c i r c u i t should not p re sen t a hazard. There are f o u r v i t a l ques t ions

which must be answered (and upon which f u e l des ign c r i t e r i a w i l l r e s t ) .

( a ) What w i l l be t h e long-term a c t i v i t y build-up on components which

need maintenance o r i n spec t ion?

11

( b ) How much deposi ted a c t i v i t y w i l l exist i n t h e c i r c u i t which could

be r e l eased under s e r i o u s acc ident condi t ions?

(c ) What a r e t h e problems of decontamination?

( d ) HOW can f a i l i n g f u e l be b e s t loca ted be fo re s e r i o u s r e l e a s e

r e s u l t s ?

The s t a r t - p o i n t f o r t hese s t u d i e s i s t h e de te rmina t ion of a f u l l

a c t i v i t y burden s p e c i f i c a t i o n f o r t h e coolant .

For a number of yea r s , f i s s i o n product a c t i v i t y l e v e l s i n the

primary c i r c u i t w e r e measured e i t h e r by an i n s t a l l e d continuous f low

P chamber o r by the counting of d i s c r e t e gas samples on a Y-spectro-

meter. The p c6unter was c a l i b r a t e d with Xe-133 and f o r t h i s reason

gas a c t i v i t i e s a r e usua l ly expressed a s "Xe-133 equiva len ts" . There

i s no doubt t h a t t h i s method y i e l d s a reasonable e s t ima te of bulk gas

a c t i v i t y which i s v a l i d f o r e s t ima t ing r a d i a t i o n l e v e l s from duc t s ,

e t c . Typical va lues i n Dragon a r e low, e.g., 1,000 pCi/cm3 o r 0.4 C i

t o t a l c i r c u l a t i n g a c t i v i t y . y-spectrometry of {gas samples r a r e l y

showed any d e t e c t a b l e a c t i v i t y o the r than rare (gases and f o r t h i s

reason the presence of 1-131, Sr-90, Cs-137, etc., i n the gas was not

suspected. Fu r the r confirmation of t h i s view w a s suggested by the lack

of 1-131 i n b o i l e r water which had been subjected t o a f low of primary

c i r c u i t gas through leaking tubes.

Deta i led s t u d i e s of a l l b o i l e r s and blowers removed from t h e

r e a c t o r however, revealed measurable q u a n t i t i e s of Iodine , Caesium and

Strontium.

c i r c u l a t i n g i n t h e coolan t b u t t h e o r i g i n a l sampling techniques f a i l e d

t o d e t e c t them - probably because those a c t i v i t i e s w e r e deposi ted on

t h e l i n e s lead ing t o the chambers o r sample b o t t l e s . A new sampling

system known a s a "Dracule" (Fig. 5 ) has t h e r e f o r e been i n s t a l l e d

which draws a sample of primary c i r cu i t gas as c lose a s p o s s i b l e t o

the core exit.

removable tube with a con t ro l l ed temperature g rad ien t . A f t e r

removal, depos i ted a c t i v i t y can be assayed with a y-spectrometer. A

There was thus no doubt t h a t s o l i d f i s s i o n products w e r e

The gas sample i s then cooled by passing it through a 15

12

number of v a r i a t i o n s are poss ib le . The tube may be f i t t e d with f i l t e r s

o r i s o - k i n e t i c heads f o r dus t sampling and may have s p e c i a l l y prepared

ac t iva t ed su r faces , e t c . A t y p i c a l p r o f i l e f o r 1-131 i s shown i n

Fig. 6 taken from . One unexpected discovery i s t h a t Xe-133 i s q u i t e

s t rongly absorbed on the tube surface.

15

By use of t h e Dracule and t h e gas sampling systems a t o t a l gas

impuri ty s p e c i f i c a t i o n can be der ived. Short-term measurements be fo re

a shut down can then be r e l a t e d t o t h e deposi ted short- l ived

a c t i v i t i e s found on t h e su r faces of r e a c t o r components removed during

the shu t down. T h i s i s of p a r t i c u l a r i n t e r e s t f o r 1-131. T h e

assessment of accumulation of long-lived i so topes such a s Cs-137

r e q u i r e s a t i m e i n t e g r a t i o n of c i r c u i t l e v e l s and the i n s t a l l a t i o n of

new b o i l e r s and c i r c u l a t o r s with known a c t i v i t y l e v e l s w i l l provide a

s u i t a b l e base l i n e f o r t h i s purpose.

Shor t ly be fo re t h e shut down f o r t h e i n s t a l l a t i o n of new b o i l e r s ,

t h e primary c i r c u i t a c t i v i t y was unusual ly high having a t o t a l

s p e c i f i c a t i o n a s i n Table 1.

A s soon a s components w e r e removed from t h e primary c i r c u i t they

were assayed f o r depos i ted a c t i v i t y . The fo l lowing r e s u l t s f o r 1-131

t y p i c a l :

From the su r face of t he s t a i n l e s s steel nose cone of t h e b o i l e r s

which r u n s a t about 735OC, 5 t o 10 x pCi/cm2 was removed by

methanol s t r y g i 1.

From t h e su r face of b o i l e r t ubes (Fig. 7 ) - mild s t e e l a t about

205-235 C - 10-20 pCi/cm . The l e v e l s of i od ine do not appear

t o vary much with t h e angle of inc idence of t h e gas.

suppor ts t h e view t h a t t h e i o d i n e i s i n gaseous r a t h e r than

p a r t i c u l a t e form; Deposi t ion of i s o t o p e s such a s Cs-137 i s

s t rong ly dependent on t h e gas f low d i r e c t i o n .

0 2

This

From t h e blower i m p e l l e r s - a l l o y steel a t about 330°C - on t h e

t r a i l i n g faces of t h e vanes 20 pCi/cm2 and on t h e lead ing f a c e s

2 p C i / c m . 2 This d i f f e r e n c e i s probably accounted f o r by the

13

s i g n i f i c a n t l y g r e a t e r d u s t deposi t i -on on the t r a i l i n g f aces .

0 ( d ) Cn the Nimonic-75 coolan t duc t runnir,g a t about 740 C no 1-131

2 could be de tec ted . The main a c t i v i t - 1 was Cs-137 a t 0.14 p C i / c m . This value i s of i n t e re s t since it r e p r e s e n t s a s i t u a t i o n s i m i l a r

t o t h a t cf a d i r e c t cyc le gas t u r b i n e blade.

The methods used t o remove a c t i v i t y f o r assay provide some c l u e s

t o t h e problems of a c t i v i t y r e l e a s e under acc ident cond i t ions and t h e

d i f f i c u l t i e s of maintenance and decontamination.

r e s u l t s obtained with methanol and phosphoric ac id s t r y g i l s and t h e

"electroleech". I n add i t ion p l a s t i c (BEX) f i l m has been used. Perhaps

t h e most s i g n i f i c a n t f a c t i s t h a t removal of a c t i v i t y from s t a i n l e s s

steel i s d i f f i c u l t compared with mild steel presumably because the

oxide f i l m i n t o which t h e a c t i v i t y d i f f u s e d i s much more adherent.

l6 de scribe s t h e

The Chemistry of H e l i u m C i r c u i t s

The choice of an i n e r t coolan t f o r t h e HTR would a t f i r s t s i g h t

seem t o e l imina te t h e normal concern regard ing cor ros ion of t h e core

assembly. Unfortunately, ( f o r two r e a s o n s ) , chemical e f f e c t s cannot

be neglected. F i r s t l y , a t i n i t i a l commissioning and a f t e r t h e loading

of new f u e l , some absorbed gases w i l l be introduced. Secondly, i f t h e

HTR i s employed with a steam cyc le , some in leakage of steam must be

expected and, i f o i l i s used as a l u b r i c a n t i n the c i r c u i t , some

hydrocarbon contamination w i l l arise.

of des ign and opt imis ing ope ra t iona l procedures, a clear under-

s tanding of t h e chemical r e a c t i o n s of t r a c e i m p u r i t i e s i n helium i s

requi red . I n CO cooled r e a c t o r s , t h e moderate temperatures lead t o a

s i t u a t i o n i n which r a d i o l y t i c r e a c t i o n s assume precedence over those

with thermally con t ro l l ed k i n e t i c s and i n - p i l e experiments are

e s s e n t i a l t o d e f i n e t h e des ign cri teria. This s i t u a t i o n needed

i n v e s t i g a t i o n f o r t he HTR.

Thus both f o r t h e purposes

2

Dragon provides an i d e a l environment f o r conducting experiments

The normal on t h e i n j e c t i o n of i m p u r i t i e s i n t o t h e primary c i r cu i t .

impuri ty l e v e l s are extremely low a s shown i n Table 2.

14

The achievement of t h e s e l e v e l s has been aided by a number of

des ign f e a t u r e s (such a s a steam pres su re below t h a t of t he primary

c i r c u i t ) some of which are no t poss ib l e i n a commercial p l a n t where

l e v e l s would be expected t o be higher . The only s e r i o u s pe r tu rba t ion

on t h e above p i c t u r e , arises from waters ide cor ros ion on t h e b o i l e r s .

When se r ious b o i l e r cor ros ion was occurr ing , t h e primary c i r c u i t

hydrogen l e v e l s rose , a t t i m e s up t o 40 vpm, b u t it i s hoped t h a t

t h e new b o i l e r s and water t rea tment methods w i l l s t a b i l i s e t h e

hydrogen below 0.5 vpm.

A l a r g e number of experiments have been c a r r i e d out12 us ing both

" s ing le shot" and continuous i n j e c t i o n s . A t y p i c a l r e s u l t from a slow

water i n j e c t i o n i s shown i n Fig. 8. Space does not permit a d e t a i l e d

analysis here , b u t the fo l lowing genera l conclusions can be drawn.

( a ) For Water I n j e c t i o n s

(i) The r e a c t i o n s a r e s imple f irst order with r e s p e c t t o water

i n agreement with l abora to ry experiments.

A (t i) The "water s h i f t " r e a c t i o n H 0 + CO - H 0 + CO can be 2 2 2 neglected under Dragon condi t ions.

(iii) Carbon depos i t i on r e a c t i o n s can be neglected (even though

'H / H 0 >lo) because of t h e long induc t ion per iod before

metal su r f aces become c a t a l y t i c a l l y a c t i v e (-lo4 hours a t

100 vpm CO) .

C 2 2

( i v ) No oxygen pick-up i s observed i n t h e gaseous cons t i t uen t s .

( v ) Mass ba lances show t h a t only about 65% of the water reacts

quickly. The rest i s absorbed and presumably r e l eased

slowly over a long period.

( b ) For Gaseous I n j e c t i o n s (H CO, C 0 2 , CH4) 2'

(i) Oxygen pick-up i s observed. The source of oxygen i s

unconfirmed b u t assumed t o be metal oxides. This e f f e c t i s

more marked with CO than H 2-

15

Table 1. Primary C i r c u i t A c t i v i t y Concent ra t ions (end of Core 7 Charge 111)

Isotope Method of Measurement Act . ivi ty Concent ra t ion 3 g i / m

~ ~~

A l l g a s e s Ion chamber 13,500

Xe-133 Y-spec. sample 9 50

Xe-135 Y-spec. sample 4,700

Kr-8 5m Y-spec. sample 2 , 900

Kr-87 y-spec. sample 4,200

1-131 "Dracule 'I 19

CS-137

Ag-111

"Dracule"

"Dracule"

Data c o l l e c t e d n o t y e t analysed

Table 2. Typica l Primary C i r c u i t Impur i ty Levels

Impuri ty P.C. Level L i m i t of Detec t ion

VPm VPm

H2°

H2

co

CH4

2 0

~

< 0.05

0.3

0.16

<0.04

<0.02

0.05

0.05

0.08

0.04

0.02

~~

Table 3 . Core Water/Graphite React ion Rate Cons tan ts

Charge Core Power

Mw

Rc h- l

I 4 20 0.35

I1 1 20 1.02

I1 2 18 0.90

I11 4 ( a ) 15 5.3

111 4 ( a ) 20 10.3

I11 6 20 4.0

I11 7 20 2.2

16

( c ) Hydrocarbon I n j e c t i o n s

(i) Cracking proceeds r a p i d l y through decreas ing molecular weight

hydrocarbons t o hydrogen and methane. The longer term

r e a c t i o n s a r e those a s soc ia t ed with these two gases. Oxygen

pick-idp occurs.

A t t h i s s t age i n t h e development of t he HTR, t he most important

conclusion i s ( a ) (i) because it confirms t h a t out-of-pile experiments

can give v a l i d des ign data .

The r a t e cons t an t f o r t he f i r s t order r e a c t i o n with water i s of

g r e a t s ign i f i cance not only because of i t s impl i ca t ions on core

cor ros ion b u t because it i s t h e v i t a l parameter i n sa fe ty s t u d i e s on

the e f f e c t s of b o i l e r f a i l u r e s producing explosive water gas. The

r e a c t i o n r a t e r ises so sharply with temperature that a f e w small a r e a s

of high su r face temperature g raph i t e can completely outweigh t h e rest

of the core. Th i s i s shown dramat ica l ly i n Table 3.

It can be seen t h a t t h e r e has been a genera l t r end towards

i n c r e a s i n g r a t e cons t an t s b u t Charge I1 Core 4 ( a ) had an outs tandingly

h igh value. This was due t o the i n t r o d u c t i o n of some c e n t r a l l y cooled

elements which had a small a r ea of g r a p h i t e su r face a t about 1200°C.

When small a r e a s of t h e core can e x e r t such a c o n t r o l l i n g in f luence

it w i l l be apprec ia ted t h a t t h e o r e t i c a l p r e d i c t i o n s of r a t e cons tan t

a r e d i f f i c u l t b u t some success i s being achieved. The ca l cu la t ed

value f o r Charge I11 Core 4 ( a ) i s 10.5 h - l ( c f . 10.3 experimental)

b u t such agreement i s admit tedly f o r t u i t o u s .

A r e g u l a r s e r i e s of i n j e c t i o n experiments i s c a r r i e d ou t f o r

success ive co res i n Dragon. These, as can be seen, from t h e above,

y i e l d o v e r a l l core f i g u r e s d i f f i c u l t t o i n t e r p r e t e i n t e r m s of b a s i c

d a t a b u t of d i r e c t re levance i n confirming t h e continued a p p l i c a b i l i t y

of t h e r e a c t o r s a f e t y c learance . They a r e b e s t descr ibed a s

" e f f e c t i v e temperature" measurements (weighted by sur f ace a rea and an

Arrhenius temperature r e l a t i o n s h i p ) .

A new series of experiments i s about t o commence us ing special

f u e l elements i n core p o s i t i o n O/O. Arrangements have been made t o

17

r eve r se t h e normal purge flow on t h i s element, so t h a t wet helium can

be passed over t h e f u e l boxes. These boxes can then be removed f o r

d e t a i l e d examination t o determine weight loss, depth of pene t r a t ion

of cor ros ion , t h e e f f e c t of i r r a d i a t i o n on r a t e cons tan t by modifica-

t i o n of t h e g raph i t e s t r u c t u r e , etc.

Spec ia l Instruments f o r HTR's

The r a p i d impuri ty r e a c t i o n s and t h e i r s e l e c t i v e a t t a c k upon

l imi t ed core a reas requires t h e maintenance of except iona l ly low l e v e l s

of impuri ty . The f i r s t ope ra t iona l requirement i s a r e l i a b l e set of

a n a l y t i c a l ins t ruments capable of ope ra t ing down to l e v e l s a t l e a s t an

order of magnitude less than those normally of i n t e r e s t .

The instrument s e l ec t ed f o r water measurement a t Dragon was the

Goldsmith-Cox e l e c t r o l y t i c hygrometer developed €o r use a t high

pressures , This instrument i s simple and r e l i a b l e , i s capable of being

made absolu te and improves i n s e n s i t i v i t y a s t h e pressure i s increased

( i n terms of vpm l e v e l s s ince it b a s i c a l l y measures absolu te water

p re s su res and c o l l e c t s more e f f i c i e n c l y a t low gas v e l o ~ i t i e s ) ' ~ .

u l t i m a t e s e n s i t i v i t y i s determined by leakage c u r r e n t s a r i s i n g from

contaminants and imperfec t ions i n t h e phosphoric ac id /g lycer ine f i lm.

Although capable of d e t e c t i n g 0.003 vpm H 0 i n 20 a t a helium i n t h e

l abora to ry , p l a n t experience suggests t h a t 0.05 vpm i s a p r a c t i c a l

f i g u r e f o r ope ra t iona l use.

explore t h e pulsed use of t hese instruments . Higher c u r r e n t s w i l l be

obtained by us ing in t e r ruped e l e c t r o l y s i s and i t i s hoped t h a t a

f u r t h e r decade of s e n s i t i v i t y w i l l be obta ined; t h a t i s a u s e f u l l i m i t

of 0.002 vpm a t a c i r c u i t p re s su re of 60 a ta . The development of a

process-type i n d i c a t i n g and supply system should not p re sen t any

d i f f i c u l t y .

The

2

Experiments a r e a t p resent i n progress t o

The above instrument has also been success fu l ly proved a s a

hydrogen monitor by preceding t h e hygrometer with a small copper oxide

furnace a t 400°C. S e n s i t i v i t i e s a r e i d e n t i c a l with those f o r water.

18

The o t h e r irnpuni.ties of i n t e r e s t (CO, CO,], 0 CH4, PJ and - 2’ 2

hydrocarbons) are measured Gsing a d i f f e r e n t i a l concen t r a t ion gas

chromatograph . I n t h i s ins t rument i m p u r i t i e s a r e absorbed on l i q u i d

n i t rogen cooled t r a p s and then r e l e a s e d thnoujn a inolecular s i e v e t o a

katharometer. This c o n s t r u c t i o n a t 0nc.e achieves concen t r a t ion f a c t o r s

of m 0 r . e thar! 100 and elirninatee t h e l e a k s and s t agnan t pockets

a s s o c i a t e d with sampling va lves (F igs . 9 and 10). Three types of

chromatograph have been b u i l t . One uses a n i t rogen c a r r i e r f o r

de t e rn in ing hydrogen and helium i n steam c i r c u i t padding gas , t h e

second a helium c a r r i e r f o r i:se i n any helium c i r c u i t app l i ca t ion .

The t h i r d type i s a p o r t a b l e multipurpose ins t rument designed f o r

d i r e c t attachment t o t h e p l a n t where t h i s was p r e f e r a b l e to t ak ing

samples t o t h e l a b o r a t o r y ins t ruments . The minimum l e v e l s d e t e c t a b l e

wi th confidence by these machines are shown i n Tahle 4.

14

rdblt. 4. ivlinim,m Impurity Levels De tec t ab le oy D i f f e r e n t i a l Concent ra t ion Chromatograplis (with Katharometer De tec to r )

Impur i ty L i m i t of Detec t ion

A 0.02 vpm i n 20 a t a heliam

O 2

2 N

0.02 vpm i n 20 a t a he l i lm

0.04 vpm i n 20 a t a helium

0.04 vpm. i n 20 a t a helium

CO 0.08 vpm i n 20 a t a helium

CH4

0.02 vpm i n 20 a t a helium c02

Such s e n s i t i v i t y i s remarkable cons ider ing t h e use of a katharo-

meter and experiments are now s t a r t i n g wi th t h e u s e of a helium

i o n i s a t i o n d e t e c t o r . This system has been used mainly with an argon

carr ier bu t should have g r e a t e r s e n s i t i v i t y wi th h igh p u r i t y helium

a s i t e x p l o i t s t h e h igh i o n i s a t i o n and e x c i t a t i o n p o t e n t i a l s of t h e

r a r e gases. Improvements of one and p o s s i b l y two decades of

s e n s i t i v i t y are hoped f o r .

19

THE MAINTENANCE OF HELIUM CIRCUITS

It i s n a t u r a l f o r o p e r a t o r s cons ider ing t h e maintenance of an HTR

primary c i r c u i t t o be concerned by t h e absence of m e t a l l i c c ladding of

t h e f u e l and p o s s i b l e consequent a c t i v i t y hazards. Fuel loaded i n t o an

HTR w i l l b e suppl ied t o a release s p e c i f i c a t i o n which i s n o t zero a s

wi th o t h e r r e a c t o r s a t p re sen t i n use. The hard l e s s o n of exper ience

i n p r e s e n t power r e a c t o r s i s , however, t h a t even i n t h e absence of some

f a i l e d f u e l (and some f a i l e d f u e l i s t h e usua l s i t u a t i o n ) , very s ign i -

f i c a n t r a d i a t i o n hazards develop from a c t i v a t e d crud o r co r ros ion

products , The minimal co r ros ion i n t h e ETR e l imina te s t h i s problem and

one i s concerned only with t h e f u e l i n t e g r i t y . Many u t i l i t i e s a r e

r i g h t l y f i r m i n t h e i r demands t h a t b o i l e r and blower i n s p e c t i o n should

be p o s s i b l e throughout t h e l i f e of t h e s t a t i o n . T h i s requirement

r e so lves i tsel f i n t o two main a reas .

c e n t r e around 1-131 and h e r e Dragon exper ience j.s very r e l evan t . On

t h e o t h e r hand long-lived i s o t o p e s such a s Cs-137 g ive r ise t o a direct

r a d i a t i o n hazard bu t Dragon exper ience cannot be taken d i r e c t l y because

of t h e r e l a t i v e l y s h o r t h i s t o r y .

The problems of i nges t ed a c t i v i t y

Between November, 1969 and March, 1970, Dragon Lias shutdown f o r a

complete replacement of t h e b o i l e r s and steam loops, A s a test case,

very c a r e f u l measurements were taken of r a d i a t i o n l e v e l s , body burdens,

and Contamination levels around the p lan t . Shortly before shutdoLm,

t h e primary c i r c u i t a c t i v i t y l e v e l was unusual ly h igh and e s t ima tes

i n d i c a t e about 1 2 C i of 1-131 e x t e r n a l t o t h e f u e l p a r t i c l e s . A t t h e

t i m e of opening t h e c i r c u i t t h i s would have decayed t o about 1.5 C i .

A s a matter of i n t e r e s t t h i s amount of 1-131 i s s u f f i c i e n t t o produce

one maximum pe rmis s ib l e concen t r a t ion f o r i n h a l a t i o n (mpc) i n about

2 x 1 0 rn of a i r o r 2.5 x 10 containment bu i ld ing volumes. Sirice

under maintenance cond i t ions wi th open a i r - locks , one a i r chdnge occurs

i n about 10 hours i t can be seen t h a t a f r a c t i o n a l release r a t e of

about 4.5 x p e r hour of t h e " f r ee" i o d i n e would g ive r i se t o

1 mpc i n t h e containment bui ld ing . The h i g h e s t genera l a i rbo rne l e v e l

was about 1 mpc - an a s ton i sh ing ly small r e l ease .

8 3 4

20

P r i o r t o opening t h e c i r c u i t , t h e helium atmcsphere was exchanged

with dry n i t rogen t o minimise convection e f f e c t s a t t h e open f l anges

(about 80 c m i n d iameter ) .

containment during removal of t h e b o i l e r s and c i r c u l d t o r s . A f t e r

removal each f l a n g e was c losed with a simple l i g h t a l l o y cover with an

i n f 1 a t a b l e seal.

Apart from t h i s no attempt was made a t

1 7 The c i r c u l a t o r s were drawn back on r a i l t r o l l e y s and sheeted

wi th PVC a s soon as c l ea rance w a s a v a i l a b l e (Fig. 11). The r a d i a t i o n

l e v e l s on t h e impe l lo r s w e r e gene ra l ly about 200 mr/h a t 1 ft.

masks were worn a s a p recau t ion dur ing t h i s phase although n o t e s s e n t i a l .

The h i g h e s t a i r b o r n e 1-131 l e v e l i n t h e a c t u a l opening was 45 mpc.

man who worked f o r about an hour around t h e open h o l e without an a i r

mask was found subsequently t o have a thy ro id burden of 170 pCi compared wi th t h e ICRP permi t ted l e v e l of 700 pCi f o r continuous working.

A i r

One

The b o i l e r s w e r e withdrawn v e r t i c a l l y i n t o PVC s l e e v e s (Fig. 32)

and gave r a d i a t i o n l e v e l s of about 50 m r / h P/y on c o n t a c t wi th t h e

p l a s t i c shee t .

a i r masks w e r e n o t worn. I n o r d e r t o minimise r e l e a s e a t t h e openings

a s m d l l s u c t i o n w a s maintained on t h e thermocouple branches lower clown

t h e duc ts . This i s a u s e f u l technique worthy of a p p l i c a t i o n elsewhere

although i t probably i n c r e a s e s t h e oxygen contamination of t h e n i t rogen

b l anke t gas.

c i r c u l a t o r s , which l a s t e d about 4 weeks, no person was exposed t o

i n t e r n a l o r e x t e r n a l r a d i a t i o n t o a l e v e l imposing any i n d i v i d u d

r e s t r i c t i o n s .

Airborne release dur ing t h i s phase was n e g l i g i b l e and

During t h e whole process of changing 6 b o i l e r s and

Contamination c o n t r o l proved excep t iona l ly s t r a igh t fo rward . A

l o c a l c o n t r o l a r e a was set up immediately ad jacen t t o t h e primary

c i r c u i t openings and a c o v e r a l l and overshoe r c u t i n e e s t ab l i shed .

t h i s means t h e spread of contamination t o t h e main containment bu i ld ing

w a s l i m i t e d t o an e x t e n t t h a t "blue" a r e a cond i t ions appl ied

By

2 pCi/cm and no p r o t e c t i v e c l o t h i n g w a s requi red . I n t h e g r e a t

ma jo r i ty of t h e containment bu i ld ing p u b l i c access would have been

permiss ib le .

2 1

Summarising, a major maintenance opera t ion has been c a r r i e d o u t on

Dragon with primary c i r c u i t condi t ions a t l e a s t a s severe a s t hose

expected on a power HTR.

e i t h e r i n r e s p e c t of t h e mechanical ope ra t ions o r t h e con t ro l of

r ad ia t ion . The c o r e remained f u l l o f i r r a d i a t e d f u e l throughout t h i s

process and t h e experience sugges ts t h a t maintenance may w e l l be

quicker and e a s i e r than on o t h e r r e a c t o r types.

During t h e per iod fol lowing shutdown, a b o i l e r tube f a i l u r e

N o s i g n i f i c a n t d i f f i c u l t i e s were encountered,

r e l eased some water i n t o the primary c i r c u i t and t h e w a t e r l e v e l reached

5.5%. I n add i t ion t o t h i s when t h e c i r c u i t was n i t rogen blanketed

wi th open f l a n g e s , some a i r in leakage occurred and a t one s t a g e t h e

oxygen concent ra t ion reached 1%. A f t e r r ec los ing , t h e c i r c u i t w a s

evacuated and f i l l e d w i t h c l ean helium. By t h i s means, t h e "free"

contaminants w e r e reduced t o a low l e v e l which was f u r t h e r improved by

t h e ope ra t ion of t h e clean-up p l a n t s during t h e l a t t e r s t a g e s of t h e

s h u t down. However, it was expected t h a t cons iderable absorp t ion of

impuri ty on t h e co re and r e f l e c t o r g r a p h i t e would t a k e place. These

i m p u r i t i e s would no t be removable u n t i l d r iven o f f by t h e c i r c u i t heat-

i n g as t h e approach t o power took place.

seen i n Fig. 13.

rises i n impuri ty l eve l . I n o rde r t o minimise co re cor ros ion each rise i s reduced by t h e clean-up p l a n t s before t h e next s t e p i s applied.

T h i s process can be c l e a r l y

Large temperature s t e p s are accompanied by sharp

A fu r the r complicat ing f a c t o r w a s t he i n j e c t i o n of hydrogen

produced from forming t h e magnet i te f i l m on t h e new bo i l e r s .

experience has shown t h a t hydrogen d i f f u s e s r a p i d l y through t h e tubes

from i t s p o i n t of formation.

above t h e equi l ibr ium H /H 0 = 8 /1 which would be expected a t f u l l

power. This equi l ibr ium i s shown as a do t t ed l i n e i n Fig. 1 3 from

which it appears t h a t t h e hydrogen no t produced from water i s f a l l i n g

slowly, e.g., from 3.0 vpm on 28th March t o 0.8 vpm on 1st April . The

carbon monoxide l e v e l i s seen t o fo l low the hydrogen l e v e l q u i t e

c l o s e l y and again t o fo l low about an 8/1 r e l a t i o n with water.

P a s t

For t h i s reason t h e hydrogen l e v e l s t a y s

2 2

22

The most s i g n i f i c a n t demonstrat ion f r c m this process i s t h e small

uptake of w a t e r and oxygen by t h e c o r e durinq open-c i rcc i t maintenance

i n s p i t e of very high contarninan’i l eve l s .

ACKNOWLEDGMENTS

The experience descr ibed i n t h i s review i s c u l l e d from 5 years

e f f o r t s by Dragon Operat ions and P r o j e c t S t a f f t oo numerous t o mention

by name.

t h e p r i v i l e g e of r epor t ing such a convincing demonstration of t h e

v i a b i l i t y of t h e HTR concept.

I thank both them and t h e Chief Execut ive, Dragon P r o j e c t f o r

REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

9.

10 .

C. Hunt and M. D. Joyce, Opera t iona l Data f o r Charge I, D.P. Report 561, (1967).

C. Hunt and L. J. Poston, Operat ional Data f o r Charge 11, D.P. Report 614, (1969).

F. Wade, Dragon P r o j e c t Helium Leak Tes t ing , D.P. Report 2 2 5 , (1963 1 . H. J. Woodley, Development Work on S e a l s - Experimental Work c a r r i e d o u t by t h e Component Development Group t o Determine t h e Ef fec t iveness of Various Sea l ing Systems Proposed f o r U s e on t h e Draqon P r o j e c t , D.P. Report 251, (1963).

W. E. Simmons, Flanqed Joints f o r Draqon Primary C i r c u i t - Pre l iminary Report , D.P. Report 197, (1963).

H. F r i c k e r , Pre l iminary Report on D r y Bearings f o r t h e Dragon Reactor Experiment, D.P. Report 24 , (1961).

G. W. Horsley and H. F r i c k e r , The S e l e c t i o n of Greases f o r t h e Lubr ica t ion of Bearinqs i n t h e Draqon Reactor , D.P. Report 51 , (1961).

G. Coast , Some Enqineerinq Problems Associated with t h e Draqon Reactor Experiment, D.P. Report 129 (1962).

H. F r i c k e r , Bearings and Gears f o r Operation i n I n e r t Gases, D.P. Report 163, (1963).

H. F r i c k e r , Bearing T e s t s - A Summary of Work Done up t o December, 1964, D.P. Report 386, (1965).

n

23

11. J. R. Dean, Control Rod System and Mechanisms f o r Hiqh-Temperature Gas-Cooled Power Reactor with Feed and Breed Fuel , P a r t 2 , D.P. Report 457, (1967).

1 2 . M. C a r l y l e and D. V. Kinsey, Resu l t s of t h e I n j e c t i o n of Impur i t i e s i n t o t h e Dragon Reactor P r i m a r y Coolant , D.P. Report 544, (June, 1969).

13. P. S. Gray and I. Gordon, J. Sci . Instrum Vol. 44, (1967).

14. P. S. Gray, D. R. O w e n s and L. 0. Green, A D i f f e r e n t i a l Con- c e n t r a t i o n G a s Chromatograph €or t h e Analysis of Low Levels of Impur i t i e s i n H e l i u m , D.P. Report 443, (1966).

15. P. R. Rowland, M. C a r l y l e and G. C. Soubel-et , The D i s t r i b u t i o n of F i s s i o n Products i n a High Temperature G a s Cooled Reactor , D.P. ReDort 702. (1970).

16. P. R. Rowland, N e w Methods f o r t h e I n v e s t i g a t i o n of Radioisotope Transport i n a G a s Cooled Reactor , D.P. Report 639, (1969).

17. P. S. Gray, J. D. Hamilton, R. L e w i s and F:. Sage, Improved Primary C i r c u i t Maintenance Techniques - The Removal of E Primary H e a t Exchanger, D.P. Report 635, (1969).

DRAGON POWER HISTORY

- u A

1

rl I 1-1 5 1-1 2 u

1 0

UTILIZATION 52.52 (6020~ TARGET) UTILIZATION 5 0 . 0 1 ( 6 7 . 5 1 0 ~ TARGET) 0 % 3 1

+ Z I I L

E I ", %

0 , * .. ,. l m 0 m Vl Yl 0 "7

CHARGE i 4 r u ~ o r c ii * b CHARGE m t:

1 1 2 1 3 1 I I I 1 2 1 3 1 1 1 1 2 I 3 1414~ 1 5 1 6 .I 7 1 COR i! CORES CORES

1. Dragon Operating History.

25

800.

700-

MASS SPECTROMETER READINGS DURING CHARGE COR'ES .5 -7

CORE 5 -1 600-

500-

1 400- IC 5 3

300-

6004

500-

1 400- IC 5 3

300-

160 -

140 -

120.

100-

$ - I

TOTAL USAGE

// UNACCOUNTABLE /

'00°1

k C O R E 6 -L(

a 2. Inner Containment Helium Concentrations.

HELIUM USAGE

SEPT OCT MAY JUN JUL AUG

3. Helium Usage.

2000

1800

I bOC

1400

I 2 0 C

IOOC

8 oc

6 0 C - 400-

26

COMPRESSOR DIAPHRAGM LIFE

- - I 1 I I

I I I I r I 1

- 1

- HIGH PRESSURE

LOW PRESSURE ---

I I I I 200-

I N I I T

T I I I I I I I I I I I I I I I I I

I 1 I 1

I I 1 I I I I I

I T

I 1 I #

1 I

t GI

/DIAPHRAGMS STILL IN USE

DATE OF REPLACEMENT OF DIAPHRAGMS

4. Compressor Diaphragm L i f e .

,HOT D U C T

/ . - O U , T C R D U C ?

T U E R M O C O U C L E I A L L V A L V E O-RING SEAL TRANSFLR TUDE O-RING S E A L

LXTlNSlOl l ROD

4 C O N T R O L V A L V E

5. Schematic Diagram of Dracule.

6. 1-131 P r o f i l e i n Dracule.

7. Sectioning a Primary Heat Exchanger.

28

co

8. Resu l t s of a Water I n j e c t i o n Experiment.

29

KATH AROM ETER

- \ -

FLOW SAMPLgARRIER CONTROLLER - - -

Cog CONCENTRATION[ CONCEIJTRATION

STAINLESS STEEL u TRAP TRAP I

- SILICA GEL

MOL E CULA R SIEVE

LIQUID N I T R O G E N

9. Flow Sheet of D i f f e r e n t i a l Concentrat ion Gas Chromatograph.

10. Process Model Chromatograph.

11. Circulator on Removal Trolley. 12 . Primary Heat Exchanger (Boiler) a f t e r Removal.

c

I

DRAGON STARTUP IMPURITIES (CHARGE a CORE8)

too vpm

IO vpm

I vpm

- - HYDROGEN LEVEL IN EQUILIBRIUM WITH WATER

(AT FULL POWER)

0.1 vpm

- - - - - - -

W c.

13. Impurity Levels dur ing Restart of the Reactor.

32

DISCUSSION

J. Kemper: What f u t u r e p lans have been made f o r Dragon?

B. G. Chapman: A s you can see, w e have l o s t considerable t ime as a

r e s u l t of b o i l e r tube failures, b u t w e b e l i e v e t h i s problem i s now behind

us. What w e want now i s two or t h r e e years of s t eady opera t ion t o con-

c e n t r a t e on t e s t i n g power r e a c t o r f u e l p ins , on f u r t h e r chemical corro-

s i o n work, and f i s s i o n product t r a n s p o r t and depos i t i on s t u d i e s .

Bender: H e l i u m leakage dur ing e a r l y per iod of ope ra t ion -was i t a problem?

B. G. Chapman: We had no s e r i o u s problem. A l l c i r c u i t components

were s t r i n g e n t l y t e s t e d a t t h e Con t rac to r ' s works by ou r own s p e c i a l i s t

teams. The r e s i d u a l problems of t e s t i n g s i t e work are not d i f f i c u l t i f

one v i t a l p recau t ion i s observed. The helium background i n the p l a n t

a r e a must be kep t very low s o t h a t l e a k l o c a t i o n i s f a c i l i t a t e d . Be-

cause concre te has a long "memory" f o r helium, a l l gas ( e i t h e r helium o r

lO$ helium i n n i t r o g e n ) used f o r p re s su re and l e a k t e s t i n g , must be

vented a f t e r t e s t ou t s ide the p l a n t bu i ld ings .

used l o c a l e x t r a c t systems should be provided.

I f " h e l i a r c " welding i s

It i s i n t e r e s t i n g t o note t h a t dur ing a r e c e n t shutdown a t Dragon

a small q u a n t i t y of helium was r e l eased (from welding) and our conta in-

ment b u i l d i n g l e v e l s are s t i l l w e l l above normal and w i l l t ake s e v e r a l

months t o c l e a r .

A . P. Fraas : To what degree would your maintenance a c t i v i t i e s have

been handicapped i f the l e v e l of r a d i o a c t i v i t y i n t h e gas c i r c u i t had

been increased by a f a c t o r of loo?

B. G. Chapman: A c t i v i t i e s would have been hampered bu t not rendered

impossible . The a i r b o r n e a c t i v i t y would have requi red airhood p r o t e c t i o n

around open holes , b u t t h e containment bu i ld ing l e v e l s would not have

been unacceptable. More urgency would have e x i s t e d f o r f i t t i n g temporary

seals on openings. Radioact ive Contamination was fo-Trd t o be very f i rmly

adherent and c o n t r o l would be not s e r i o u s l y more d i f f i c u l t . Di rec t r a d i a -

t i o n would r e q u i r e some s m a l l amount of temporary s h i e l d i n g or f a i r l y

t i g h t working t ime l i m i t a t i o n .

33

R. Huddle: I would l i k e t o comment on W. Chapman's remark regard ing

f a i l e d f u e l .

which i s designed t o f a i l - t o o b t a i n f a i l u r e da t a . The f a i l e d f u e l

which Chapman r e f e r r e d t o was when t h e purge on one o f t h e s e elements

ceased t o be e f f e c t i v e . The important po in t i s t h a t it shows t h a t w e do

not n e c e s s a r i l y need p e r f e c t f u e l f o r H T R ' s .

We have a very l a r g e amount of experimental f u e l i n Dragon

~

Paper 2/103

QPERATIKG' -EXPERIENCE-WZ"E, VR - EXPERIMENTAL POWER STATION

H. J. Hantke, Brown Boveri/Krupp Reaktorbau GmbH G.,,Ivens , Arbeitsgemeins c h a f t Versuchsreaktor GmbH E. A. Nephew, Oak Ridge Nat iona l Laboratory

ABSTRACT

This r e p o r t desc r ibes t h e ope ra t ing performance of t h e AVR experimental power s t a t i o n dur ing i t s commissioning pro- gram as w e l l as power ope ra t ions fo l lowing customer accep- t ance of t he p l a n t i n May 1969. The p l a n t outages, which have occurred s i n c e t h e power commissioning program began on December,18, 1967, a r e enumerated and eva lua ted accord- i n g t o t h e i r na tu re and dura t ion . The o v e r a l l p l a n t a v a i l - a b i l i t y , t o March 31, 1970, has been g r a t i o i n g l y h igh and amply demonstrates the feasibility of the pebble bed reac- t o r concept.

INTRO DUCT1 ON

The AVR experimental s t a t i o n employs a h igh temperature , helium-

cooled r e a c t o r and i s t h e f i r s t power p l a n t t o use s p h e r i c a l f u e l e le-

ments, The core c o n s i s t s of a randomly packed pebble bed conta in ing

approximately 100,000 spheres , each having a diameter of 6 cm. The con-

t r a c t f o r t he cons t ruc t ion of t h i s demonstrat ion power p l a n t was s igned

on August 13, 1959 by t h e Arbei tsgemeinschaft Versuchs-Reaktor GmbH (Am) and t h e Brown Boveri/Krupp Reaktorbau GmbH (BBK).

o f t h e p l a n t began n e a r l y two years l a t e r , having been delayed by d i f f e r -

ences of opinion concerning t h e t e c h n i c a l des ign of t h e r e a c t o r founda-

t i o n and by t h e l a c k of r e g u l a t o r y l a w s i n t h e Federa l Republic of

Germany governing t h e cons t ruc t ion and ope ra t ion of nuc lea r power s ta - t i o n s . The AVR s t a t i o n i s shown i n F ig . 1.

Actual cons t ruc t ion

F i r s t c r i t i c a l i t y was achieved on August 26, 1966 and t h e f i r s t

product ion of e l e c t r i c i t y began on December 18, 1967 fol lowing a compre-

hensive program o f zero power experiments. The t o t a l e l e c t r i c a l energy

63 generated by March 31, 1970 i s 138.1 X l o6 kWh. The f u e l elements have

34

35

a t t a i n e d a n average burnup of 43.3% f i f a , corresponding t o 7.0% fima.

The p l a n t a v a i l a b i l i t y f a c t o r , t aken as t h e r a t i o of gene ra to r ope ra t ion

t i m e t o t o t a l ca lendar t ime, has been q u i t e good. The o v e r a l l p l a n t

a v a i l a b i l i t y f a c t o r f o r t h e 27 month per iod of p l a n t ope ra t ion i s 64.6%.

The p l a n t a v a i l a b i l i t y achieved f o r s e l e c t e d per iods of ope ra t ion are:

51.4% during t h e y e a r 1968,

92.0% during t h e win te r months from October 1968 t o March 1969,

71.6% dur ing t h e yea r 1969, and

95.45 during t h e win te r months from October 1969 t o March 19'70.

Because of t h e good ope ra t ing experience obtained, t h e power s t a t i o n

was turned over t o t h e AVR organ iza t ion by BBK i n May 1969. P r i o r t o t h e

acceptance of t h e p l a n t by t h e customer, a f i n a l thorough i n s p e c t i o n of

t h e va r ious p l a n t components w a s made by t h e cons t ruc to r f i rm. The i n -

s p e c t i o n and maintenance program, conducted from March 1, 1969 t o A p r i l 15,

1969, revea led t h a t t h e degree of r a d i o a c t i v e contamination of primary

c i r c u i t components was smal le r t han had been expected, f o r example, d i s -

assembly of t h e f u e l handl ing f a c i l i t y was poss ib l e without s p e c i a l

s h i e l d i n g precaut ions . The a c t i v i t y l e v e l of t he primary coolan t gas

has been c o n s i s t e n t l y low, about 300 c u r i e s , throughout r e a c t o r opera-

t i o n s t o da t e . This i n d i c a t e s good f u e l element performance wi th no

d e t e r i o r a t i o n of t h e i r f i s s i o n product r e t e n t i o n c a p a b i l i t y occurr ing

due t o fas t neutron exposure and burnup.

The operational experience acquired w i t h the AVR Experimental Sta-

t i o n over t h e p a s t two years v e r i f i e s t h e v a l i d i t y of i t s b a s i c des ign

and e s t a b l i s h e s t h e pebble bed concept as a v i a b l e a l t e r n a t i v e wi th in

t h e HTR r e a c t o r l i n e . There has been no evidence of f u e l spheres be ing

unduly damaged by t h e severe environmental condi t ions of t h e core or by

t h e mechanical l oads imposed upon them dur ing normal f u e l handl ing opera-

t i o n s . The b a s i c f e a s i b i l i t y of t h e pebble bed r e a c t o r concept has been

amply demonstrated and t h e o v e r a l l performance of t h e power s t a t i o n has

been such as t o j u s t i f y t h e cons t ruc t ion of a l a r g e r , p ro to type pebble

bed power s t a t i o n . Present p lans c a l l for t h e cons t ruc t ion of a 300 Mwe

high temperature , pebble bed power s t a t i o n t o begin l a t e t h i s year .

A

'ig .

Fig. 1.

2. View

The AVR Power Station in Julich.

of Reactor Building.

Stbra- chonnel Ibr Spent Fuei Elements

37

DESCRIPTION OF THE AVR POWER STATION

A b r i e f d e s c r i p t i o n of t h e AVR power s t a t i o n i s provided here t o

c l a r i f y t h e d i scuss ion which fo l lows .

r e a d i l y a v a i l a b l e i n t h e open l i t e r a t u r e .

of t h e r e a c t o r bu i ld ing . The core i s f i l l e d wi th approximately 100,000

s p h e r i c a l f u e l , g raph i t e and boron elements. The AVR f u e l element con-

s i s t s of an unfueled g r a p h i t e s h e l l f i l l e d w i t h coated uranium-thorium

d ica rb ide p a r t i c l e s which a r e uniformly d ispersed i n an inne r g r a p h i t e

mat r ix .

of thorium. The core i s doubly contained i n two s t e e l p re s su re v e s s e l s ,

with pure helium maintained a t a s l i g h t overpressure i n t h e volume

between t h e two vessels t o prevent p o s s i b l e outward leakage of t h e

contaminated coolant gas .

More d e t a i l e d information i s

F igure 2 shows a s e c t i o n view

Each f u e l element con ta ins 1 g of 92% enr iched uranium and 5 g

The coolan t gas e n t e r s t h e core through r a d i a l s l o t s i n t h e bottom

r e f l e c t o r and streams upward through t h e core . Two helium blowers c i r -

c u l a t e t h e coolan t gas through a carbon b r i c k b r idge , l o c a t e d above t h e

su r face of t h e pebble bed, t o t h e steam genera tor . The carbon b r i c k

b r idge se rves t o s h i e l d t h e steam genera tor from f a s t neutrons. T h i s i s

important because t h e steam genera tor i s not r ep laceab le due t o t h e

i n t e g r a t e d cons t ruc t ion which has been used. The steam genera tor i s a

forced-flow Benson system wi th four p a r a l l e l s e c t i o n s , each having i t s own r e g u l a t o r y and c o n t r o l devices .

The f u e l handl ing system i s comprised of gas l o c k s , mechanical

swi tches , and t r a n s p o r t t ubes through which t h e f u e l spheres are charged

i n t o the core by means of helium p res su re . During r e a c t o r ope ra t ion , t h e

f u e l spheres are recyc led cont inuously from t h e bottom of t h e core back

i n t o t h e t o p i n order t o provide f u e l mixing and a t t a i n uniform burnup

and a f l a t power d i s t r i b u t i o n .

f o r c e of g r a v i t y and t h e pneumatic energy r equ i r ed t o e l e v a t e t h e spheres

i s provided by t h e p re s su re d i f f e r e n t i a l genera ted by t h e main helium

blowers. Thus, t h e f u e l handl ing system i s simple and rugged and posses-

s e s a minimum number of components wi th moving mechanical p a r t s .

Spheres a r e removed from t h e core by t h e

38

AVR PLANT OPERATING EXPERIENCE

Power Program

A summary of AVR power g e n w a t i o n achievements s i n c e December 1967

i s presented i n Tables L &nd 2. A s of March 31, 1970, a t o t a l o f 12,912

hours of genera tor a i l - l ine t i m e had been achieved and t h e t o t a l e l e c t r i -

cal- energy produced amounted to 138,075 Mwh. The f u e l elements have a t - t a ined a n average hurnup of 43.328 f i f a , corresponding t o 7.08 f i m a . A t

t h e p re sen t time, t h e core and f u e l handl ing c i r c u i t s con ta in 47,729 f u e l

spheres . This number i s g radua l ly inc reas ing as f r e s h f u e l elements a r e

loaded and dummy g raph i t e spheres a r e discharged. The maximum power l e v e l

reached thus fa r i s 50 Mwth, or 15.6 Mwe. This was achieved by inc reas -

i n g t h e helium blower speed from 4000 t o 4400 rpm and r a i s i n g t h e coolan t

gas p re s su re from 1 0 t o 11 a t m . The i n s u f f i c i e n t shutdown worth of t h e

rods had p rev ious ly prevented ope ra t ion a t t h e nominal des ign l e v e l ,

15 Mwe.

The power h i s t o r y of t h e AVR s t a t i o n i s depic ted i n Fig. 3 . During

t h i s 27 month per iod, some 60 i n t e r r u p t i o n s i n power gene ra t ion occurred

due t o r e a c t o r scrams and scheduled p l a n t shutdowns. I n Fig. 3 , t h e

scheduled p l a n t shutdowns are i d e n t i f i e d by t h e c a p i t a l l e t t e r s A through

Q, and t h e unscheduled shutdowns i n i t i a t e d by t h e automatic s a f e t y c i r - c u i t s are i d e n t i f i e d by numbers. I n t h e case of a n a c t u a l commercial

power s t a t i o n , t h e scheduled p l a n t shutdowns would have allowed s u f f i -

c i e n t t i m e t o b r i n g r e se rve s t a t i o n s on l i n e whereas t h e automatic scrams

would have r e s u l t e d i n a temporary r educ t ion i n t h e t o t a l power a v a i l -

a b l e t o t h e g r i d . This d i s t i n c t i o n , however, i s not app l i cab le t o a n

experimental power s t a t i o n , p a r t i c u l a r l y one such as t h e AVR undergoing

i t s commissioning t e s t s . Here, it se rves merely t o d i f f e r e n t i a t e between

outages occurr ing unexpectedly dur ing r o u t i n e r e a c t o r ope ra t ion and those

which were c l e a r l y fo re seeab le .

Scheduled P l a n t Shutdowns.--The scheduled p l a n t shutdowns shown i n

Fig. 3 are grouped according t o t h e i r causes and l o c a t i o n i n t h e p l a n t

system and l i s t e d i n Table 3 . Por t ions of t h e t o t a l outage per iods a re

somewhat a r b i t r a r i l y ass igned t o s p e c i f i c components and systems even

c

Table 1. Summary of AVR Operations During the Year 1968

C

Jan Feb Mar APT h Y June J u l y Aug Sept Oct Nov D e C

Calendar hours Generator hours Cumulative generator hours Core thermal operation, h

Gross e l e c t r i c a l generation,

Plant requirements, Mwh N e t e l e c t r i c a l production, Mwh Thermal energy produced, Mwh

Cum. thermal energy (s ince

cum. gross e l e c t r i c a l (since

Cum. e l e c t r i c a l t o power gr id ,

Max. e l e c t r i c a l power l eve l ,

Mwh

9-19-6'7) (Mwh)

12-1'7-6'7) (Mwh)

Mwh

Mw

Max. thermal power l eve l , Mw Total charged f u e l elements

Fifa, $ Fima, $ Plant e l e c t r i c a l a v a i l a b i l i t y

f ac to r , $

744 349 646 350

1820 726 1093 7972

853

3086

2352

5.9

23.2 23139

3.87 0.63

47.04

69 6 744 720 744 720 744 744 720 744 720 744 285 725 374 - 259 427 180 - 514 680 708 931 1656 2030 2030 2289 2715 2895 2895 3409 4089 4797 293 741 374 - 321 454 193 - 550 702 722

2841 7269 4087 - 2336 4061 1586 - 4682 6307 6558 771 1023 549 - 383 648 254 - 840 1120 1185 20'70 6246 3538 - 1954 3413 1332 - 3842 5187 5373 9299 23376 13056 - 9195 139'73 5907 - 17434 22524 23477

0 W

1241 2215 2759 2759 3142 3724 3970 3970 4697 5635 6613

5965 13197 17284 17284 19620 23681 25267 25267 29949 36257 42815

4792 10951 14380 14380 16381 19771 21087 21087 24885 29994 35277

10.1 10.4 14.2 - 9.8 10.0 9.8 - 9.5 9.4 9.3

32.4 33.9 45.4 - 33.2 33.8 31.9 - 34.1 32.9 33.0 25589 25589 25589 25589 25736 25736 26310 26429 27150 27745 28150

5.48 9.1 11.32 11.32 12.83 15.21 15.86 15.79 18.19 21.35 24.70 0.89 1.4'7 1.83 1.83 2.08 2.46 2.57 2.56 2.95 3.46 4.00

40.95 97.45 51.94 - 35.97 57.39 24.19 - 69.09 94.44 95.16

Table 2. Summary of AVR Operations During the Year 1969-70

Jan 69 Feb Mar Apr May June July Aug Sept Oct Nov Ikc Jan 70 Feb Mar

Calendar hours Generator hours Cumulative genera tor hours Core thermal opera t ion , h

Gross e l e c t r i c a l generation,

P lan t requirements, Mwh Net e l e c t r i c a l production, Mwh Thermal energy produced, Mwh

Cum. thermal energy ( s ince

cum. gross e l e c t r i c a l ( s ince

Cum. e l e c t r i c a l t o power g r id ,

Max. e l e c t r i c a l power l e v e l ,

Mwh

9-19-67) (Mwh)

12-17-67) (Mwh)

Mwh

Mw

Max. thermal power l e v e l , Mw Tota l charged f u e l elements

F i f a , $ Fima, $ Plan t e l e c t r i c a l a v a i l a b i l i t y

f a c t o r , $

744 672 744 720 744 720 744 744 720 744 720 744 744 672 744 687.5 629 - 220 564 626 710 60.5 595 738 720 727 602 671 565

5485 611C 6114 6334 6898 7524 8234 8294 8889 9627 10347 1107~ 11676 12347 1.2912 705.5 650 19 309 585.5 645 7ll 208 630 743.5 720 743.5 611 672 571

6694 6567 - 2596 6855 7590 8319 746 7098 8388 8293 10122 7997 ‘7539 6456 1125 1039 - 352 913 986 1117 78 975 1212 1192 1320 1008 1133 965 5569 5528 - 2244 5942 6604 7202 667 6123 7176 7101 8802 6989 6406 5491

22963 21867 219 9101 22075 24634 2’7340 3179 24201 28053 26389 32’781 25879 24L26 21620 rp 0

7570 8481 8490 8869 9789 10816 11955 12087 13096 14265 15364 16730 1’7808 le826 19726

49509 56077 56077 58672 65527 73117 81437 82182 89281 97668 105962 116083 124080 131619 138075

40693 46058 46058 48246 54038 60475 67490 68120 7coao 81057 87967 96554 1 0 3 3 ~ 4 1 0 9 5 ~ 7 114867

11 .8 12.84 - 12.65 12.90 13.17 13.17 12.9 12 .7 12.43 12.38 15 .6 13.28 11.13 11.43

38.6 40.6 11 .4 39.2 39.73 41.L7 41.44 41.08 40.78 40.60 38.54 50.76 42.36 36.35 37.86 32597 34095 34155 34855 35895 36917 38015 38065 39115 40925 42615 44415 45435 45955 47875

2C.42 26.15 26.13 26.76 28.67 30.80 33.06 33.38 35.20 36.64 37.90 39.60 41.21 43.07 63.32 3.96 4.24 4.23 4.34 4.64 4.99 5.36 5.41 5.70 5.94 6 .14 6.42 6.68 6.98 7.02

92.40 93.60 - 30.56 75.80 86.94 95.43 8.13 82.63 99.19 100 97.71 80.91 99.85 75.34

Q

41

though t h e maintenance a c t i v i t i e s were c l o s e l y i n t e r r e l a t e d and usua l ly

performed concurrent ly . While t h e apportionment of t h e outage per iods

t o p a r t i c u l a r components i s admi t ted ly imprecise and depends upon an

a r b i t r a r y e x e r c i s e of personal judgment, a q u a l i t a t i v e l y v a l i d p i c t u r e

of t h e l o c a t i o n and r e l a t i v e importance of t h e va r ious malfunct ions

emerges. I n a s ses s ing t h i s information, it i s important t o note t h a t

85% of t h e t o t a l p l a n t downtime occurred dur ing t h e ttshakedown" phase of

p l a n t run- in opera t ions p r i o r t o customer acceptance of t h e experimental

power s t a t i o n . A f u l l d e s c r i p t i o n of t h e na tu re of each scheduled shut -

down i s provided i n t h e Appendix.

cs

Unscheduled P l a n t Shutdowns. --The unscheduled shutdowns caused by

the a c t i o n of automatic s a f e t y c i r c u i t s dur ing r e a c t o r ope ra t ion a r e

numbered and sho'wn i n Fig. 3. The lo s s of components such as diaphragm

compressors, main feedwater pump, e t c . , which a r e necessary for continued

r e a c t o r ope ra t ion are a l s o included i n t h i s category. The 43 i n c i d e n t s

of t h i s na tu re which have occurred a r e grouped i n Table 4 according t o

t h e i r causes and l o c a t i o n i n t h e p l a n t system. The most f requent and

annoying cause o f power outages i s diaphragm rup tu res o f t h e l a r g e ,

water-dr iven, helium compressors. Although t h e p l a n t des ign provides f o r

a s tandby r e se rve u n i t , t h e compressors a r e un fo r tuna te ly loca t ed i n a

r a d i a t i o n area s o t h a t maintenance cannot be c a r r i e d ou t while t h e reac-

t o r i s a t power. This s i t u a t i o n w a s g r e a t l y a l l e v i a t e d by the i n s t a l l a t i o n of a p i s -

t o n type Su lze r compressor i n May 1969. T h i s compressor, l oca t ed i n an

a r e a which i s se rv iceab le dur ing r e a c t o r opera t ion , has exh ib i t ed super-

i o r ope ra t ing performance. A t o t a l of 3500 hours of ope ra t ion was a t -

t a i n e d be fo re t h e f irst maintenance and r e p a i r were requi red . The Su lze r

compressor has now been i n ope ra t ion f o r a t o t a l o f 7800 s e r v i c e hours,

wi th only 100 hours o f downtime. It i s expected t h a t t he cu r ren t main-

tenance i n t e r v a l o f 3000 hours can be cons iderably lengthened as ope ra t -

i n g experience i s acquired. The i n t r o d u c t i o n of t h i s machine t o t h e AVR

gas p u r i f i c a t i o n p l a n t system has l a r g e l y e l imina ted one of t h e most

f requent causes of p l a n t outages.

~ ~~~

42

Table 3. Breakdown of Scheduled AVR Plant Shutdorms fran Dec. 18, 1967 to March 31, 1970

A B C D E F G H I J K L M N O P Q

60 130

Outage Period Totals

Fuel Handling Facility: &rap Separator Severalizer Singulizer Selector heel Dosierrad Counter coils

Gas Purification Plant: RA-4 tube rupture Diaphragm compressors RA-4 installation Water ingress RA-4 Oil ingress to RA-4

Secondary Circuit: Turbine balance Feedwater plmp Pressure regulator Turbine inspection

Other : Blower lubrication

oy~mics experiments Inspection, maintenance Rod drive motor Cmnissloning Fuel loading Fuel recycling operator error

system

Generator Off-the 01)

420 1509

14 5 145

530

72 27

1 10

79

110

72 370

450 111

72 740 1537 302 1401 1 1245 10 72 27 79 15 177 611 2 139 178

3681

6608

100 370 577

120 1190 k: 48 1 15 77

100

Table 4. Breakdown of Unscheduled AVR Plant Shutdowns (Dec. 1967 to April 1970)

Cause of Power Outage Generator Designation of Malfunction (Nature of Malfunction) Off-time (h)

Fuel Handling Facility:

Difficulties with the fuel hand- ling facility were either repaired on-line or resulted in a scheduled shutdown listed in Table 3.

Gas Purification Facility:

3,6,12,16,17,21,25 and 33

1,27 2 18,30,36,37

7,8,14,26,34,35 19,20,23 28

4,5,10,15,29,32,41,43 11.24.39 9,iz . 22,38 13,40

Diaphragm compressor failure

Secondary Circuit:

Loss of main feedwater pump Oil circuit for turbine Live steam valve malfunctions

Electrical Circuits:

Circuit breaker overload Grid voltage fluctuations Defective signal amplifier (in L)

Others:

Operator error Scrams of unknown or external origin Loss of blowers Brush sparking of turboalternator Defective rod drive system

255 h 17 m

27 h 10 m 13 m

109 h 46 m 137 h 09 m

82 h 49 m 11 h 59 m

94 h 48 m 0

9 h 4 5 m 17 h 22 m

54 m 3 h 3 3 m 56h 3 m 87 h 37 m

43

13 I4 6 16 17 1Bx9 20 21 /

19 68 J

32 333435 36 37 383940 41 44.3 2223 2% 25 26 27 28 2930

19 69 1970 2 Fig. 3. AVR Power Generat ion Record.

1

- 0 40 80 I20 - 160 - 200- 240- 280 320- 360 TIME (DAYS)

Fig. 4. AVR Performance Diagram for 1968/69.

44

AVR P lan t A v a i l a b i l i t y

The a v a i l a b i l i t y of t h e AVR power s t a t i o n , c a l c u l a t e d as t h e r a t i o

of genera tor on-line t ime t o t o t a l ca lendar t ime, has been except iona l

f o r an experimental power s t a t i o n undergoing i t s commissioning tes t s .

can be seen i n Fig. 4, t h e p l a n t performance has improved s t e a d i l y s i n c e

power ope ra t ion began i n l a t e 1967. a b i l i t y w a s 51.4%, i n s p i t e of extended p l a n t shutdowns f o r f u e l loading

and maintenance of t h e gas p u r i f i c a t i o n and f u e l handl ing f a c i l i t i e s .

The a v a i l a b i l i t y f a c t o r f o r t h e year 1969 w a s 71.6%. months from October 1968 t o March 1969, t h e o v e r a l l p l a n t a v a i l a b i l i t y

w a s 92%, inc reas ing t o 95.4% f o r t h e same t ime per iod i n 1969-70. t a k e s i n t o account a l l power ope ra t ions t o d a t e , inc luding both t h e com-

missioning and run-in phases of operation prior to customer acceptance of

t h e p l a n t , t h e o v e r a l l a v a i l a b i l i t y amounts t o 64.6%.

As

I n 1968, t h e o v e r a l l p l a n t avail-

During t h e win ter

If one

AVR Reactor Core

Fuel Behavior.--Although t h e average f u e l burnup has reached 7% fima,

t h e f i s s i o n product r e t e n t i o n of t h e f u e l has remained l a r g e l y unchanged

and good.

of t h e coolan t gas has remained e s s e n t i a l l y cons tan t a t 300 c u r i e s .

a c t i v i t y a r i s e s mainly from uncoated uranium p resen t - i n t h e f u e l elements.

The R / B r a t i o f o r 1 3 3 X e i s one t e n t h of t h e s p e c i f i e d va lue of 5 x lo?

A p o r t i o n of t h e f i r s t charge f u e l has a l r eady experienced a burnup of 1 2 %

f i m a , cons iderably more than t h e f u e l s p e c i f i c a t i o n va lue of 9% f i m a .

Never the less , it i s planned t o r e t a i n t h e s e h igh ly burned f u e l elements i n

t h e core f o r some t i m e longer and t o d e l i b e r a t e l y recharge them t o t h e

h ighes t f l u x zone during r ecyc l ing ope ra t ions . The purpose of t h i s a c t i o n ,

which w i l l probably l e a d t o a p a r t i a l d e s t r u c t i o n of t h e f u e l p a r t i c l e

coa t ings , i s t o demonstrate t h e o p e r a b i l i t y of a pebble bed r e a c t o r even

when some of t h e f u e l elements become d e f e c t i v e .

Throughout t h e two yea r s of power ope ra t ion , t h e a c t i v i t y l e v e l

This

Primary C i r c u i t Impurities.--The concent ra t ions of gaseous impuri-

t i e s i n t h e primary c i r c u i t , a r i s i n g from t h e outgass ing of core s t r u c -

t u r a l material dur ing power ope ra t ion , have dec l ined s t e a d i l y due t o t h e

c l eans ing a c t i o n of t h e gas p u r i f i c a t i o n p l a n t .

t h e s e chemical contaminants a r e : CO - 30 vpm, H2 - 9 vpm, CH4 - 2 vpm,

The present l e v e l s of

45

C 0 2 - 5 vpm, and H 2 0 - 0.1 vpm. The presence of t hese i m p u r i t i e s i n t h e

primary c i r c u i t has not r e s u l t e d i n any apprec iab le co r ros ive damage t o

t h e f u e l elements.

Refuel ing Program. --Fuel loading and r ecyc l ing i s c a r r i e d out dur ing

power ope ra t ion a t a ra te s e t mainly by core r e a c t i v i t y cons idera t ions .

The German l i c e n s i n g a u t h o r i t i e s r e q u i r e t h a t t h e core must no t be loaded

t o a r e a c t i v i t y which exceeds t h e worth of t h e f o u r shutdown rods l e s s

1/2 Ni le , as measured wi th r e s p e c t t o t h e r e fe rence shutdown temperature

of 105°C. The complete AVR f u e l element c i r c u i t p r e s e n t l y conta ins

107,381 f u e l , boron, and g r a p h i t e spheres . O f t hese , 13,200 spheres a r e

l o c a t e d i n t h e sphere d ischarge tube a t the base of t h e core . Table 5

l i s t s t h e p re sen t composition of t h e r e a c t o r core and a s s o c i a t e d f u e l

c i r c u i t s . O f t h e 47,729 f u e l spheres i n the AVR c i r c u i t , some 30,000 are t h e f irst charge U. S. -manufactured, machined f u e l spheres , 4390 are t h e

"wallpaper" type, and t h e res t a r e spheres manufactured by new molding

techniques developed by t h e German f i rm N u k e m .

Table 5. AVR Core Composition, March 31, 1970

Complete Fuel Discharge C i r c u i t Tub e Sphere Type Core

Fuel 47729 42840 4889

Graphi te 57354 49363 7991

Boron 2298 1978 3 20

T o t a l 107381 94181 13200 .-

The f u e l loading ra te i s chosen 'on t h e b a s i s of t h e o r e t i c a l p red ic-

t i o n s obtained from two-dimensional, mult iple-zone burnup c a l c u l a t i o n s

which inco rpora t e t h e known load ing and r e c y c l i n g program as we l l as t h e

flow behavior of t h e spheres w i t h i n t h e pebble bed. Deviations from t h e I pred ic t ed behavior are manifested i n t h e form of changing gas e x i t t e m -

pe ra tu re s from t h e core. When t h i s occurs , t h e r e f u e l i n g program i s a l -

t e r e d t o r e - a t t a i n t h e des i r ed ope ra t ing condi t ions . However, a time

AVR 70.13-1

Fig. 5. Sphere Flow Characteristics in the AVR Pebble Bed Core.

AVR 70.13-2

I BBC/KRUPP

A

Fig. 6. Example f o r a Representation of AVR Core in 2-D Erebus Burnup Calculation.

47

delay of 4 t o 6 weeks e x i s t s be fo re t h e a l t e r e d r e f u e l i n g and r ecyc l ing

program t a k e s e f f e c t , There i s t h e r e f o r e a h igh i n c e n t i v e t o o b t a i n

q u a n t i t a t i v e accuracy i n t h e burnup c a l c u l a t i o n s so t h a t t h e opt imal f u e l

loading program can be s e l e c t e d .

Burnup c a l c u l a t i o n s f o r a pebble bed r e a c t o r are complicated by t h e

f a c t t h a t t h e zonal nuc l ide concent ra t ions i n the core va ry bo th wi th

f u e l burnup and f u e l movement. Figure 5 shows t h e flow p a t t e r n of spheres

as they t r a v e r s e t h e core .

sphere v e l o c i t y depends s t r o n g l y upon i t s r a d i a l p o s i t i o n i n t h e core .

The power d i s t r i b u t i o n c a l c u l a t i o n s must adequate ly account f o r t h e con-

t i nuous ly changing core conf igu ra t ion as w e l l as t h e long term f i s s i o n

product accumulation. Figure 6 shows how t h e movement of f u e l spheres

through t h e core i s s imulated i n t h e EREBUS burnup program by mul t ip l e

zone core c a l c u l a t i o n s a t d i s c r e t e t i m e s t e p s .

The sur face p r o f i l e l i n e s c l e a r l y show t h a t t h e

Core Physics Measurements

Rod Drop Experiment.--Several rod drop experiments have been per-

formed t o determine t h e r e a c t i v i t y worth of t h e f o u r rod bank. The fou r

rods are scrammed during r e a c t o r power ope ra t ion and t h e r e s u l t i n g f l u x -

t ime t r a c e i s recorded. An a c o u s t i c a l record of t h e no i ses generated by

t h e rack and p in ion gears of each rod i s used t o determine t h e depth of

rod i n s e r t i o n as a func t ion of t ime. The f lux- t ime t r a c e i s analysed by an inve r se k i n e t i c s program developed by M r . K. J. K a l k e r o f the KFA

Nuclear Research Center a t J ' a i c h . Although t h e t h e o r e t i c a l b a s i s of t h e

experiment i s not f u l l y understood, t h e experimental results ag ree w e l l

wi th estimates o f t h e t o t a l rod worth obta ined by o t h e r methods. The

rod drop experiment has been adopted as a s tandard means f o r p e r i o d i c a l l y

i n v e s t i g a t i n g changes i n rod worth due t o f u e l burnup and a l t e r e d core

conf igura t ion .

Simulated Rod Fai lure . - -The marginal shutdown worth of t h e AVR rod

bank makes it important t o know t h e consequences of a poss ib l e s i n g l e

rod fa i lure . For t h i s reason, experiments have been performed t o measure

t h e nuc lear behavior of t h e ho t core when he ld s u b c r i t i c a l by only t h r e e

of t h e f o u r rods. I n t h e s e experiments, t h e r e a c t o r core was allowed t o

48

cool by n a t u r a l convect ion f o r a per iod s u f f i c i e n t l y long t o e s t a b l i s h

t h e cool ing down ra te . The blowers were then used t o f o r c e f u l l y cool

t h e core u n t i l it aga in reached c r i t i c a l i t y by v i r t u e of t h e negat ive

temperature c o e f f i c i e n t of r e a c t i v i t y . This was done t o expedi te t h e

cool ing process which would otherwise r e q u i r e s e v e r a l weeks. It was

found t h a t t h e r e a c t o r power l e v e l s ou t t o about 30 kw, a f t e r undergoing

o s c i l l a t i o n s t o 50 kw, fo l lowing i t s r e t u r n t o c r i t i c a l . The experiment

demonstrates t h a t a t l ea s t s e v e r a l weeks would be a v a i l a b l e t o provide

a d d i t i o n a l shutdown r e a c t i v i t y i f one rod f a i l s t o opera te during a

scram a t f u l l r e a c t o r power.

Temperature Coef f i c i en t . --The temperature c o e f f i c i e n t of r e a c t i v i t y

has been determined by means o f per iod measurements of a given core con-

f i g u r a t i o n made a t two d i f f e r e n t temperatures . A t t h e r e fe rence shutdown

temperature of 105"C, t h e temperature c o e f f i c i e n t of r e a c t i v i t y was found

t o be -18.9 mN/"C.

10% due t o e r r o r s i n measuring t h e temperature , i s t h e average of t h r e e

s e p a r a t e measurements and ag rees f a i r l y w e l l wi th ca l cu la t ed r e s u l t s .

The experimental procedure employed was t o e s t a b l i s h a s l i g h t l y super-

c r i t i c a l rod p o s i t i o n a t a core temperature of 103°C and t o then measure

t h e r e a c t o r per iods of t h i s core conf igu ra t ion f o r core temperatures of

103"C, llO"C, and 114°C. The core temperature was va r i ed by a d j u s t i n g

t h e temperature of t h e feedwater t o t h e steam genera tor .

This . va lue , conta in ing a n es t imated unce r t a in ty of

T r i t i u m Measurements i n t h e AVR Reactor.--The AVR coolant gas con-

t a i n s ve ry l i t t l e tritium dur ing normal power opera t ion , on t h e o r d e r of

about 0 .5 c u r i e s .

r e a c t i o n wi th Li-6, which i s p resen t as a n impur i ty i n the g raph i t e and

carbon b r i c k s t r u c t u r a l r a t e r i a l , and d i r e c t l y as a f i s s i o n product.

T r i t i u m has a l s o been de tec t ed i n t h e AVR secondary c i r c u i t i n concen-

t r a t i o n s of about 2 . 5 microcur ies p e r l i t e r o f feedwater, corresponding

t o a n o v e r a l l a c t i v i t y of about 0.14 c u r i e s i n t h e steam c i r c u i t , The

o r i g i n of t h e tritium found i n t h e secondary c i r c u i t i s not y e t c l e a r .

It i s suspected t h a t a t l eas t a p o r t i o n of t h e secondary c i r c u i t tritium

i s produced by neutron a c t i v a t i o n of nitrogen-14, a n i so tope present i n

The primary sources of tritium i n t h e AVR a r e t h e ( n , a )

t h e feedwater from t h e ammonia and hydrazine used t o prepare and process

t h e feedwater.

49

The main po r t ion of the t r i t i i m found i n the secondary c i r c u i t ,

however, must a r i s e e i t h e r from ano the r more product ive ac t iva tLon proc-

ess or frorn f i i f fus ion of t h e primary c i r c u i t tritium through t h e s t ee l

tu-bing o f t h e steam genei-ator. Seve ra l e x p r i m e n t s have been performed

t o a s c e r t a i n which of t h e s e r ep resen t s t h e primary mechanism. These ex-

pe r inen t s a r e based on t h e de te rmina t ion of t h e ra te of tritium bui ldup

i n t h e steam c i r c u i t . I f t h e tritium i n t h e feedwater a r i s e s from neu-

t r o n a c t i v a t i o n of some impuri ty , t h e time requi red f o r i t s bui ldup t o

equ i l ib r ium concent ra t ion should be r e l a t i v e l y long. If, on t h e o t h e r

hand, t h e tritium d i f f u s e s from t h e primary c i r c u i t i n t o the secondary

c i r c u i t , i t s concent ra t ion i n t h e steam c i r c u i t should b u i l d up i n a manner similar t o t h a t i n t h e helium c i r c u i t .

During a r e a c t o r shutdown per iod , i n which one would expect n e i t h e r

a c t i v a t i o n or d i f f u s i o n t o occur , t h e e n t i r e feedwater was rep laced wi th

f r e s h water conta in ing no tritium. Following t h e r e a c t o r s t a r t u p , t h e

tritium concent ra t ions i n t h e helium and steam c i r c u i t s were c a r e f u l l y

monitored. It was found t h a t tritium b u i l t up i n bo th c i r c u i t s propor-

t i o n a l l y , i n d i c a t i n g t h a t a d i f f u s i o n process wi th a de lay t i m e of only

a few minutes w a s t ak ing p lace . However, i n another experiment, some

5 c u r i e s of tritium toge the r wi th s e v e r a l hundred l i t e r s of hydrogen were

i n j e c t e d i n t o t h e primary c i r c u i t . The tritium and hydrogen concentra-

t i o n s i n t h e steam c i r c u i t were then monitored. As tonish ingly , no i n -

c rease i n t h e i r l e v e l s could be de t ec t ed .

The reason f o r t h e nega t ive result of t h e tritium i n j e c t i o n expe r i -

ment i s not c l e a r l y understood a t t h e p re sen t t i m e . I t i s t h e r e f o r e

planned t o r epea t t h i s experiment, under more c a r e f u l l y c o n t r o l l e d condi-

t i o n s and using more s e n s i t i v e measuring procedures, sometime i n t h e near

f u t u r e . It i s p r e s e n t l y be l i eved t h a t on ly d i r e c t d i f f u s i o n of tritium

through t h e steam gene ra to r t ub ing can account f o r t h e concent ra t ion ob-

served i n t h e secondary c i r c u i t . The y i e l d s of o t h e r poss ib l e r e a c t i o n s

a r e considered t o be too s m a l l t o s u f f i c i e n t l y e x p l a i n t h e e f f e c t .

50

Amnionia Injection Experiment.--In laboratory tests, ammonia when

admixed to the helium coolant gas has been shown to act as a gaseous

lubricant, reducing the coefficient of friction between graphite spheres

in a pebble bed reactor. In anticipation of possible future requirements

in large pebble bed reactors, ammonia was injected into the AVR primary

circuit in an experiment conducted on January 23, 1969. The purpose of

the experiment was to determine the stability or decomposition rate of

NH3 in a realistic reactor environment of radiation and temperature. decomposition rate of the ammonia in the primary circuit was determined

by observing the growth of the decomposition products, hydrogen and nitro-

gen. The concentrations of these impurities were measured by gas chroma-

tography at regular intervals following the injection of the ammonia.

The decomposition half-life for ammonia was measured to be from 8 to 12 minutes in the AVR core environment.

The

Dynamics Experiments.--Reactor operating parameters such as the

feedwater flow rate, the coolant gas blower speed, and the position of

the shutdown rods have been varied stepwise to determine the dynamic

response characteristics of the plant. Further tests have been conducted

to determine the consequences of the loss of sensitive components such

as the main feedwater pump and the helium circulators. In the case of

loss of connection to the electrical grid, it was demonstrated that the

generator can supply the plants own needs without difficulty. In all the

simulated incidents, the automatic safety circuits operated according to

plan and functioned reliably to prevent any unsafe reactor condition from

arising .

SUMMARY AND CONCLUSIONS

The successful operation of the AVR experimental power station over

the past 27 months verifies the validity of its basic design and estab-

lishes the pebble bed concept as a viable alternative within the HTGR

reactor line.

tion, which includes the power commissioning program, has been excep-

tionally good for an experimental power station.

and significant is the fact that the plant availability record has

The availability of the plant during this period of opera-

Even more gratifying

improved s t e a d i l y as f a m i l i a r i t y and experience wi th complicated p l a n t

systems, such as t h e gas p u r i f i c a t i o n p l a n t and t h e f u e l handl ing f a c i l -

i t y , were acqui red .

down phase of power opera t ions have e l imina ted many weak s p o t s and g r e a t l y

improved t h e o v e r a l l r e l i a b i l i t y of t h e power s t a t i o n .

Improvements and r e v i s i o n s made dur ing t h e shake-

I n our opinion, t h e promising p o t e n t i a l i t y of pebble bed r e a c t o r s

and t h e good performance of t h e AVR power s t a t i o n j u s t i f i e s t he planned

cons t ruc t ion of a 300 h e pro to type power s t a t i o n based on the pebble

bed concept. The prospec ts f o r f u l l y automating t h e f u e l element pro-

duc t ion process , using sphere molding techniques developed by N u k e m could

g r e a t l y reduce f u e l element manufacturing cos t s , a key i t e m i n a s s e s s i n g

t h e economic p o t e n t i a l of pebble bed r e a c t o r s . It should be kep t i n mind

t h a t a l though good performance has been obtained i n a c c e l e r a t e d capsule

t e s t s , adequate f u e l element behavior a t t h e requi red burnup and f a s t

neut ron exposure must be demonstrated i n r e a c t o r experiments. The ex-

c e l l e n t s a f e t y c h a r a c t e r i s t i c s , good f u e l cycle economy and h igh e f f i -

c iency of pebble bed power s t a t i o n s combine t o make t h e prospec ts q u i t e

promising t h a t t h i s type of power s t a t i o n w i l l emerge as a s e r i o u s com-

p e t i t o r i n t h e f u t u r e energy market.

52

APPENDIX

This appendix provides a d e t a i l e d d e s c r i p t i o n of t h e scheduled p l a n t

shutdowns depic ted i n Fig. 3 , i nc lud ing t h e da t e , na tu re and dura t ion of

t he va r ious outage per iods .

Scheduled P l a n t Shutdowns

A. December 19 t o 22, 1967 - A p re sc r ibed i n s p e c t i o n of t h e t u r b i n e

bear ings was performed as p a r t of t h e commissioning program. P l a n t

down t i m e , 72 hours.

B. January 15 t o February 15, 1968 - This shutdown period served as p a r t

o f t h e commissioning program f o r the f u e l handl ing f a c i l i t y . During

t h e shutdown period, some 2450 a d d i t i o n a l f u e l elements and 6370

g raph i t e elements were loaded i n t o t h e core thereby inc reas ing t h e

core r e a c t i v i t y from 8 .68 t o 10.61 Ni l e s . A s i s t h e case f o r a l l

prolonged p l a n t shutdowns, t h e outage per iod w a s a l s o u t i l i z e d t o

c a r r y o u t r e p a i r s , r ev i s ions , and improvements on va r ious components.

P l an t down time, 740 hours.

A p r i l 16 t o June 19, 1968 - The p l a n t was shut down f o r a comprehen-

s i v e i n s p e c t i o n and maintenance of p l a n t components and t o concur-

r e n t l y c a r r y out a major r e c y c l i n g of t h e in-core f u e l elements under

cold r e a c t o r condi t ions . During t h e r e a c t o r shutdown, acceptance

t e s t s were performed on secondary c i r c u i t components and experiments

t o s imula te t h e loss of e l e c t r i c a l l oad and l o s s of t h e main feed-

water pump were conducted. The dynamic response of t h e p l a n t t o

t h e s e s imulated i n c i d e n t s w a s determined and the automatic s a f e t y

system was found t o perform according t o design.

C.

The sphere r ecyc l ing program was delayed by d i f f i c u l t i e s which

a r o s e wi th t h e f u e l handl ing f a c i l i t y . Mechanical components such as

t h e s e v e r a l i z e r , s i n g u l i z e r , and sphere locks l o c a t e d i n t h e f u e l

t r a n s p o r t tubes jammed f r equen t ly because of t h e higher c o e f f i c i e n t

of f r i c t i o n i n a d r y helium atmosphere.

reso lved and by June 12, 1968 some 6120 spheres had been recyc led ,

r e s u l t i n g i n a r e a c t i v i t y loss of 560 mN as t h e c e n t r a l boron zone

was pu l l ed deeper i n t o t h e core . Malfunctioning of t h e f u e l handl ing

The problems were g radua l ly

53

f a c i l i t y became less f r equen t as f a m i l i a r i t y and experience wi th

t h e system were acquired.

By June 14, 1968, thermal power ope ra t ion could be resumed and

f i v e days l a t e r t h e coolant gas impur i t i e s dropped t o a l e v e l per-

m i t t i n g h igher gas o u t l e t temperatures from the core. On June 19,

1968, steam condi t ions a l lowing t u r b i n e ope ra t ion were a t t a i n e d and

e l e c t r i c a l power product ion resumed. P lan t down time, 1537 hours.

J u l y 15 t o J u l y 27, 1968 - The r e a c t o r was shu t down i n o rde r t o i n -

s p e c t t h e low temperature s t ages of t h e W-4 gas p u r i f i c a t i o n p l a n t .

Four days e a r l i e r l a r g e helium l o s s e s had been noted a f t e r t hese

u n i t s had been charged wi th helium a t ope ra t ing pressure . The i n -

s p e c t i o n revea led t h a t t h e helium tub ing i n t h e low temperature u n i t s

had ruptured because it had been mounted too r i g i d l y t o accommodate

t h e f o r c e s of con t r ac t ion . The low temperature s e c t i o n s of t h e gas

p u r i f i c a t i o n p l a n t were dismantled and removed f o r r e p a i r ou t s ide t h e

r e a c t o r b u i l d i n g and t h e p l a n t resumed ope ra t ion a t 8 Mwe. The

forced shutdown period was u t i l i z e d t o i n s t a l l a d d i t i o n a l thermo-

couples on t h e o u t e r p re s su re v e s s e l l i d and t o perform acceptance

t e s t s on t h e f u e l burnup and i d e n t i f i c a t i o n device. Other small re-

p a i r s and improvements, such as t h e i n s t a l l a t i o n of microphones

a long t h e t r a n s p o r t tubes of t h e f u e l handl ing f a c i l i t y t o provide

a c o u s t i c monitoring, were e f f e c t e d . P l an t down t i m e , 302 hours. E. August 9 t o October 6, 1968 - This p l a n t shutdown, of n e a r l y two

D.

months d w a t i o n , was i n i t i a t e d by a diaphragm rup tu re i n t h e helium

compressor of t he gas p u r i f i c a t i o n p l a n t . The d e c i s i o n w a s t aken

t o extend t h e outage per iod f o r two weeks t o i n s t a l l t h e RA-4 gas

p u r i f i c a t i o n u n i t s which had been r epa i r ed i n t h e meantime. I n

p a r a l l e l wi th t h e M-4 i n s t a l l a t i o n , core dry ing ope ra t ions were

undertaken t o remove moisture from t h e primary c i r c u i t which had

been admit ted as a result of t h e diaphragm rup tu re of t h e water d r iven compressor MK-2. This was done by c i r c u l a t i n g t h e primary

gas through a B a O f i l t e r f o r f ive days and subsequent ly ope ra t ing

t h e r e a c t o r a t a low thermal power l e v e l of 1 M w .

t h e core dry ing opera t ions and t h e RA-4 i n s t a l l a t i o n work we, r e com-

p l e t ed , b u t resumption of power ope ra t ion was delayed by a new series

By August 25, 1968,

F.

G.

54

of d i f f i c u l t i e s wi th t h e f u e l handl ing f a c i l i t y . n

The r ecyc l ing of i n -co re spheres , which had cont,nued d r i n g t h e

i n i t i a l phase of t h e shutdown per iod , was h a l t e d on August 17, 1968

by weak e l e c t r o n i c s i g n a l s f r o n t h e sphere d e t e c t i o n c o i l s l oca t ed

along t h e pneumatic t r a n s p o r t tubes . These c o i l s d e t e c t t h e passage

of a sphere through t h e t r a n s p o r t tubes and provide con t ro l s i g n a l s

t o t h e Z u s e 32 computer used t o d i r e c t and record f u e l handl ing op-

e r a t i o n s . I n v e s t i g a t i o n revea led t h a t t h e weakened s i g n a l s a r o s e

from t h e a l t e r e d phys ica l p r o p e r t i e s of g raph i t e caused by neutron

i r r a d i a t i o n . The a m p l i f i c a t i o n of t h e s e s i g n a l s was improved and t h e

sphere r ecyc l ing program resumed on September 14, 1968.

FYequent mechanical d i f f i c u l t i e s wi th components of t h e f u e l

handl ing f a c i l i t y , however, s eve re ly hindered t h e r ecyc l ing ope ra t ions .

The p o r t s e l e c t o r wheel f a i l e d t o p o s i t i o n proper ly t o t h e t r a n s p o r t

tube p o r t s and on September 19, 1968, t h e Dosierrad jammed completely.

The removal and disassembly of t h e s e u n i t s i s t ime consuming because

they a re loca ted i n s i d e t h e barr ier gas envelope surrounding t h e p r i -

mary system.

wheel and a s t ronge r motor-clutch d r i v e u n i t was provided f o r t h e

Dosierrad. Thermal power ope ra t ion of t h e AVR s t a t i o n began aga in

on October 4 , 1968 and e l e c t r i c a l genera t ion resumed on October 6,

1968. P l a n t down time, 1401 hours .

October 24, 1968 - This scheduled shutdown w a s made t o in spec t poss i -

b le damage t o t h e steam t u r b i n e dur ing a t o o r ap id res tar t i n c i d e n t

which occurred s i x days e a r l i e r . The i n s p e c t i o n revea led only minor

damage t o t h e t u r b i n e and r e p a i r was de fe r r ed t o t h e next shutdown

per iod scheduled f o r March 1, 1969. P l a n t down time, 1 hour and 6

minutes.

February 28 t o A p r i l 18, 1969 - This major p l a n t shutdown f o r r e v i -

s i o n and maintenance was scheduled t o provide a f i n a l thorough i n -

spec t ion of t h e p l a n t f a c i l i t i e s p r i o r t o customer acceptance of t h e

experimental nuc lea r s t a t i o n . The i n s p e c t i o n and maintenance a c t i v i -

t i e s were c a r e f u l l y organized i n advance t o minimize t h e l eng th of

t h e outage per iod . The shutdown period was ushered i n a day prema-

t u r e l y by a d e f e c t i v e s i g n a l a m p l i f i e r i n the s a f e t y c i r c u i t s which

New end-switches were i n s t a l l e d on t h e p o r t s e l e c t o r

55

t r i g g e r e d a r e a c t o r scram. During t h e ensuing outage per iod , t h e

d e f e c t i v e a m p l i f i e r w a s r epa i r ed and a h o s t of o the r maintenance and

r e v i s i o n a c t i v i t i e s were c a r r i e d out on the steam t u r b i n e , t h e f u e l

handl ing system and numerous components of t h e primary and secondary

c i r c u i t s .

Thermal power ope ra t ion of t h e p l a n t resumed on A p r i l 18, 1969

a f t e r t h e major r e v i s i o n and i n s p e c t i o n program was completed on

schedule .

while t h e gas p u r i f i c a t i o n system cleansed t h e primary c i r c u i t o f gas-

eous impur i t i e s admit ted during t h e r e p a i r and i n s p e c t i o n a c t i v i t i e s .

A f t e r t h r e e days, t h e impur i ty concent ra t ions dec l ined t o a l e v e l

where h igher coolant gas temperatures could be accepted. The r e a c t o r

power l e v e l w a s t hen r a i s e d t o 20 Mwth g iv ing a h o t gas temperature

o f 600°C and pe rmi t t i ng t h e t r a n s i t i o n t o steam product ion and turbo-

a l t e r n a t o r opera t ion . P l a n t down t i m e , 1245 hours.

I n i t i a l l y , t h e thermal power l e v e l w a s r e s t r i c t e d t o 10 Mw

H. A p r i l 21, 1969 - Following t h e r e p a i r of t h e steam t u r b i n e , t h e ro-

t a t i o n of t h e t u r b i n e s h a f t was c a r e f u l l y checked and found t o be

f ree of imbalances. On A p r i l 21, 1969, t h e t u r b i n e was shu t down

f o r a s h o r t t i m e so t h a t t h e in s t rumen ta t ion f o r checking i t s b a l -

ance could be removed. Following t h i s , t h e r e a c t o r power l e v e l was

g radua l ly r a i s e d t o 39 Mwth and ope ra t ion o f t h e e l e c t r i c a l genera-

t o r resumed l a t e t h e same day. P l a n t down time, 10 hours .

May 10 t o May 13, 1969 - The r e a c t o r was shu t down t o r e p a i r t he main

coolant water pump which had been damaged by i n s u f f i c i e n t bea r ing

l u b r i c a t i o n . P l a n t down t i m e , 72 hours .

May 1 5 t o May 16, 1969 - This shutdown w a s made t o r ep lace a defec-

t i v e steam pressure r e g u l a t o r f o r t h e t u r b i n e . P l an t down t i m e ,

26 hours and 31 minutes.

I.

J.

K. June 9 t o June 13, 1969 - This shutdown was r equ i r ed t o r e p a i r t h e

p o r t s e l e c t o r wheel o f t h e f u e l handl ing f a c i l i t y .

79 hours.

June 18, 1969 - Power ope ra t ion was i n t e r r u p t e d t o conduct a rod drop

experiment t o measure t h e r e a c t i v i t y worth of t h e four shutdown rods.

P l a n t down time, 1 5 hours and 8 minutes.

P l a n t down time,

L.

56

M. J u l y 30 t o August 6, 1969 - This p l a n t shutdown was made t o c a r r y

out dynamics experiments and t o provide t i m e f o r needed maintenance

work. The dynamics experiments cons is ted of a rod drop experiment

t o measure t h e worth of t h e f o u r shutdown rods. Following t h i s , one

of t h e rods w a s f u l l y withdrawn from t h e core t o s imula te a "jammed

rod" i n c i d e n t and t o a s c e r t a i n how long t h e r e a c t o r would remain sub-

c r i t i c a l i f on ly t h r e e rods were a v i l a b l e f o r i n s e r t i o n during f u l l

power opera t ion . This information, t oge the r wi th knowledge of t h e

power l e v e l t o which t h e r e a c t o r w i l l s t a b i l i z e a f t e r r e t u r n t o c r i t i -

ca l , i s important i n a s s e s s i n g t h e se r iousness of an a c t u a l acc iden t

of t h i s type. It was concluded t h a t a t l eas t s e v e r a l weeks would be

a v a i l a b l e t o provide supplementary shutdown r e a c t i v i t y i n t h e event

of a n a c t u a l s i n g l e rod fa i lure . P l a n t down t i m e , 177 hours.

N. August 9 t o September 3, 1969 - A gas flow blockage developed i n t h e

low temperature s e c t i o n s of t h e RA-4a and RA-4b p u r i f i c a t i o n s t ages

on August ET, 1969 due t o f rozen moisture . The u n i t s were taken out

of ope ra t ion when t h e p re s su re drop ac ross them reached t h r e e atmos-

pheres and r e a c t o r power ope ra t ion continued f o r a s h o r t time wi th-

ou t t h e s e s t ages of t h e p u r i f i c a t i o n f a c i l i t y . The r i s i n g concen-

t r a t i o n of water vapor i n t h e primary c i r c u i t , however, made it nec-

e s s a r y t o s h u t down t h e r e a c t o r on August 9, 1969. The source of

t he water i s be l ieved t o have been from t h e b a r r i e r gas system which

enc loses t h e primary r e a c t o r p re s su re v e s s e l . Shor t ly be fo re t h e

flow blockage developed, a po r t ion of t h i s gas had been routed t o

t h e RA-4 u n i t f o r p u r i f i c a t i o n .

The gas i n t a k e l i n e s t o t h e RA-4 u n i t were d r i e d by a u x i l l i a r y

hea t ing and on August 19, 1969 t h e r e a c t o r was brought t o a power

l e v e l of Mwth t o f a c i l i t a t e dry ing of t h e primary c i r c u i t . On

August 26, 1969, dur ing t h e removal of t h e auxi l l ia ry hea t ing equip-

ment, t r a c e s of hydrau l i c o i l were found on some gas va lves . The

r e a c t o r was s h u t down and t h e source of t h e o i l w a s found t o be rup-

tu red diaphragms i n compressors MK-6a/b.

carbon t e t r a c h l o r i d e and a n i t rogen drying s t ream began a t once, This

work was completed on September 3, 1969 and e l e c t r i c a l power genera-

t i o n resumed a t t h a t t i m e . P l a n t down t i m e , 611 hours.

Cleansing opera t ions wi th

57

0 . January 19, 1970 - A rod drop experiment was performed t o measure t h e

r e a c t i v i t y worth of t h e fou r shutdown rods. P l a n t down t i m e , 2 hours

and 23 minutes.

P. January 20 t o January 25 , 1970 - The p l a n t was shu t down f o r r o u t i n e

maintenance work, no tab ly t h e replacement of e l e c t r i c a l c o i l s on t h e

magnetic valves of gas c i r c u i t s i n s i d e t h e containment v e s s e l . I n

add i t ion , t h e s u b c r i t i c a l i t y of t h e r e a c t o r , wi th a l l f o u r shutdown

rods i n s e r t e d and t h e core a t a temperature of 105OC, was measured.

The s c r a p s e p a r a t o r u n i t of t h e f u e l handl ing f a c i l i t y w a s inspec ted

and found t o be not func t ion ing properly. It was then disassembled

and 350 undamaged spheres were found t o have accumulated i n t h e hopper

arrangement loca t ed beneath t h e r o t a t i n g drum of t h e u n i t .

The spheres were removed from t h e hopper b p means of a provisory

s u c t i o n device , The malfunct ion w a s found t o have a r i s e n from t h e

increased c o e f f i c i e n t of f r i c t i o n between t h e g r a p h i t e spheres and

s t e e l caused by t h e d r y helium atmosphere and excess ive a x i a l p l ay

i n t h e r o t a t i n g drum. When it became apparent t h a t ex tens ive r e -

p a i r s were requi red , i nc lud ing t h e cons t ruc t ion and i n s t a l l a t i o n of

a completely new and improved r o t a t i n g drum u n i t , t h e r e a c t o r was

brought back t o power t o avoid a prolonged shutdown per iod .

work on t h e sc rap s e p a r a t o r u n i t proceeded concurren t ly wi th r e a c t o r

power opera t ion . P l a n t down t i m e , 139 hours and 20 minutes. March 6 t o March 13, 1970 - The r e a c t o r power g radua l ly dec l ined from

14.5 Mwe i n January t o 9 Mwe i n e a r l y March because of f u e l burnup.

Since t h e f u e l handl ing f a c i l i t y w a s ou t o f commission, it was i m -

poss ib l e t o c a r r y ou t normal f u e l loading and r ecyc l ing and on March 6, 1970 t h e power s t a t i o n had t o be s h u t down.

Repair

Q.

The r e p a i r s t o t h e s c r a p s e p a r a t o r were completed and completely

s a t i s f a c t o r y ope ra t ion of t h e f u e l handl ing f a c i l i t y has s i n c e been

obtained. The outage pe r iod was prolonged by t h e n e c e s s i t y of pump-

i n g t h e r e a c t o r coolant gas inventory i n t o s t o r a g e so t h a t a d e f e c t i v e

va lve i s o l a t i n g t h e s c r a p s e p a r a t o r from t h e primary c i r c u i t could be

rep laced . P l an t down t ime, 177 hours and 48 minutes.

58

DISCUSSION

R. C . Ihh lberg : P lease comment on t h e type of opera tor e r r o r s which

cause unscheduled shutdowns.

H. Knt fer : F a i l u r e s are not only caused by opera tors t h a t use s h i f t

people, b u t about 2/3 by maintenance people and 1/3 by s h i f t people.

Example by maintenance people: a n e l e c t r i c i a n drew t h e e l e c t r o n i c card

f o r t h e main blowers i n s t e a d of a n a u x i l i a r y blower. Example by s h i f t

people: a n ope ra to r opened a wrong va lve f o r dep res su r i s ing a small room

of t h e blocking-gas-system. This caused a pressure drop f o r t h e helium

pressure c o n t r o l system and dropped a rod. Some poss ib l e ways t o mini-

mize human f a i l u r e s ( t o a b o l i s h them i s imposs ib le ) : 1) Minimizing

maintenance work t h a t could in f luence opera t ion , 2 ) i n s t a l l a t i o n o f more

warning s i g n a l s , and 3 ) i n t r o d u c t i o n of more check lists.

John Kemper: W y t h e sea rch f o r t r i t i u m ? I thought t h a t HTGR's were

e s p e c i a l l y f o r t u n a t e i n t h i s r e s p e c t .

G. Ivens : Y e s , you are q u i t e c o r r e c t i n say ing t h a t tritium produc-

t i o n i s e s p e c i a l l y low i n HTGR's . The tritium presen t i n t h e primary

c i r c u i t can be e a s i l y handled by t h e gas p u r i f i c a t i o n p l a n t . O u r i n t e r -

e s t was t o f i n d out t he o r i g i n of t h e s m a l l amount of tritium which has

been de tec t ed i n t h e steam c i r c u i t .

P. Cohen: I n f u t u r e experiments on tritium it might be u s e f u l t o

add tritium as HTO.

E. Hoinkis : What i s t h e iod ine concent ra t ion i n t h e primary coolan t

c i r c u i t ?

G. Ivens : There i s no d e t e c t a b l e l e v e l of i od ine i n t h e AVR primary

coolan t gas. Furthermore, no iod ine could be de t ec t ed e i t h e r on g raph i t e

spheres which are removed from t h e core or on s t e e l su r faces of components

i n d i r e c t contac t w i th t h e coolant gas. Unfortunately, t h e most in te res t - i n g su r faces , which are i n con tac t wi th t h e h o t primary gas, a r e not ac-

c e s s i b l e f o r i n spec t ion .

P. S. Weltevreden: Comparing t h e impur i ty l e v e l s of AVR an?. Dragon

it appears t h a t t h e AVR l e v e l a r e o r d e r s of magnitude h ighe r t han those of

Dragon.

59

G. Ivens: The gaseous impurities in the AVR primary circuit were

initially high because of the presence of more than 1QCJ trjns of carbon

brick structural material. This ungraphitized carbonaceous material

emits large quantities of E20 and C 0 2 when heated,

the ear ly AVR operations was restricted because of the presence of these

oxidizing impurities. In the meantime, outgassing has proceeded to the

point that the AVR power level is no longell limited by these impurities.

It should be noted, however, that in a pebble bed reactor corrosion is

distributed evenly amongst the fuel elements because they are in contin-

uous motion.

The povzr level of

Paper 3/138

OBJECTIVES AND PLANS FOR FEXESJTN?

IN THE PEACH- B-OTTOM HTGR* --"-w.-4 ___ --..

K . P . Steward

Gulf General Atomic Incorporated

San Diego, C a l i f o r n i a

ABSTRACT

The Peach Bottom r e a c t o r o f f e r s unique c a p a b i l i t i e s as a t es t f a c i l i t y f o r HTGR f u e l s . It i s t h e only r e a c t o r i n t h e United States t h a t can i r r a d i a t e HTGR f u e l i n a proper neutron spectrum, i n a pressur ized helium environ- ment, and i n an HTGR temperature regime. I n a d d i t i o n , t h e Peach Bottom r e a c t o r can accommodate kilogram q u a n t i t i e s of t es t f u e l s f o r long p e r i o d s , and complete f u e l element assemblies can be t e s t e d t o e v a l u a t e t h e i n t e r a c t i o n s of f u e l p a r t i c l e s , f u e l beds, and g r a p h i t e s t r u c t u r e s . One of t h e fundamental o b j e c t i v e s of Peach Bottom Core 2 , t h e r e f o r e , i s t o e x p l o i t t h e unique c a p a b i l i t i e s of t h i s r e a c t o r as a test f a c i l i t y f o r i r r a d i a t i o n of HTGR f u e l concepts .

Gulf General Atomic p lans i n i t i a l l y t o des ign and f a b r i c a t e f o u r types of tes t elements f o r i n s e r t i o n wi th Core 2 . Up t o 40 tes t elements may be i r r a d i a t e d w i t h i n t h e c o r e a t any one t i m e . The major objec- t ives of each element type a r e as fol lows:

1. Fuel T e s t Elements (FTEs). To e v a l u a t e t h e performance of HTGR f u e l s over a wide range of o p e r a t i n g condi t ions

2 . Recycle Fuel Test Elements (RTEs). To provide i r r a d i a t e d HTGR f u e l i n s u f f i c i e n t q u a n t i t i e s t o develop r e c y c l e processing technology, and t o tes t r e c y c l e p a r t i c l e s

3 . Fuel Bed Test Elements (FBTEs). To provide l a rge - scale tes ts of 1100-MW(e) HTGR f u e l

4 . Proof Test Elements (PTEs). To demonstrate i r r a d i a t i o n performance of F o r t S t . Vrain f u e l

*Work supported i n p a r t by t h e U.S. Atomic Energy Commission under Contracts AT(04-3)-633 and AT(04-3)-167, P r o j e c t Agreement No. 1 7 .

60

61

Emphasis i s being placed on t h e f i r s t t h r e e element types s i n c e t h e s e have t h e g r e a t e s t re levance t o 1100-MW(e) HTGR technology. I n p a r t i c u l a r , t h e FTEs w i l l be used (1) t o determine t h e time-at-temperature l i f e t i m e s of v a r i o u s types of f u e l , (2) t o o b t a i n d a t a t o i d e n t i f y mechanisms of p a r t i c l e f a i l u r e and thereby t o improve f u e l performance and l i f e t i m e s , (3) t o t es t l o o s e p a r t i c l e g r a p h i t e i n t e r - a c t i v e f o r c e s i n blended beds, and (4) t o conduct c o n t r o l l e d i r r a d i a t i o n experiments on f u e l p a r t i c l e thermal s t a b i l i t y , meta l l ic d i f f u s i o n rates, f i s s i o n product release, and g r a p h i t e mechanical p r o p e r t i e s .

Experiments w i l l be conducted as func t ions of temperature and f l u e n c e up t o 4.2 x 1021 n/cm2; burnups w i l l b e Q20% FIMA i n t h e mixed f i s s i l e and Q60% FIMA i n t h e UC2 f i s s i l e p a r t i c l e s a t end of l i f e . s t a b i l i t y samples w i l l b e i r r a d i a t e d a t temperatures from 1000° t o 16OOOC i n thermal g r a d i e n t s up t o 1000"C/in. i n order t o determine time-temperature l i m i t s f o r coated p a r t i c l e s under i r r a d i a t i o n . Extensive p o s t i r r a d i a t i o n examination of t h e FTEs w i l l be performed. One of t h e FTEs t o b e i r r a d i a t e d f o r 3 y r i s being sponsored by t h e USAEC.

Some thermal

Object ives of t h e FBTEs, i n a d d i t i o n t o la rge-sca le t e s t i n g of 1100-MW(e) f u e l , are (1) t o eva lua te v a r i o u s 1100-MW(e) bonded bed concepts , and ( 2 ) t o determine t h e dimensional changes of blended beds of r e f e r e n c e 1100-MW(e) f u e l by p e r i o d i c gamma scanning. A l l of t h e s e elements w i l l be i r r a d i a t e d f o r 3 y r and w i l l r e c e i v e a dose of approximately 4 x 1 O 2 I n/cm2 dur ing t h i s t i m e . Most of t h e FBTEs w i l l c o n t a i n only one f u e l combination and may be monitored c o n t i n u a l l y during i r r a d i a t i o n by sampling t h e i n d i v i d u a l purge g a s streams.

The RTEs are sponsored by t h e USAEC as an i n t e g r a l p a r t of t h e Nat iona l HTGR Recycle Program. They inc lude both r e f e r e n c e and advanced concepts f o r 1100-MW(e) r e c y c l e f u e l , p r i m a r i l y as bonded r o d s , a l though some blended beds w i l l a l s o be included, tests of oxide f u e l produced by t h e sol-gel process and w i l l c o n s t i t u t e proof tests of t h e r e c y c l e 4:2(Th,U)02 B I S O p a r t i c l e s . I n a d d i t i o n , 2:O(Th,U)02 B I S O p a r t i c l e s w i l l b e i r r a d i a t e d , a t t a i n i n g t h e burnup a f t e r 3 y r i n t h e Peach Bottom r e a c t o r t h a t t h e o t h e r p a r t i c l e s would receive a f t e r 4 y r i n t h e 1100-MW(e) HTGR. The RTEs w i l l provide i r r a d i a t e d material f o r head end reprocess ing s t u d i e s a t ORNL a f t e r 1-, 2-, and 3-yr i r r a d i a t i o n s i n Core 2.

These elements w i l l provide la rge-sca le

62

1. I N T R O D U C T I O N

The Peach Bottom r e a c t o r , operated by Phi lade lphia E l e c t r i c Company,

o f f e r s unique c a p a b i l i t i e s as a tes t f a c i l i t y f o r high-temperature gas-

cooled r e a c t o r (HTGR) f u e l s . It i s t h e only r e a c t o r i n t h e United States

t h a t can i r r a d i a t e HTGR f u e l i n a r e p r e s e n t a t i v e HTGR neutron spectrum

and helium coolant environment. I n a d d i t i o n , t h e Peach Bottom r e a c t o r

can accommodate kilogram q u a n t i t i e s of tes t f u e l s f o r long i r r a d i a t i o n

p e r i o d s , and f u l l - s c a l e f u e l element assemblies can be t e s t e d t o e v a l u a t e

t h e i n t e r a c t i o n s of f u e l p a r t i c l e s , f u e l beds , and g r a p h i t e s t r u c t u r e s . One

of t h e fundamental o b j e c t i v e s of Peach Bottom Core 2 ( t h e second co re t o

b e loaded i n t o Peach Bottom), t h e r e f o r e , i s t o u t i l i z e t h e unique capabi-

l i t i e s of t h i s r e a c t o r as a tes t f a c i l i t y f o r i r r a d i a t i o n of HTGR f u e l

concepts . Table 1 compares l i g h t water r e a c t o r and HTGR environments

and i n d i c a t e s t h e s u i t a b i l i t y of t h e Peach Bottom r e a c t o r f o r i r r a d i a t i n g

HTGR f u e l s .

Gulf General Atomic (GGA) has designed and i s f a b r i c a t i n g fou r types

of t es t elements f o r i n s e r t i o n wi th Core 2. Up t o 40 t es t elements may be

i r r a d i a t e d w i t h i n t h e c o r e a t any one t i m e . The element t ypes are l i s t e d

below, and t h e ch ief o b j e c t i v e s of each are given i n Table 2 .

Fuel test elements (FTEs)

Recycle f u e l tes t elements (RTES)

Fuel bed tes t elements (FBTEs)

Proof t e s t elements (PTES)

The f i r s t t h r e e element types have b a s i c a l l y t h e same des ign , w i t h

t h e f u e l being contained i n c y l i n d r i c a l g r a p h i t e bodies w i t h i n r e g u l a r

Peach Bottom f u e l element sleeves. I n c o n t r a s t , t h e PTE des ign s imula tes

a s e c t i o n of a large-HTGR f u e l block and i s hexagonal i n shape. Emphasis

i s being placed on t h e FTEs, R T E s , and FBTEs, s i n c e t h e s e elements have

g r e a t e r f l e x i b i l i t y f o r t e s t i n g f u e l under s p e c i a l l y c o n t r o l l e d condi t ions .

These elements are sponsored both p r i v a t e l y by GGA and by t h e Atomic

63

Table 1. Comparison of Reactor Operating Conditions

H20 Test Reactor (ETR) Peach Bottom HTGR 1100-MW(e) HTGR

Environment

Neutron spectrum

Thermal flux (n/cmz-sec)

Fast flux (n/cm2-sec)

Core Th:U ratio

Peak fuel temp. ("C)

Thermal gradient

Burnup rate

Low-pressure static helium

Light-water reactor (Lm)

14 2x10

14 4x10

2 : l

>1300

High

Hieh

High-pressure flowing helium

HTGR

4xio13

4~10'~

8:l

>1300

Moderate

Moderate

High-pressure flowing helium

HTGR

13 4x10

13 4x10

4:l

1340

Moderat e

Moderat e I

Table 2. Chief Objectives of Various Test Elements

Fuel test elements (FTES)

1. Evaluate the performance of HTGR fuels over a wide range of operating conditions and lifetimes

a. Determine the time-at-temperature lifetimes of various types of fuel

b. Obtain data to identify mechanisms of particle deteriora- tion under extreme conditions and thereby to improve fuel performance and lifetimes

C. Evaluate various production processes

2. Conduct controlled irradiation experiments on:

a. Fuel particle thermal stability

b. Metallic diffusion rates in an HTGR environment

c. Fission product release of various types of fuel particles

d. Graphite mechanical and creep properties

e. Various fuel rod fabrication concepts

and bonded rods under HTGR conditions 3. Evaluate the long-term, in-pile performance of blended beds

1. Demonstrate performance of recycle type fuel and beds unaer actual RTLK operating conaitions

2 . Produce significant quantities of irradiated APGR fuel for reprocessing scudies at UKNL, and subsequently to:

a. Demonstrate separability of particles

b. Prove out reprocessing equipment

1. Demonstrate performance of large-HTGR fuel beds i b. Periodic gamma scanning for dimensional changes

under actual reactor operating conditions

fuel combinations for:

a. Periodic monitoring for fission gas release

Recycle fuel test elements (RTEs)

Fuel bed tes 2. Provide bonded and blended bed elements containing single elements (FBTEsJ

Proof test elements (PTEs)

1. Demonstrate irradiation performance of reference Fort St. Vrain fuel

64

Energy Commission. I n a d d i t i o n , t e s t i n g of PTE-2, which has received 150

equiva len t full-power days (EFPD) of i r r a d i a t i o n i n Core 1, w i l l cont inue

i n Core 2 . Another proof t es t element (PTE-3) i s scheduled f o r i n s e r t i o n

a f t e r 300 EFPD. Both of t h e s e PTEs a re designed t o f u l f i l l t h e proof test

requirements f o r F o r t . S t . Vrain (FSV) f u e l , and are being funded under

t h e P u b l i c Serv ice of Colorado (PSC) development c o n t r a c t .

2 . D E S I G N OF TEST ELEMENTS

Fuel Test Elements

The FTEs are designed t o r e p l a c e instrumented and non-instrumented

Peach Bottom Core 2 f u e l elements. E x t e r n a l l y , t h e s e elements have t h e same appearance as s tandard C o r e 2 f u e l elements, and their b a s i c arrange-

ment and des ign are a l s o s imilar (F igs . 1 and 2 ) .

The top r e f l e c t o r , sleeve, and bottom connector components, which

form t h e o u t e r containment envelope of each tes t element, are s tandard

Core 2 f u e l element components. The lower r e f l e c t o r and i n t e r n a l t r a p are very s i m i l a r t o t h e s tandard f u e l element components. The act ive f u e l

p o r t i o n i s made up of l ong , c y l i n d r i c a l H-327;k g r a p h i t e f u e l bodies which

r e p l a c e t h e f u e l compacts i n t h e Core 2 f u e l element. Each tes t element

comprises e i t h e r t h r e e 31-in.-long f u e l bodies o r s i x 15.5-in.-long f u e l

bodies , depending on t h e s p e c i f i c t e s t o b j e c t i v e s . Eight a x i a l purge

grooves a r e p r e s e n t on t h e o u t s i d e diameter of each f u e l body. The f u e l

bodies conta in e i g h t l /Z-in.-diameter f u e l h o l e s equal ly spaced on a

c i r c u l a r p a t t e r n and are f i l l e d wi th coated f u e l p a r t i c l e s , e i t h e r i n t h e form

of f u e l rods o r blended beds (F ig . 1). The sleeve, t o p r e f l e c t o r , and

bottom connector assembly c o n s t i t u t e a completely closed containment around

t h e f u e l bodies .

A c e n t r a l (spine), 1.1-in.-diameter h o l e i s provided f o r c o n t r o l l e d

i r r a d i a t i o n t e s t i n g of v a r i o u s sample types. These c e n t e r (spine) samples

a r e enclosed w i t h i n t h e i r own c o n t a i n e r s and inc lude f u e l p a r t i c l e s and

“Manufactured by Great Lakes Carbon Corporat ion.

6 5

@ fuel rods, graphite samples, metallic diffusion samples, and fission product

release samples.

hole contains a graphite spine.

When samples are not included, the center irradiation

Each test element is cooled by the flow of helium in the tricusp

channels between fuel elements in the same manner as the Core 2 elements,

and is purged in the same basic manner as the Core 2 fuel elements.

the purge gas flows in the purge grooves and the annular clearance gap between

the fuel body outside diameter and sleeve inside diameter. Seven test

elements will be located in special positions within.the core at which

the purge gas stream can be sampled periodically for release of fission

products.

A l l

Approximately 12 of the test elements occupy instrumented locations

within the core. Each of these elements has a tungsten-rhenium and a Chromel-

Alumel thermocouple located at the plane of maximum temperature of the

element. The core locations of the remaining uninstrumented test elements

have been selected to meetthe objective of maintaining relatively constant

power de ty within these elements during their irradition lifetime. Axial

and radial temperature profiles in a representative element are shown in

Figs. 3 and 4 .

Proof Test Elements

The PTEs are designed to replace standard instrumented Core 2 fuel

elements. Externally, they are different from Core 2 elements, having a

hexagonal shape over the fueled region (Fig. 1). Also , these elements are

cooled both internally and externally, and there is no purge stream to remove

released fission products.

Each element consists of seven sections joined together. From

bottom to top,these sections consist of a bottom connector, a bottom

reflector, four fuel zones, and a top reflector. The bottom portion of the

element is cylindrical with a transition to a hexagonal shape in the

bottom reflector section. The active fuel zones are hexagonal, with the 6d

66

width a c r o s s t h e f l a t s equal t o t h e diamerer of t h e in te rmedia te spacers

of t h e r e g u l a r elements. The tes t element l e n g t h i s t h e same as f o r a

s tandard Peach Bottom element. A s shown i n p lan view i n Fig. 1, t h e element

has seven in te rna l - coolan t channels and 1 2 f u e l rod holes d i s t r i b u t e d

w i t h i n t h e hexagonal g r a p h i t e body.

seven i n t e r n a l coolant channels and t h e s i x modified t r i c u s p channels

ad jacent t o t h e element. The elements are n o t purged, but a r e instrumented

w i t h two thermocouples whose j u n c t i o n s are loca ted 53 i n . above t h e bottom

of t h e act ive core. A Chromel-Alumel thermocouple i s loca ted i n t h e

g r a p h i t e body, and a tungsten-rhenium thermocouple i s loca ted i n t h e i n n e r

r i n g of ho les w i t h i n an annular f u e l rod.

Cooling i s provided both by t h e

The a c t i v e p o r t i o n of t h e f u e l element i s composed of fou r f u e l zones

j o i n e d t o g e t h e r a x i a l l y by cemented threaded j o i n t s . Each fuel zone

conta ins 1 2 b l i n d f u e l h o l e s d r i l l e d from t h e top of t h e zone t o w i t h i n

about 0.4 i n . of t h e bottom. The diameter of t h e r e g u l a r f u e l ho les i s

0.470 i n . The f u e l h o l e s conta in ing t h e annular f u e l r o d s wi th t h e

thermocouples have a diameter of 0’.517 i n . The seven coolan t channels are

continuous from t h e top t o t h e bottom of t h e zone, and they are a l igned with

t h e channels i n a d j a c e n t zones t o form continuous i n t e r n a l coolan t

channels over t h e f u l l l e n g t h of t h e f u e l element.

PTE-2, o r i g i n a l l y i r r a d i a t e d i n Core 1, w a s n o t removed from t h e

c o r e and w i l l cont inue i r r a d i a t i o n i n Core 2 .

p r o f i l e s dur ing o p e r a t i o n are shown i n F igs . 5 and 6 . Axia l and r a d i a l temperature

3 . HTGR FUEL LOADING COMBINATIONS

Present HTGRs employ carbon-coated uranium and thorium carb ide spheres

as t h e i r f u e l .

i n diameter , are bonded i n t o f u e l rods o r compacts and a re subsequently

loaded i n t o g r a p h i t e elements o r blocks. A p o s s i b l e a l t e r n a t i v e f o r

f u t u r e HTGRs i s t o load t h e b locks wi th blended beds of l o o s e p a r t i c l e s .

I n genera l , i t i s d e s i r e d t o g a i n a b e t t e r understanding of HTGR

f u e l des ign technology i n t h e s e areas and assess t h e p r o p e r t i e s and

These f u e l p a r t i c l e s , which can vary from 250 t o 1000 pm

67

performance of v a r i o u s types of coated p a r t i c l e s , f u e l r o d s , and

blended beds under i r r a d i a t i o n . The loadings of t h e test elements are

t h e r e f o r e d i c t a t e d by t h e f u e l loadings proposed f o r c u r r e n t and f u t u r e

HTGRs .

@

The f u e l p a r t i c l e s considered a re of t h e same genera l t y p e , having

s p h e r i c a l c a r b i d e o r oxide k e r n e l s , a porous "buffer" l a y e r of p y r o l y t i c

carbon (PyC) t o absorb f i s s i o n gases , a t t e n u a t e f i s s i o n r e c o i l s , and

al low f o r k e r n e l swel l ing , and an o u t e r l a y e r of r e l a t i v e l y dense

i s o t r o p i c PyC t o r e t a i n f i s s i o n gases . Such p a r t i c l e s a re termed B I S O

by GGA; T R I S O p a r t i c l e s a re s imilar b u t inc lude an e x t r a l a y e r of dense

p y r o l y t i c s i l i c o n c a r b i d e sandwiched between two l a y e r s of i s o t r o p i c PyC

t o r e t a i n meta l l ic f i s s i o n products . Cross s e c t i o n s of B I S O and T R I S O

p a r t i c l e s are shown i n Fig. 7 . P a r t i c l e s may be e i t h e r f i s s i l e (conta in ing 2 3 2 ) U235) o r f e r t i l e (conta in ing only Th

k e r n e l type and p a r t i c u l a r r e a c t o r a p p l i c a t i o n .

, and t h e i r s i z e depends on t h e

The s tandard elements i n Peach Bottom Core 2 w i l l be loaded wi th B I S O

f i s s i l e p a r t i c l e s i n t h e form of compacts. The F o r t S t . Vrain co re w i l l

b e loaded wi th TRISO f i s s i l e and TRISO f e r t i l e p a r t i c l e s i n t h e form of

f u e l rods.

and B I S O f i s s i l e p a r t i c l e s and BISO f e r t i l e p a r t i c l e s i n t h e form of

The 1100-MW(e) HTGR w i l l i n i t i a l l y be loaded wi th both TRISO

rods (see F i g . . 8 ) . T h e test element program i s aimed p r i m a r i l y toward

f u r t h e r i n g 1100-MW(e) HTGR technology, a l though advanced knowledge of t h e

performance of F o r t S t . Vrain f u e l s w i l l be obtained from t h e bonded

TRISO/TRISO f u e l included i n t h e program.

r e s t r i c t e d t o t h e 1100-MW(e) HTGR s t a r t u p and r e c y c l e modes of opera t ion .

The d i s c u s s i o n below i s

I n t h e 1100-MW(e) HTGR, two types of g r a p h i t e block loadings w i l l

b e used ( see Fig. 8 ) . The p a r t i c l e s i n t h e A-type block can be processed

t o g e t h e r a f t e r i r r a d i a t i o n , bu t t h e p a r t i c l e s i n t h e B-type block r e q u i r e

s e p a r a t i o n . This s e p a r a t i o n i s necessary because f i s s i o n a b l e U i s

bred i n t o t h e f e r t i l e p a r t i c l e s and i s recycled i n t h e (Th,U233)02 B I S O

r e c y c l e p a r t i c l e s .

233

However, l a r g e q u a n t i t i e s of U236 are produced i n t h e

68

U235C2 T R I S O p a r t i c l e s , and continuous r e c y c l e of t h i s m a t e r i a l i s no t

d e s i r e d . The amount of U236 bred i n t o t h e U235C2 B I S O p a r t i c l e s i n t h e

s t a r t u p A block i s r e l a t i v e l y s m a l l and i s progress ive ly d i l u t e d wi th

every r e c y c l e ; t h e s e p a r t i c l e s t h e r e f o r e do not need separa t ing . I n t h e

FTEs and FBTEs, emphasis i s placed upon t h e performance of t h e s t a r t u p A

and B block combinations, both as rods and loose p a r t i c l e s . The RTEs

emphasize only t h e v a r i o u s r e c y c l e block loadings and p o s s i b l e a l t e r n a t i v e s

t o t h e r e f e r e n c e loadings shown i n Fig. 8. The loading combinations being

i n v e s t i g a t e d i n t h e v a r i o u s elements and t h e j u s t i f i c a t i o n s f o r t h e i r

i n c l u s i o n are given i n Table 3.

4 . RECYCLE FUEL TEST ELEMENTS

Eight RTEs a re being f a b r i c a t e d f o r i n s e r t i o n i n Peach Bottom Core 2.

These elements are funded by t h e USAEC a s an i n t e g r a l p a r t of t h e Nat ional

HTGR Recycle Program, and are being f a b r i c a t e d j o i n t l y by GGA and Oak

Ridge Nat iona l Laboratory (ORNL). The main o b j e c t i v e s of t h e elements

are l i s t e d i n Table 2 . Two of t h e RTEs are t o be i r r a d i a t e d f o r 1 y r ,

two f o r 2 y r , and fou r f o r 3 y r ( s e e F ig . 9 ) . A l -yr i r r a d i a t i o n i n t h e

Peach Bottom HTGR i s roughly 300 EFPD. P e r i o d i c v i s u a l and gamma scanning

w i l l be conducted on t h e elements , bu t they w i l l n o t be instrumented. The

elements w i l l be shipped t o ORNL f o r p o s t i r r a d i a t i o n examination and subsequent

s t u d i e s a f t e r d i scharge from t h e Peach Bottom r e a c t o r .

Each element c o n s i s t s of s i x 15.5-in. g r a p h i t e bodies i n which t h e

uranium loading is determined by t h e p o s i t i o n i n t h e r e a c t o r . The f u e l

p a r t i c l e combinations and l o c a t i o n s i n t h e elements have been s e l e c t e d t o

produce i r r a d i a t e d f u e l bed materials which w i l l have been exposed t o a

wide range of r e a c t o r environmental condi t ions . These f u e l bed m a t e r i a l s

w i l l be used segment by segment t o t e s t and develop f u e l reprocess ing

equipment and processes . The u n i t o p e r a t i o n a l processes which w i l l be

i n v e s t i g a t e d inc lude s i z e r e d u c t i o n , s i z e s e p a r a t i o n , burning, chemical

d i s s o l u t i o n , leaching , and s o l i d s t r a n s p o r t . I n a d d i t i o n , t h e elements

w i l l provide proof tes ts of so l -ge l oxide f u e l p a r t i c l e s and, i n p a r t i c u l a r ,

t h e r e f e r e n c e r e c y c l e (Th,U)02 f u e l p a r t i c l e s .

69

Tab le 3 . F u e l Loading Combina t ions i n Advanced T e s t E lements

J u s t i f i c a t i o n D e s i g n a t i o n Comb i n a t i o n Type of Loading

RTE s

(4 .2 Th,U)02 B I S O

U02 BISO + ThC2 BISO

(2 .0 Th,U)02 B I S O + Tho2 B I S O

(2 .0 Th,U)C2 B I S O + ThC2 BISO

UC2 B I S O + ThC2 BISO

U C 2 TRISO + ThC2 B I S O

U02 BISO + Tho2 BISO

UC2 B I S O + Tho2 BISO

UC2 TRISO + ThC2 TRISO

Loose & bonded

Bonded

Bonded

Bonded

Loose & bonded

Loose & bonded

Bonded

Bonded

Bonded

Refe rence r e c y c l e p a r t i c l e ; w i l l check o u t bed s t i c k i n g , chemica l s e p a r a t i o n , e t c .

Backup r e c y c l e A b l o c k l o a d i n g i f Th i s e l i m i n a t e d from (Th,U)O p a r t i c l e s

A l t e r n a t e r e c y c l e A b l o c k l o a d i n g ; 2:l r a t i o t o o b t a i n e n d - o f - l i f e burnup (20%) i n 3 y r . Tho2 proposed i f o x i d e p r o c e s s adop ted .

Recyc le A b l o c k l o a d i n g ; 2:l r a t i o t o o b t a i n 20% burnup i n 3 y r

S t a r t u p A b l o c k l o a d i n g

S t a r t u p and r e c y c l e B b l o c k l o a d i n g

Backup B b l o c k l o a d i n g i f Th is e l i m i n a t e d and i f s o l - g e l Tho2 i s used

Backup A o r B b l o c k l o a d i n g ; s e p a r a b l e chemica l ly o r by s e l e c t i v e b u r n i n g

A l t e r n a t e B b l o c k l o a d i n g i f c l e a n e r pr imary c i r c u i t s r e q u i r e d

2

~

FTEs and FBTEs

e UC2 BISO + ThC2 BISO Loose & bonded S t a r t u p A b l o c k l o a d i n g

f UC2 TRISO + ThC2 BISO Loose & bonded S t a r t u p and r e c y c l e B b l o c k l o a d i n g

k (2 .0 Th,U)C2 BISO + ThC2 B I S O Loose & bonded Recycle A b l o c k l o a d i n g w i t h (Th,U)C2 s u b s t i t u t i n g f o r (Th,U)02

1 (2 .0 Th,U)C2 TRISO + ThC2 TRISO Loose & bonded Refe rence F o r t S t . V r a i n l o a d i n g

Tab le 4 . RTE Loading Combinationsa

F i r s t D i scha rge Second Discha rge F i n a l D i scha rge Cen te r L i n e

P o s i t i o n of Temp RTE 1, RTE 3 , RTE 5 , RTE 7 , Fuel Body Range A l l Bonded A l l Bonded A l l Bonded RTE 6 , A l l Bonded i n Element (OF) (Quar t e red ) RTE 2 (Quar t e red ) RTE 4 (Quar t e red ) A l l Bonded (Quar t e red ) RTE 8

6 2050- b d e i f b d e i b d e i g a c f g f b

( t o p of 1950 r e a c t o r )

5 2250- bdeh bdeh d bdeh f ac f g i b

2050

4 2250- a c f g e a c f g f a c f g f bdeh d

3 2050- a c f g f b a c f g a c f g d b d h i e

(max. f l u x ) 2300

b

(max. f l u x ) 2250

2 1650- ac f g d ac f g f b a c f g 2050

f b d h i i

1 1050- ac f g ac f g e a c f g e b d h i f b b

(bot tom of 1650 r e a c t o r )

aLoadings are i n d i c a t e d by letters a , b , c , d , e , f , g, h , and i, and are d e f i n e d i n T a b l e 3 . bBlended beds .

70

A

The proposed loadings of t h e RTEs a re shown i n Table 4 . I n R T E s 1,

3 , 5 , and 7 , a l l s i x bodies are "quartered," and c o n t a i n a d i f f e r e n t f u e l

combination i n every two h o l e s of a given body. These elements w i l l be

used f o r small-scale reprocess ing i n t h e ORNL h o t c e l l . RTEs 2 , 4 , 6 , and

8 , which conta in only s i n g l e loadings i n a given body, w i l l provide l a r g e r

amounts of t h e ch ie f combinations of i n t e r e s t f o r h o t c e l l work and a l s o

w i l l provide t h e i n i t i a l i r r a d i a t e d material requi red f o r reprocess ing i n

t h e Thorium-Uranium Recycle F a c i l i t y (TURF). A d d i t i o n a l l y , some blended

beds w i l l be i r r a d i a t e d i n RTEs 2 , 4 , and 8 , s i n c e t h e s e elements c o n s t i t u t e

a backup des ign f o r f u e l rods i n t h e HTGR.

Fuel f o r t h e RTEs i s being f a b r i c a t e d by both ORNL and GGA. ORNL has

produced and coated a l l t h e so l -ge l oxide k e r n e l s , and GGA has produced

and coated t h e v a r i o u s c a r b i d e k e r n e l s . The p a r t i c l e s have been made t o

t e n t a t i v e 1100-MW(e) HTGR s p e c i f i c a t i o n s . Fuel rods wi th d i f f e r e n t matrices

are be ing f a b r i c a t e d f o r t h e RTEs both a t GGA and ORNL by means of t h e

h o t i n j e c t i o n molding process . Blended beds are being loaded a t GGA. A l l

element assembly w i l l t a k e p l a c e a t GGA, and i t i s planned t o s h i p t h e

elements t o Peach Bottom by May 15.

5 . FUEL BED TEST ELEMENTS

S i x FBTEs being f a b r i c a t e d f o r i n s e r t i o n i n Peach Bottom Core 2.

The main o b j e c t i v e s of t h e s e elements , which are funded p r i v a t e l y by

GGA, are given i n Table 2. It i s planned t o i r r a d i a t e a l l of t h e s e elements

f o r 3 y r (900 EFPD), and t o conduct p e r i o d i c v i s u a l and gamma scanning.

Gamma scanning w i l l reveal dimensional changes of t h e beds, p a r t i c u l a r l y i n

t h e case of FBTEs 5 and 6 , which are l o o s e bed elements. A l l s i x FBTEs

w i l l be instrumented and w i l l be i n s t a l l e d on stand-off p i n s where t h e

purge gas stream can b e monitored p e r i o d i c a l l y dur ing i r r a d i a t i o n . In-p i le

f i s s i o n produce release d a t a can t h u s b e obtained from t h e s e elements. The

mixed element (FBTE-5), which i s complementary t o FTEs 1 and 2 (see Sec t ion 6 ) , w i l l c o n t a i n g r a p h i t e s p i n e samples and w i l l receive p o s t i r r a d i a t i o n

examination; t h e o t h e r elements w i l l conta in no s p i n e samples and may b e used f o r reprocess ing s t u d i e s a t ORNL a f t e r d i scharge .

71

Each element c o n s i s t s 'of t h r e e 31-in. bodies r a t h e r than s i x 15.5-in.

bodies as i n . t h e R T E s . The proposed f u e l p a r t i c l e loading combinations a r e

shown i n Table 5 , and i n each case t h e s e are r e l e v a n t t o t h e 1100-MW(e) HTGR

loadings.

b i n a t i o n s r e p r e s e n t t h e s t a r t u p mode f o r t h e 1100-MW(e) HTGR (F ig . 8 > , and

(Th,U)C B I S O + ThC B I S O r e p r e s e n t s t h e r e c y c l e A block loading w i t h

carb ide r a t h e r than oxide f i s s i l e k e r n e l s .

T R I S O p a r t i c l e s are t o be used i n F o r t S t . Vrain,and t h i s p a r t i c u l a r

element w i l l t h e r e f o r e provide advanced knowledge of t h e performance of such

bonded f u e l . I n a d d i t i o n , t h e f i s s i o n gas release d a t a obtained by purge

stream sampling w i l l provide cont inuing s u r v e i l l a n c e of F o r t S t . Vrain

opera t ing condi t ions . Only a s i n g l e f u e l combination i s t o be used throughout

FBTEs 1, 2, 3 , 4 , and 6 i n order t h a t t h e f i s s i o n gas release d a t a can be

d i r e c t l y r e l a t e d t o t h e performance of a p a r t i c u l a r type of f u e l .

The U C 2 B I S O + ThC2 B I S O and UC TRISO + ThC2 B I S O bonded com- 2

2 2 Bonded (Th,U)C2 TRISO + ThC2

The o u t e r bed loadings of t h e mixed element FBTE-5, which are t h e

s a m e as i n FTEs 1 and 2, are shown i n F ig . 10. This i s a blended bed element

i n which t h e p a r t i c l e s are mixed w i t h c a l c i n e d petroleum coke ('L0.15 g/cm

of f u e l h o l e ) . Body 1 conta ins p a r t i c l e s wi th propylene-derived coa t ings

which are r e l e v a n t t o t h e F o r t S t . Vrain and 1100-MW(e) HTGRs. These

p a r t i c l e s have r e l a t i v e l y high PyC coa t ing d e n s i t i e s (1.70 t o 1 .80 g/cm

f o r TRISO and 1.80 t o 1 .90 g/cm3 f o r BISO). I n bodies 2 and 3 , t h e e f f e c t of PyC d e n s i f i c a t i o n r a t e on t h e r h e o l o g i c a l problems of blended beds w i l l

be i n v e s t i g a t e d . The high-densi ty PyC i n t h i s case is 1.80 t o 1 . 9 0 g/cm

and t h e low-density PyC i s 1.55 t o 1 .65 g/c:m , The ra te a t which

stresses i n blended beds b u i l d up may v a r y wi th t h e PyC d e n s i f i c a t i o n r a t e

on t h e p a r t i c l e s , t h e l a t te r rate being h i g h e r , t h e lower t h e PyC d e n s i t y 21 2 f o r doses up t o about 4 x 10 n/cm . Owing t o such u n c e r t a i n t y of

blended bed behavior , t h e 3-yr i r r a d i a t i o n s p i n e samples i n FTE-3 w i l l

b e contained i n a bonded element, and FBTE-5 w i l l demonstrate t h e 3-yr

i r r a d i a t i o n behavior of such beds.

i n t e r a c t i o n stresses w i l l n o t b u i l d up s i g n i f i c a n t l y u n t i l a t least t h e

t h i r d year of i r r a d i a t i o n ,

from t h e r e a c t o r f o r p o s t i r r a d i a t i o n examination a t any s u i t a b l e t i m e ,

s i n c e i t w i l l c o n t a i n no c r i t i c a l s p i n e samples.

3

3

3

It i s a n t i c i p a t e d t h a t p a r t i c l e / g r a p h i t e

I f necessary , however, FBTE-5 could be removed

72

6. FUEL TEST ELEMENTS

S i x FTEs are being f a b r i c a t e d f o r i n s e r t i o n with Peach Bottom Core 2 .

The o v e r a l l o b j e c t i v e s of t h e s e elements a r e given i n Table 2 ; more s p e c i f i c

o b j e c t i v e s of t h e v a r i o u s s p i n e samples a r e d iscussed i n t h e following

s e c t i o n s . The elements w i l l c o n t a i n both bonded rods and blended beds as

t h e d r i v e r f u e l , and c o n t r o l l e d experiments w i l l be conducted i n t h e s p i n e s

Two of t h e FTEs w i l l be i r r a d i a t e d f o r 1 y r , two f o r 2 y r , and two f o r 3 y r

( s e e F ig . 9 ) . This schedule was chosen p r i m a r i l y t o a s s e s s t h e e f f e c t s

of i r r a d i a t i o n dose on t h e performance of t h e blended beds, bonded r o d s ,

and s p i n e samples. A l l t h e elements w i l l b e instrumented, and FTE-6 w i l l be a t a purge sampling p o s i t i o n . P o s t i r r a d i a t i o n examinations of a l l t h e

elements w i l l be conducted a t GGA.

Outer Bed Loadings

Each element c o n s i s t s of t h r e e 31-in. bodies . The loadings of FTEs

3 , 4 , 5 , and 6 are shown i n F ig . 11; and t h e o u t e r bed loadings of FTEs

1 and 2 are shown i n Fig. 1 0 . FTEs 1 and 2 use blended beds as descr ibed i n Sec t ion 5, whi le FTE-3 uses bonded rods conta in ing t h e same

p a r t i c l e s , The loadings of FTEs 4 , 5, and 6 a r e i d e n t i c a l , and

t h e p a r t i c l e s are t h e same as those i r r a d i a t e d i n FBTEs 1-4. f u e l rods i n a given body w i l l be bonded wi th an i s o t r o p i c coke + p i t c h ,

and t h e remainder w i l l be bonded wi th a n a t u r a l f l a k e g r a p h i t e + p i t c h .

Since any p a r t i c u l a r combination and mat r ix runs t h e complete l e n g t h of

any given element, i t w i l l be p o s s i b l e t o determine t h e e f f e c t s of

temperature , f l u x , and f l u e n c e up t o 4 . 2 x 1021 n/cm

performance of t h e s e rods. Likewise, t h e e f f e c t s of t h e same v a r i a b l e s

on blended bed performance can be checked i n FTEs 1 and 2 and FBTE-5.

Half of t h e

2 on t h e i r r a d i a t i o n

73

diameter s p i n e ho le .

i n t o f i v e c a t e g o r i e s according t o t h e samples i n s e r t e d and t h e tes t o b j e c t i v e s :

I n g e n e r a l , t h e s p i n e sample program can be divided

1. Thermal s t a b i l i t y of f u e l p a r t i c l e s

2 . F i s s i o n product release from f u e l p a r t i c l e s

3 . Metallic f i s s i o n product d i f f u s i o n through g r a p h i t e

4 . Isothermal proof t e s t i n g of f u e l rods

5. Phys ica l p roper ty changes of g r a p h i t e s

The specimens d i f f e r i n d e s i g n , bu t they a re a l l contained w i t h i n 1-in.-

diameter c y l i n d r i c a l c r u c i b l e s having s u f f i c i e n t c l e a r a n c e w i t h i n t h e f u e l

body c e n t e r h o l e t o prevent i n t e r f e r e n c e w i t h t h e g r a p h i t e body a t any

t i m e dur ing i r r a d i a t i o n . By us ing t h e s e c r u c i b l e s , a l l t h e c e n t e r samples

are i n e f f e c t t r i p l y contained, and t h e experiments r e q u i r i n g more p r e c i s e

p o s t t e s t i n g a n a l y s i s ( thermal s t a b i l i t y and f i s s i o n product r e l e a s e ) employ

smaller c o n t a i n e r s i n s i d e t h e s e c r u c i b l e s . The purpose and d e s c r i p t i o n of

each type of experimental sample are presented below, and t h e p r i n c i p a l

experimental v a r i a b l e s being i n v e s t i g a t e d w i t h each type of sample are

d iscussed . I n a l l cases, t h e experiments are repeated i n 1-, 2- , and 3-yr

i r r a d i a t i o n elements , o f t e n a t d i f f e r e n t temperatures s o t h a t t h e e f f e c t s

of i r r a d i a t i o n dose and temperature can be assessed .

Fuel P a r t i c l e Thermal S t a b i l i t y Samples

The purposes of t h e s e experiments are:

1. To expose f u e l p a r t i c l e s t o a w i d e range of o p e r a t i n g v e r i a b l e s ,

p a r t i c u l a r l y high temperatures and temperature g r a d i e n t s

2 . To determine t h e time-at-temperature l i f e t i m e s of v a r i o u s types of

f u e l

3 . To o b t a i n d a t a which can, when i n t e g r a t e d with o t h e r work, lead t o

a b e t t e r understanding of p a r t i c l e d e t e r i o r a t i o n and f a i l u r e under

extreme condi t ions

I n g e n e r a l , u s e f u l data concerning p a r t i c l e performance and des ign l i m i t s

can be obtained o n l y by exposing v a r i o u s p a r t i c l e types t o condi t ions severe

74

enough t o cause f a i l u r e of some of t h e p a r t i c l e s . This can be achieved by

combining abnormally high i r r a d i a t i o n temperatures and thermal g r a d i e n t s *

which can cause k e r n e l migra t ion , kernel-coat ing i n t e r a c t i o n , and subsequent

p a r t i c l e coa t ing f a i l u r e .

The most s a t i s f a c t o r y way t o achieve high temperatures and high thermal

g r a d i e n t s i n t h e tes t elements i s t o use t h e sample des ign shown i n F ig . 1 2 .

P a r t i c l e s are loaded i n d i v i d u a l l y i n t o s e p a r a t e holes of t h e inne r g r a p h i t e

c r u c i b l e s . These c r u c i b l e s s l i d e i n t o t h e c y l i n d r i c a l s p i n e c r u c i b l e , which

i s then loaded wi th l o o s e p a r t i c l e s , and a l i d i s screwed i n t o p l ace . The

amount of uranium i n t h e p a r t i c l e s and t h e a x i a l p o s i t i o n of t h e c r u c i b l e

i n t h e element determine t h e maximum temperature and temperature g r a d i e n t

experienced by t h e p a r t i c l e s i n t h e i n n e r c r u c i b l e s . Several of t h e i n n e r

crucibles are assembled within each 1-in.-diameter crucible. The maximum

temperature of t h e s e c r u c i b l e s w i l l b e about 1 5 O O 0 C , and temperature g r a d i e n t s

w i l l be up t o 1000"C/in.

o t h e r s are bonded i n p l ace . The experiment i s designed t o provide condi t ions

under which k e r n e l migra t ion can occur and p o s s i b l y r e s u l t i n t h e f a i l u r e of

some of t h e p a r t i c l e coa t ings i n t h e i n n e r c r u c i b l e s , c e r t a i n of which have

been e s p e c i a l l y s e l e c t e d because of high migra t ion ra tes observed i n out-of-

p i l e tests. Some o t h e r , s impler c r u c i b l e s a r e a l s o being used t o i r r a d i a t e

p a r t i c l e s from t h e same ba tches under almost i so thermal condi t ions .

Some p a r t i c l e s i n t h e s e c r u c i b l e s are l o o s e and

Var iab les o t h e r than f l u e n c e and temperature t h a t are being i n v e s t i g a t e d

can b e convenient ly divided i n t o two c a t e g o r i e s :

v a r i a b l e s . Kernel v a r i a b l e s inc lude :

k e r n e l v a r i a b l e s and coa t ing

Kernel type . . . . . . U C 2 , (Th,U)C2, ThC2, U02,(Th,U)02, Tho2

Th:U r a t i o . . . . . . Zero, 2:1, 4 : 1 , 18:l

Impurity con ten t . . . . Various amounts

Hydrogen content . . . . Kernels d e l i b e r a t e l y hydrolyzed before coa t ing

Coating v a r i a b l e s inc lude :

B I S O and TRISO type coa t ings

D i f f e r e n t c o a t i n g gases

High and low c o a t i n g d e n s i t i e s

75

The c r u c i b l e s were radiographed before and w i l l be radiographed a f t e r

. i r r a d i a t i o n i n hopes of r e v e a l i n g k e r n e l and coa t ing dimensional changes,

k e r n e l - m i g r a t i o n , and p a r t i c l e f a i l u r e . Other p o s t i r r a d i a t i o n examination

tests w i l l i n c l u d e photomacrography, gamma scanning, autoradiography,

metal lography, and e l e c t r o n microprobe a n a l y s i s .

Fuel P a r t i c l e F i s s i o n Product Release Samples

The purpose of t h e s e experiments i s t o o b t a i n t h e fol lowing information:

1. I n - p i l e and out-of-pi le f i s s i o n product r e l e a s e d a t a f o r B I S O and

TRISO f u e l p a r t i c l e s as a func t ion of burnup and temperature

2 . D i s t r i b u t i o n c o e f f i c i e n t s of f i s s i o n products between k e r n e l s and

coated p a r t i c l e s

3 . Kernel release d a t a a s a func t ion of burnup and temperature

4 . The e f f e c t of d i f f e r e n t types of k e r n e l s ( f i s s i l e , f e r t i l e , ox ides ,

carb ides) on f i s s i o n product r e l e a s e

5 . Thorium and uranium migrat ion i n k e r n e l and coa t ing as a f u n c t i o n

of temperature 6 . The e f f e c t of m e t a l doping of t h e coa t ings on t h e f i s s i o n

product r e l e a s e

T h e des ign of g r a p h i t e c r u c i b l e s f o r f i s s i o n product release samples i s

shown schemat ica l ly i n F ig . 1 3 . A few hundred l o o s e p a r t i c l e s are loaded i n t o

each c r u c i b l e a long w i t h g r a p h i t e f l o u r t o absorb f i s s i o n products .

c r u c i b l e i s i n t u r n contained w i t h i n an o u t e r g r a p h i t e c r u c i b l e approximately

1 i n . i n diameter by 1 . 2 5 i n . long. Some of t h e small c r u c i b l e s a re contained

w i t h i n thick-walled niobium cans ( % 1 / 4 i n . t h i c k ) , as shown i n Fig. 1 4 . The o b j e c t of t h e niobium cans i s t o r e t a i n a l l t h e f i s s i o n products r e l e a s e d

during i r r a d i a t i o n w i t h i n t h e c r u c i b l e , and t h i s can be achieved only w i t h

a s e a l e d can. However, r e a c t o r phys ics c o n s i d e r a t i o n s l i m i t t h e amount

of metal which can be placed i n t h e c o r e , and t h e r e f o r e g r a p h i t e o u t e r

c r u c i b l e s have been used f o r most of t h e s e samples. The v a r i a b l e s being

i n v e s t i g a t e d w i t h t h e f i s s i o n product release samples are:

The

76

BISO versus TRISO coatings

uc2, (Th,U)C2, ThC2, U 0 2 , (Th,U)02, Tho2 kernels

Effect of doping of fissile particle coatings

Postirradiation examination of the fission product release samples,

which will be carried out in the Chemistry Department facilities at GGA

rather than the Hot Cell, will include:

1. Fission gas release and fission product release measurements

2. Determinations of fission product concentrations between kernels,

coatings, graphite, and charcoal, leading to in-pile release data

3 . Postirradiation anneals to confirm release data 91 4 . Postactivation analyses for Sr release

5. Bare kernel release measurements

6. Fission product release measurements on doped particles

Metallic Diffusion Samples

The purpose of the metal diffusion studies is to obtain:

1. Migration rates for strontium, cesium, barium, samarium, etc., in

graphite under reactor conditions as a function of metal concentra-

tion and temperature

2 . Data on the effects of the presence of other fission product metals

on the migration rate of individual metals (especially strontium)

3 . Data on the ability of getter materials to reduce metal migration.

(Getter materials combine with fission products such as strontium

to inhibit their diffusion into the reactor system.)

4 . Data on the integrity of getter materials

A typical graphite crucible for the diffusion samples is shown in

Fig. 15. The graphite crucibles are about 1.25 in. long and 1.0 in.

in outside diameter. The annular hole contains various metals (such as

strontium, cesium, and barium) in the form of carbides, mixed in known

proportions in a graphite matrix material.

77

Variables to be investigated include different fission product elements

and element concentrations, combinations of various elements, and getters. crs

I Postirradiation examination will include:

1. Sectioning of crucibles and determination of metal concentrations

across the crucible walls

2. Analyses of the source materials to determine total metal l o s s

3 . Calculations of migration rates for each metal

4 . Determination of the effects of getters 5. Investigations of getter integrity

Bonded Rod Samples

Bonded rod samples are included to evaluate the performance of various

types of bonded fuel rod concepts as functions of temperature and irradiation

dose, and thereby to determine the best overall fuel rod concept for use

in future HTGRs.

Bonded rods are approximately 1 1 2 in. in diameter and from 314 in.

to 2 in. long.

with a screw-on top.

type, production method, and packing fraction.

Each rod is inserted in a 1-in.-diameter graphite crucible

The variables being investigated include filler

Postirradiation examination of the samples will include visual

examination, dimensional change measurement, photomacrography, metallography,

and possibly burdleach testing for Sic integrity.

Graphite Samples

The purposes of the graphite sample irradiations are as follows:

1. TO determine the effect of irradiation dose and temperature on the

tensile properties of various graphites

2. To investigate the dimensional change behavior of various graphite

s amp 1 es

78

3. To e v a l u a t e t h e t r a n s i e n t creep c h a r a c t e r i s t i c s of v a r i o u s g r a p h i t e s

as func t ions of i r r a d i a t i o n dose and temperature

Graphi te samples are e i t h e r c y l i n d r i c a l ( t e n s i l e t es t p i eces ) o r f l a t

(dimensional change p i eces ) o r of t h e genera l des ign shown i n F ig . 1 6

( t r a n s i e n t creep samples). I n each case, t h e samples are contained i n

g r a p h i t e c r u c i b l e s much as i n t h e case of t h e bonded rod samples descr ibed

above. I n each of t h e d i f f e r e n t sample t y p e s , i t i s proposed t o i n v e s t i g a t e

several d i f f e r e n t v a r i a b l e s o t h e r than i r r a d i a t i o n dose and temperature ,

inc luding:

1. I s o t r o p i c and needle-coke g r a p h i t e s

2. P a r a l l e l and t r a n s v e r s e o r i e n t a t i o n i n t h e l o g

3 . Edge and c e n t e r of l o g

4 . Various sample s i z e s

The p o s t i r r a d i a t i o n examination of t h e g r a p h i t e samples w i l l i nc lude :

1. Examination and photomacrography

2. T e n s i l e tests a t v a r i o u s temperatures

3. Dimensional change measurements

4 . Measurement and t e n s i l e t e s t i n g of t r a n s i e n t creep samples

5. Some metallography

7 . PROOF TEST ELEMENTS

One PTE (PTE-2) w i l l be i n Core 2 a t s t a r t u p . This element w a s

i r r a d i a t e d i n Core 1 and has n o t been removed from i t s o r i g i n a l p o s i t i o n .

It i s intended t o leave PTE-2 i n Core 2 f o r a f u r t h e r 300 EFPD. Another

PTE (PTE-3) i s being considered f o r i n s e r t i o n i n Core 2 a f t e r 300 EFPD

f o r a 300-EFPD i r r a d i a t i o n p e r i o d , a f t e r which t i m e i t w i l l be discharged f o r

p o s t i r r a d i a t i o n examination a t GGA.

t h a t f u e l elements conta in ing bonded T R I S O / T R I S O f u e l p a r t i c l e s , such as

t h o s e t o be used i n t h e F o r t S t . Vrain HTGR, w i l l m e e t t h e i r des ign

o b j e c t i v e s . They conta in r e f e r e n c e F o r t S t . Vrain TRISO p a r t i c l e s bonded

by h o t i n j e c t i o n molding using carbonaceous mat r ix materials.

These elements w i l l demonstrate

79

PTE-2, which is now i n t h e Peach Bottom r e a c t o r , conta ins both f u e l

rods and "piggyback" specimens c o n s i s t i n g of t h e component materials of

t h e f u e l rods.

t o p re l iminary F o r t S t . Vrain s p e c i f i c a t i o n s , and were bonded by h o t

i n j e c t i o n molding us ing a phenol ic r e s i n / g r a p h i t e powder mat r ix . Twenty-

f o u r annular rods , 5.93 i n . l ong , and 70 s o l i d r o d s , 11.53 i n . l ong , were

f a b r i c a t e d us ing t h r e e b a s i c types of f u e l p a r t i c l e s .

have d i f f e r e n t coa t ing types:

The rods c o n t a i n product ion TRISO p a r t i c l e s which were made

These p a r t i c l e s

1. H T I (high-temperature i s o t r o p i c )

2. Low-density L T I (low-temperature i s o t r o p i c )

3. High-density L T I

The piggyback l o c a t i o n s c o n t a i n l o o s e p a r t i c l e s from each of t h e 1 4

ba tches o r blends used, H T I PyC and S i c coa t ing s t r i p s , and mat r ix material

of t h e type used i n t h e rods. The piggyback materials are contained i n

g r a p h i t e c r u c i b l e s loca ted i n t h e c e n t e r s of annular f u e l r o d s . These

samples are 118 i n . i n diameter and occupy a t o t a l of 1 7 i n .

of i r r a d i a t i o n w i l l be 1150" t o 12OO0C, and t o t a l f a s t neutron dose rece ived

Temperatures

w i l l b e about 1 . 3 x 1021 n/cm2. Some g r a p h i t e d i s k s are a l s o being

i r r a d i a t e d t o approximately t h e same temperature and dose as t h e o t h e r

samples.

8 . CONCLUSIONS

I n t h e p a s t , GGA has emphasized t h e sys temat ic t e s t i n g of f u e l p a r t i c l e s ,

f u e l r o d s , and g r a p h i t e t o f u l l HTGR exposures i n i r r a d i a t i o n capsules . By

combining observed i r r a d i a t i o n performance and t h e o r e t i c a l c a l c u l a t i o n s ,

i t has been p o s s i b l e t o d e f i n e t h e g r o s s l i m i t s of acceptab le des ign condi-

t i o n s . GGA i s now emphasizing t h e understanding of t h e behavior of coated

p a r t i c l e s , r o d s , e t c . , and i s a t tempt ing t o d e f i n e more p r e c i s e l y t h e l i m i t s

of opera t ing condi t ions f o r t h e v a r i o u s c o r e components. For t h e s e

purposes, t h e unique t e s t i n g c a p a b i l i t i e s of t h e Peach Bottom r e a c t o r are

d e s i r a b l e , s i n c e i t o f f e r s a r e p r e s e n t a t i v e HTGR environment and neutron

spectrum and can tes t l a r g e f u e l samples and tes t assemblies a t HTGR

burnup rates and temperature g r a d i e n t s . 63

80

GGA p lans t o i n i t i a l l y i r r a d i a t e s e v e r a l tes t elements i n Peach Bottom

Core 2, inc luding FTEs, RFTEs, FBTEs, and PTEs . These elements have

d i f f e r e n t o b j e c t i v e s , bu t are a l l designed t o f u r t h e r large-HTGR technology.

I r r a d i a t i o n t e s t i n g and eva lua t ion of tes t elements w i l l cont inue f o r

approximately 3 y r .

FBTE a No.

1

2

3

4

5

6

i a D i e 3 . Loaaings 01

Loading Combination

(2.0 Th,U)C2 B I S O + ThC2 B I S O

UC B I S O + ThC2 B I S O 2

UC2 T R I S O + ThC2 B I S O

(2.0 Th,U)C2 TRISO + ThC2 TRISO

Mixed element ( i d e n t i c a l t o FTEs 1, 2; see Fig. 10)

(2.0 Th,U)C2 B I S O + ThC2 B I S O

Fuel Configurat ion

Bonded r o d s ; n a t u r a l f l a k e g r a p h i t e mat r ix + p i t c h b inder

Bonded rods ; n a t u r a l f l a k e g r a p h i t e mat r ix + p i t c h b inder

Bonded r o d s ; n a t u r a l f l a k e g r a p h i t e mat r ix + p i t c h b inder

Bonded r o d s ; n a t u r a l f l a k e g r a p h i t e mat r ix + p i t c h b inder

Blended beds ( p a r t i c l e s + coke)

Blended beds ( p a r t i c l e s + coke)

a Other FBTEs may be i n s e r t e d i n t h e Peach Bottom r e a c t o r a t l a t e r d a t e s .

81

FUEL OR F I S S I O N PRODUCT SAMPLE PURGE CRUCIBLE, GRaOVE

PTE CROSS SECTION

/ FTE CROSS SECTION

GRAPH I TE BODY

Fig. 1. Cross Sections of Fuel Test Element and Proof Test Element.

I SECTION 0-D

t - ACTIVE CORE 90"

I ELEMENT - INTERNAL rA /TRAP FUEL BODY -

/ 3 OR 6

D+l ATHERHOCOUPLE

D 4 SECTION A-A

Fig. 2. Fuel Test Element Assembly.

82

25

20

N

0

x Y

-

u- I5 w 3 + IL w &

6

10

5

EUEL 1

I F U E L BOOY / SURFACE F U E L BOOY

SLEEVE I . D .

Axial Temperature Distribution in a Typical Fig. 3. Axial Temperature Distribution in a Typical I - Fuel Test Element.

0 IO 20 30 40 50 60 70 80 90 I

A C T I V E CORE E L E V A T I O N . INCHES

D I S T R I B U T I O N THROUGH F U E L , S E C T I O N A t f D I S T R I B U T I O N THROUGH LIGAMENT, S E C T I O N 8

Fig. 4. Radial Temperature Distribution at the Point of Axial Peak Temperature in a Typical Fuel Test Element.

F U E L ELEMENT CROSS S E C T I O N I I i

0.55 1 .oo 1.50

R A D I U S OF ELEMENT, INCHES

1 .EO

a3

2200

2000

1800

h

LL e 1600 W Lz 3 l- a E 1400 n. r W I-

1200

1000

800

2500

2400

2300

2200

2100

2000 h

," 1900 v

2 1800

2 I700 4 1600

1500

1400

1300

1200

1100

3 I-

W

I-

MAXIMUM FUEL

AVERAGE FUEL SURFACE -- TEMPERATURES

RODS

RODS

/ I I 1 I 1 I 1 1 I I 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

NORMALIZED A X I A L DISTANCE FROM BOTTOM OF ACTIVE CORE

F i g . 5 . Fuel Rod Axial Temperature P r o f i l e s i n m.

CHANNEL

\ F U E L ROO

I I I I I 1 1 I 1 I 1 1000 0 0.2 0.4 0.6 0.8 1 .o 1.2 1.4 1 .6 1.8 2.0 2.2

R A D I A L D I S T A N C E ( I N C H E S )

Fig . 6. Radial Temperature Prorile i n PPE a t Plane of Maximum Fuel Temperature.

BISO

i

' j i

i . I '

I -

/ OUTER ISOTROPIC

\ PYROLYTIC CARBON

SILICON CARBIDE BARRIER COATING\

0

1

INNER ISOTROPIC a

PYROLYTIC CARBON

/ 'BUFFER PYROLYTIC CARBON

t

TRISO Fig . 7. Cross Sec t ions of Gulf Atomic Coated

P a r t i c l e s .

I N I T I A L CORE m

NUMBER 2

No separation required

LENGTH OF IRRADIATION (YR) 1 2 3

"235

TRISO

ThC2 BISO

c 2 BLOCK B

Separation required

85

RECYCLE CORE

F i g . 8 .

Fig. 9.

I rrad ia t ion Schedule for T e s t Elements.

ACETYLENE BUFFER: MIXED GAS I SOTROPIC COATINGS

Coated P a r t i c l e s for the 1100-MW(e) HTGR. The reference f u e l i n the Fort S t . Vrain and llOO-MW(e) HTGRs is i n the form of bonded rods.

TRISO

ELEMENT TYPE RT E

FBTE FT E

PTE

(Th,U)C2 BISO + ThC2 BISO (LOW DENSITY)

(Th,U)C2 BISO + ThC2 BISO (HIGH DENSITY) ___---- ------------

00 W c/ 00 uc2

uc2

-- BISO + ThC2 BISO (LOW DENSITY)

B ISO + ThC2 BISO (HIGH DENSITY) __ - - - - - ------- --

ACETYLENE PROPYLENE BUFFER: ISOTROPIC \3 0 (Th,U)C2 TRISO + ThC2 TRISO

COATI NTS

PROPYLENE BUFFER: 0 (Th,U)C2 BISO + ThC2 BISO (HIGH DENSITY) PROPYLENE ISOTROPIC COATINGS ALL BLENDED BEDS

_ _ _ _ _ _ _ _ _ _ - _ - B O O V I-_- _ _ _ _ - _ - - - - _ - -_- -_-

LOW DENSITY = 1.55-1.65 G/CM3 HIGH DENSITY = 1.80-1 -90 G/CM3

Fig . 10. Outer Bed Loadings of FBTE 5 ( a l s o FBTEs 1 and 2 ) .

86

FTE 3 I I

UC2 + ThC2 (HIGH DENSITY)

I I I

ACETYLENE BUFFERS: MIXED GAS ISOTROPIC COATINGS

FTE 4,5,6 I I

(Th,U)C2 + ThC2 (HIGH DENSITY)

(Th,U)C2 TRISO + ThC2 TRISO

I I I

PROPYLENE BUFFERS: PROPYLENE ISOTROPIC COATINGS

Fig . 11. Outer Bed Loadings of Fuel Test Elements. For FTEs 1 and 2 see F ig . 8 ; loading of a l l three bodies i n each element i s i d e n t i - c a l ; a l l bonded rods.

-1

P A R T I C L E S

INNER CRUCIBLE (MINUS L I D )

r 3" T O 6"

";I

L I D (SCREWS I N ) /

,OUTER C R U C I B L E - 1 " O . D . MAX

/CRUCIBLE AS SHOWN AT LEFT

/ U N BO ND E D P A R T I C L E S

F i g . 12.

OUTER CRUCIBLE

Design of Thermal S t a b i l i t y Spine Samples.

C I NNER GRAPH I T E CRUC I B L E

COKE \ F I L L E R

GRAPH I T E CRUC I B L E

1 .

0"

Fig. 13. Design of Fission Product Release Spine Samples.

N I O B I U M N I O B I U M F O I L S H I M SPACER W I R E AND GRAP-HO I L ( P y C )

FOIL S H I M

GRAPH I T E CRUC I B L E

WELD F U E L PART I C L E S

rig. 14. Design of Alternative Fission Product Release Spine Samples.

88

/

\J

a, d a

co c d

$ w

w

w c M

d

rn

d

89

DISCUS SI ON

S. I. Kaplan: P lease desc r ibe t h e removal of t h e broken f u e l e l e -

ment t h i s p a s t month.

John Kemper: During t h e removal of t h e Peach Bottom Core I f u e l

elements t h a t had developed cracked g raph i t e s l eeves , one of t h e f u e l

elements f e l l back i n t o t h e core . When a second a t tempt w a s made t o

remove t h e f a l l e n element only t h e upper s e c t i o n could be removed. A

s p e c i a l removal t o o l and jacking device weredeveloped and t e s t e d . The

bottom s e c t i o n was jammed on t h e s t andof f p in , b u t with t h e new t o o l i t

w a s s u c c e s s f u l l y removed without d i f f i c u l t y . A s of t h i s d a t e , a l l o f

t h e f u e l elements t h a t had experienced cracked s l eeves have been success-

f u l l y removed, and over 200 new Core I1 type elements have been loaded.

It i s planned TO have Core I1 loaded and the p l a n t r e tu rned t o power by

e a r l y t h i s summer. It i s a c r e d i t t o t h e r e f u e l i n g t e a m and t h e HTGR

concept t h a t wi th cons iderable unforeseen f u e l element removal problems,

t h e elements were a l l removed from t h e core i n a r e l a t i v e l y s h o r t per iod

of e lapsed t ime,

G. Meijer : Refer r ing t o your i r r a d i a t i o n experiments of 1100 Mwe

H E R f u e l , what are t h e des ign burnup, f luence and temperature condi t ions

f o r t he 1100 Mwe HTGR f u e l ?

K. P. Steward: M a x i m u m burnup i n m i x e d f i s s i l e particles i s 20%.

Maximum burnup i n UC2 b u f f e r d i l u t e d f i s s i l e p a r t i c l e s i s approximately

75% maximum; fast f luence expected i s approximately 8 x lo2' nvt .

mem temperatures experienced by t h e f u e l w i l l be 1350°C.

Maxi-

D. Tytga t : 1. What w i l l be t h e d i f f e r e n t pyrocarbon d e n s i t i e s

adopted f o r t h e coated p a r t i c l e s ? 2. The f u e l element experiments i n

Peach Bottom a r e supposed t o s t a y 1, 2, and 3 yea r s . Are these years

f u l l power ope ra t ing years or ca lendar y e a r s ?

K. P. Steward: 1. On BISO p a r t i c l e s t h e "high" d e n s i t y PyC w i l l be

Outer PyC 1.80-1.90 g/cm3 and the "low" d e n s i t y w i l l be 1 .55-1.65 g/cm3.

d e n s i t i e s on TRISO p a r t i c l e s w i l l be 1.70-1.80 g/cm3.

per iods r e f e r r e d t o are increments of 300 e f f e c t i v e f u l l power days. Each

2. The ope ra t ing

90

such increment w i l l t a k e up approximately one ca lendar year of Peach

Bottom i r r a d i a t i o n .

Hugh B. Stewart : John KemRer has pointed out t h e e f f e c t i v e way i n

which t h e cracked Core No. I f u e l elements have been removed. It i s

worth not ing t h a t the cracked s l eeves i n Core No. I occurred as a result of compact expansion due t o t h e very p r imi t ive coated p a r t i c l e s t h a t were,

used i n t h a t core. I n f a c t , Roy Huddle has pointed out t ha t t h e s e p a r t i -

c l e s were not coated p a r t i c l e s ; they were "covered" p a r t i c l e s . Core No. I1 w i l l use t h e h igh ly developed coated p a r t i c l e s now a v a i l a b l e t o us and t h e phenomenon tha t was observed i n Core No. I w i l l not occur i n

Core No. 11.

91

PLANT CONSTRUCTION EXPERIENCE

(Session 11)

92

Chairman : G. E. Locket t , OECD High Temperature Reactor P ro jec t

Co-Chairman: M. Bender, Oak Ridge Nat iona l Laboratory

Paper 1/105

FORT ST. V R A I N CONSTRUCTION PROGRESS -* - -' ~ ~ _. ."..--.=f*-"-h.

"-II_._ -_._-.- .-- .- * *- - H . N . Wellhouser

A s s i s t a n t P r o j e c t Manager For t S t . Vrain

San Diego, C a l i f o r n i a Gulf General Atomic

ABS TRACT

The 330-Mw(e) F o r t S t . Vrain Nuclear Generating S t a t i o n i s being b u i l t by Gulf General Atomic Incorporated a s prime c o n t r a c t o r f o r t h e P u b l i c Serv ice Company of Colorado as p a r t o f t h e USAEC Power Reactor Demonstration Program. Among t h e s i g n i f i c a n t design f e a t u r e s incorporated i n t h e p l a n t are t h e p r e s t r e s s e d concre te r e a c t o r v e s s e l , once-through modular steam g e n e r a t o r s with i n t e g r a l superhea ters and r e h e a t e r s , steam- d r i v e n axial-f low helium c i r c u l a t o r s , and hexagonal g r a p h i t e f u e l elements incorpora t ing improved coated f u e l p a r t i c l e s . Construct ion of t h e p l a n t f e a t u r e d t h e u s e of ground f r e e z i n g t o provide a cofferdam t o keep ground water from t h e excava- t i o n and preplaced aggregate concre te f o r t h e bottom head of t h e p r e s t r e s s e d concre te r e a c t o r vsse l . A s of March 31, 1970 , c o n s t r u c t i o n i s est imated t o be over 62% complete and on schedule f o r a completion d a t e of June 1 9 7 1 .

I N T R O D U C T I O N

The 330-Mw(e) For t S t . Vrain Nuclear Generating S t a t i o n , now under con-

s t r u c t i o n , i s being b u i l t by Gulf General Atomic Incorporated f o r t h e Publ ic

Service Company of Colorado and i n cooperat ion with t h e USAEC as p a r t of t h e

Power Reactor Demonstration Program. The s t a t i o n i s loca ted approximately

35 m i l e s n o r t h of Denver, Colorado, and i s scheduled t o go i n t o commercial

o p e r a t i o n i n 1 9 7 2 . Gulf General Atomic h a s r e t a i n e d Sargent & Lundy as

Architect-Engineer and EBASCO Services as c o n s t r u c t o r .

The F o r t S t . Vrain p l a n t i n c o r p o r a t e s a number of s i g n i f i c a n t des ign

f e a t u r e s new t o power r e a c t o r systems. The most prominent are t h e p r e s t r e s s e d

concre te r e a c t o r vessel (PCRV), once-through modular steam genera tors wi th

0 i n t e g r a l superhea ters and r e h e a t e r s , steam-driven ax ia l - f l o w helium c i r c u l a t o r s ,

93

94

and hexagonal g r a p h i t e f u e l e lements i nco rpora t ing improved pyrocarbon-si l i -

con ca rb ide coated f u e l p a r t i c l e s .

The flow diagram shown i n F ig . 1 p o i n t s ou t t h e major opera t ing parameters .

The helium coolant a t a p re s su re of about 700 p s i a flows downward through t h e

r e a c t o r core where i t i s hea ted t o 1430°F. The coolan t flow can be trimmed

by t h e use of o r i f i c e va lves loca t ed a t t h e top of t h e core t h a t are i n t e g r a l

wi th t h e c o n t r o l rod d r i v e mechanisms. From t h e r e a c t o r c o r e , t h e coolan t

f lows through t h e steam gene ra to r s . A f t e r pass ing through t h e steam gene ra to r s ,

t h e helium i s r e tu rned t o t h e co re a t a temperature of about 760°F by fou r

steam-turbine-driven helium c i r c u l a t o r s . Two i d e n t i c a l loops are used , each

inc luding a six-module steam gene ra to r and two helium c i r c u l a t o r s . Each loop

c o n t r i b u t e s h a l f t h e t o t a l ou tput of t h e nuc lear steam supply system, which produces s t e a m a t 2400 p s i g and 1000°F wi th s i n g l e r ehea t t o 1000°F. The

helium c i r c u l a t o r s are d r iven by t h e exhaust steam from t h e high-pressure

tu rb ine . This steam i s then rehea ted and r e tu rned t o t h e intermediate-pres-

s u r e t u r b i n e . The c i r c u l a t o r s are a l s o equipped with a Pe l ton water wheel

d r i v e so t h a t they may be d r iven us ing t h e b o i l e r feed pumps f o r emergency

cond i t ions .

The gene ra l r e a c t o r arrangement can be seen i n F ig . 2 . The p res t r e s sed

concre te r e a c t o r v e s s e l (PCRV) i s 31 f t i n i n t e r n a l diameter with a 75-ft

i n t e r n a l h e i g h t . The upper and lower heads a re nominally 15 f t t h i c k , and

t h e w a l l s have a nominal t h i ckness of 9 f t . Thus t h e PCRV provides t h e dua l

func t ion of conta in ing t h e coolan t a t ope ra t ing p res su re and a l s o providing

r a d i o l o g i c a l s h i e l d i n g . The e x t e r i o r v e r t i c a l s u r f a c e of t h e v e s s e l may be

descr ibed as an hexagonal prism wi th v e r t i c a l p i l a s t e r s a t each co rne r . I t

i s 61 f t ac ross p i l a s l t e r s , 49 f t ac ross f l a t s , and 106 f t h igh .

The conc re t e w a l l s and heads of t h e PCRV are cons t ruc ted around a carbon-

s t e e l l i n e r which i s nominally 314 i n . t h i c k . The l i n e r i s anchored t o t h e

concre te and provides a hel ium-t ight membrane. A s y s t e m of water-cooled tubes

welded t o t h e concre te s i d e of t h e l i n e r provides a heat-removal s y s t e m t o

c o n t r o l concre te temperature . I n a d d i t i o n t o t h e coolan t t ubes , a thermal

b a r r i e r i s provided on t h e i n s i d e su r face of t h e l i n e r t o l i m i t t he flow of

hea t t o t h e l i n e r w a l l s .

95

The top head has r e f u e l i n g p e n e t r a t i o n s t h a t a l s o house t h e c o n t r o l rod

d r i v e s . It a l s o inco rpora t e s p e n e t r a t i o n s and w e l l s t o house t h e helium

p u r i f i c a t i o n system and neut ron d e t e c t i o n chambers. The bottom head has

pene t r a t ions f o r each of t h e steam-generator modules and helium c i r c u l a t o r s

p lus a l a r g e c e n t r a l opening f o r access t o t h e main c a v i t y .

All of t h e p e n e t r a t i o n s through t h e PCRV are provided wi th two indepen-

dent c l o s u r e s . The PCRV i n n e r c a v i t y and t h e primary c losu res serve as primary

containment f o r t h e r e a c t o r ; t h e massive PCRV and t h e secondary c losu res a c t

as t h e secondary containment.

The v e s s e l i s p r e s t r e s s e d t o p l ace t h e conc re t e s t r u c t u r e i n compression

p r i o r t o s e r v i c e . The p r e s t r e s s i n g system used is known as a l i n e a r tendon

pos t t ens ion ing system. During cons t ruc t ion , s teel tubes are embedded i n t h e

v e s s e l conc re t e f o r later i n s e r t i o n of t h e tendons, each of which c o n s i s t s of

up t o 170 1/4-in.-diameter "thermalized" w i r e s .

means of cold-deformed bu t ton heads t o a washer assembly t h a t t r a n s f e r s and

d i s t r i b u t e s t h e loads over s p l i t shims and a s teel bear ing p l a t e i n t o the

concre te . Three tendon arrangements are used: 90 l o n g i t u d i n a l ( v e r t i c a l )

tendons, 310 c i r cumfe ren t i a l tendons , and 48 crosshead tendons.

The w i r e s are anchored by

I n s i d e t h e PCRV i s t h e core suppor t f l o o r , a water-cooled s t r u c t u r e of s teel and r e in fo rced conc re t e 5 f t t h i c k . It is supported from t h e bottom

of t h e PCRV c a v i t y by 1 2 water-cooled s tee l columns. Twelve duc t s conduct

r e a c t o r - o u t l e t helium through t h e co re support f l o o r t o t h e 1 2 steam gene ra to r

modules loca t ed below.

CONSTRUCT I ON

One of t h e f i r s t t h ings done a f t e r moving onto t h e cons t ruc t ion s i t e

i n Apr i l of 1968 w a s t o pour a conc re t e pad ad jacen t t o t h e area where t h e

r e a c t o r bu i ld ing would be loca ted .

e r e c t i o n of t h e PCRV l i n e r bottom head and o t h e r l i n e r components, thus allow-

ing work t o be c a r r i e d ou t concurren t ly wi th t h e cons t ruc t ion of t h e r e a c t o r

This provided a working area f o r t h e

@

36

b u i l d i n g foundat ion and t h e support r i n g f o r t h e r e a c t o r v e s s e l . Figure 3

shows t h e l i n e r assembly area and t h e r e a c t o r bu i ld ing excavat ion. By

car ry ing ou t t h e PCRV l i n e r f a b r i c a t i o n concurren t ly with t h e c o n s t r u c t i o n

of t h e r e a c t o r b u i l d i n g foundat ion, t h e schedule w a s shortened s i g n i f i c a n t l y .

Concurrently w i t h t h e s e two o p e r a t i o n s a plywood mockup of a s e c t i o n of t h e

PCRV bottom head w a s cons t ruc ted f o r use i n t r a i n i n g t h e c o n s t r u c t i o n f o r c e

and t o h e l p i n planning f u t u r e opera t ions .

Construct ion of t h e r e a c t o r b u i l d i n g r e q u i r e d an excavation t o bedrock,

which w a s l o c a t e d some 50 t o 55 f t below ground leve l . The excavat ion w a s

t o be roughly 100 f t x 150 f t x 50 f t deep w i t h a d d i t i o n a l p e n e t r a t i o n i n t o

bedrock of 25 f t i n some areas. This excavat ion requi red t h e removal of about

30,000 cu yd of sand and g r a v e l t o expose t h e P i e r r e s h a l e bedrock and some-

what over 3,200 cu yd of s h a l e . Excavation work w a s complicated by the f a c t

t h a t t h e w a t e r t a b l e w a s a t a depth of 20 t o 25 f t i n sands having a high

t r a n s m i s s a b i l i t y and w a s f u r t h e r complicated by t h e c h a r a c t e r i s t i c of t h e

P i e r r e s h a l e t o decompose upon exposure t o a i r . For normal excavat ion of

t h i s type of sandy s o i l , s i d e s l o p e s would have had t o be a t l eas t 2-112 t o

1. This would have increased t h e yardage t o be excavated and would a l s o have

caused a l a r g e p o r t i o n of t h e area f o r t h e t u r b i n e b u i l d i n g t o be u n a v a i l a b l e

f o r work as long as t h e excavat ion w a s open.

Although s h e e t p i l i n g and convent ional w e l l p o i n t dewatering w a s considered,

t h e underground water flow w a s s o s t r o n g t h a t i t w a s ques t ionable whether t h i s

method could have s u c c e s s f u l l y kept t h e excavat ion dry. The ground f r e e z i n g

technique w a s s e l e c t e d as a b e t t e r method and because i t enabled n e a r l y ve r t i -

cal s i d e excavat ion and provided a w a l l capable of support ing heavy loads ,

t hus permi t t ing c l o s e access t o t h e work.

The freeze-wal l method of providing a cofferdam, shown i n Fig. 4 , i s

r e l a t i v e l y new i n t h i s country. For an oblong excavat ion such as For t S t .

Vrain, t h e w a l l , i n p lan view, is roughly an e l l i p s e . The th ickness of t h e

f rozen w a l l averaged about 6 f t a t t h e t o p , and a t rock e l e v a t i o n , about 8

f t . (Four l o c a t i o n s were d e l i b e r a t e l y thickened t o serve as crane p la t forms . )

97

To c o n s t r u c t t h i s w a l l , 180 U-tube p i p e assemblies c o n s i s t i n g of two

55-ft s e c t i o n s of 1-1/2-in. p ipe connected by a r e t u r n bend a t t h e bottom,

w e r e sunk i n t o t h e ground on t h e per imeter i n p r e d r i l l e d 4-in. h o l e s f i l l e d

wi th a b e n t o n i t e s l u r r y as a d r i l l i n g a i d . Ninety more assemblies were sunk

on t h e i n n e r pe r ime te r , and an a d d i t i o n a l 90 assemblies were used f o r c rane

pads and gan t ry t r a c k pads. All of t h e U-tubes were connected t o common

b r i n e supply and r e t u r n headers , which r a n from t h e r e f r i g e r a t i o n compressor

house near t h e n o r t h e a s t norner of t h e excavat ion . Af t e r t h e w a l l w a s f rozen ,

t h e b r i n e (wi th a f r e e z i n g po in t of -45°F) w a s cont inuous ly c i r c u l a t e d through

a l l of t h e p i p e s a t a supply temperature of -11°F and a r e t u r n temperature

of -6°F.

s ing le -cyc le ammonia compressors. Each r e f r i g e r a t i o n u n i t had i t s own cool ing

tower. I n t h e event of a power f a i l u r e , a standby d i e s e l gene ra to r of 625

kVA capac i ty w a s connected t o t h e system.

The r e f r i g e r a t i o n system cons i s t ed of two 85-ton and one 66-ton

Wall f r e e z i n g w a s s t a r t e d on May 6 , 1968, and by June 3 , 1968, most of

t h e w a l l w a s f rozen . I n o rde r t o exped i t e c l o s u r e i n some areas, a d d i t i o n a l

U-tubes w e r e i n s t a l l e d , and l i q u i d n i t r o g e n w a s in t roduced . I n o t h e r areas

t h a t proved d i f f i c u l t t o c l o s e , a d d i t i o n a l ho le s w e r e d r i l l e d and a c l ay

s l u r r y w a s in t roduced t o impede t h e flow and permit f r e e z i n g t o occur.

To p r o t e c t t h e sha le- type bedrock from d e t e r i o r a t i o n by exposure t o a i r , the shale had t o be covered w i t h conc re t e o r g u n i t e w i t h i n e i g h t hour s . It

w a s imposs ib le t o p r o t e c t t h e s h a l e and t o l i n e d r i l l and b l a s t s imul taneous ly .

A s a r e s u l t t h e s h a l e w a s excavated only t o rough l i m i t s , l e av ing about 9 t o

1 2 i n . of s h a l e i n p l ace . The s h a l e w a s t hen c u t back t o f i n i s h l i m i t s and

p ro tec t ed wi th g u n i t e on t h e v e r t i c a l s u r f a c e s and conc re t e on t h e h o r i z o n t a l

s u r f a c e s .

The excavat ion w a s completed i n t i m e t o permit t h e f i r s t permanent con-

crete t o be poured on September 18, 1968, fo l lowing t h e award of t h e cons t ruc-

t i o n permit on September 17 , 1968. The f reeze-wal l system remained i n ope ra t ion

u n t i l t h e r e a c t o r b u i l d i n g basement w a l l s were completed and b a c k f i l l e d t o

grade i n February 1969.

98

The suppor t r i n g f o r t h e PCRV, which i s a c y l i n d e r 50 f t i n o u t s i d e

diameter , 3 f t 6 i n . t h i c k , and 33 f t h igh , requi red 575 cu yd of concre te .

When a l l t h e s tee l w a s complete, t h e concre te w a s placed i n a continuous pour

over a per iod of 18 hours us ing Pumpcrete machines. The f i n a l seismic

des ign f o r t h e r i n g requi red a much l a r g e r q u a n t i t y of r e i n f o r c i n g s teel

( s e e F ig . 5) than w a s o r i g i n a l l y a n t i c i p a t e d . Because of t h i s , a 14-week

de lay occurred i n t h e completion of t h e r i n g , which took p lace on February

13, 1969.

Meanwhile, work on t h e bottom head assembly c o n s i s t i n g of t h e s teel

l i n e r , p e n e t r a t i o n s , and t h e co re suppor t f l o o r columns, which w a s s t a r t e d

i n June 1968, w a s completed, and t h e assembly w a s moved t o i t s permanent posi-

t i o n over t h e PCRV support r i n g i n February 1969. This assembly, which weighed

s o m e 400 tons inc lud ing a temporary e r e c t i o n j i g , s t ee l f o r m s f o r t h e b o t t o m

head, and some of t h e r e i n f o r c i n g s teel , w a s moved on r a i l s by a modified

t u r b i n e g e n e r a t o r s t a t o r l i f t i n g r i g and w a s lowered i n t o p l a c e on temporary

support s tee l e r e c t e d w i t h i n t h e support r i n g . F igure 6 shows t h e assembly

during t h e move.

The c o n s t r u c t i o n sequence r e q u i r e d t h a t t h e bottom head l i n e r p l a t e be

i n p l a c e p r i o r t o c o n c r e t e placement, i n o r d e r t o a s s u r e i n t i m a t e c o n t a c t

between t h e concre te and t h e unders ide of t h e bottom head l i n e r and penetra-

t i o n s . It w a s t h e r e f o r e decided t o u s e t h e Preplaced Aggregate Concrete

(PAC) technique i n t h e bottom head t o ensure s a t i s f a c t o r y q u a l i t y . A s t h e

name i m p l i e s , t h e PAC technique c o n s i s t s of p lac ing c l e a n coarse aggregate

i n t o t h e forms. The aggrega te i s placed i n l a y e r s t o avoid formation of

l a r g e v o i d s i n t h e aggrega te mass, and t o achieve i n t i m a t e contac t wi th a l l

embedments. Grout i s pumped under p r e s s u r e i n a continuous opera t ion through

previous ly embedded p i p e s t o f i l l t h e v o i d s e x i s t i n g i n t h e aggregate m a s s .

For t h e F o r t S t . Vrain PAC, t h e v o i d s amounted t o about 48 per cen t of t h e

aggregate volume.

A f u l l - s c a l e mockup of a s e c t i o n of t h e bottom head w a s made us ing PAC

t o i n v e s t i g a t e t h e concre te s t r e n g t h p r o p e r t i e s as w e l l as o t h e r v a r i a b l e s

of PAC and t o t es t t h e c a p a b i l i t y of PAC g r o u t pumping equipment, t h e capabi

99

of t h e b a t c h p l a n t t o mix t h e grout w i t h i n t h e s t r l n g e n t l imi - t a t ions of t h e

s p e c i f i c a t i o n s , and t h e c a p a b i l i t y of monitoring equipment t o be used f o r

grout l eve l i n d i c a t i o n , and t h e a b i l l t y t o p l a c e t h e PAC without v o i d s . The

r e s u l t s of t h e mockup were h i g h l y encouraging a r d provided information t h a t

w a s v a l u a b l e i n o p e r a t i o n a l planning f o r t h e PAC f o r t h e bottom head.

A f t e r a l l t h e bottom head embedments, inc luding tendon tubes and r e i n -

f c r c i n g s t ee l w e r e i n s t a l l e d (Fig. 7 ) , 1400 cu yd of 1-1/2-in. aggregate

w e r e p laced i n t o t h e bottom head forms. The aggregate w a s screened and washed

before being loaded on t r u c k s f o r d e l i v e r y t o t h e PCRV. A t t h e d e l i v e r y p o i n t ,

i t w a s aga in passed through a r o t a r y washer and discharged i n t o a holding

hopper. From t h e holding hopper t h e aggregate w a s placed i n t o p o s i t i o n by

hand (Fig. 8) .

Before t h e grout ing opera t ion w a s s t a r t e d , t h e aggrega te w a s cooled by

f l u s h i n g and p a r t l y f i l l i n g t h e forms with cold water and i c e .

i n g r e d i e n t s , sand, f l y a s h , cement, i n t r u s i o n a i d ( a combination of f l u i d i f i e r

and expansion a g e n t ) , w a t e r , and i ce w e r e batched and measured by weight 21%

by a Gulf General Atomic automatic concre te batching p l a n t , which i s loca ted

on t h e s i t e and provides a l l of t h e j o b concre te . The p l a n t i s r a t e d a t

100 cu y d l h r (with provis ions f o r h e a t i n g o r cool ing t h e concent r ic mix) and

i s capable of handl ing a l l requi red des ign mixes.

l a b o r a t o r y , which inc ludes a 600,000-lb Forney t e s t i n g machine, permits t e s t i n g

of a l l concre te and r e i n f o r c i n g s t ee l and inc ludes t h e c a p a b i l i t y of t e s t i n g

t o f a i l u r e a s i z e 18s b a r .

The grout

A well-equipped t e s t i n g

The ba tches w e r e loaded i n t o concre te t r a n s i t mixing t r u c k s t h a t premixed

t h e g r o u t .

a g i t a t o r u n i t s and 12 pumps, w a s e r e c t e d a t t h e PCRV s i t e . The mixer-agi ta tor

discharged t h e grout i n t o a common manifold t o which t h e 1 2 h o r i z o n t a l p i s t o n

pumps were connected.

a c t u a l l y i n u s e during t h e bottom head grout ing opera t ion , t h e remaining equip-

ment being on s tandby.

wi th cool ing c o i l s and i n s u l a t i o n t o maintain t h e grout a t 45" t 3°F.

A mixing and pumping p l a n t , c o n s i s t i n g of t h r e e s t a t i o n a r y mixer-

Only two of t h e mixer -ag i ta tors and e i g h t pumps w e r e

The mixer -ag i ta tors and t h e manifold were provided

P r i o r

100

t o release of t h e g rou t from t h e mixe r -ag i t a to r s i n t o t h e manifold, t h e t e m -

p e r a t u r e and f l u i d i t y of t h e g r o u t were t e s t e d . Each pump served one i n s e r -

t i o n p o i n t a t a t i m e . The i n s e r t i o n p o i n t s were changed as t h e g rou t ing

ope ra t ion proceeded.

Grout flowed under pump p r e s s u r e through t h e g rou t p ipes i n t o t h e

aggrega te .

h o r i z o n t a l l a y e r s . The number and l o c a t i o n of t h e h o r i z o n t a l g rou t p i p e s

w e r e s u f f i c i e n t l y redundant t o provide f u l l and uniform grout coverage of

t h e PCRV bottom head.

Hor i zon ta l l-in.-diameter g r o u t p ipes w e r e placed i n several

Grout i n j e c t i o n s t a r t e d from t h e lowest p o i n t i n t h e form a t t h e cen te r

of t h e aggrega te mass. A s t h e g rou t ing o p e r a t i o n proceeded t h e g rou t t ubes

w e r e s lowly r e t r a c t e d t o produce an upward and outward g rou t f l o w from the

c e n t e r to t h e per iphery . The s l o p e of t h e advancing g rou t w a s such t h a t

entrapment of a i r w a s prevented, and displacement of l a i t a n c e and water

toward t h e ven t ing areas w a s achieved.

Shor t v e r t i c a l g rou t p i p e s w e r e employed f o r topping o f f a t t h e construc-

t i o n j o i n t . A s u r p l u s of g rou t w a s i n j e c t e d t o more than f i l l t h e aggregate

m a s s u n t i l overf low of und i lu t ed g rou t w a s observed a t t h e s u r f a c e of t h e

aggrega te . D i lu t ed g rou t w a s c o n t i n u a l l y broomed o f f t h e s u r f a c e . The grout-

i ng o p e r a t i o n took 5 4 hours and w a s completed on June 2 1 , 1969.

The u s e of sounding o r obse rva t ion w e l l s , which are u s u a l l y employed

f o r g rou t - l eve l d e t e c t i o n and c o n t r o l i n preplaced aggregate conc re t e work,

w a s n o t f e a s i b l e f o r t h e PAC i n t h e PCRV because of t h e requirement t h a t no

openings were allowed through t h e bottom l i n e r p l a t e .

f o r determining g rou t levels w a s t h e r e f o r e developed, u s ing t h e p r i n c i p l e of

t i m e domain r e f l ec tomet ry . The p r i n c i p l e i s based upon sending a f a s t - r i s e

p u l s e down a c a b l e and observing t h e amplitude c h a r a c t e r i s t i c s of t h e r e f l e c t e d

pu l se . A s t h e t r ansmi t t ed p u l s e encounters d i s c o n t i n u i t i e s i n t h e c a b l e ,

marked changes i n amplitude are d e t e c t e d . One of t h e cab le d i s c o n t i n u i t i e s

t h a t can be d e t e c t e d i s t h e p o i n t i n t h e cab le where t h e surrounding medium

changes from a i r t o water o r g r o u t .

An e l e c t r o n i c method

The s i g n a l w a s d isplayed on an o s c i l l o s c

101

There were 94 p a i r s of sensor w i r e s , each w i r e running t h e f u l l depth of

t h e aggrega te m a s s . The complete p r o f i l e of t h e g rou t a t any t i m e dur ing t h e

g rou t ing ope ra t ion could thus be observed and displayed i n a three-dimensional

p l a s t i c model. The continuous v i s u a l d i s p l a y i n t h e model of t h e g rou t l e v e l s

allowed a c o n t r o l l e d grout ing ope ra t ion . A f t e r completion of t h e g rou t ing

ope ra t ion , j u s t below where t h e i n i t i a l s e t of t h e g rou t w a s expected, t h e

w i r e mesh on t h e c o n s t r u c t i o n j o i n t s u r f a c e w a s removed i n p repa ra t ion f o r t h e

"green cu t t i ng ' ' o p e r a t i o n , which w a s s t a r t e d a f t e r t h e i n i t i a l s e t b u t p r i o r

t o a f u l l hardening of t h e g rou t . Green c u t t i n g removed t h e s u r f a c e g rou t

and exposed t h e aggregate without loosening o r breaking t h e aggregate p a r t i c l e s .

The c o n s t r u c t i o n j o i n t s u r f a c e w a s wet-cured t h e r e a f t e r .

The l a r g e mass of t h e bottom head PAC, approximately 1600 cu yd, r equ i r ed

c a r e f u l temperature c o n t r o l dur ing t h e hydra t ion phase. One hundred and f o r t y

thermocouples measured t h e conc re t e tempera tures . The cool ing system w a s de-

s igned t o ma in ta in conc re t e cu r ing temperatures below 130°F. The a c t u a l mea-

sured maximum temperature w a s 117°F. Cooling w a s provided by (1) c i r c u l a t i n g

r e f r i g e r a t e d water through 70 s e l e c t e d v e r t i c a l tendon tubes and through

supplemental p ipes ( 2 i n . d iameter ) embedded i n t h e c o n c r e t e , ( 2 ) c i r c u l a t i n g

r e f r i g e r a t e d water i n t o t h e bottom head l i n e r p e n e t r a t i o n s , and (3) maintaining

t h e ambient temperature of t h e s u r f a c e s t o about 60°F by a system of coo l water

sp rays and w e t bu r l ap . A 190-ton cool ing system provided t h e necessary r e f r i g - e r a t e d water . Postcool ing w a s commenced a t t h e completion of g rou t ing and w a s

terminated when t h e average conc re t e temperature w a s down t o 70" & 1 0 ° F .

The remainder of t h e conc re t e f o r t h e PCRV w a s convent ional ly mixed and

p laced . The conc re t e , which i s designed f o r a s t r e n g t h of 6000 p s i , con ta ins

aggregate t h a t i s c a r e f u l l y c o n t r o l l e d f o r g r a d a t i o n , f r i a b l e p a r t i c l e s , and

o rgan ic con ten t . The conc re t e temperature i s l i m i t e d t o 130°F dur ing cu r ing

t o minimize shr inkage and high-temperature-gradient c racks . An ex tens ive

i n s p e c t i o n and t e s t i n g program w a s developed f o r t h e vessel conc re t e t o ensure

compliance w i t h t h e s p e c i f i c a t i o n .

I n p r e p a r a t i o n f o r pouring t h e s i d e w a l l conc re t e l i f t s , t h e l i n e r b a r r e l

s e c t i o n s were completed i n t h e assembly area, moved i n t o t h e r e a c t o r b u i l d i n g

102

(F ig . 9 ) , and welded i n p l ace . Rebar, tendon tubes , and forms w e r e placed

f o r t h e 1 4 success ive l i f t s of about 5 f t each making up t h e PCRV s idewal l .

The f i r s t l i f t w a s poured on J u l y 17 , 1969, with t h e four teenth on January 7 ,

1970. I n t h e meantime, t h e top head l i n e r has been completed and i n s t a l l e d

(Fig. lo), and prepara t ions w e r e under way f o r t h e t h r e e l i f t s of t h e top head.

The f i r s t of t h e s e l i f t s w a s poured on January 30, and t h e f i n a l pour t o com-

p l e t e t h e PCRV w a s made on February 1 3 , one week ahead of schedule (F ig . 11 ) .

This i s p a r t i c u l a r l y impressive because i t inc ludes 1 4 weeks l o s t on t h e

schedule during t h e c o n s t r u c t i o n of t h e support r i n g . The t o t a l concrete

placed i n t h e PCRV i s about 6500 cu yd, inc luding t h e 1600-cu yd PAC i n t h e

bottom head.

The i n s t a l l a t i o n of t h e tendons w a s s t a r t e d i n October 1969. The tendons

are p a r t i a l l y f a b r i c a t e d when they a r e rece ived a t t h e s i t e . Af te r they are

pul led through t h e in-place tendon tubes , they are buttonheaded and then pre-

s t r e s s e d wi th 1000-ton j a c k s from both ends of curved tendons and one end of

t h e s t r a i g h t tendons (F ig . 1 2 ) . The f i r s t tendon w a s s t r e s s e d on January 16 ,

1970. A s of March 31, a l l 4 4 8 tendons i n t h e PCRV have been i n s t a l l e d , and

1 4 2 have been s t r e s s e d .

A s t h e concre te was being placed f o r t h e PCRV s i d e w a l l s and top head,

workers continued wi th t h e PCRV i n t e r n a l s . The i n t e r n a l s inc lude t h e l i n e r

i n s u l a t i o n , support columns f o r t h e co re support f l o o r , t h e co re support

f l o o r , and c o r e b a r r e l . S team g e n e r a t o r i n s t a l l a t i o n w a s s t a r t e d i n January

1970. This o p e r a t i o n w a s begun by i n s t a l l i n g t h e secondary modules, which

r e s i d e w i t h i n t h e p e n e t r a t i o n s i n t h e PCRV bottom head (F ig . 1 3 ) .

I n s t a l l a t i o n of t h e helium c i r c u l a t o r s and t h e f u e l t r a n s f e r machine

i s scheduled t o begin i n J u l y - August of t h i s yea r . The convent ional p a r t

of t h e p l a n t i s progress ing a t an e q u a l l y r a p i d pace and i s on schedule .

S ince moving onto t h e c o n s t r u c t i o n s i t e i n A p r i l 1968, over 50,000 cu yd

of e a r t h have been excavated, n e a r l y 38,000 cu yd of concre te have been poured,

and over 3,700 tons of s t ee l have been e r e c t e d o r i n s t a l l e d . A s of March 31,

1970, t h e c o n s t r u c t i o n i s es t imated t o be over 62% complete and on schedule .

F igure 1 4 i s a r e c e n t o v e r - a l l view of t h e c o n s t r u c t i o n s i t e .

Cons t ruc t ion i s on schedule f o r a completion d a t e of June 1971.

Fig. 1. Simplified flow diagram.

104

Fig . 2 . General arrangement of the primary reactor components.

F i g . 3 . V i e w of construct ion s i te showing l i n e r assembly area t o the l e f t of reactor bui ld ing excavat ion.

I 105

NOTE: Tvpical Only. A l l Fraaze Points Ara

Nat Shown

0 E X C A V A T I O N , ’ L I M I T S

c

Fig. 4 . Plan view of freeze-wall cofferdam.

Fig. 5. Top of support r i ng rebar s t r u c t u r e showing PCRV support hinge mounting corbels.

106

Fig. 6. PCRV bottom head assembly (approximately 400 tons) being moved to pos i t ion on temporary support s t e e l i n s ide the support r i ng (covered with p l a s t i c ) .

F ig . 7 . PCRV bottom head re inforc ing s t e e l and tendon tubes.

Fig . 8. S p e c i a l aggrega te be ing p rep laced i n t h e bottom head p r i o r t o t h e pumping of t h e g r o u t .

F ig . 9 . L i n e r s e c t i o n s two and t h r e e be ing lowered i n t o p o s i t i o n on t o p of t h e f i r s t l i n e r s e c t i o n . This double s e c t i o n is 34 f t h igh .

108

Fig. 10. PCRV l i n e r top head i n its permanent loca t ion p r i o r t o t h e pouring of the 14th and f i n a l s idewal l l i f t .

Fig. 11. Fina l concrete placement i n the PCRV top head

109

Fig. 12. 1000-ton jack in place for stressing a circumferential tendon

Fig. 13. One of the secondary steam generator modules being moved into position for installation in a bottom head penetration.

i?

Fig. 14 . Over-all view of c o n s t r u c t i o n s i t e showing switch-yard ( r i g h t foreground) , r e a c t o r b u i l d i n g - t u r b i n e bui ld ing ( c e n t e r ) , and main cool ing tower ( l e f t r e a r ) .

111

0 DISCUSS I ON

D. B. Trauger: Futuhe r e a c t o r s a r e of t he pod design which i s a

more complex conf igura t ion .

t i o n experience e x t r a p o l a t e t o t h e f u t u r e v e s s e l s , p a r t i c u l a r l y w i t h

placement of concrete and p r e s t r e s s i n g sequence?

c u l t o r e a s i e r ?

How does t h e For t S t . Vrain v e s s e l construc-

W i l l t hey be more d i f f i -

H. N. Wellhouser: We b e l i e v e t h e pod design i s s impler t o cons t ruc t .

The concrete placement experience

The concrete placement

There a r e many fewer tendons t o stress. gained w i t h F o r t S t . Vrain i s d i r e c t l y r e l a t e a b l e .

requirements f o r t h e 1100 Mwe v e s s e l should be cons iderably e a s i e r than

For t S t . Vrain because of t h e absence of c ross head and c i r cumfe ren t i a l

tendons.

G. Mei je r : Can you give the cons t ruc t ion time being es t imated f o r

t h e 1100 Mwe HTGR?

H. N. Wellhouser: Actual cons t ruc t ion t i m e for t h e nuc lear steam

supply system i s est imated t o be 37 months.

G. E. Locket t : I understood you t o say t h a t t h e maximum l i f t was about 190 tons and th i s was done by a 160 t o n crane.

e s t e d t o know how t h i s was achieved.

I should be i n t e r -

H. N. Wellhouser: I n t h e U.S . , crane manufacturers a l l o w f o r a 25%

overload provided t h e overload l i f t s a r e done inf’requently. I n t h i s case

w e worked c l o s e l y wi th t h e manufacturer so as not t o void t h e warranty.

M. Bender: I n t h e pas t , t h e r e have been claims t h a t concrete ves-

s e l s would shor t en the cons t ruc t ion schedule f o r a r e a c t o r p l a n t over

t h a t requi red wi th s teel v e s s e l s because of t h e d e l i v e r y per iod needed

f o r steel vesse l s . Could you comment on the v a l i d i t y of t h a t claim on

t h e b a s i s of Fo r t S t . Vrain exper ience? Could a concrete v e s s e l be con-

s t r u c t e d i n t h e equiva len t per iod of t i m e needed f o r a concrete contain-

ment v e s s e l ?

H. N. Wellhouser: It seems t h e cons t ruc t ion t ime f o r t h e PCRV i s

somewhat s h o r t e r than f o r v e s s e l s common t o water r e a c t o r s . With the

112

except ion of t he r e a c t o r i n t e r n a l s , the For t St . Vrain PCRV could be

completed and s t r e s s e d i n about 24 months.

B. G. Chapman: What techniques have been used f o r providing the

l e a k t i g h t n e s s o f t h e PCRV l i n e r ?

H. N. Wellhouser: A l l welds on the PCRV l i n e r a r e 106 radiographed

and helium l eak checked.

G. D. Whitman: What i s the s t a t u s of t h e p r e s t r e s s i n g ope ra t ion?

H. N. Wellhomer: A l l 448 tendons have been i n s t a l l e d and s t r e s s e d .

The las t tendons were stressed on A p r i l 24, 1970.

T. A. J aege r : H a s the des ign of t h e p re s t r e s sed concrete r e a c t o r

pressure v e s s e l been developed on the b a s i s of t h e convehtional s tandard

s p e c i f i c a t i o n s f o r p r e s t r e s s e d concrete o r d id you apply s p e c i f i c a l l y

developed des ign c r i t e r i a ?

Concerning the u t i l i z a t i o n of the ground breathing technique, I

would l i k e t o know whether a d e t a i l e d d e s c r i p t i o n of t h e experiences, es-

p e c i a l l y of t h e t roub le s experienced, i s a v a i l a b l e .

H. N. Wellhomer: Concrete s tandards were used where a p p l i c a b l e as

w a s ASME Sec t ion I11 nuclear code.

I don ' t know of any publ ished r e p o r t tha t covers t h i s work i n t h e

d e t a i l I th ink you a r e looking f o r .

Paper 2/120

THE RELATIONSHIP OF THE HTR WITH EARLIER GAS-COOLFD RFACTORS

C.S. Lowthian

ABSTRACT

Many of t h e p o t e n t i a l advantages of t h e HTR stem from t h e concept of p a r t i c l e f u e l and t h e use of helium as a cool- an t . It is t h e r e f o r e s i g n i f i c a n t l y d i f f e r e n t i n seve ra l a spec t s from t h e type of r e a c t o r a l ready developed i n t h e UK, but it is s t i l l b a s i c a l l y a gas-cooled r e a c t o r system. The design and opera t ing experience gained with t h e magnox r e a c t o r s and t h e AGR, even i f not immediately app l i cab le , i s d i r e c t l y re levant t o determining t h e type of problems l i k e l y t o be encountered and t o f i n d i n g t h e economic so lu t ions . This paper reviews t h e design, experimental and cons t ruc t ion- a1 experience of e a r l y gas-cooled r e a c t o r s which is app l i c - a b l e t o t h e HTR.

INTRODUC T I ON

A nuc lear power s t a t i o n i s a l a r g e and t e c h n i c a l l y complicated in-

vestment made wi th t h e s i n g l e objec t of producing e l e c t r i c i t y economically.

Innovations a r e requi red t o reduce both t h e c a p i t a l cos t and t h e running

cos t o f new power s t a t i o n s , but must not be introduced at t h e expense of

reliability. An investment will only pay for itself if a high l o a d fac-

t o r can be achieved.

continued r e l i a b i l i t y i s t o develop a family o f r e a c t o r s , each drawing

on t h e experience of i t s forbears .

One way of achieving successfu l innovat ion with

I n t h e UK t h e gas-cooled r e a c t o r has been developed over 20 years .

The i n i t i a l dec is ion t o adopt gas cool ing and g raph i t e moderation w a s taken f o r economic reasons r e l a t i n g t o t h e a v a i l a b i l i t y of d i f f u s i o n

p lan t and heavy water. The e a r l y magnox r e a c t o r s have now been operat-

i ng success fu l ly f o r a number of y e a r s with high load f a c t o r s and, des-

p i t e t h e i r high i n i t i a l c a p i t a l c o s t , have met t h e i r t a r g e t genera t ion

cos ts .

113

114

The High Temperature Reactor (HTR) i s t h e l a t e s t development i n

t h e design of gas-cooled r eac to r s .

ment and power s t a t i o n cons t ruc t ion a r e going ahead rap id ly .

UK t h e ex tens ive experience of t h e design and cons t ruc t ion of gas-

cooled r e a c t o r s is a v a i l a b l e t o t ake maximum advantage of t h e p o t e n t i a l

of t h e HTR. This paper reviews some of t h e r e l evan t experience and

i n d i c a t e s how t h e des ign i s be ing evolved.

I n t h e US and i n Germaqy develop-

In t h e

COOLANT GAS

General P r o p e r t i e s

The coolant gas is t h e heat t r a n s f e r and heat t r a n s p a t medium

wi th in t h e r e a c t o r system. The o v e r a l l p r o p e r t i e s of helium a r e l i t t l e

d i f f e r e n t from those of carbon dioxide i n t h i s r e spec t . The low dens i ty

is o f f s e t by t h e high s p e c i f i c heat so t h a t f l o w v e l o c i t i e s a r e not

g r e a t l y d i s s i m i l a r , while dynamic f o r c e s a r e decreased. I n l o c a l a r e a s

t h e p a r t i c u l a r p r o p e r t i e s of helium l e a d t o no t i ceab le d i f f e rences . For

example, t h e high thermal conduct iv i ty o f helium r e q u i r e s a th i ckness

of i n s u l a t i o n pack up t o t h r e e t imes t h a t i n carbon dioxide. I n gene ra l ,

however, because che p r o p e r t i e s a r e s o wel l known, t h e e f f e c t s can be

c a l c u l a t e d and t h e experience wi th carbon d ioxide can be r e a d i l y u t i l i s e d .

The chemical p r o p e r t i e s of helium a r e markedly d i f f e r e n t from those

of carbon d ioxide , t h e oxygen content of which has l e d t o cor ros ion

problems wi th l o w grade m i l d s t e e l s . Helium i s chemically i n e r t , f o r

p r a c t i c a l purposes, i n a r e a c t o r c i r c u i t , but t h e i n i t i a l experiences

at Dragon and Peach Bottom have suggested t h e increased importance of

t r a c e impur i t ies . The c i r c u i t gas w i l l have a hydrogen l e v e l o f t h e

order of 10 ppm, because of t h e chemical r educ t ion of water f rom minute

l e a k s from t h e b o i l e r s and from d i f f u s i o n of hydrogen through t h e b o i l e r

tubes.

I n t h e r e s u l t a n t reducing atmosphere t h e metal components o f t h e

c i r c u i t may l o s e t h e i r normal p r o t e c t i v e oxide l a y e r , l eav ing them open

t o a t t a c k from f u r t h e r impur i t i e s . I n p a r a l l e l , t h e su r face p r o p e r t i e s

of t h e m a t e r i a l s w i l l be d i f f e r e n t t o those i n an ox id i s ing atmosphere,

and f r i c t i o n and wear phenomena must be c a r e f u l l y inves t iga t ed .

115

Chemical Tropert i e s

A nuc lea r power s t a t i o n has t o be b u i l t n e c e s s a r i l y f o r at l e a s t a 20 y e a r l i f e . The s t r eng th , and p a r t i c u l a r l y t h e c reep performance, of

most common engineer ing m a t e r i a l s a r e known f o r such long opera t ing

per iods , o r can at l e a s t be conf ident ly predicted. The e f f e c t of t h e

environment wi th in a nuc lea r r e a c t o r is u n c e r t a i n however, so du r ing the.

e a r l y design per iod of t h e magnox r e a c t o r s ex tens ive experimental pro-

grammes were i n i t i a t e d . of p a r t i c u l a r i n t e r e s t w a s t h e chemical r e a c t i o n

between t h e coolant , carbon dioxide, and t h e ma te r i a l s of those compon-

e n t s i n t h e r eac to r , which because of r a d i a t i o n e f f e c t s could only be

r ep laced with g rea t d i f f i c u l t y .

0

There w a s a h o w n r e a c t i o n between t h e gas and t h e g raph i t e modera-

t o r , t h e r e a c t i o n r a t e be ing dependent on bo th t h e temperature and t h e

neutron f lux . This r e a c t i o n w a s i n v e s t i g a t e d u s i n g complex in-p i le rigs

and t h e design information produced has beery e n t i r e l y s a t i s f a c t o r y , as

shown by t h e successfu l opera t ion of t h e Calder r e a c t o r s f o r I 5 years .

The r e a c t i o n s wi th o the r m a t e r i a l s were a l s o i n v e s t i g a t e d and similar design information w a s produced. Unfortunately t h e e f f e c t o f break-away

cor ros ion i n low s i l i c o n semi-kil led s t e e l s was not f u l l y appreciated.

O f l i t t l e concern on open su r faces , it can produce s t r a i n s i n mechanically

r e s t r a i n e d assemblies and t h i s was not n o t i c e d u n t i l inspec t ion of t h e

Bradwell r e a c t o r s after they had been running success fu l ly f o r 6 years .

It has been found necessary f o r t h e t ime be ing t o reduce gas temperatures

at Bradwell, and a l l succeeding magnox s t a t i o n s , t o con t ro l t h e r a t e of ox ida t ion , and inev i t ab ly t h e output has been reduced. Nevertheless a load f a c t o r (based on guaranteed output ) of 82.3% w a s achieved at

Bradwell during 1969 d e s p i t e t h e r e s t r i c t i o n .

The research programme on s t e e l s has been increased s ince t h e d is -

covery of t h e s t r a i n phenomena, p a r t i c u l a r l y t o cover t h e higher pressure

and temperature condi t ions of t h e AGR.

much more use of s t a i n l e s s s t e e l s , which have a higher r e s i s t a n c e t o

oxidat ion and no d i f f i c u l t i e s a r e expected. Research i s cont inuing,

however, t o confirm e x i s t i n g r e s u l t s .

I n general t hese r e a c t o r s make

116

The chemical p r o p e r t i e s of helium a r e e n t i r e l y d i f f e r e n t t o those of

carbon dioxide, but similar research is i n v e s t i g a t i n g any cor ros ion

e f f e c t s of impur i t i e s c a r r i e d by t h e gas. A new phenomenon is t h e e f f e c t

of t h e r e a c t o r gas on t h e su r face p r o p e r t i e s o f ma te r i a l s , p a r t i c u l a r l y

t h e c o e f f i c i e n t o f f r i c t i o n . Table 1 g ives some t y p i c a l r e s u l t s of

dynamic f r i c t i o n c o e f f i c i e n t s measured i n pure helium.

Table 1. Dynamic F r i c t i o n Fac tors

Mater ia l C ornb i n a t ion

C r Carbide on s e l f

Graphit e on s e l f

316 Stain- l e s s s t e e l on s e l f

Helium Pressure

Atmospheres

1 50

1 50

1 50

Temperature C 0

800 800 800 800

6 50 6 50

Contact Pressure kg/cm2

65 65 65 65 21 21

F r i c t i o n f a c t o r min max mean

0.62 1.29 1.17 0.37 0.56 0.47

0.24 0.41 0.34

1.85 2.83 2.65 0.22 0.34 0.30

1.08 2.92 1.6

These r e s u l t s a r e very d i f f e r e n t f r o m t h e corresponding p r o p e r t i e s

i n a i r and show an i n t e r e s t i n g tendency f o r t h e c o e f f i c i e n t t o decrease

wi th i n c r e a s i n g gas pressure. The abso lu te l e v e l s a r e high however, and

emphasise t h e ca re which w i l l have t o be t aken i n ma te r i a l s e l ec t ion .

Vibra t ions and Noise

O f t h e order of 5% of t h e gross e l e c t r i c a l power produced i n a gas-

cooled nuc lear r e a c t o r s t a t i o n is used t o c i r c u l a t e t h e gas around t h e

system. There is t h e r e f o r e a cons iderable amount of energy i n t h e gas

stream which, i n t h e event of a v o r t e x shedding frequency co inc id ing with

a n a t u r a l frequency w i t h i n a contained volume, could cause s i g n i f i c a n t

mechanical damage. Several e f f e c t s such as t h i s have been r e p o r t e d

du r ing t h e h i s t o r y of gas-cooled r eac to r s ' and i l l u s t r a t e t h e growing

s o p h i s t i c a t i o n i n commissioning experimentat ion r e q u i r e d as gas pressures

and f l o w dynamic heads increased wi th r e a c t o r development.

A t Dungeness 'A' e a r l y d i f f i c u l t i e s were experienced during t h e

prel iminary commissioning t e s t s of a bypass c i r c u i t . This c i r c u i t , which

117

i s 60 f t long and 4 f t :diameter bypasses t h e c i r c u l a t o r o u t l e t gas back

t o a point mid-way along t h e l eng th of t h e b o i l e r s adjacent t o t h e evap-

o r a t o r bank of tubes and enables t h e c i r c u l a t o r s t o be run independently

of t h e main r e a c t o r gas c i r c u i t . During t h e c i r c u l a t o r commissioning it w a s found t h a t at speeds above a c r i t i c a l value t h e noise l e v e l and duct

v i b r a t i o n became i n t o l e r a b l e . The c i r c u i t w a s ins t rumentated w i t h s t r a i n

gauges, accelerometers and pressure t ransducers and t e s t s showed t h a t t h e

f requencies at which t h e duct w a s v i b r a t i n g were 133 Hz and 158 Hz. The

maximum s t r e s s l e v e l i n t h e duc t ing w a s measured at 780 lb/ in .2 peak-to-

peak; at 133 Hz t h e maximum a c c e l e r a t i o n recorded w a s 9 g and t h e maximum 2 gas pressure pu l sa t ion w a s 0.7 lb/ in . peak-t o-peak.

Af t e r cons5derable inves t iga t ion , which included measuring t h e pre-

dominant response f requencies of t h e duct , t h e phenomenon w a s t r a c e d t o resonance between an aerial v i b r a t i o n i n t h e duct and a vor t ex shedding

frequency from t h e t r a i l i n g edges of two cruciforms suppor t ing t h e tongue

of a duct bellows unit. The severe v i b r a t i o n s were e l imina ted by modify-

i n g t h e cruciform t r a i l i n g edges, when t h e maximum duct a c c e l e r a t i o n l e v e l

w a s reduced t o 0.64 g.

A similar and p o t e n t i a l l y more s e r i o u s resonance w a s found during

b o i l e r commissioning between an a e r i a l v i b r a t i o n i n t h e b o i l e r s h e l l and

pa r t of t h e a c t u a l tube assembly, i . e . t h e t o p rows of t h e superhea ter

and t h e support beam system. I n t h i s case it w a s not poss ib le t o modify

t h e i n i t i a t i n g member, s o an attempt w a s made t o f i t b a f f l e s wi th in t h e

b o i l e r s h e l l t o change t h e frequency of t h e a e r i a l v ib ra t ion .

Tests at s i t e became s o time-consuming and expensive t h a t it w a s decided t o adopt wind tunnel techniques t o f i n d t h e form of a s u i t a b l e

b a f f l e , wi th in t h e confines of t h e e x i s t i n g tube banks.

amount of modelling work w a s necessary before condi t ions i n t h e b o i l e r

could be reproduced, but eventua l ly seve ra l forms of s o l u t i o n were

evolved which, i n t h e wind tunnel , suppressed t h e resonance. Not a l l

of t hese s o l u t i o n s were successfu l i n t h e a c t u a l b o i l e r however, f o r

reasons which a r e n o t f u l l y understood, a l though it may have been e i t h e r

an over s i m p l i f i c a t i o n i n t h e modelling technique o r a d i f f e rence i n

A considerable

118

some unappreciated c r i t i c a l dimension between t h e model and t h e f i n a l

app l i ca t ion .

behind some of t h e tube banks w a s success fu l , however, i n reducing t h e

The f i n a l s o l u t i o n of f i t t i n g t r a n s v e r s e b a f f l e s immediately

maximum a l t e r n a t i n g s t r e s s measured on a tube t o -0.4 + t / i n . 2 , which i s s i g n i f i c a n t l y l e s s t h a n t h e f a t i g u e s t r e n g t h a s ses sed on t h e l i f e o f t h e

p l an t .

During t h e design s t a g e f o r Oldbury an a n a l y s i s w a s made of t h e

poss ib l e v i b r a t i o n s of t h e many components. Ca lcu la t ions showed t h a t t h e

b o i l e r s could be s u s c e p t i b l e t o e f f e c t s similar t o those found at Dungeness ' A ' , s o t h a t des ign w a s changed t o incorpora te a continuous

v e r t i c a l b a f f l e through t h e tube banks. As a precaut ion , commissioning

tes t s were c a r r i e d out first wi th co ld air at atmospheric pressure and

then wi th hot p r e s s u r i s e d air at a range of cond i t ions chosen t o g ive

t h e r equ i r ed son ic v e l o c i t i e s and gas dens i ty .

l e v e l w i th in t h e b o i l e r s was found t o be i n s i g n i f i c a n t , a l though a d i s c r e t e p re s su re pu l se w a s p resent throughout t h e e n t i r e c i r c u l a t o r

speed range at a frequency o f w i t h i n 5% of t h a t p red ic ted .

I n t h e event , t h e s t r e s s

Oldbury w a s indeed a very ' q u i e t ' r e a c t o r i n terms of v i b r a t i o n s

but dur ing t h e commissioning a new phenomenon r e l a t e d t o t h e n o i s e pro-

duced by t h e c i r c u l a t o r w a s found. This w a s a cons iderable amount of

sound energy appear ing at very l o w f r equenc ie s ( o f t h e order o f 50 Hz)

i n a r e a s very c l o s e t o t h e c i r c u l a t o r o u t l e t and r ep resen t ing , t h e r e f o r e ,

a p o t e n t i a l hazard t o t h e i n s u l a t i o n cover p l a t e s .

For t h e AGR design at Hinkley 'B' t h e design o f t h e cover p l a t e s

has been modified i n case similar e f f e c t s occur. The c i r c u l a t o r d r i v e

i s a cons tan t speed e l e c t r i c motor, and t h e impel lor and d i f f u s e r have

been d e s i d e d t o avoid a l l s i g n i f i c a n t t r a c e o f blade-passing frequency.

The pre l iminary t e s t s at atmospheric p re s su re have ind ica t ed low over-all

no i se l e v e l s , but a f u l l t e s t programme i s planned t o i n v e s t i g a t e t h e

f i n a l l e v e l s i n t h e r e a c t o r .

To complement t h e AGR commissioning t e s t s , ex t ens ive experimental

f a c i l i t i e s a r e be ing developed by TNPG.

check t h e p r e d i c t i o n s on no i se performance wi th helium c i r c u l a t o r s . No

These w i l l be used a l s o t o

119

@ immediate problems are apparent, for although the velocity of sound is

very much higher the acoustic impedance (sound velocity x density) is

little different to that of carton dioxide. Noise, as a general subject,

has only a limited theoretical basis however, especially in enclosures

where the dimensions are less than the wave-length of the sound waves,

and a considerable amourit of experimental work will be necessary.

CIRCUIT ENGINEERING

Def ini t ion

The term circuit engineering in a gas cooled reactor includes all

the plant which is necessary to translate the heat generated in the

reactor into steam to drive the turbo-generators, i.e. the reactor pres-

sure vessel and its associated liner, insulation and cooling equipment,

the boilers, and the gas circulators.

The pre-stressed IconcrGte pressure vessel is the basis of the cir-

cuit engineering of an integrated concept giving the designer freedom

in specifying vessel shape and gas pressure and yet always guaranteeing

reactor safety because of the massive redundancy in the number of stres-

sing cables. The cloncrete must be isolated from the high reactor gas

temperatures, however, and one of the major problems which has arisen is

the design and erection of the liner insulation.

Insulat ion

Metallic foil and wire mesh insulation and fibre insulation are the

two main types which have been used in the concrete pressure vessels

built in the UK.

for both types.

T"G have'been heavily involved in the development work

Metallic Foil and Wire Mesh Insulation

Feasibility studies to prove the integral concrete pressure vessel

concept for the Magnox reactors were completed by TNPG in 1960 and the

insulation of the vessel liner was then realised to be a major problem.

There was virtually no published information relevant to the thermal per-

formance of insulants in a high pressure gas environment and most conven-

tional insulants were excluded because of the compati bility requirements. @

120

Development work was therefore started by TNPG. Subsequently it

was learned that Darchem Ehgineering Ltd were working on lqwrallel lines

and from 1962 with the Oldbury tender awarded to TNPG, the two research and design teams worked together to develop what is now called metallic

foil and wire mesh insulation.

Oldbury reactor were insulated with 30 layers of 0.004 in. stain- less steel foil separated by a wire mesh (0.048 in. dia x 2 in. pitch), the whole pack being compressed by a mild steel cover plate in. thick

to a pack thickness of 2.00 in.

A s an example of this material the

Similar insulation has since been used for the Wylfa (Magnox) and

for Dungeness *B* (AGR) reactors. Considerable commissioning and opera-

tional experience has therefore been obtained in carbon dioxide at temp-

eratures up to 400 C and pressures up to 400 psia. The major problems

associated with surface pressure gradient effects have been fully

reported .

0

2 , 3

Fibre Insulation

With the completion of the Oldbury insulation, work on all aspects

of the AGR insulation was accelerated.

of the AGR were less severe than for Magnox reactors and a much wider

range of materials could therefore be used. Higher temperatures and

pressures (650 psia and 650°C for Hinkley *B*) had to be considered. TNPG decided to review the whole field in an attempt to develop an insul-

ant to meet the following specification:

The compat ibility requirements

(a) The required thermal performance under natural convect ion over

the full range of pressures and temperatures likely to be used

in AGRs and HTRs.

(b) A high resistance to surface pressure gradients.

(c) A low installed cost. This involves both low initial cost and a

relatively small site cost. Installation time was to be kept to

a minimum.

(d) The insulant must be capable of accommodating local irregularities

of the cold fade (reactor liner). A

121

(e) Isotropic properties. Insulants with highly anisotropic properties

cannot readily be used for penetrations and regions with complex

boundaries . A s a result of the review it was clear that a fibrous insulation had

many of the required properties and this type of insulation was eventually

chosen for Hinkley 'B' and Hunterston 'B'. The development work for both

these stations is being carried out in conjunction with the insulation

contractors, Delaney Gallay Ltd. A s an example, the insulation for the liner walls consists of layers of Triton Kaowool fibre compressed by a

7 in. thick stainless steel cover plate to 2 in. thickness overall. The

cover plate is mounted on a single bimetallic stud welded to the liner.

1

Thermal Performance

4,59697 In the last ten years work in England, France and elsewhere

has enabled an adequate understanding of the thermal performance of

insulants in a high pressure gas environment to be established. The bulk

of the experimental work has been carried out in high pressure carbon

dioxide but the limited work in helium is in fair agreement with the

correlations developed.

879

Metallic Foil and Wire Mesh - Natural Convection. - In any analysis of experimental data the convective contribution is first obtained by

deducting the metal conduction and radiation terms" from the measured

conductivity of the insulant. The convective contribution, which in

most realistic assemblies in carbon dioxide is the dominant contribution,

can then be correlated for a given insulant assembly against the pack

Rayleigh number (Fig 1 ). where

N = Nusselt number U

- heat out of cold face when gas movement occurs

N-- = Mean Nusselt number over cold face

- heat out of cold race under static gas conditions -

U C P fm 2 d 3 9 A T

4 R = Rayleigh number = a

P, k3 h lw\ For values of Ra above 100 convection within the insulation becomes

122

significant. Two lines are shown on Fig 1 which indicate the improvements

that are possible as a result of internal sealing features. The lower

line can be considered typical of the performance of well designed insula-

tion incorporating back lap seals2' 39 plane of the foils.

to reduce the permeability in the

A s a guide to thermal performance, complex flow pressure drop measure-

ments are carried out on representative insulation details. These measure-

ments have shown that foil and wire mesh is highly anisotropic; the perm- eability measured in the plane of the foils being between I O 4 and 10 5

times that measured normal to the foils.

The most successful theoretical model, developed by kvidson 4 , treats the whole pack as an anisotropic permeable solid.

Fibres and Granular Materials - Natural Convection. - For isotropic materials the Davidson theory is equally applicable6 and a comparison

between predicted and measured mean Nusselt number is given in Fig 2.

Data from are also included.

p y = Permeability in the vertical (y) direction

D = Pack thickness

H = Pack height

K = Thermal conductivity of a porous body with no overall gas movement through the body.

Otherwise the same notation has been used as defined on Fig 1.

The importance of one parameter which is normally overlooked,namely

the sealing characteristics of the hot face, is clearly demonstrated.

This correlation is equally applicable to HTR conditions where it should be possible to reduce the value of R a D/H to below the value for which

convection and hot face sealing effects are important.

Surface Pressure Gradient Wfects. - In gas cooled reactors it is

inevitable that the hot face of the insulation is exposed to surface

pressure gradients. The insulation must be capable of satisfactory

123

opera t ion when r a p i d pressure changes (due f o r example t o a change i n

r e a c t o r power) t ake place and t o achieve acceptab le pressure d i f f e r e n t i a l s

a c r o s s t h e hot f a c e it must be permeable. Surface pressure g r a d i e n t s can

t h e r e f o r e inf luence t h e pressure d i s t r i b u t i o n wi th in t h e in su lan t .

Davidson's theory has been extended t o cover t h i s case6 and t h e

inf luence of a uniform pressure grad ien t on t h e mean and l o c a l pack

Nusselt numbers is given on Fig 3 f o r t h e case of an open face. where

&)eo CP PA-. o2

F = = j t m 4 H = t h e constant p r e s s u r e > g r a d i e n t over t h e hot f a c e = v e r t i c a l d i s t ance measured from t h e high pressure region

( ' p 4 ) , , y

Otherwise t h e same n o t a t i o n has been used as def ined on F ig 1.

The theory has now been extended s o t h a t t h e inf luence of t h e hot face

s e a l i n g c h a r a c t e r i s t i c s can be inves t iga t ed . A considerable programme of

b a s i c experimental work is i n progress at " P G t o provide s u b s t a n t i a t i o n of t h i s theory. I n i t i a l r e s u l t s obtained on r e a c t o r prototype i n s u l a t i o n f o r

t h e f l o o r of Hinkley 'B' t e s t e d i n high pressure carbon dioxide a r e

encouraging.

Mechanical Design Aspects

I n both t h e f o i l and wire mesh and t h e f i b r e i n s u l a n t s developed f o r

t h e Oldbury and Hinkley 'B' r e a c t o r s t h e i n s u l a t i o n pack w a s he ld i n posi-

t i o n and p ro tec t ed by cover p l a t e s . D i f f e r e n t i a l expansion problems a r e severe and i n order t o rninimise s l i d i n g at supports s i n g l e s tud supports

a r e used i n both designs. Adequate space between t h e cover p l a t e when co ld

ensured t h a t t h e r e w a s no i n t e r a c t i o n between hot cover p l a t e s . However, s l i d i n g problems cannot be e l imina ted as t h e gaps between cover p l a t e s

must be br idged t o conta in o r p ro tec t t h e in su lan t . The cover p l a t e s pro-

v ide t h e in su lan t compressive loads and t h e f r i c t i o n a l f o r c e s between t h e s l i d i n g components a r e r e a c t e d by t h e s tud.

Liner Studs. - Studs a r e e i t h e r gun welded o r hand welded t o t h e l i ne r . The number, s i z e and ma te r i a l s a r e subjec t t o t h e fol lowing cons t r a in t s :

(a) Thermal

( i) Stud dens i ty as expressed by t h e t o t a l s t u d c ross -sec t iona l a r e a per

u n i t a r e a of l i n e r is r e s t r i c t e d by heat loading. I n t h e "I%

124

t h i s r e s t r i c t i o n is l e s s severe because of t h e r e l a t i v e l y high

conduct iv i ty of t h e b a s i c i n su lan t .

(ii) Single s t u d s i z e in f luences t h e maximum l i n e r temperature and

temperature grad ien ts . The in su lan t t h i c k n e s s is g r e a t e r i n t h e

HTR and l a r g e r s t u d diameters a r e t h e r e f o r e acceptable .

( i i i ) W e l d i n g problems a r e eased if a mild s t e e l s t u d is used. However,

thermal conduction and t h e hot f a c e temperature condi t ions r equ i r e

t h e use of s t a i n l e s s ma te r i a l s . Bi-metall ic s tuds have t h e r e f o r e

been used extensively.

( b ) Mechanical

( i v ) Pack compression f o r c e s r e s u l t i n t e n s i l e s tud loads. Also due

t o t h e non-uniformity of t h e pack t h e e f f e c t i v e cen t r e of t h e

pack loading is unlikely to coincide with t h e stud c e n t r e l i n e

and bending loads a r e t h e r e f o r e introduced.

( v ) Bending loads on t h e s tuds r e s u l t from t h e f r i c t i o n f o r c e s between

t h e s l i d i n g su r faces and a l s o from t h e weight of t h e cover p l a t e s

on v e r t i c a l sur faces .

( v i ) Qnamic loads a r i s e because t h e sound pressure l e v e l w i th in t h e

r e a c t o r can be high enough t o in t roduce v i b r a t i o n i n t h e i n s u l a t i o n

component s

Cover P la tes . - The dimensions o f t h e cover p l a t e s a r e inf luenced

by d e f l e c t i o n , s t u d loading and d i f f e r e n t i a l expansion problems. Edge

d e f l e c t i o n must be r e s t r i c t e d as t h e s e a l i n g e f f e c t i v e n e s s of t h e ma te r i a l

b r i d g i n g t h e cover p l a t e gaps is a f f ec t ed .

The acous t i c response of t h e cover p l a t e s can in t roduce random c y c l i c

p l a t e s t r e s s e s which w i l l e x i s t throughout r e a c t o r l i f e and cover p l a t e

s i z e , t h i c k n e s s and m a t e r i a l must c a t e r f o r t h e problem of f a t i g u e and

creep.

Fibre. - It is very important t h a t t h e f i b r e used t o r e s t r i c t t h e

gas movement should have adequate long term compression c h a r a c t e r i s t i c s

over t h e f u l l range of r e a c t o r temperatures. Compatibi l i ty and dust

content a r e a l s o important c o n s i d e r a t i o n s i n t h e choice of f i b r e .

i

125

Insulation for HTRs crs Theoretical and experimental work have enabled correlations to be

produced for both the natural and forced convection situations. Because

of the relatively low pack Rayleigh number, the thermal problems of the

insulants in a helium environment are eased and height effects and hot

face sealing characteristics are not so important.

Analytical procedures of the kvidson type may not be necessary for

HTR insulants. It is not difficult to design the insulation so that the

effects of convection are negligible but this does not necessarily result

in the most economic design. Consideration of the F parameter indicates

that the lower density and lower pressure gradients in the HTR (due to the

reduced gas circulation) are likely to reduce the effect of surface pres-

sure gradients relative to the AGR.

However, if after an overall cost assessment of the cooling system

and insulation designs the same conductance as used in the AGR is re-

quired then, because of the high thermal conductivity of the gas, packs

of two to three times the thichess of those used in the AGR are likely.

For higher temperature applications, fibres with a higher resilience

than Triton are required and a dust free fibre would be an advantage.

However, permeability requirements can be relaxed as a value of Ra D/H

(Fig 2) less than 2 can be easily achieved and a wide range of fibres

and fibre sizes can be considered for use in the HTR. A considerable programme of physical property measurements on fibrous materials has

therefore been initiated by TNPG.

Many of the mechanical problems investigated during the development

of the AGR insulation have direct relevance to the HTR insulation. Acous-

tic effects and stud heat loading are likely to be less severe but stic-

tion and sliding problems demand sophisticated studies. Ideally thermal

cycling and similar proving tests should be carried out in a helium

environment of the reactor purity.

Elsewhere special solutions are proposed (Fig 4), as some of the materials used to date would not be suitable. These problems are dis-

cussed in detail later under Basic HTR Design. 6d

126

PENETRATIONS

The i n t e g r a l b o i l e r concre te pressure v e s s e l must be pene t ra ted at v a r i o u s po in t s t o a l l o w t h e loading and discharge of f u e l , t h e i n s e r t i o n

of con t ro l rods, ins t rumenta t ion e t c .

arrangement i s proposed and t h i s g r e a t l y s i m p l i f i e s t h e water and

steam p e n e t r a t i o n problems.

For t h e HTR a podded b o i l e r

The major i n s u l a t i o n p e n e t r a t i o n problem involves t h e s tandpipes

( F i g 4) through which f u e l and con t ro l rods a r e passed.

t i o n s a r e very numerous and from t h e l i n e r coo l ing and thermal i n s u l a t i o n

a r e a Etignificant pa r t of t h e t o t a l problem.

These penetra-

Under normal opera t ion t h e s tandpipes conta in a plug and t h e l i n e r

However, t h e plug i s re- coo l ing and i n s u l a t i o n problem i s then s m a l l . moved p r i o r t o r e f u e l l i n g on load and each s tandpipe can t h e n be consid-

e r e d t o be a h igh aspect r a t i o i n s u l a t e d v e r t i c a l pipe z l w e d at t h e t o p

and open at t h e bottom t o an i n f i n i t e high pressure , high temperature

environment. It is t h e empty s t a t e which d i c t a t e s t h e design of t h e

i n s u l a t i o n and cool ing system.

TNPG have c a r r i e d out very ex tens ive i n v e s t i g a t i o n s of t h i s problem

and t h e inf luence of gas, gas pressure , gas temperature and e x t e r n a l

thermal r e s i s t a n c e has been measured f o r a range of pipe s i z e s inc luding

prototype Hinkley 'B' s tandpipes .

s tandpipe i n t e r n a l d e t a i l s on heat loading and temperature d i s t r i b u t i o n

has been covered. Tests on t h e Hinkley 'Bv s tandpipes us ing helium a r e

now i n progress and t e s t s on t h e much l a r g e r diameter s tandpipes pro-

posed f o r t h e HTR a r e be ing planned.

I n t h e l a t t e r case t h e inf luence of

Only r e s u l t s f o r t h e empty s tandpipe case with a high thermal re -

s i s t a n c e t o t h e heat s i n k w i l l be given i n k h i s paper.

Standpipe Tests

On F i g 5 t e s t r e s u l t s a r e given f o r two s tandpipes , one of which

was t e s t e d i n helium and carbon dioxide, t h e o the r i n carbon dioxide

only. 0 The maximum pressures and temperatures were 400 p s i g and 500 C.

The heat t r a n s f e r c o e f f i c i e n t i n t h e Nusselt number is obtained

127

@ from l o c a l gas and s tandpipe temperatures and heat f l u x e s and t h e mean

va lue over t h e height of t h e s tandpipe i s used. The v a r i a t i o n of l o c a l

Nusselt number with height w a s not large. - ' y- D = Nusselt number

R; = Rayleigh number L Cp pm2D39 A7

u k T = heat t r a n s f e r c o e f f i c i e n t between gas

and s tandpipe

b = standpipe bore

AT = temperature drop between gas and s tandpipe

T = mean abso lu te gas temperature

Otherwise t,he same n o t a t i o n as f o r F i g 1 w a s used.

Basic s t u d i e s have a l s o been conducted i n which f l o w v i sua l i&\ ion

techniques were used t o i n d i c a t e t h e n a t u r a l convection flows f o r t h i s

s i t u a t i o n .

work and a l s o by gas temperature measurements i n t h e high pressure gas

t e s t s suggest t h a t a complete unders tanding of t h e problem i s u n l i k e l y

at t h e present t ime.

The complexity and i n s t a b i l i t y of t h e f l o w ind ica t ed by t h i s

HTR Standpipes

Designs o f HTR s tandpipes d i f f e r i n two important and opposi te

F i r s t l y , i n order t o a l l o w t h e core f u e l r e s p e c t s from t h e AGR designs.

b r i c k s t o be removed a much l a r g e r diameter of s tandpipe is proposed.

Secondly, t h e use o f a downward flow core exposes t h e s tandpipe t o t h e

low temperature gas. Under present design condi t ions t h e n e t t e f f e c t of

t h e s e d i f f e rences i s t o reduce t h e Rayleigh number i n t h e HTR by at l e a s t

one order of magnitude r e l a t i v e t o t h e AGR.

The thermal loading problem of t h e HTR s tandpipe is t h e r e f o r e of

t h e same order as t h e AGR and it i s p a r t l y f o r t h i s reason that f u l l

s c a l e t e s t s a r e planned. An even more important reason, however, is that it i s e s s e n t i a 1 , t o know t h e temperatures of a l l t h e charge chute compon-

eats and t h i s can bes t be done by t e s t i n g prototgpes.

component temperatures due t o t h e downward f l o w core is very s i g n i f i c a n t .

The reduct ion of

C i r c u l a t o r s

I n t h e e a r l y magnox r e a c t o r s , which were housed i n s t e e l p ressure

128

v e s s e l s with ex te rna l duc ts t o t h e b o i l e r s , t h e gas c i r c u l a t o r s were

housed wi th in t h e ducts. They were dr iven by s h a f t s p e n e t r a t i n g t h e

pressure c i r c u i t through a gas s e a l . Many forms o f d r ive were used,

from steam tu rb ine t o e l e c t r i c motor with f l u i d coupling.

t h e first of t h e i n t e g r a t e d designs with t h e whole of t h e pressure c i r -

c u i t w i th in a s i n g l e v e s s e l , t h e concept w a s r e t a ined , with t h e c i r c u l a t o r

be ing dr iven by an extension s h a f t l o c a t e d i n a tunnel through t h e ves se l

w a l l . For Hinkley 'B' however, a new concept of encapsulated motor d r ive

was introduced, based on t h e success fu l ful ly-enclosed design pioneered

by Howdens at Winscale AGR, wi th t h e motor l o c a t e d wi th in t h e ves se l w a l l and opera t ing under f u l l r e a c t o r gas pressure, s o e l i m i n a t i n g t h e need f o r

a s h a f t s ea l .

A t Oldbury,

The design incorpora tes many of t h e d e s i r a b l e f e a t u r e s which have

been proved on e a r l i e r designs. F i r s t l y , t h e d r ive is a constant speed

e l e c t r i c motor, which has cons iderable advantages i n f l e x i b l e commission-

ing and i n providing emergency suppl ies . Var i a t ion of r e a c t o r mass f l o w

at Hinkley 'B' i s achieved us ing v a r i a b l e i n l e t guide vanes, an arrange-

ment which i s wel l proven i n conventional c i r c u l a t o r design, but which

of course remains t o be t e s t e d under r e a c t o r condi t ions.

motor and t h e impel lor form a s i n g l e compact u n i t which can be removed

r e l a t i v e l y quickly from se rv ice , and be r ep laced equal ly quickly a f t e r

maintenance and r epa i r .

Secondly, t h e

Although t h e r o t a t i n g s h a f t gas s e a l has been el iminated, t h e whole

of t h e s h a f t b e a r i n g assembly i s i n t h e gas c i r c u i t , and ex tens ive dev-

elopment work has been necessary t o design s e a l i n g arrangements f o r t h e

l u b r i c a t i o n o i l system. I n p a r a l l e l , much of t h e a n c i l l a r y equipment

i n t h e motor compartment, such as t h e water coolers f o r c o n t r o l l i n g t h e

l o c a l gas temperatures , have had t o be designed and manufactured t o a

very high s tandard t o ensure minimum contamination o f t h e r e a c t o r gas.

The f i r s t Hinkley 'B' c i r c u l a t o r i s now ready f o r prototype t e s t i n g

under s imulated r e a c t o r condi t ions before f i n a l i n s t a l l a t i o n . The form

o f design has been adopted f o r t h e o the r AGR s t a t i o n s now on order , and

r ep resen t s a s o l i d b a s i s f o r f u t u r e HTR designs.

129

Boilers c.$ The boilers in gas-cooled reactor stations are much more like gas to

water cross-flow heat exchangers in conventional coal or oil-fired boilers.

Throughout, there has been a compromise between the desire for large face

area, and so small gas-side pressure drops and dynamic forces, and the re-

quirements for fitting them into expensive pressure shells. Manufacturing

costs have influenced the compromise to a considerable extent.

Early designs were assemblies of straight tubes, tightly packed in

staggered or in-line banks, connected at each end by forged bends. Welding

costs were inevitably high, so methods have been developed for manipulation

of continuous lengths of tube to achieve the same low horizontal and vert-

ical pitching which is necessary in producing compact boilers.

In principle the inter-connections between tubes are relatively

straightforward because of the regular pitching and both welded and clamp-

ing designs have been used. Clamps have the particular advantage of pro-

viding damping of any tube vibration and worked well with low alloy steel

boilers;

tures have risen, galling problems have been encountered and T " G now

use welded connections. The lack of damping is allowed for in the ex-

tensive vibration survey now carried out during the design stage.

Relatively little trouble has been experienced by TNPG w i t h

as higher alloy material has been introduced as gas tempera-

nuclear boilers, but occasional tube failures have, of course, been

experienced. In general, they have become apparent early in the life

of the station, and can often be traced to individual defective welds.

The tubes are blanked-off external to the reactor vessel and, while

this means a small loss of heat transfer surface, no measurable opera-

tional penalty has been experienced. The siting of the boilers within

the reactor pressure vessel, as at Oldbury or Hinkley 'B', inevitably

raises the question of what can be done in the event of massive tube

failure because of a systematic design fault.

occurring is extremely small as gas temperatures are rigorously con-

trolled at a low level (relative to those in fossil-fired boilers) and

it is now general practice to install full-flow treatment plant for the

The probability of this

130

b o i l e r feedwater.

however, so t h e b o i l e r s a r e p r o t e c t e d from d i r e c t r a d i a t i o n from t h e

core by a shield-wall. The c a l c u l a t e d a c t i v a t i o n is such t h a t access

f o r inspec t ion w i l l be a v a i l a b l e throughout t h e l i f e of t h e p lan t . If

massive f a i l u r e i s d e t e c t e d t h e n t h e b o i l e r s could be r ep laced under an

extreme emergency procedure which would involve a major breach of t h e

vesse l . This would t ake a cons iderable t ime, but not longer than t h a t

r e q u i r e d t o d e s i g n and manufacture new b o i l e r s .

Experience wi th once-through b o i l e r s is l imi t ed ,

The ex ten t of t h e opera t ion is such t h a t it would only be t a c k l e d

under extreme condi t ions. The main emphasis i n t h e design i s placed

not on removeabi l i ty but on i n v e s t i g a t i n g and t e s t i n g e x i s t i n g fo rms of

b o i l e r t o redrice t h e r i s k of a major systematic f a i l u r e t o an absolu te

minimum.

FUEL HANDLING

One of t h e e a r l i e s t dec i s ions i n t h e evolu t ion of gas-cooled

r e a c t o r s w a s t o design f o r on-load f u e l handling.

achieve a p lan t which could be operated at t h e h ighes t poss ib l e a v a i l a -

b i l i t y wi th planned outage f o r t h e r e a c t o r be ing wi th in t h a t necessary

f o r t h e turb ine .

t h e wish t o use t h e b e n e f i c i a l e f f e c t s on r e a c t i v i t y of t h e build-up of

plutonium and t h e r e l a t i v e l y low f u e l l i f e .

The objec t w a s t o

With magnox r e a c t o r s t h e d e c i s i o n w a s r e i n f o r c e d by

The r e s t r i c t i o n of n a t u r a l uranium as a f u e l p l aces a high premium

on core r e a c t i v i t y .

plutonium i n f u e l of low i r r a d i a t i o n can be used t o o f f s e t t h e low re-

a c t i v i t y of highly i r r a d i a t e d f u e l .

With a continuous r e - f u e l l i n g cycle t h e build-up of

There a r e over 3000 f u e l channels per r e a c t o r i n t h e first two s t a t i o n s at Berkeley and Bradwell and with an average f u e l dwell time

of f o u r yea r s , corresponding t o an i r r a d i a t i o n l i f e of 3000 MWD/te, t h e

annual r a t e of channel replacement approached 800. a v a i l a b i l i t y of off-load f u e l handl ing w a s , t he re fo re , considerable and

extremely expensive because of t h e very high cos t of f u e l i n t h e a l t e r -

n a t i v e f o s s i l - f u e l l e d s t a t i o n s .

The e f f e c t on r e a c t o r

With t h e i n t r o d u c t i o n of enr iched f u e l i n t o t h e AGR design t h e

131

benefits of on-load refuelling on fuel life were reduced, but equivalent

advantages soon became apparent. Because the reactivity of the reactor

is an average of the reactivities of the individual channels, the irra-

diations of which are uniformly distributed over the range new to fully

irradiated fuel, the overall reactivity is almost independent with time.

There is no need therefore to include an excess number of control rods

to control the inevitable high reactivity of the new fuel in a reactor

which is re-fuelled on a yearly off-load basis.

is the fact that all the replacement fuel is of an enrichment which is

known well in advance of it being required on site. If a station, for

any reason, runs only at a poor load factor, then the refuelling is ad-

justed by reducing the rate at which channels are changed. The level

of enrichment is also reduced compared to a fuel cycle with yearly re-

fuelling which reduces the fuel costs by $4/kW to $5/kW.

Of pmticular importance

The decision to adopt on-load refuelling was taken in full reali-

sation of the mechanical problem involved and the successful achievement

has not been without substantial difficulty. The basic problem on a

magnox reactor is to penetrate the reactor pressure vessel, locate a

channel some 50 feet away, and grab and remove a fully irradiated, deli- cately designed, fuel element. The handling mechanisms must operate in

high pressure carbon dioxide at a temperature of 400 C, replace the new

fuel precisely in the right position without damaging the fuel beneath

with any impact loads, then retract from the vessel leaving the pressure

vessel closure secure.

0

During the early fuel-handling trials difficulties were experien-

ced with the operation of some of the mechanisms because of distortions

due to the uneven rates of heating, while the fuel element grabs had a

very limited life. None of the reactor stations designed by TNPQ has ever suffered a loss of availability because of deviatioh from the

planned fuel cycle and the final success of the operations can be seen

from the history of the Berkeley and Bradwell stations during 1967 (lhbles 2 and 3).

132

Total number of channels refuelled in 1967 Total number of channels refuelled to 31/12/67 Deviation from programme on 31/12/67( channels) Maximum channel average fuel irradiation - MwD

Te

Table 2 Fuelling Performance at Berkeley during 1967

Reactor 1 Reactor 2

8 20 799 3518 3517 - 3 + 4 3555 3530

T o t a l number of channels refuelled in 1967

Total number of channels refuelled to 31/12/67

Maximum channel average fuel irradiation - MwD

Deviation from programme on 31 /I 2/67 ( channels)

Te

Table 3 Fuelling Performance at Bradwell during 1967

Reactor 1

1271

2384 0

3696

Year 1963 1964 1965 1966 1967

Berkeley 45.7 53.1 89.3 76.9 86.8 Bradwell 66.7 79.3 86.9 82.3 90.3

Reactor 2

3450

1968 1969

91.9 89.4 81.3 82.3

I

The performance during 1967 has been chosen to illustrate the re- fuelling rates which can be achieved after the initial difficulties have

been overcome. The very high rate on Reactor 1 at Bradwell is indeed

due to the final clearing up of a residual back-log from earlier years.

During these early years there were considerable difficulties at both

stations, but without detriment to the overall load factors (based on

guaranteed output) which are given in Table 4.

Table 4 Annual Percentage Load Factors

~~~ ~

The advantages of on-load refuelling will be as important for the

HTR as for the earlier gas-cooled reactors, The reactor is being spec-

ifically designed to achieve these advantages, for example by adopting

133

downward gas flow such that the refuelling equipment will operate at the

relatively low temperature of 300 C.

earlier designs in that the mechanisms must work in helium, and the fuel

element is heavier and must be loaded into free-standing columns. With the

experience already gained, however, there is every chance that the early

operational difficulties will be significantly less than those experienced

at Berkeley and Bradwell.

0 There are major differences from

THE BASIC "IT DESIGN

A sketch of the reactor section is given on Fig 4 and illustrates the main features of the design. The object of the design is to exploit

the natural advantages of the HTR - high ratings and gas temperatures - while taking full account of existing experience on gas-cooled reactors

in order to reduce risks and produce a reliable station.

The basis of the pressure circuit design is the pre-stressed concrete

pressure vessel, but in a new departure for TNPG the boilers are located in vertical pods in the vessel wall. The main reason for this is economy,

for the high gas temperatures have so reduced the boiler heat transfer

surface that we nu longer have to arrange the units in an annulus round

the core in order to get sufficient frontal area. The boilers are very

compact and fit into four of the eight available pods, the other four

being used for helium storage.

Many detail .designs of boiler are being investigated but the prefer-

red arrangement at the moment is very similar to that used for the AGR.

Alternative boiler designs are being investigated based on the principle

of helically coiled elements, which give a more compact overall unit.

They represent a marked change in manufacturing procedure, however, and

until much more experimental work has been done there are several tech-

nical unknowns. These are concerned with the variable pitch between the

tubes and thus the uncertainty of predicting heat transfer and vibration

characteristics, as well as the erection of the preferred welded tube-to-

tube connect ions.

The coolant gas flow is downwards through the reactor to avoid 0 0 having hot gas (750 C) anywhere near the standpipes in the top of the

134

A

v e s s e l , and t 6 enable on-load r e f u e l l i n g i n an ambient temperature of

30OoC. The s tandpipes a r e d i f f i c u l t t o i n s u l a t e i n themselves, and they

conta in t h e v i t a l con t ro l rod mechanisms which must at a l l t imes be pre-

vented f rom over-heat ing.

The o v e r a l l i n s u l a t i o n wi th in t h e ves se l has been divided i n t o two

Cooling f l o w s at 3OO0C have been designed s o q u i t e d i s t i n c t d iv is ions .

t h a t a l l t h e i n s u l a t i o n p r o t e c t i n g t h e v e s s e l l i n e r s e e s gas of a maximum temperature of 4OO0C.

similar t o t h a t used at Hinkley 'B' AGR, t a k i n g due allowance f o r t h e

helium atmosphere.

r e a c t o r i n l e t gas temperature constant during a l l normal running

sequences, t h i s i n s u l a t i o n w i l l experience a minimum of thermal cyc l ing

and can t h e r e f o r e be guaranteed f o r long l i f e .

This i n s u l a t i o n w i l l be designed i n a manner

A s t h e r e a c t o r con t ro l system tends t o keep t h e

The f u n c t i o n of t h e high temperature i n s u l a t i o n , which covers a

r e s t r i c t e d Pegion from t h e core o u t l e t t o t h e b o i l e r i n l e t , w i l l be

l i m i t e d as f a r as poss ib l e t o r e s t r i c t i n g t h e passage of heat from t h e

hot t o t h e c o l d gas streams r a t h e r t h a n t o maintain component tempera-

t u r e s .

and t h e r e f o r e s t a t i o n e f f i c i e n c y , and represent no hazard t o t h e s a f e t y

of t h e p l an t . It is hoped t o use s o l i d in su lan t s , because of doubts

about t h e r e s i l i e n c e o f f i b r e ma te r i a l s , at high temperatures. The

r e a c t o r core , f o r example, w i l l s t and on a f i r e b r i c k hear th .

Any breakdown w i l l r e s u l t only i n a l o s s of temperature p o t e n t i a l ,

The c i r c u l a t o r s a r e l o c a t e d i n p e n e t r a t i o n s through t h e t o p s l a b of

t h e ves se l above t h e b o i l e r pads.

l a r g e t o a l l o w t h e passage of a b o i l e r , during e r e c t i o n , and as a remote

p o s s i b i l i t y du r ing removal f o r replacement, t h e r e is s u f f i c i e n t space t o accommodate a l l t h e c i r c u l a t o r a u x i l i a r y se rv ices . The design of t h e

c i r c u l a t o r w i l l most probably f o l l o w t h a t be ing developed f o r t h e AGR,

but a l l of t h e assumptions i m p l i c i t i n t h a t design a r e be ing re-examined.

For example, it may prove p r e f e r a b l e t o go back t o a v a r i a b l e speed

e l e c t r i c d r ive if f r i c t i o n and wear experiments i n helium show any pot-

e n t i a l d i f f i c u l t i e s with an i n l e t guide-vane mechanism.

A s t h e s e p e n e t r a t i o n s a r e s u f f i c i e n t l y

The HTR i s s i g n i f i c a n t l y d i f f e r e n t from e a r l i e r gas-cooled r e a c t o r s

135

63 i n t h e use of p a r t i c l e f u e l and t h e helium coolant . Outside t h e

immediate environment o f t h e core , however, much of t h e engineer ing is

l i t t l e d i f f e r e n t i n cha rac t e r t o t h a t o f previous gas-cooled r eac to r s .

The experience gained on t h e s e r e a c t o r s is be ing used t o evolve an HTR

design with t h e same r e l i a b l e high load f a c t o r opera t ion t h a t has a l ready

been achieved at Berkeley and Bradwell. By t h e end of 1970 TNPG w i l l have

completed such a design f o r a 600 W(E) power s t a t i o n t o be i n opera t ion

i n 1975.

ACKNOWLJ3EEMENTS

The work descr ibed i n t h i s paper has been c a r r i e d out by many

people wi th in The Nuclear Power Group Ltd. The au thors acknowledge

t h e advice and a s s i s t a n c e given by t h e i r co l leagues and would a l s o thank t h e i r Di rec tors f o r permission t o publ i sh t h e information.

136

REFERENC E S

1. J. Derbyshire and R. Stead, Vibration problems in Nuclear Power Plant, Paper No 2, Symposium or$ High Pressure Gas as a Heat Transport Medium, I. Mech. E., Vol. 181, March 1?67. -

2. J.W. Hughes et al, Insulation design and development for the Oldbury vessels, Conference on Prestressed Concrete Pressure Vessels, 1.C .E. Paper No.60, March 1967

3. J. W. Hughes and C. O'Tallamhain, Metallic foil insulation, Research Report RI, p. 235, Symposium of High Pressure Gas as a Heat Transport Medium, I. Mech.E., Vo1.181, - March I967

4. J. Davidson, Heat transfer behaviour of metallic foil insulation in high pressure gas, Paper No 13, Symposium on High Pressure Gas as a Heat Transport Medium, I. Mech.E., Vo1.181, - March 1967

5. R. Grossin et al, Heat transfer by natural convection in a porous medium, Paper No - 27 Second Conference - November 1969

6. B.N. Furber and J. Davidson, The thermal performance of porous insulants in a high pressure gas environment, Paper No 28, Second Conference on Prestressed Concrete Reactor Vessels and their Thermal Insulation, Brussels 18-20th November 1969

7. G. Mordchelles Regnier et al, Quelques Recherches Rgcentes Effectuees en France sur 1'Isolation Thermique des RBacteurs Nucleaires, Paper SM 1-11/76, Proceedings of a Symposium on Advanced and High Temperature Gas Cooled Reactors, I.A.E.A., Julich, October 1968

8. P.G. Cowap and B.N. Furber, The performance of Stainless Steel Foil insulation in a helium atmosphere, D.P. Report $2, 1967

9. B.N. firber, Contribution to discussion, p. 744, Conference on Prestressed concrete pressure vessels, I.C.E. March 1967

IO. B.N. Furber and R. Broderick, The development of an apparatus to measure the normal emissivity of small plane samples in a controlled atmosphere, Paper l2,Bristol 'Convention, I.Mech.E., 1968

11. J.C. Grebetz, Compression and Permanent deformation characteristics of Fibrous insulation, Materials Research and Standards, Jan. 1970, p. 26-31 and 61-63

12. W. J. Green, Temperature distributions along thermally insulated enclosed vertical pipes containing a pressurised gas - to be pub1 is hed

. . . - . . -

137

NOT*IIOH

CI i S f E C l F l C HEAT OF G A S ( C 0 N S T . PRESSURE)

P m - G A S D E N S 1 1 1 A T 1.

p.= G A S V I S C O S I T Y A T 1.

L , - C A S C o N n u c r i v i r i AT r. 4 - M E A N GAS C A f T H I C K N E S S . i N U L I N E I O f GAS G A P S

1. - M E A N GAS T E Y I E I A T U R E A T - T E M P E R A T U R E D I F I E R E N C E A C R O S S P A C K

S P E C I M E N DETLlLS ND O F FOILS 1 0 10 I I P A C K T H I C K N E S S I . 4 0 T O 1 . 2 1 I N . X 0 A W I T H I H T E R N A L S E A L I N G F E A T U R E S 0 V W I T H N O I N T E R N A L S E A L I N G F E A T U R E S

M A X PRESSURE 5 1 0 . t i .

M A X T E M P E R A T U R E b I O ‘C

rfsr CONDITIONS ( C A R N O N DIOXIDE)

t P A C K V T E S T E D I N H E L l U I l

1 I I 1 I I l l 1 I 1 3 4 5

I 0’ lol 2 3 4 5 ,ol 2 3 4 5

1. Foi l and Wire Insulat ion - Variation of i”, with Ra - Natural Convection.

2. I so t ropic Insulat ion - Variation of with R D,” - Natural Convection.

138

F H/y 10

2 3 I O I O I . -

I S H O W E V E R SMALL

I I O I OL F

IO '

I s o t r o p i c I n s u l a t i o n - Local and Mean Nusse l t Number - Forced Convect ion.

I CORE 5 CHARGE MACHINE 1 CORE SUPPORT 6 CHARGE MACHINE C H U T f 3 CIRCULATOR 7 CONCRETE PRESSURE VESSEL 4 BOILER 8 STANDPIPE

4. . Reactor E l e v a t i o n - HTR.

n

MEAN RAYLEICH NUMBER R o

5. S t a n d p i p e s - V a r i a t i o n of N w i t h R . u a

140

DISCUSSION

G. Meijer : Is your evapora t ion i n t h e steam genera tor downhill or

u p h i l l ?

Do you p lan t o do any tes t f o r downhill evapora t ion?

B. N. Furber: The flow i n the b o i l i n g s e c t i o n i n t h e TNPG design i s

u p h i l l .

Tests are i n progress t o i n v e s t i g a t e t h e heat t r a n s f e r and s t a b i l i t y

a spec t s . It may a l s o be of i n t e r e s t t o r e p o r t that Professor W. B. Hall (Manchester Un ive r s i ty ) is i n v e s t i g a t i n g s u p e r c r i t i c a l bo i l i ng . Resul t s

t o da t e sugges t t h a t downhill s u p e r c r i t i c a l b o i l i n g i s more c o n s i s t e n t t han

u p h i l l s u p e r c r i t i c a l b o i l i n g .

M. Dal le Donne: You mentioned t h a t t h e Nusselt number a t t h e i n s u l a -

t i o n and a t the s tandpipe i s about 10 t i m e s lower w i t h helium than w i t h

C 0 2 .

tha t of C 0 2 , s o t h a t t h e hea t t r a n s f e r c o e f f i c i e n t i s about t h e same and

t h e h e a t l o s s e s through t h e i n s u l a t i o n a r e about t h e same.

I n r e a l i t y t h e thermal conduc t iv i ty of helium i s 7 times h igher than

B. N. Furber: The r educ t ion of Raleigh number by a f a c t o r of 10

I n t h i s des ign a a p p l i e s approximately t o t h e TNPG s tandpipe design.

l a r g e s tandpipe diameter i s requi red t o enable f u e l b r i c k s t o be changed.

Assuming t h i s f a c t o r of 10, then t h e Nussel t number i s reduced by approxi-

mately 2.5. This does not compensate f o r t h e increased thermal conduc-

t i v i t y of helium. Nevertheless , t he main po in t i s that t h e increased hea t

t r a n s f e r c o e f f i c i e n t i n t h e TNFG s tandpipe des ign can be accommodated.

R. Huddle: What m a t e r i a l s do you in tend t o use f o r t h e hot f ac ings

of duc ts and l i n e r s ? Do you th ink t h e r e i s a material problem?

B. N. Furber: I n t h e low temperature regions t h e ho t f ace materials w i l l be similar t o those used i n the HTR. I n t h e high temperature reg ions

ceramic materials ( f o r example, s i l i c o n n i t r i d e ) a r e be ing s e r i o u s l y con-

s ide red .

M. Bender: The HTGR r e a c t o r s of t h e Dragon type opera te a t tempera-

t u r e s t h a t may be s e v e r a l hundred degrees above those of t h e AGR type.

141

0 Could you i n d i c a t e w h a t e f f e c t t h e s e e l eva ted temperatures would have on

t h e i n s u l a t i o n requirements and c h a r a c t e r i s t i c s .

Many people do not r e a l i z e t h a t concrete v e s s e l s are usua l ly designed s o

that coolant gas ad jacen t t o t h e concre te i s a t r e a c t o r i n l e t temperatures

and t h e i n s u l a t i o n performance i s t h e r e f o r e not very s e n s i t i v e t o core

o u t l e t temperature condi t ions . Nevertheless , t h e r e i s some inc rease i n

coolan t temperatures over t hose i n t h e AGR systems, and my impression i s

that t h e s e do t a x t h e c a p a b i l i t y o f r e f l e c t i v e i n s u l a t i o n s because the

seals between i n s u l a t i o n panels are d i f f i c u l t t o hold t o low leakage

l e v e l s . This i s t h e main reason why f i b e r i n s u l a t i o n s have become a t t r a c -

t i v e . The speaker ev iden t ly d i d no t understand t h e ques t ion and it d i d

not seem worthwhile t o -be labor t h e po in t .

(Supplementary note :

M.B. )

B. N. Furber: There i s no b a s i c thermal problem r e s u l t i n g from t he

h igher gas temperatures of t h e inc rease of thermal conduct iv i ty i f tempera-

t u r e i s not l i n e a r . The des ign conductance results from a s tudy and it i s probable tha t i n s u l a t i o n th i ckness could be 4 t o 6 i n . o r 2 t o 3 t i m e s

A. G. R.

143

HTGR N E W AND ADVANCED DESIGNS

(Session 111)

Chairman : J. D. Thorn, U n i t e d Kingdom Atomic E n e r g y A u t h o r i t y R i s l e y , W a r r i n g t o n , LanCS.

Co-Chairman: G. D. Whitman, Oak Ridge N a t i o n a l L a b o r a t o r y

144

Paper 1/101

THE H I CH,,TE~-EE-VJTURE REACT-OR.,DE-S?E LOP.EEuT . I N .&E RMANX- (PRESENT SITUATION, PROCRAM AND FUTURE ROLE)

ABSTRACT

Although HTR-development i n t h e German F e d e r a l Repub l i c i s b e i n g s u p p o r t e d by t h e government t o abou t t h e same e x t e n t as f a s t - b r e e d e r deve lopment , ana about one thousand m i l l i o n DM w i l l be s p e n t by 1 9 7 5 a c c o r d i n g t o t h e p r e s e n t p l a n s , t h e u t i l i t i e s c o n t i n u e t o show r e l a t i v e l y l i t t l e i n t e r e s t i n t h e HTR system.

N e w developments , however, e s p e c i a l l y i n t h e f i e l d o f r e a c t o r s a f e t y and p o l l u t i o n i n d i c a t e a poten- t i a l l y broad f u t u r e marke t f o r t h e HTR a l o n g s i d e LWR’s, as w e l l as f o r o t h e r r e a c t o r t y p e s n o t d i r e c t - l y i n v o l v e d i n t h i s development program. These market chances are coupled w i t h t h e s p e c i f i c c h a r a c t e r i s t i c s of t h e HTR-system, namely; f a v o u r a b l e economy, h i g h i n h e r e n t s a f e t y , a v e r y much b e t t e r f u e l u t i l i z a t i o n , and less c o o l a n t water demand t h a n f o r t h e LWR’s. Some o f t h e s e c h a r a c t e r i s t i c s w i l l become g r e a t l y i m p o r t a n t i n t h e F e d e r a l Repub l i c of Germany because of t h e r e s t r i c t e d number o f s i t e s w i t h s u f f i c i e n t f r e s h water c o o l i n g and because t h e h i g h p o p u l a t i o n d e n s i t y means t h a t s i tes v e r y n e a r t o c i t i e s must be t a k e n i n t o c b n s i d e r a t i o n more and m o r e .

I

With t h e r e a l i z a t i o n of t h e f i r s t l a r g e HTR-plant i n t h e F e d e r a l Repub l i c o f Germany, t h e HTR development w i l l e n t e r t h e d e c i s i v e phase i n t h i s c o u n t r y . T h e f i n a l d e c i s i o n f o r t h i s p l a n t , t h e 300 MWe THTR, i s e x p e c t e d by t h e midd le o f May 1 9 7 0 . Of g r e a t impor t ance a l s o i s t h e fo r thcoming a s s o c i a t i o n of t h e f i r m s GHH, KRUPP, and BBC.

The work on t h e HTR i s p r o c e e d i n g i n t h r e e s t e p s . During t h e f i r s t s t e p , work w a s c o n c e n t r a t e d on two c y c l e s p l a n t s . T h i s i n i t i a l development work, now e s s e n t i a l l y comple ted , was d i r e c t e d towards t h e con- s t r u c t i o n of t h e AVR-reactor and t h e d e s i g n o f t h e THTR. During t h e c o n s t r u c t i o n phase f o r t h e THTR, f u r t h e r component and f u e l e lement t es t s w i l l be made w i t h i n t h e framework of a s p e c i f i c i n d u s t r i a l d e v e l - opment program.

145

146

I n t h e second s t e p , t h e c o u p l i n g o f t h e HTR i n c l o s e d c y c l e d i r e c t l y t o a he l ium t u r b i n e w i l l be i n v e s t i - g a t e d . Such a development program h a s a l r e a d y s t a r t e d and w i l l be f i n i s h e d i n 1 9 7 4 w i t h t h e p r e p a r a t i o n o f t h e c o n s t r u c t i o n documents f o r a 600 MWe p l a n t . Some v e r y i m p o r t a n t p r o j e c t s w i t h i n t h i s program are:

1.

2 .

3 .

C o n s t r u c t i o n and o p e r a t i o n of t h e 2 2 MWe KWSH-plant. The commissioning of t h i s power p l a n t i s schedu led f o r 1 9 7 4 .

C o n s t r u c t i o n and o p e r a t i o n o f a f o s s i l f i r e d 5 0 MWe power p l a n t employing a he l ium t u r b i n e .

E s t a b l i s h i n g a m u l t i p u r p o s e t e s t f a c i l i t y which p a r t i c u l a r l y i n c l u d e s a t e s t s e c t i o n w i t h a he l ium t u r b i n e r e p r e s e n t a t i v e o f a 300 M W e p l a n t , w i t h which f u l l scale components of t h e pr imary c i r c u i t can be t e s t e d u n d e r r e a l i s t i c o p e r a t i n g c o n d i t i o n s .

T h i s development p r o g r a n i s e x p e c t e d t o r e a c h t h e s t a g e of commercial a p p l i c a t i o n by 1 9 8 0 .

During t h e t h i r d development s t e p , t h e d i r e c t u s e o f t h e r m a l e n e r g y from H T R ’ s f o r chemica l p r o c e s s e s w i l l be i n v e s t i g a t e d . Such p r o c e s s e s are t h e p r o d u c t i o n of hydrogen for u s e i n s t e e l manufac tu re by c r a c k i n g methane, heavy o i l or c o a l . Another a p p l i c a t i o n i s t h e p r o d u c t i o n o f r a w materials f o r t h e chemica l i n d u s t r y . T h i s work i s p r e s e n t l y i n t h e p r o j e c t d e f i n i t i o n phase . We assume t h a t i n 1 9 7 1 t h e development work can be s t a r t e d on f u l l scale.

The above mentioned a c t i v i t i e s w i l l b e s u p p o r t e d by a broad materials development program.

The p a p e r g i v e s a s h o r t r e p o r t on t h e p r e s e n t s t a t u s o f HTR development i n Germany. The main problems which are s t i l l open are p o i n t e d o u t and t h e p lanned a c t i v i - t i e s are mentioned.

147

I N T R O D U C T I O N

The p r imary and s t i l l most i m p o r t a n t g o a l of t h e p r e s e n t HTR development i s t o p r o v i d e t h e u t i l i t i e s w i t h an economic and safe n u c l e a r h e a t s o u r c e , which a l l o w s t o r e a l i z e a modern steam t u r b i n e p r o c e s s . We know t h a t t h e s a t u r a t e d steam t u r b i n e does n o t r e p r e s e n t a f i n a l s o l u t i o n , i n s p i t e o f t h e f ac t t h a t t h e y have been improved v e r y much i n t h e frame of t h e LWR development . On t h e o t h e r hand t h e i n v e s t i g a t i o n s of t h e l a s t f e w y e a r s have shown t h a t HTR h a s f u r t h e r development p o t e n t i a l beyond t h e r e a l i z a T i o n o f a modern steam c y c l e . T h i s p o t e n t i a l con- s i s l s f i r s t i n t h e u s e of t h e more p r o g r e s s i v e and i n p r i n c i p l e , more s i m p l e gas t u r b i n e p r o c e s s for e l e c t r i c i t y p r o d u c t i o n . Another p o t e n t i a l development i s t h e d i r e c t conve r s ion of n u c l e a r h e a t i n t o chemica l ene rgy i n form of a steam r e f o r m i n g p r o c e s s .

Th i s means, t h a t t o d a y w e have t o c o n s i d e r t h e HTR as a r e a c t o r f a m i l y more as a s i n g l e p l a n t t y p . The d i f f e r e n t v a r i a t i o n s o f f e r due t o t h e i r s p e c i f i c p r o p e r t i e s broad p o s s i b i l i t i e s o f a p p l i c a t i o n n o t o n l y on t h e e l e c t r i c a l ene rgy marke t b u t a l s o on t h e p r imary ene rgy marke t . T h i s p o t e n t i a l i s of g r e a t i m - p o r t a n c e because of t h e f ac t t h a t t h e p r imary ene rgy market t o d a y i s n e a r l y 4 t i m e s l a r g e r t h a n t h e marke t f o r e l e c t r i c a l e n e r g y t o which n u c l e a r ene rgy h a s t h u s f a r been l i m i t e d ex- c l u s i v e l y , and t h a t i n 30 y e a r s as F ig . 1 shows, it w i l l s t i l l be as l a r g e as t h e marke t f o r e l e c t r i c a l energy .

CHARACTERISTIC ITEMS OF THE GERMAN HTR-PROGRAM

Without any change i n t h e most u r g e n t t a s k , namely t o push t h e f i r s t HTR-concept ( w i t h steam t u r b i n e ) from t h e e x p e r i m e n t a l i n t o t h e commercial phase .

crs

148

I n West Germany development work f o r t h e more advanced con- c e p t w i t h d i r e c t c y c l e and f o r t h e i n d u s t r i a l p l a n t h a s a l - r e a d y been s t a r t ed of c o u r s e w i t h d i f f e r e n t p r i o r i t i e s and d i f f e r e n t ra tes .

The i n i t i a t i v e f o r t h i s r e s u l t s f rom t h e f o l l o w i n g r e a s o n s : I n West Germany t h e l i g h t water r e a c t o r h a s meanwhile e s t a b - l i s h e d v e r y s t r o n g i n d u s t r i a l s u p p o r t and h a s r e a c h e d a n as- t o n i s h i n g p o s i t i o n i n t h e e n e r g y marke t . I t has been e s t i - mated , f o r example , t h a t f o r t h i s y e a r a p p r o x i m a t e l y h a l f o f t h e new power p l a n t i n s t a l l a t i o n s w i l l b e s h a r e d by PWR and BWRs. Thus new economic c r i t e r i a have been e s t a b l i s h e d a g a i n s t which e a c h f u r t h e r n u c l e a r deve lopment i n West Germany must be matched. A g e n e r a l s u c c e s s f o r r e a c t o r companies s u p p o r t i n g t h e HTR for t h a t r e a s o n w i l l o n l y be a c h i e v e d i f extreme e f f o r t s are u n d e r t a k e n and i f t h e y are f o c u s s e d as soon as p o s s i b l e t o HTR c o n c e p t s w i t h t h e h i g h e s t economic p o t e n t i a l .

I may r e c a l l t h a t t h e H T R c o n c e p t h a s had i n West Germany r a t h e r modest s u p p o r t by t h e u t i l i t i e s u n t i l 1 r e c e n t l y . T h i s s i t u a t i o n i s c h a n g i n g s l o w l y now. One of t h e r e a s o n s are t h e growing s a f e t y r e q u i r e m e n t s f o r n u c l e a r p l a n t s i n h i g h dea- s i t y p o p u l a t e d areas s u c h as West Germany.

A f u r t h e r i m p o r t a n t r e a s o n i s g i v e n by t h e s i t u a t i o n f o r c o o l i n g water p r o v i s i o n i n West Germany. S e v e r a l a n a l y s e s i n d i c a t e a t e r m i n a t i o n of f r e s h water c o o l i n g i n f o r e s e e a b l e t i m e . I n LWRs t h e u s e of c o o l i n g towers l e a d s t o e s s e n t i a l l y h i g h e r c o s t p e n a l t i e s t h a n i n o t h e r c o n c e p t s and would c lear- l y a f f ec t t h e p o s i t i o n o f t h i s p l a n t t y p e i n i n d u s t r i a l c o m p e t i t i o n .

As f a r as w e know t o d a y H T R s w i t h h e l i u m t u r b i n e s h o u l d have t h e l a r g e s t p o t e n t i a l o f a l l n u c l e a r c o n c e p t s w i t h res-

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23% / portion tor elcctrual energ- production

1 1 . 1950 1960 I970 )980 1990 m

F i g . 1 P r i m a r y Energy Demand i n West Germany u n t i l 2000

-I -- I hydrocraainrl_ r---------- I I I I H,S I I I I I

J I

CK *KO

1 react or 2 steam turbine cycle 3 heat exchanger 4 convertion CO+H.O+CXh + HI 5 C a -wash 6 methan separation 7 direct reduction 8 condensation: Ha0 9 H2 - compressor 10 hydrocracker 11 tube cracking furnace

CH, + HI 0 -CO+3H2 -49Kcallmol

u\ pldtr stctl

F i g . 2 Schemes for I n d u s t r i a l HTR-Plants

150

p e c t t o r e q u i r e m e n t s f o r c o o l i n g water. Even a i r c o o l i n g cou ld come i n t o c o n s i d e r a t i o n . A q u i c k l y and comple t e ly

e x h a u s t i n g t h e d i r e c t c y c l e sys tem a l r e a d y t o d a y shou ld t h e r e f o r e make t h e marke t ing of t h e HTR e s s e n t i a l l y easier .

t h e p o t e n t i a l o f t h e HTR concep t by developinR

The i n t e r e s t i n development work f o r t h e t h i r d g e n e r a t i o n o f H T R p l a n t s , i . e . i n d u s t r i a l complexes comes from i n d u s t r i a l groups o u t s i d e of t h e e l e c t r i c i t y i n d u s t r y whose economic and p o l i t i c a l i n f l u e n c e can have a s t r o n g impact on t h e whole HTR development . The r e a s o n f o r t h i s i n t e r e s t l i e s p a r t l y i n t h e hope o f m a i n t a i n i n n t h e b a s i s for t h e i r i n d u s t r i - a l e x i s t e n c e ( coa l ) p a r t l y i n t h e p r o s p e c t of b r o a d e n i n g t h e

market for t h e i r p r o d u c t s (brown c o a l , n a t u r a l g a s ) , and a l s o i n t h e p o s s i b i l i t y of u s i n g more new economic i n d u s t r i a l p r o c e s s e s ( s t e e l making, p r o d u c t i o n of chemical r a w materials e tc . 1.

I n F i g , 2 as examples two p r o c e s s e s are ahown which cou ld be

r e a l i z e d w i t h an HTR i n d u s t r i a l complex. The lower drawing shows a scheme for s t ee l manufac tu re : n a t u r a l g a s is c racked i n t o hydrcgen and carbon d i o x i d e i n a t u b e c r a c k i n g t u r n a c e a t 900'- 95OoC u s i n g n u c l e a r h e a t . The hydrogen i s p i p e d t h e n t o a p l a n t for d i r e c t r e d u c t i o n of i r o n o x i d e i n t o i r o n sponge t h a t w i l l be f u r t h e r c o n v e r t e d t o s t e e l i n a n e l e c t r i c arc. The n e c e s s a r y e l e c t r i c ene rgy for t h e a rc i s produced i n t h e same H T R p l a n t by a steam t u r b i n e d y c l e . The steam gene- r a t o r f o l l o w i n g t h e t u b e c r a c k i n g f u r n a c e i n t h e p r imary he l ium c i r c u i t .

S i m i l a r i n d u s t r i a l complexes are p o s s i b l e f o r making a c e t y l e n e , e t h y l e n e o r o t h e r r a w materials from n a t u r a l gas . I n s t e a d of n a t u r a l gas a l s o o i l or p o s s i b l e g o a l cou ld be used i n t h e s e p r o c e s s e s .

151

6d The upper drawing i n F ig . 2 shows a p o s s i b l e scheme f o r h y d r o c r a c k i n g of o i l t o g a s o l i n e .

HTR WITH STEAM T U R B I N E

F ig . 3 shows a s u r v e y of t h e t o t a l program of West Germany s u b d i v i d e d a l o n g t h e l i n e s of development a l r e a d y mentioned. The most i m p o r t a n t s o u r c e o f c o n s t r u c t i o n - and o p e r a t i o n e x p e r i e n c e f o r t h e t w o - c i r c u i t l i n e and s i m u l t a n e o u s l y t h e s t a r t i n g p o i n t f o r t h e o t h e r development l i n e s i s t h e 1 5 M W e e x p e r i m e n t a l power s t a t i o n AVR, t h a t a t t h e end o f 1 9 6 7 s t a r t e d power o p e r a t i o n and s i n c e t h e n h a s o p e r a t e d w i t h s a t i s f a c t o r y , r e s u l t s as d e s c r i b e d i n p a p e r 2/103.

On t h e b a s i s o f t h e main d e s i g n p r i n c f i p l e s of t h i s p l a n t d u r i n g t h e y e a r s 1 9 6 4 - 6 8 , t h e 300 MW THTR p l a n t h a s been developed by BBK and KFA i n a s s o c i a t i o n w i t h EURATOM.

More t h a n a y e a r ago BBK made a b i d t o H K G , a c o n s o r t i a of s e v e r a l u t i l i t i e s f o r t h i s THTR power p l a n t . The c o n t r a c t n e g o t i a t i o n s f o r t h e c o n s t r u c t i o n o f t h e p l a n t , i n t h e c o u r s e of which n o t i n c o n s i d e r a b l e d i f f i c u l t i e s a r o s e are i n t h e v e r y f i n a l s t a g e n o w . The l i c e n s i n g p r o c e d u r e h a s been r a n n i n g s i n c e

t h e b e g i n n i n g of t h i s y e a r and h a s p r e s e n t e d no a p p a r e n t d i f f i - c u l t i e s t h u s fa r .

I n t h e THTR p r o j e c t s p h e r i c a l f u e l e l e m e n t s c o n t a i n i n g uranium/ thor ium o x i d e w i l l be used i n an o n - l i n e r e f u e l l i n g scheme. The e l e m e n t s have become so w e l l deve loped i n t h e l as t y e a r s t h a t t h e s u p p l y i n g company, NUKEM, i s i n t h e p o s i t i o n t o d a y o f g i v i n g f a r - r e a c h i n g g u a r a n t e e s on t h e a c h i e v a b l e burn-up . I n p a r a l l e l t o t h e c o n s t r u c t i o n of t h e p l a n t a R and D-pro- gram w i l l be carried o u t , i n which a l l t h e main components

I . Generation H T R-ST

Construct +

152

E. Generation HTR-HT

Commercial p lonb

I//////' H T R - marerid and fue l development

1. Generation HTR-PH

+J definition

1

Fig. 3 Main a c t i v i t i e s Wi th in The HTR-Development Program

Tab le 1. T y p i c a l Datas of F i r s t - G e n e r a t i o n HTR P l a n t s

Electrical output (Generctor net power)

Gas temperature core in let core outlet

Gas-pressure (maximum

Power density in the core

Specific Burn-up

Type of Fuel

Diameter o f fuel kernel

Direction of gas flow in the core

Core-diameter

Core-heigh t

Prestressed Concrete PV

inner diameter inner height

Live-steam pressure temperature

MW

OC

OC

kg/cm 2

MW/m3

MWd/t U

CL

m

m

m m

2 k d c m OC

THTR

FR-Germany

307,5

250 750

40

6

115.000

uo2-Tho2

400

d o w n w a r d s

5,6

6

cylindrical

15,9 15,3

181 530

~

Fort St.Vrain

USA

330

404 776

49,l

6,3

100.000

uc2-Thc2

400

l o w n w a r d

4,7

6

cylindrical

9,45 22,8

169 538 I

Mark Ill

;reat Britain

640

300 800

56,25

6 3

6o.Ooo

uc2-Thc2

800

d o w n w a r d s

8,79

6 8

cy1 .with pods

9,5(core-part) 16 (core-part)

1 74 565 -_

n

153

w i l l b e t e s t e d i n d e t a i l . Among t h e s e components are t h e f u e l c h a r g i n g d e v i c e , t h e r e g u l a t i n g and shut-down r o d s , p a r t s of t h e g r a p h i t e s t r u c t u r e , the h o t g a s c a n a l between core and t h e steam b o i l e r s , t h e burn-up measurement p l a n t , t h e b o i l e r s and blowers.

G

A p a r t f rom t h e f u e l e l e m e n t s and t h e p e r t i n e n t f u e l c h a r g i n g d e v i c e t h e THTR shows as i s descr ibed i n more d e t a i l i n t h e

n e x t p a p e r (2/113) a wide - sp read agreement i n t h e d e s i g n w i t h c o r r e s p o n d i n g c o n c e p t s of t h e GCA and €hose o f t h e B r i t i s h companies ( see t a b l e 1). The t i m e s c h e d u l e f o r t h e f i r s t commerc ia l HTR p l a n t i n West Germany w i l l be f i x e d e x c l u s i v e l y by t h e r e s p o n s i b l e f i r m s B B C , BBK and GHH. Up t o now no b f f i c i a l s t a t e m e n t s have been d i v u l g e d . Q u i t e con- s i d e r a b l e e f f o r t s f o r f u r t h e r deve lopment on an p u r e l y i n - d u s t r i a l scale has y e t t o be done and i t i s c e r t a i n t h a t an order for t h e f i r s t commercial p l e n t i s n o t t o be e x p e c t e d before a t r u s t w o r t h y a p p r a i s a l on t h e v a l i d i t y of t h e c o s t c a l c u l a t i o n s f o r THTR have been made. The e a r l i e s t d a t e for t h i s seems t o be t w o y e a r s a f t e r t h e b e g i n n i n g o f the con- s t r u c t i o n p h a s e f o r t h e THTR, b u t p r o b a b l y an even l o n g e r p e r i o d w i l l b e n e c e s s a r y ,

I t i s a p p a r e n t t h a t t h e main e f f o r t s w i t h i n t h e s t e a m c y c l e l i n e t o r e a c h t h e commercial s t a g e i n West Germany are con- c e r n e d w i t h t h e c o n c e p t of s p h e r i c a l f u e l e l e m e n t s . However i n v e s t i g a t i o n s are b e i n g made a lso f o r reactor c o r e s w i t h b l o c k t y p e f u e l e l e m e n t s . The q u e s t i o n , which f u e l c o n c e p t w i l l have t h e l a r g e r l o n g t e r m p o t e n t i a l i n Germany i s q u i t e open , i t also depends on t h e f u r t h e r r e s u l t s o f t h e deve lop - ment f o r an HTK d i r e c t c y c l e p l a n t ( H H T ) which i s b e i n g p e r - formed f o r t h e t i m e b e i n g i n t w o p a r a l l e l l i n e s one f o r t h e b a l l and one f o r t h e b l o c k t y p e f u e l c o n c e p t .

I

I

154

A

HTR WITH HELIUM T U R B I N E ( H H T )

HT development work i s b a s e d on t h e f o l l o w i n g main p r o j e c t s .

1.) The e x p e r i m e n t a l p l a n t KWSH, a 2 2 MW n u c l e a r p l a n t w i t h h e l i u m t u r b i n e u s i n g a n o n - i n t e g r a t e d a r r angemen t and rod - shaped f u e l e l e m e n t s .

2 . ) A 50 M W e h e l i u m t u r b i n e p l a n t i n Oberhausen w i t h a f o s s i l f u e l f i r e d heater.

3 . ) A t es t s i t e w i t h 4 i n s t a l l a t i o n s which have been p l a n n e d i n s u c h a way t h a t t h e y supp lemen t e a c h o t h e r w i t h r e s p e c t t o magni tude and a v a i l a b i l i t y . A l though t h e e x p e r i m e n t a l p l a n t KWSH which h a s been o r d e r e d one y e a r ago s h o u l d f u r n i s h i m - p o r t a n t datas on t h e o p e r a t i o n a l b e h a v i o r o f a t y p i c a l d i r e c t c y c l e H T R p l a n t , i n p a r t i c u l a r on i t s r e g u l a t i n g and f a u l t b e h a v i o r , and a l t h o u g h it s h o u l d i n a d d i t i o n o f f e r t h e p o s s i - b i l i t y of t e s t i n g p r i s m a t i c f u e l e l e m e n t s , KWSH h a s come u n d e r d i s c u s s i o n a g a i n . T h i s d i s c u s s i o n i s s t i l l i n f u l l swing. F i n a l d e c i s i . o n s w i l l b e made o n l y i n c lose c o o r d i n a t i o n w i t h t h e s p e c i a l o b j e c t i v e s of t h e g e n e r a l HHT development .

I n t h e 5 0 MW p l a n t i n Oberhausen t h a t w i l l b e o r d e r e d by RWE and t h e c i t y o f Oberhausen w i t h i n t h i s y e a r w i l l o v e r t a k e t h e

f u n c t i o n of a d e m o n s t r a t i o n p l a n t . Q u e s t i o n s of t h e long- t ime b e h a v i o r , a v a i l a b i l i t y and m a i n t e n a n c e o f h e l i u m t u r b i n e s w i l l i n v e s t i g a t e d t h e r e . S i m u l t a n e o u s l y t h i s p l a n t o f f e r s t h e p o s s i - b i l i t y o f e v a l u a t i n g t h e t r a n s i e n t b e h a v i o r of t h e main compo- n e n t s i n t h e h e l i u m t u r b i n e c i r c u i t e x p e r i m e n t a l l y .

The d e s i g n of t h e d i f f e r e n t machines of t h e HHT t e s t s i t e and t h e i r s p e c i f i c t a s k s are d e s c r i b e d i n p a p e r 6/117. I s h o u l d s a y h e r e t h a t t h i s t e s t s i t e w i l l be t h e c e n t r a l p o i n t f o r e x t e n s i v e component work.

155

The main a c t i v i t i e s are d e a l i n g w i t h problems o f t h e t u r b i n e , ( t h e b e a r i n g , s e a l i n g and t h e r o t o r and b lade c o o l i n g : ) , and problems of t h e h o t g a s d u c t s between t h e r e a c t o r c o r e ou t - l e t and t h e t u r b i n e e n t r y ( i n s o l a t i o n , expans ion d e v i c e s , v a l v e s and f i t t i n g s ) .

crs

The mass f low for t h e d i f f e r e n t p l a n t s of t h e t e s t s i t e c o v e r s t h e r a n g e from 3 t o 250 k g / s e c . The t empepa tu res v a r y i n t h e r e g i o n from 400 t o 85OoC p r o b a b l y even 1000°C. T h e a v a i l a b l e diameter of t h e t e s t s e c t i o n s l i e s between 0 . 2 and 2 meters. I t i s b e i n g e s t a b l i s h e d a t KFA by t h e i n d u s t r i a l f i r m s BBC and GHH. These companies w i l l o p e r a t e a l s o t h e p l a n t t o g e t h e r w i t h KFA. F i r s t expe r imen t s can be s t a r t ed w i t h i n t h e n e x t f e w months, t h e comple t ion o f t h e whole t e s t s i t e w i l l n o t be ach ieved b e f o r e end of 1 9 7 2 .

Pa ra l l e l t o t h e c o n s t r u c t i o n and o p e r a t i o n o f t h e above men- t i o n e d i n s t a l l a t i o n s , t h e o r e t i c a l and e x p e r i m e n t a l i n v e s t i - g a t i o n s have been s t a r t ed i n meantime. These w i l l f o r t h e most p a r t be concerned w i t h two problem zreas , 1. d e f i n i n g t h e l ay -ou t o f t h e p l a n t and 2 . t h e p l a t e - o u t of f i s s i o n pro- d u c t s i n t h e t u r b i n e c i r c u i t , a knowledge which i s of d e c i - s i v e s i g n i f i c a n c e f o r t he e v a l u a t i o n of H T R d i r e c t cyc le power

p l a n t s .

The p o s s i b l e l a y - o u t s are s t i l l judged i n a r a t h e r d i f f e r e n t way from c o u n t r y t o c o u n t r y , as t h e mee t ing of t h e BNES i n London a g a i n showed j u s t r e c e n t l y , and are unde r much d i s - c u s s i o n . The q u e s t i o n o f t h e bes t d e s i g n i s n o t y e t d e c i d e d even i n s i d e i n d i v i d u a l working groups . Under d i s c u s s i o n are non-, h a l f - , and f u l l y i n t e g r a t e d c o n c e p t s . I n s i d e o u r working groups p r e s e n t l y a c i r c u i t o u s and d e t a i l e d e v a l u a t i o n scheme i s b e i n g set up t h a t can be a p p l i e d s y s t e m a t i c a l l y t o t h e d i f f e r e n t c o n c e p t s . F i n a l r e s u l t s can n o t be expec ted u n t i l

156

t h e end o f t h i s y e a r . P a r t i c u l a r d e t a i l s on it w i l l be p r e s e n t e d i n pape r No, 5 / 1 1 5 .

A s u r v e y o f t h e work p lanned f o r s t u d y i n g t h e b e h a v i o r o f f i s s i o n p r o d u c t s i n t h e c o o l i n g c i r c u i t i s g iven i n pape r No. 7 / 1 2 7 .

At. t h e c e n t r a l p o i n t of t h i s work i s an i n - p i l e expe r imen t t h a t w i l l be s t a r t e d i n t h e r e a c t o r PEGASE (Cadarache) i n t h e middle of t h i s y e a r and t h a t w i l l s i m u l a t e as w e l l as p o s s i b l e t h e o p e r a t i n g c o n d i t i o n s o f a o n e - c i r c u i t power p l a n t .

The knowledge from these i n v e s t i g a t i o n s as w e l l as t h a t worked o u t i n o t h e r a f o r e mentioned p r o j e c t s w i l l be u t i - l i z e d f o r t h e d e s i g n work o f a 600 M W e p l a n t whose uppe r he l ium gas t e m p e r a t u r e h a s a l r e a d y been e s t a b l i s h e d a t 85OoC. The comple t ion of t h e s e d e s i g n s i s expec ted by 1 9 7 4 . T h i s means t h a t t h e commissioning of t h e f i r s t l a r g e HHT p l a n t i n West Germany can n o t be e x p e c t e d b e f o r e t h e end of t h i s decade .

THE I N D U S T R I A L HTR - PLANTS

The development work f o r an i n d u s t r i a l p l a n t , t h e t h i r d g e n e r a t i o n o f H T R s , was s t a r t e d more t h a n one y e a r ago and h a s now reached t h e s t a t u s o f a p r o j e c t d e f i n i t i o n phase . We assume t h a t t h e p r o p e r development program can s t a r t a f t e r a p o s i t i v e c o n c l u s i o n of t h i s phase i n t h e c o u r s e of n e x t y e a r . T h i s program w i l l encompass t h e f o l l o w i n g a c t i v i t i e s : 1.) Exper imen ta l and t h e o r e t i c a l thermodynamic s t u d i e s i n c o n t e x t w i t h t h e d e s i g n work f o r a t u b e c r a c k i n g f u r n a c e , b e g i n n i n g a t a s i n g l e t u b e (10 m l o n g , 0 , l m d i a m e t e r ) Later a t a whole t u b e bundle .

I

157

2.) Coal g a s i f i c a t i o n expe r imen t s u s i n g hydrogen a t 100 b a r s i n s e v e r a l i n s t a l l a t i o n s o f d i f f e r e n t s i z e . ( L a b o r a t o r y scale - 1 kg c o a l / h , - h a l f t e c h n i c a l scale - 50 kg c o a l / h , and f i n a l l y i n a p i l o t p l a n t o f 1 t o 2 t c o a l / h )

0

3. Expe r imen ta l i n v e s t i g a t i o n s t o t h e k i n e t i c s of r e a c t i o n s between coal and steam (steam r e f o r m i n g ) and a b o u t t h e r e a c t i v i t y of c o a l s hav ing d i f f e r e n t q u a l i t i e s .

4 . ) F u r t h e r development work for r e a c t o r components w i t h r e s p e c t t o r e a c h h i g h e r gas t e m p e r a t u r e s ( 9 5 O o C ) .

I n a d d i t i o n t o KFA, t h e companies Rhe in i sche Braunkohlewerke, Cologne, and Bergbauforschung Essen are i n v o l v e d i n t h e de-

velopment work d u r i n g t h e f i r s t phase.

HTR MATER I AL- AND FUEL ELEMENT DEVELOPMENT

The development work mentioned so f a r i s , as shown i n F ig . 3 , accompanied by a broad H T R - m a t e r i a l and f u e l e lement development program. T h i s program i n c l u d e s bas ic work on t h e u n d e r s t a n d i n g of t h e i r r a d i a t i o n b e h a v i o r o f a l l f u e l e lement components - ( p a r t l y i n c l o s e c o o p e r a t i o n w i t h DRAGON) , the

c o l l e c t i o n of material d a t a s t h a t are r e q u i r e d f o r t h e f u e l e l e m e n t s d e s i g n work, and t h e development o f manufac tu r ing t e c h n i q u e s f o r s p e c i f i c f u e l e lement components. A f u r t h e r i m p o r t a n t p o i n t o f t h i s program, which i s s u p p o r t e d by NUKEM and KFA, i s r e l a t e d t o t h e p r e p a r a t i o n , per formance and e v a l u a t i o n o f s p e c i f i c a t i o n t es t s f o r t h e THTR-, KWSH- and t h e HHT-project. S i n c e d e t a i l s o f t h i s program w i l l be r e p o r t e d i n p a p e r No. 7/128, t h e p a r t i c u l a r p o i n t s and t h e s p e c i a l problems can be d i s p e n s e d w i t h h e r e .

158

RE PRO CE S S IN G

I n orPder t o g i v e a comple t e p i c t u r e of HTR development i n West Germany, one must s t i l l t o u c h upon t h e i m p o r t a n t work i n t h e area o f r e p r o c e s s i n g ( g i v e n i n d e t a i l by Dr. Merz i n p a p e r No. 1 / 1 3 6 ) . T h i s work i n c l u d e s t h e head-end and t h e r e c o v e r y of f i s s i o n materials, t h e l a t t e r b e i n g r e l a t e d o n l y t o t h e uran ium-thor ium-cycle . Out of a g r e a t number o f d i f f e r e n t t e c h n i q u e s t h a t were worked t h r o u g h i n t h e f i r s t p h a s e o f t h i s p r o j e c t t h e b u r n - l e a c h method as head end and t h e t h o r e x p r o c e s s f o r t h e uranium s e p a r a t i o n have been f i n a l l y s e l e c t e d as t h e most f a v o r a b l e ones f o r d e t a i l e d i n v e s . t i g a t i o n s . Dry t e c h n i q u e s are recommended t o l a t e r de- ve lopment work. The i n v e s t i g a t i o n s t o d a t e have s t i l l t h e s t a t u s of l a b o r a t o r y e x p e r i m e n t s , however t h e y are b e i n g p e r - formed a l r e a d y w i t h h o t mater ia l and w i t h i n a comple t e scheme. The r e s u l t s o b t a i n e d have been t h u s f a r s a t i s f a c t o r y , s o t h a t d e s i g n work f o r a p i l o t p l a n t c o u l d be s t a r t e d meanwhile. T h i s p l a n t s h o u l d r e a c h a t h r o u g h p u t of 2 - 3 k g / d a y i n 1973 which i s enough t o p r o c e s s t h e bu rned f u e l of t h e AVR.

The re i s n o t y e t a d e f i n i t e c o n c e p t i o n on t h e c o n s t r u c t i o n of a b i g p r o t o t y p e p l a n t f o r r e p r o c e s s i n g t h o r i u m c o n t a i n i n g HTR f u e l e l e m e n t s . S e v e r a l v a r i a t i o n s are p o s s i b l e and are b e i n g a n a l y s e d p r e s e n t l y . The t o t a l deve lopment o f n u c l e a r e n e r g y i n West Germany w i l l i n f l u e n c e t h e d e c i s i o n b u t a l s o t h e q u e s t i o n of how q u i c k l y t h e uran ium-thor ium c y c l e c a n s u b s t i t u t e t h e l o w e n r i c h e d uranium c y c l e s w i l l p l a y a r o l e a t l ea s t i n Europe .

PROS PE CT S

Only a small p a r t of t h e a c t i v i t i e s men t ioned are a l r e a d y comple t ed , namely , t h e AVR-project and t h e deve lopment o f t h e THTR. The o t h e r s are s t i l l i n p r o g r e s s and a p a r t of

‘I

159

6d them h a s j u s t been s ta r ted .

Because of t h e r e l a t i v e l y f a r - r e a c h i n g g o a l s v e r y much and u n f o r t u n a t e l y t h e most d e c i s i v e work i s s t i l l i n f r o n t of u s . The f o r e s e e n f u n d s which are q u i t e c o n s i d e r a b l e f o r Germany are, a t l eas t i n p r i n c i p l e , ag reed t o by t h e p e r t i n e n t federal m i n i s t r y . I n fac t t h e HTR-development i n West Germany has

meanwhile r eached t h e same p r i o r i t y as t h e development o f t h e l i q u i d coo led f a s t breeder.

These facts o b l i g e u s t o a n a l y s e r e g u l a r i l y and v e r y s e r i o u s - l y t h e whole program t h a t means t o p u t up t h e q u e s t i o n : Whether t h e e x i s t i n g f i n a n c i a l and p e r s o n e l b a s e i s s u f f i - c i e n t f o r a s u c e s s f u l comple t ion of t h e mentioned a c t i v i t i e s and whether these a c t i v i t i e s are i n a r e a s o n a b l e r e l a t i o n t o t h e a i m s . During t h e n e x t weeks w e w i l l u n d e r t a k e such a n a n a l y s i s once more.

A s t h e HTR l i n e can n o t r e l y on e x t e n s i v e m i l i t a r y deve lop- ments , which w a s t h e case f o r t h e commercial e x p l o i t a t i o n of t he LWR-systems, i n Germany w e are wor ry ing abou t a p o s s i b l e o v e r c h a r g e o f t h e d i s p o s a b l e f o r c e s a l r e a d y i n t he c o u r s e of i n t r o d u c i n g t h e f i r s t HTR g e n e r a t i o n i n t o t h e f r e e e n e r g y marke t as it e x i s t s i n our c o u n t r y . The f u s i o n of a l l german i n d u s t r i a l companies s u p p o r t i n g t h e HTR which i s b e i n g com- p l e t e d w i t h i n t h e n e x t months, g i v e s g r e a t hopes i n t h i s r e s p e c t b u t i t i s p o s s i b l e t h a t o n l y an even g r e a t e r , t h a t means i n t e r n a t i o n a l i n d u s t r i a l base can g a r a n t e e a good chance f o r t h e commercial e x p l o i t a t i o n o f t h e HTR. An effec- t i v e i n t e r n a t i o n a l c o o p e r a t i o n between t h e competent c o u n t r i e s shou ld t h e r e f o r e be p u t i n t o c o n s i d e r a t i o n much more i n t h e f u t u r e .

180

DISCUSSION

F. L. Culler: 1. Is 1600°F (approximately 850°C) high enougk tem-

perat .De todri .ve t h e water gas r e a c t i o n ? W i l l t h e h e l l m i not have t o be

a t cons iderably h ighe r temperature to o b t a i n good y i e l d s of w e t f u e l gas

wi th t h e steam carbon r e a c t i o n ? 2. Have i r r a d i a t e d g raph i t e f u e l element

saniples been burned and leached ?

H. Kr'kner: 1. 850°C i s not a s u f f i c i e n t l y h igh temperature for a steam-reforming process , 900-950°C w i l l be necessary. We are hopeful t h a t

such gas temperatures can be reached w i t h i n t h e next 5-7 years . 2. Y e s ,

i n ho t c e l l experiments, d e t a i l e d d e s c r i p t i o n s of t h e experiment w i l l be

given i n t h e papers by Merz and Hackstein.

I Paper 2/113

DESIGN, FEATURES AND ENGINEERING STATUS OF THE THTR 300 MWe -I- . -d - * I io

PROTOTYPZ POWER STATION - -nmw-=w,- -A

H. Oehme J. Schoning -3

Brown Boveri/Krupp Reaktorbau Ltd. is developing a line of high-temperature helium-cooled pebble-bed reactors, with completely integrated primary sys- tem. The feasibility of the concept has been dem- onstrated by the AVR experimental reactor, which has been supplying electricity to the grid since December 1967. The next stage in the development is the 300 MWe THTR, which has the same design characteristics as the AVR. However, various com- ponents were modified because of the increased dimensions of the THTR, such as the prestressed concrete pressure vessel, The design of the THTR, its safety concept, and supporting development tests are described, as well as the status of engineering and construction arrangements.

GENERAL CHARACTERISTICS OF THE 300 MWe THTR

The main activity of the Brown Boveri/Krupp Reaktorbau L t d .

is concentrated on a thorium high temperature reactor line based on spherical fuel elements. iTh e main characteristics o f this reactor line are the following:

- spherical fuel elements - helium as coolant - high primary gas temperatures up to 800 C - primary system completely integrated

0

The spherical fuel element permits continuous fuel loading

during operation. Each individual fuel element has a negligible

amount of reactivity.

161

162

The feasibility and advantages of this reactor concept have

been demonstrated by the construction and operation of the AVR

experimental reactor ( 15 NWe) which was brought to criticality

for the first time in August 1966. Since December 1967 AVR has supplied electricity to the grid, and during 1969 the plant achieved an availability of 72 5. The thorium high temperature reactor (THTR) possesses essentially the same construction characteristics

as the AVR (Slide 1). However, based on the experience gained with

A V R , and in view of the higher power of THTR the following compo-

nents were introduced or changed:

- the prestressed concrete pressure vessel - the core structure - the shut-down system - the fuel element circulating system

The 300 MWe THTR prototype reactor is designed and optimized

as a direct predecessor of a reactor of commercial size (Slide 2).

THE INTEGRATED PRIMARY SYSTm

The reactor core and all main components of the primary system - blowers, steam generators, shut-down system -, excluding the fuel circulating system and gas purification plant, are completely inte-

grated in a prestressed concrete pressure vessel. Access to these

components is not foreseen although they can be removed in case of

failure or of inspection or maintenance work. All other components

and facilities are located outside the prestressed concrete pressure

vessel and are accessible during reactoa operation (Slide 3) .

The reactor core consists of approximately TOO 000 spherical fuel elements of 6 cm diameter. The cylindrical core of 5,6 m diameter and 6 m height is formed by a graphite wall which acts as reflector and surrounds the core on all sides. The bottom re-

flector is conically shaped with an inclination of 30° and opens

into a central discharge tube of 800 mm dia.

163

Graphite is a well-known material proven in many reactors. The

THTR reflector is exposed to a higher radiation dose (maximum total

dose 2 nvt) and to a considerably higher temperature. Apart

from that and in addition to its usual function as a reflector, and

as nuclear and thermal shielding, it must perform the mechanical

function of a supporting and containing structure for the pebble bed.

By an appropriate choice of the graphite (considering the important

influence of the cementing resin) it is possible to remain within

controllable dimension changes of the structure since the determining

fast neutron flux decreases very rapidly in the reflector so that

a noticeable shrinkage can occur only up to a depth of 10 - 20 cm of the inner graphite layer. With the selected graphite type and

the total maximum dose to be expected a maximum shrinkage of 2 $ and an corresponding growth to the initial dimensions was taken as

a design basis. Because of all these requirements, special measures

for the core structure are necessary, to provide the necessary

flexibility of the graphite structure for temperature- and radiation-

induced dimensional changes, thus avoiding possible compressive

forces while insuring the stability of the total structure. These

requirements were met by relatively small blocks specially shaped

and arranged in columns and by connecting and fixing the blocks

with wedges and dowels (Slide 4 and 5).

The prestressed concrete pressure vessel acting at the same time as pressure vessel and as gas- and pressure tight safety

containment, includes all main components of the primary circuit.

It is a cylindrical vessel of- 16 m diameter and 15 m height with

a wall thickness of approximately 5 m. Prestressing is achieved

with annular and vertical stressing cables having a guaranteed rupture

sheaths embedded in the concrete and are grouted after prestressing

to insure sufficient protection against corrosion.

load of 890 Mp. The prestressing cables are inserted in

164

For the pressure vessel a relatively conservative design was

selected, which has been proven in numerous large-scale constructions

and has been and will be tested in a number of model tests ( l :5 , 1:45, 1:20 ). The test result will be incorporated in the large- scale construction. For further Frojects, while retaining the same

vessel shape, various technical and cost-saving improvements are

foreseen, such as transition to a horizontal wire winding pre-

s+,ressing system and reduction of the inner dimensions by an optimum

arrangement of components. It is possible that further economic

advantages may result from the podded design; the pressure vessel

cost and the technical safety requirements are approximately equal

for both vessel types.

For the introduction of blowers, steam generators, absorber r o d s ,

pipes, etc. the prestressed concrete pressure vessel is equipped

with penetrations. These penetrations are reenforced by ,shutter-

tubes designed t o absorb the forces in the area near the penetrations.

The largest openings of 2,25 m diameter for the steam generators

are located in the top slab of the vessel.

Gas-tightness is achieved by application of a metal liner of

20 - 30 mm thickness serving at the same time as the inner shutter-

ing for the prestressed concrete pressure vessel during construction.

For the liner material special properties are required, such as

high elasticity, good weldability, and high yield strength.

An insulation on the inner wall and a water cooling system

welded on the outside of the liner protect the steel liner and the

concrete against excessive temperature effects. A s insulation a

metal foil insulation (stainless steel) proved to be the optimum

solution since with this insulation purity requirements can be

easily met, irradiation behaviour of the materials is sufficiently

known, mechanical properties are satisfactory, and aging and

corrosion are not to be expected. The thickness of the heat

insulation of the liner is approximately 70 mm.

165

Six parallel single-stage radial blowers each supplying one

steam generator, circulate the coolant gas. The blower is designed

as an insertable unit. The impeller is mounted in an overhung po-

sition and is oil-lubricated. A baffer-gas system avoids the pene- tration of oil into the primary gas. Each blower is equipped with

a combined bypass and shut-off valve system to allow connection of a single blower while other blowers are operating, trimming of

coolant gas flow through one steam generator under constant blower

speed, and shut-off of one line in the event of failure of one steam

generator. A s drive a bipolar ?-phase squirrel cage motor was se- lected which is characterized by simple design and high operational

safety. Blower power is controlled by variable frequency. Three

blowers are supplied by one turbine generator set. In the event of

failure of the two turbine sets, operation can be maintained with

constant speed by the mains power supply. In addition, transformer

sets are provided, permitting start up and emergency operation of

individual blowers (Table 1 and Slide 6).

0

Table 1. Blower Data

ITumber of units 6 Delivery per circulator 49,25 kp/sec

Pressure at suction duct 38,9 ata

14 m 3 /sec Delivery in volume

Pressure increase 1 , l ata

Temperature at suction duct 25OoC

Circulator power 1780 kW

Six steam generators connected in parallel transfer the heat

generated in the core to the secondary water-steam-circuit. The

main steam generator date are listed in the table (Table 2).

166

Table 2. Steam Generator Data

Number of units

Diameter of tube bundle

Height of bundle

Gasflow through the generator

Total length

Output per unit

Hot gas temperature Cold gas temperature

Gas pressure

Pressure drop at gas side Gas m a s s flow

Live steam conditions

Feed water conditions

Rehe at ing

at entry at exit

Steamflow

6 2 000 + 55 mm max. 12 500 mm upwards

17 800 mm 128 MW

750°c

39,35 ata max. 0,45 ata

49,25 Kr/sec 335OC / 190 ata 18OoC / max. 240 ata

250°C

The helium flows through the steam generators in an upward direction.

The tube bundles of superheater, reheater, evaporator, economizer

are arranged in gas flow direction. The steam generator operates

in a once-through-system. The water steam-flow through economizer,

evaporator, and reheater is in counter-flow direction, the super-

heater in uniflow direction. Three steam generators are connected

in one main group, which is equipped with a common feed-water line,

a start up flush-tank, and a common cold reheater line. To insure safe emergency cooling and removal of decay heat from the core

each steam generator can be operated independently of the other

steam generators, i.e. each is equipped with separate shut off and

control valves. Each tube bundle of the steam generator is sub-

divided into forty independent systems which are individually led

through the top slab of the prestressed concrete pressure vessel

16’7

0 and connected outside of the vessel. Hence, in the event of a tube

failure each system can be isolated from outside. The steam generator

is so designed that it can be replaced.

The control and shut-down system is designed so that the reactor load can be varied between 40 and 100 $ and the shut-down reactivity required to keep the reactor in the cold critical state is available.

Fine control is carried out by 36 rods which are freely moved in bore holes in the reflector. Shut-down of the reactor under both

normal and fault conditions is effected by 42 core rods which are

directly inserted into the pebble bed. This procedure was verified

by exterrsive tests on a complete model 1:6, 1:lO and on sectional

models in 1:1, 1:2, 1:3. This proved that the rods can be directly

inserted into the pebble bed under all operating conditions of the

reactor and neither the fuel elements nor the absorber rods them-

selves are exposed to excessive stresses. Each core rod is driven

by a pneumatic step drive, achieving safe insertion at a step size of 2 cm and a step frequency of 0,3 sec. For rapid shut-down a

long-stroke pneumatic system is provided, with an insertion speed

of 30 cm/sec. Each drive has a separate supply system with re-

dundancy (Slide 7).

The THTR fuel element circulating system is of special interest. It is not integrated in the primary circuit and in contrast to

reactors with rod type fuel elements, requires no machinery inside

the core for charging, exchanging, and discharging fuel elements.

For the downward motion of the fuel elements from the core (fuel

element discharge) gravity is used and upward movement (fuel element charge) is achieved with pneumatic energy by helium from the primary

circuit. For the construction of the fuel element circulating system

a building block system, composed of conventional units was used

(Slide 8 and 9 ) .

168

Because of their special requirements, the components involved

in the helium system required the development of a new technology

which can be roughly divided into three problem areas:

- helium purification and purity - helium leak-tightness - lubrication and friction in an extremely pure and dry helium atmosphere

According to the present state of technology, these problems

can be considered essentially solved. For selection and application

of components, however, a number of aspects must be taken into

account, which have proven to be advisable, on the basis of know- ledge and experience gained up to now, especially through con-

struction and operation of the AVR. The following aspects are of importance:

- good accessibility to the components needing maintenance (location outside of radiation area)

- good maintenance facilities for systems with flanged and screwed joints (plastic material)

- protection of the sensitive instruments against dust and dirt caused by assembly, through use of suitable filters

components (oil, air, etc.)

- protection of the helium against pollution from the operating

- simple and conservative design of systems - well-defined and separated tasks for individual systems, thus allowing uncomplicated design

- use of conventional proven components These fundamental aspects partly result from special newly

developed designs which were carried out to achieve an optimum

solution of the reactor concept of the gas cooled high temperature

line

169

The mechanical impurities which are mainly produced by the gas

circulation plant the fuel element abrasion in the core and the

assembly protection and which impair the leak tightness and the

control exactness of a number of valves, can be controlled by

mechanical filters but even more effectively by restricting or re-

moving the impurity sources. The impurities from the gas circulation

facilities can be largely avoided by barrier gas systems or oil free

blowers and compressors (gas-bearings blowers, membrane compressors

and dry operation compressors). The impurities produced by assembly work are reduced to a minimum by careful assembly (freedom of

grease, scale, and corrosion of all helium-carrying systems) in

appropriate clean conditions standards. For this purpose pre-assembly

in the workshops of the suppliers must be demanded. The dust which

is unavoidable due to fuel element abrasion is eliminated by appro-

priate mechanical filters in the gas purification plant.

0

The wear problems occuring under these specific reactor condi-

tions are solved by special constructions (cageless ball bearings,

special bearing clearances, dry lubricated structures) with appro-

priate auxiliary measures such as barrier gas systems.

For safety and economy reasons, the problem of leak tightness

is of special importance for the HTR. Although welding is the most simple, most economic and safest solution, it is only appl icable

for static seals and this applies for operational reasons only in

some cases. The solution adopted for dynamic seals is to be de-

monstrated for two especially critical cases, that is gas tightness by valves and gas tightness in the event of rotary motion (momentum

of rotation). In both cases a satisfactory solution could be achieved only by redesigning the components in parallel with sealing material

and lubricant developments for these components. Because of the

favourable development results of plastics materials such as Viton,

Kel-F, Vespel, RCH 1000 all now available, which in addition to sufficient radiation and temperature resistance, have satisfactory

mechanical properties; the same applies for lubricants.

c3

170

The first example concerns a simple, safe, and economic gas shut-off bellows valve (leak tightness in the valve cap is achieved

by a specially mounted plastic material). (Slide 10)

The second example concerns a gas-tight rotary drive. Also in

this case, special importance was attached to safety and economy.

In this type of solution, the gas tightness is achieved by using grease which forms a considerably less complicated fluid seal.

Higher pressure relative to system pressure, grease expansion due

to temperature changes, and re-lubrication are effected by pressure

reservoirs (Slide 11 ) .

SAFETY CONCEPT

In this connection only those facts will be mentioned, in which the safety concept of THTR differs from those of other reactors. The decisive safety aims are:

Complete integration of the primary coolant circuits in the pre-

stressed concrete pressure vessel

- hence limited l o s s of coolant in the event of credible accidents - release of activity via ventilation stack in the maximum credible accident

- no sudden depressurization of core - integrity of core structure The penetrations in the concrete vessel are designed to prevent

activity release by either

- two pressure-tight covers or - an outer pressure-tight cover, and an inner flow limiting cover

(about 33 cm2 corresponding to a coolant pipe diameter of 65 mm) which, in the case of the outer cover being damaged, restricts

the escaping coolant flow to a precise value.

A

171

Those primary coolant pipes which lead from the concrete pressu-

re vessel to the auxiliary circuits are 6 6 5 mm diameter. These pipes are double walled as far as the fast action safety release

valves. The safety container has no safety valves and is designed

for the pressure of the maximum credible accident. The total primary

building is divided into three categories of rooms, according to

their different activity levels (Slide 12).

- Rooms continuously accessible during operation - Rooms accessible only for a short time during operation,

0

for inspection purposes

- Rooms no% accessible during operation, access possible only after partial or total shut-down of plant

The third category includes primarily the room for the installation

of the fuel circulating plant and part of the room for the gas purification plant as well as the space between air duct and pre-

stressed concrete pressure vessel. According to their room catego-

ries, these rooms are separately ventilated and are connected to the ventilation stack via the normal exhaust air system and in

addition, by a pressure safety system.

The safety analysis of this concept showed that, the maximum

credible accident, with reference to endangering the surroundings, requires two independent faults to occur, namely

- a break in a primary coolant pipe ( 6 65 mm f8) outside the concrete pressure vessel

- the simultaneous malfunctioning of the safety release valve in that pipeline

In this case the escape of coolant gas is relatively slow, without

a noticeable pressure rise in those rooms and can easely be vented.

men in the case of this maximum credible accident, the allowable

contamination of the surroundings is not reached.

The largest realistic reactivity excursion through

- the accidental withraval of the two most effective control rods from the core

the largest realistic coolant disturbance by 6id

- the simultaneous breakdown of three blowers, and the largest realistic pressure change through

- the burst of a steam generator pipe and causing water ingress into the core

have such a limited effect on the reactor, that the surroundings

are not endangered.

THE STATUS OF THE 300 Mlrle THTR-PROTOTYPE

Construction documents for this prototype plant have been

completed and an offer has been submitted. The licensing procedure

has been initiated early 1970 ( l 5 / l ) . A utility group for the operation of this reactor plant has been founded and construction

work is to be started in the beginning of 1971. The construction

period is 5 years, including a test run of the plant of three month.

173

I A V R 69 2-6

I BBCIKRUPP

1 AVT Reactor Sectional V i e w

4 Graphite Structure-Ceiling 3 Prestressed Concrete Pressure

Vessel (~odel) of THTR Power Plant with Steam Generators, Blowers, Core, Absorber Rod, Facility etc.

175

5 Graphite Structure-Bottom

6 Blower Section

176

S e c t i o n of Core Rod Drive 7

THTR 701-18

FLIEOSCHEMA BESCHICKUNG

BBC / KRUPP

.ow Diagram)

*% (Photo KO.) 6- Sing-ul izer 7- Demaged Sphere Separa tor €3- Demaged Sphere Container

1- Fue l Element Charge Equipment 2- 3urn-up PIeasurement 3- Switch 9- Buffer-zone 4- Pneumatic E leva t ing Device 5- Core

IO- Discharge of Spent Fuel 11- Process Computer

177

, Fail-safe Antrieb Fail-safe Drive

Handantrieb Monual Drive

Funktionsteilanordnung THTR BBC KRUPP Refuelling Mechanisms Assembly 6 8 3 6 - 9

Dreh tei le ~ Functional Ports I

1

Funkt8 ,.dilblock Integra iody

Stopfbuchse Gland

I

THTR 59.25 - 20

VENTILDICHTKOPF FUR

GASABSPERR - ARM AT U R E N

B B C l KRUPP

10 Gas shut-off Valve

178

1-

5 W F l PENElRATIOII WllH GLANDS w

GLANDLESS SHAFT PENETRAIYIU

I THTR

11 Rotary Drive

THTR

R l ROOYCATEGORY - NOT ACCESSIBLE DURING REACTOR OPERATION

R t RWUCAIEGORI - ACCESSIBLE TEMPORARILY

R 3 RMYCATEURV- ACCESSIBLE PERMANENTLY

I CORE

2 STEAM GENERATOR

3 CIRCULATOR

4 PRESTRESS0 PRESSURE VESSEL

5 FUEL ELEMENT OUTLET

6 CONTROL RW

BBC / KRUPP

12 Safe ty Concept

179

DISCUSSION

B. N. Furber: I would l i k e t o have fur ther d e t a i l s of the meta l l ic

insulat ion. I n the experience of TNPG t h i s insu la t ion suf fers from a t least two weaknesses: c u l t t o design t o accept in te r face pressure gradients. I n the c i r c u l a t o r

o u t l e t these two weaknesses a r e l i k e l y t o ser iously a f f e c t the insu la t ion

performance.

(1) It is highly anisotropic , ( 2 ) it i s very d i f f i -

What t e s t work has been carr ied out i n t h i s a r e a ?

J. Schzning: We are aware of the problems raised by you and our

people have a concept t o overcome them, the detai ls of which we are not

ab le t o present. work going on t o confirm the concept t h a t has been selected.

There are extensive experiments and de ta i led design

R. D. Vaughan: The THTR reac tor sec t ion shows the b o i l e r s t o be of

cy l indr ica l plan form, but located i n the pressure vesse l vaul t along w i t h

the reac tor core. May I conclude t h a t the designers have looked a t the a l t e r n a t i v e with pods i n t h e w a l l of the pressure vessel and found it would be more expensive?

J. Sch'dning: The\design of the THTR pressure vesse l and t h e arrange- ment of the b o i l e r s within the vesse l was already decided upon i n 1965.

A t t h a t time the pod-type vesse l did not ye t seem t o be evaluated.

t h i s time a change of the concept i s not possible due t o the advanced state of development and design. It is important, however, that the THTR-

vesse l allows the removal of the b o i l e r s i n t h e same way as the pod-type vesse l does. s t a t i o n a de ta i led comparison of the two vessels and t h e i r influence upon the p lan t i s under way.

At

Within the development work f o r a l a r g e commercial power

M. Dalle Donne: Coming back t o t h e question of Dr. Furber, I think

he answered h i s question himself with h i s paper. With helium one i s i n the region of Raleigh number where the Nusselt number i s constant, so t ha t

a var ia t ion of helium v e l o c i t y does not change the heat t r a n s f e r coeff i -

c ien t of the insu la t ion ; although t h i s may be the case w i t h C 0 2 , where

the heat i s mainly t ransfer red by convection, while with helium it i s

t ransfer red by conduction.

Paper 3/112

TRE LAYOUT OF T m CORE AND FUEL ELEMENTS OF THl3 THTR -”-%.

- -------- - - -- - - - - w l L - - r . - S - - ~ - ~ ~ , ~ * r W - c ” c l l r r r - -._. __ K. Ehbers (Brown Boveri/Krupp Reaktorbau GmbH,

Cologne, Federal Republic of Germany)

H. Harder (Brown Boveri/Krupp Reaktorbau GmbH, Cologne, Federal Republic of Germany)

E. Schroder (Brown Boveri/Krupp Reaktorbau GmbH, Cologne, Federal Republic of Germany)

\A‘ ABSTRACT

The continuous loading and circulation of the fuel elements in pebble bed reactors results in a number of performance differen- ces from reactors with prismatic fuel elements. Their influence on the layout of the core and fuel elements of the THTR is de- scribed.

Introduction

The THTR is a high-temperature gas cooled pebble bed reactor, the construction of which is expected to be licensed within 1970. The THTR is operated on an uranium/thorium cycle, with fissile and fertile material mixed within each fuel element in a ratio of 1 : l O . The first batches of fuel elements discharged from the reactor will not undergo immediate reprocessing but will be stored for an unspecified period. Although an uranium/ plutonium cycle operating at a lower enrichment would, under these conditions, result in equal or slightly lower fuel cycle costs, the uranium/thorium cycle has been selected because of its greater potential when reprocessing facilities w i l l be available. Optimization calculations have resulted in the fol- lowing design of fuel elements:

180

181

Content of fissile material ( ~ 2 3 5 Content of thorium

Enrichment

6d 0.96 g/sphere

9.62 g/sphere

0.93 Mean residence time in reactor 3.0 years

Mean fast neutron dose (E 2 0. I MeV) 4.8-102’ nvt

Maximum power per sphere

Mean burn-up [fima]

Mean burn-up [fifa)

3.5 kW

0.12

1.34 Mean number of circulations through core 6

The spherical fuel element has a diameter of 6 cm and con- sists of an exterior fuel-free graphite shell with a minimum thickness of 5 mm. Inside of this shell the coated particles are embedded in a graphite matrix of the same composition as the exterim shell.

Core Design

The initial core of the THTR consists of a mixture of approxi- mately 350 000 of such fuel elements and an approximately equal number of graphite spheres, of the same diameter, containing no fuel. The same fuel element type is used for the initial core and the running-in period as well as for the equilibrium core, which allows a considerable reduction in the fuel element fabri- cation cost. The total uranium inventory of 359 kg U235 in the initial core is approximately equivalent to the sum of the con- tents of U235 and U233 in the equilibrium core. Because in the initial core, the thorium to uranium ratio is only h a l f that of the equilibrium core and fission products have not yet been generated, approx. 40 000 absorber spheres must be added to the initial core, each containing 4 g of natural hafnium. Hence no trim rods are required for the initial core and the running-in period.

The core is subdivided in two radial zones. The central zone of 4 m dia is surrounded by an outer zone of 0.80 m width. In order to flatten the power and to obtain a sufficient reactivity worth of control rods inserted into the radial reflector, the ratio of fissile isotope density between the.outer and inner zone is 1.5 and the burnable poison density ratio 0.66. This is achieved by mixing fuel, absorber, and graphite spheres in the two zones accordingly.

182

In this way a reasonable power flattening and a reactivity worth of 4.5-Nile of 36 control rods in the radial reflector is obtained. Such a worth of the reflector rods -is needed to com- pensate for reactivity changes during load-changing between :OO and 40 $ (Xe-override) and for other adjustments.

The neutron flux increase in the outer zone of the reactor core required for this purpose is close to the economically optimal neutron flux distribution. Neutron leakage is increased only slightly. During steady load operation, a major number of these rods are inserted in the reflector to compensate for 1.7 to 2.6 Nile excess reactivity which must be available for con- trolling partial load operation and fine adjustment. If a daily load curve is to be followed, a few additional rods must be in- serted or withdrawn for a few hours. The periods during which all reflector rods are either extracted or inserted are both very short.

When power operation starts the pebble bed is circulated. The spheres pass through the core in the downward direction within average transit time of six months. By the conical shape of the core bottom, the spheres are directed towards the central dis- charge tube, through which they are extracted from the core. Each sphere subsequently passes through a measuring equipment, by which the type of sphere - fuel element, graphite or absorber sphere -, and the burn-up state of the fuel elements can be de- t ermine d .

Initially, some of the graphite and absorber spheres entering into the measuring equipment but no fuel elements are discharged from the system. An equal number of new fuel elements are added to the core to compensate fuel burn-up. The new fuel elements, together with the other spheres not discharged from the system, are lifted pneumatically through guiding tubes and drop on the pebble bed from a low height, thus closing the circulation loop.

The flow lines on which the spheres pass through the core* and the time required for passing through the core on different core radii have been experimentally determined in models of similar geometry. Specifically, it has been verified that the separation of the initial core into an inner and outer zone is maintained sufficiently distinct in spite of the circulation process. A mixing of the two zones which occurs in a region of two or three sphere diameters at the zone boundary is desirable, since steep gradients of power at the boundary are avoided.

A

183

In order to maintain the subdivision of the core into zones, three refuelling tubes point at the core center, and twelve at the outer zone. By an appropriate distribution of fresh and re- circulated spheres to both of these tube systems any desired width of the core-zones can be established, but cones of diffe- rent heights over both core-zones must be accepted.

crs

With increasing burn-up the recirculated fuel elements, graphite and absorber spheres are distributed to the inner and outer zone of the core such as to maintain the power distribu- tion of the initial core. After approximately one and a half years of full power operation - i.e., at a mean fima value of approximately 4.5 $ - all graphite spheres will have been re- placed by new fuel elements. At this time the overall thorium: uranium ratio has largely been shifted towards the equilibrium value, and fission products have accumulated to a considerable degree

As power operation is continued, the first burnt fuel ele- ments must be discharged from the core and replaced by new fuel elements. The burn-up measuring facility insures the discharge of fuel elements with highest burn-up; during the running-in phase these are the fuel elements of the initial core.

After a total of 3 years of full power operation the equi- librium phase in reactor fuelling is almost achieved. From then on the core composition and hence also the power distribution remains constant. Every day 621 new fuel elements must be added to compensate fuel burn-up.

To maintain the desired different fuel concentrations in both core zones during the equilibrium phase the fuel elements are divided into 3 classes, as determined by the measuring facility. Fuel elements having remained in the reactor more than 2 . 9 years of full power are discharged. Because of the different times for passing through the core at different radii this value corresponds to a mean residence time of the fuel elements of 3.0 years at full power. Fuel elements with a burn-up corresponding to a residence time between 1.6 and '2.9 years are subsequently added to the inner zone of the core. Fuel elements with a re- sidence time of less than 1.6 years and all fresh elements are added at a ratio of 79 % to the outer zone and 21 % to the inner zone.

184

Figure 1 shows the residence time spectrum of the discharged fuel elements. Since it is required that only a negligibly small share of the burnt fuel elements (approx. 10-4) remains in the reactor longer than 4.2 years of full power (corresponding to 14 $ fima), the burn-up measuring facility must determine the uranium content of these elements to an accuracy of approx. f 15 mg U235 equivalent. The fissile material content at dis- charge amounts to approx. 50 mg U235 and 250 mg U233. It has been confirmed by experiments that this condition can be met by measuring and evaluating the reactivity increase caused by the fuel elements passing through a measuring reactor of some hun- dred Watts of power. When the operation of the TRPR or further irradiation tests permit an increase of the maximum residence time, the accuracy of burn-up measurement can be reduced.

The distribution of the fuel residence time round the mean value of three years is of small influence on the reactivity and hence on fuel cycle costs.

By recirculating the fuel elements with higher bum-up into the inner zone and by adding the new and the fuel elements of low burn-up preferably to the outer zone, the reactivity worth of the reflector rods of 4.5 Nile can be maintained also in the equilibrium core. Figure 2 shows the radial power distribution taken from a two-dimensional calculation at the core height of maximum power for completely withdrawn, completely inserted re- flector rods, and for a rod position corresponding to full load operation. At full power operation leakage losses are 8.5 $. The maximum fluctuation of the local gas temperature caused by the shifting of the reflector rods are 200 - 250 OC, however with a gradient of less than 10 OC/min. The maximum fuel and gas tem- peratures have been calculated at 1250 OC, and 935 OC respec- tively.

The fuel temperatures in the THTR are favourably influenced by the mode of fuelling the reactor. Since the fuel elements are added from the top and part of their fissile material is burnt during their transit through the core, the power maximum is shifted towards the upper half of the core, that is towards the cold gas side.

Because of the continuous flow of the fuel elements the maxi- mum temperatures in the fuel elements occur only during a short period, compared to their life time. The maximum temperatures occur at the gas stagnation area between the main coolant gas flow and the bypass which cools the element discharge tube. This area occupies approx. 10-4 of the core volume.

185

The fuel elements pass through this region within approx. 20 days. The maximum temperatures are generated in the fuel ele- ments only during this time and only if all margins left for calculation errors on the reflector rod effectiveness sum up to

0

I I During later transits through the core the fuel elements have

the pessimistic side, which then requires insertion of all rods.

already reached a partial burn-up so that lower fuel temperatures result after the first transit.

During operation no reactivities must be compensated other then the reactivity fluctuations resulting essentially from partial load operation, which are to be compensated by the re- flector rods. Thus the 42 absorber rods located above the core are required for reactor shut-down exclusively, and can remain in withdrawn position during almost the entire power operation. Only for stdrt-up after an extended shut-down, several absorber rods must remain inserted to compensate the positive reactivity due to the reduced xenon concentrations and to the decay of pro- tactinium into uranium-233 during the shut-down period.

The requirement of the reactivity worth of these 42 rods must be calculated for the end of a shut-down period of several months. Then the protactinium has completely decayed into U233. A balance at that time is shown in Table 1.

Calculation Procedures

The calculation methods applied for the design of pebble bed reactors largely coincide with the procedures for other high temperature reactors. In addition to these standard methods com- puter programs were developed by which the unique method of charging pebble bed reactors can be taken into account.

The isotope distribution within the core, essential for cal- culating criticallity, power distribution and control rod worth results only if the burn-up state and the local position of the individual fuel elements are known, which however depend on the neutron flux distribution, the flow behaviour of the spheres, and the charging strategy. Therefore, for calculating the equi- librium isotope distribution, a flux distribution must be assessed, and a charging strategy assumed.

186

For several fictive concentric flow channels, the increase of the fuel element burn-up is determined f o r each transit. By averaaing the nuclide concentrations over appropriate volume fractions, an impoved qeutron flux distribution is calculated. If after several steps of itsration this distribution differs too much from the desire6 distrihtion, the charging strategy is varied.

The computer program which calculates this position-dependent core composition for a given charging strategy, takes into account the different transit times of the fuel elements in different flow channels, the influence of the burn-up moasuring accuracy on the distribution of the recirculated fuel elements to the inner and outer core, and their final discharge from the reactor circuit . The 2-dimensional f lux calculations in r,z- geometry must be carriec! out with a rather fine subdivision of the core volume into material zones, i n order to represent the flow paths of the elements suffjciently.

1

During the running-in period the core composition and the power distribution are time dependent. Therefore, burn-up cal- culations are carried out starting with the power distribution of the initial core. The core is subdivided into a number of sphere packages. Using the neutron flux averaged over such a sphere package, the burn-up in the fuel elements of this package is calculated f o r a time increment with the assumption that the circulation movement can be neclected. Subsequently %he indivi- dual sDhere packages are shifted according to the paths ahancet! due to their movement during this time increment. The calcula- tion of' tbe further increase in burn-up is then performed with the neutron flux at the new position of each sphere package. After a number of time increments, Cukie flux distribution must be recalculated. A difficult data storaqe problem within this pro- cedure is the follow-up of the nuclide concentration in those sphere packages which are recirculated after a transit through the core and where individual spheres are either assigned to different core zones o r partly discharTed. The nuclide concen- trations must be distributed accordincly.

The temperature distribution is also calculated in r,z- 'Zeometq. The eeometry is described by a mesh layout. The mass flow distribution is determined by similar aethods as developed in electrical network theory. The mass flow and temperature de- pendence of the flow resistance is taken in$o account.

A

187

For determinirig the release of fission products the tem- perature and burn-up history of one representative fuel slement during its residence time is calculated. As transport mechanism of solid fission products, temperature-activated solid-state diffusion and evaporation from the eiirface of the fuel element into the coolant gas are considered. The time-dependent diffu- sion equation is solved numerically under variable temperature and power conditions for the fuel particles, the fuel coating, the graphite matrix, and the fuel-free shell of the fuel ele- ment. The contamination of the coating and the matrix by fission material during fabrication and the diffusion of inert gas pre- decessors for various radioactive decay chains are taken into account .

The results are time-dependent release rates of one repre- sentative element and - by integration over the core volume and time - the total release of the core.

With similar procedures corrosion rates and atresses are calculated, the latter program makes use of an existing com- puter code "STRETCH", adapted to the requirements of a moving fuel bed.

Consequences for the Fuel Elements Layout

The major part of activity in the primary circuit is due to gasous fission products caused by the contamination of the fuel elements with fissile material, either directly or indirectly by decay of a gaseous predecessor.

The favourable influence of the moving fuel on the total re- lease from the core can be demonstrated by comparing a static core with a continuously flowing core of identical temperature distribution and residence times.

The release rates of long-lived fission products from the moving fuel elements are far below those of the static core; for strontium 90 e.g. they are lower by approx. 3 orders of magnitude. Therefore, even without having a secondary contain- ment around the primary circuit no layer of Sic is required in the coating of particles. This is important for improving the neutron economy, and lowering both fabrication and reprocessing costs.

188

Since the fuel elements will remain within the region of high temperatures for relatively short periods only, no stringent re- quirements are needed for the irradiation resistance of the gra- phite. The graphite selected for the fuel elements (see Figure 3 ) starts expanding at temperatures of approx. 900 OC and doses of 8 x 102l nvt (E t 0.1 MeV). This has to be compared with the maximum dose of 6 . 3 x 1021 nvt in the reactor, which is only reached by a fraction of 10-4 of all elements discharged.

The results of stress analysis for the fuel elements in reactor operation are shown in Figure 4. The maximum tensile stresses occur after the first transit through the core, since then the temperature gradient disappears because of the sharp reduction of power production. In the core the temperature ea- dient partially compensates the stresses caused by dimensional changes due to irradiation. The sudden change of tension de- creases with increasing burn-up. A compariscjn with calculated stresses of irradiation experiments shows (see Figure 5) that the maximum stresses occuring in these experiments are much higher than those to be expected in the THTR. Therefore, there will be no damage of the fuel elements by irradiation-induced and thermal stresses.

REFERENCES

1. L. Massimo, Nukleonik 11, 72 (1968)

189

Table 1. Reactivity Balance

Reqiiirement

1.7 Nile le-everride between full and 40 $ load (normally compensated by reflector rods)

2.0 Nile Temperature

3.8 Nile Xe-equilibrium

3.6 Nile Pa-233 decay and U233 build up

0.5 Bile Margin to compensate fluctuations due t o deviations from refuelling plan

11.6 + 1.1 Total

Rod efficiency

17.3 Nile Calculated worth of 42 rods

1.3 Nile Worth of the two most efficient rods

16.0 + 2

+ 4:;4,;=2:?: Shut-down margin

Table 2. Fission Product Release into Primary Circuit

moving core 1 ~~

Isotope 1. from. I from impurities kernel

Xe 133 Xe 135 Kr 88 Kr 85m J 131 J 132

J 133 Sr 90

Cs 137

1300 Ci

150 Ci

1350 Ci 750 Ci

ci

ci

ci -

260 Ci

static core

from impurities

1300 Ci

150 Ci

1350 Ci

750 Ci 50 Ci

ci

0.1 Ci -

1500 Ci

from kernel

- - - - - - - 5 ca.10 Ci

E-10000 Ci

190

\

rei. units 1

P (T

10

10'

1 0 - ~

io-' 2.0 3.0 L .O 5.0

T [years]

Fig. 1. THTR Fuel Element Residence Time Bistribution

F'unc t ion.

region boundory

Fig. 2. Radial Power Distribution in THTR Equilibrium Core.

1 2 3 4 5 6 7 e 10"""t I 1 1 1 I I I 1 )

E 70,l MeV

( '!. 1 I

Fig. 5. Dimensional Changes of THIrR Fuel Element Graphite.

191

IRRADIATION TIME (DAYS I

Fig. 4. Reference Fuel Element: TangenCial Stresses and

Power in THTR.

ANGENTIAL ;TRESS kp lcm2)

x)

0

- 10 - 2 0

-a

-40

-50 II II I I J IRRADIATION TIME [DAYS I

POWER I k W I

Fig. 5 . Reference Fuel Element: Tangential Stresses and

Power in Irradiation Experiment.

192

DISCUSSION

J. D. Thorn:

H. Harder: 1 X

What i s the design value of R/B?

L. W. Graham: I n looking a t t h e s t r e s s e s i n the b a l l , have you con-

s idered t h e e f f e c t s of the d i f f e r e n t i a l dimensional changes between the

f u e l and un:fueled p a r t s ?

H. Harder: No. The s t r e s s e s shown i n t h e s l i d e concern a sphere of homogeneous graphi te matrix.

J. D. Thorn: What changes i n hea t t r a n s f e r a r e expected around the

f u e l element b a l l ?

H. Harder: Not s i g n i f i c a n t .

F. P. 0. Ashworth: There i s a complex vec tor f i e l d of b a l l ve loc i - t i es and swel l times i n the core. Is the re a p robab i l i t y of a b a l l s tay- i n g i n the core long enough for t he r e l ease t o exceed the t o l e r a b l e core t o t a l f o r a p a r t i c u l a r f i s s i o n product?

H. Harder: There i s only an extremely sma,ll p robab i l i t y t h a t spheres remain i n the core f o r such a long time. Even then t h i s would r e s u l t i n a neg l ig ib l e e f f e c t on t o t a l a c t i v i t y of t h e primary coolant.

T. A. Jaeger: This ques t ion concerns the diagram showing t h e r e s u l t s of a thermoviscoelastic ana lys i s of the sphe r i ca l f u e l element. Could you

give some d e t a i l s on the underlying assumptions and the method of ana lys i s u t i l i z e d , e .g . , d i d you include considerations on the a f f e c t s of contact pressures and l o c a l hea t t r a n s f e r disturbances ?

H. Harder: The temperature d i s t r i b u t i o n due t o the power produced i n a f u e l element during i t s seve ra l t r a n s i t s through t h e core and the

s t r e s s e s r e s u l t i n g from the time-dependent temperature gradients i n the sphere have been calculated. The stresses due t o i r r a d i a t i o n a f f e c t s Lave been determined by piecewise eva lua t ion of t he curves representing

shrinkage of graphi te versus neutron dose as measured i n i r r a d i a t i o n ex- periments a t t h e corresponding temperatures. l l i s includes some approxi- mation because the temperature i n each of t hese experiments has been

193

@ confined during t h e whole i r r a d i a t i o n times. For creep, p r o p o r t i o n a l i t y

The e f f e c t s o f contac t between d/dt (fill) and stress has been assumed.

p re s su res and l o c a l heat t r a n s f e r d i s turbances have not been considered

i n t h i s ca l cu la t ion .

W. R. Martin: What i s t h e absorber m a t e r i a l t o be used i n t h e 42

con t ro l r o d s ?

H. Harder: B4C.

W. R. Martin: How i s t h e H f and B being incorpora ted i n t o t h e f u e l

elements as a burnable poison?

H. Harder,: The burnable poisons w i l l not be incorpora ted i n t o t h e

f u e l elements, bu t will be contained i n absorber spheres , which a r e f r e e

of f u e l .

W. P. Ear thold : How we l l do you measure the burnup and what i s the

peak-to-average burnup r a t i o ?

H. Harder: The burnup can be measured wi th an accuracy of 10-12 mg

U235 equiva len t .

i n g r e a c t o r .

mean burnup 12% fima.

This has been demonstrated i n a prototype of t h e measur- The maximum burnup i n t h e equi l ibr ium core i s 145 f i m a , t h e

B. G. Chapman: Was the f i g u r e of 2.0 n i l e s r e a c t i v i t y change f o r

temperature, t h e ful l value from f u l l power t o a cold core, o r t o a warm core wi th decay h e a t after shutdown?

H. Harder: 2 .0 n i l e s i s t h e r e a c t i v i t y change from f u l l power t o a cold core. This f i g u r e concerns t h e equi l ibr ium core. I n the i n i t i a l

core , t he corresponding va lue i s about 4.0 n i l e s .

L. A. Lys: Could you desc r ibe t h e des ign p r i n c i p a l of t he c o n t r o l

rod, i n p a r t i c u l a r t h e i r mode of o p e r a t i o n . f o r r e a c t o r scram.

H. Harder: The con t ro l rods a r e i n s e r t e d i n t o the pebble bed w i t h -

ou t guiding tubes.

has been demonstrated by a s e r i e s of experiments i n models of similar geometry that such a procedure i s poss ib l e without having too high loads

It has been descr ibed by Sch'dning i n h i s paper. It

194

J. D. Hart: What i s the design l i m t t f o r corrosion of the f u e l

spheres?

t h a t t h i s would cause dust t o be generated?

Should t h i s l e v e l of corrosion occur i n pract ice , i s i t expected

H. Harder: The exact spec i f ica t ion of the corrosion r a t e w i l l be

given by Dr. Hackstein i n h i s paper. Concerning dust, we can r e l y on

the AVR experience, where only negl igible amounts of dust have been found

up t o now.

G. Ivens: Because of the la rge amounts of chemical impurit ies i n

the AVR primary c i r c u i t , we should have expected considerable corrosion

and dimensional changes. I n f a c t , we have found no evidence of such

changes i n f u e l spheres taken from the AVR core.

A. Chamberlain: I n paper 1/101 De. Kr'ber referred t o extensive guarant,ees on THTR f u e l from the f u e l manufacturer. T h i s presmbly i m -

p l i e s t h a t the i r r a d i a t e d f u e l elements a r e monitored f o r f i s s i o n product

re lease before being fed back i n t o the reactor . I n t h i s context what con-

s t i t u t e s a f u e l element which has not m e t the guarantees?

H. Krgmer: The THTR has divided f o r measuring the burnup of each

b a l l leaving the core; and it a l s o has a device f o r measuring the f i s s i o n

gas re lease of each b a l l .

f i s s i o n gas re lease of more than 5 X

guarantee given gy Nukem.

Balls having a burnup l e s s than 8$ fima and a

R/B w i l l be replaced under the

Paper 4/121

LARGE HTGR D E B

A . J . Goodjohn Gulf General Atomic I n c o r p o r a t e d 40

San Diego, C a l i f o r n i a

A ABSTRACT -5i

The d e s i g n s t a t u s of a n u c l e a r steam supply system employing a h igh- tempera ture gas-cooled r e a c t o r and c a p a b l e of producing s u f f i c i e n t h i g h - q u a l i t y steam t o g e n e r a t e approximate ly 1100 m ( e ) i n a c o n v e n t i o n a l r e h e a t steam c y c l e i s d i s c u s s e d . T h i s design--the r e s u l t of more t h a n two y e a r s of e n g i n e e r i n g e f f o r t by a l a r g e f r a c t i o n of t h e d e s i g n e n g i n e e r i n g s t a f f of Gulf Genera l Atomic Incorpora ted- - i s based on t h e s a m e technology and development work as t h e 330-MW(e) F o r t S t . V r a i n Nuclear Genera t ing S t a t i o n which Gulf Genera l Atomic i s p r e s e n t l y c o n s t r u c t i n g f o r t h e P u b l i c Service Company of Colorado under t h e United States Atomic Energy Commission Power Reac tor Demonstrat ion Program. S e v e r a l d e s i g n improvements have been i n c o r p o r a t e d which e i t h e r improve t h e s a f e t y o r economics o r f a c i l - i t a t e t h e o p e r a t i o n and maintenance of t h e p l a n t . The r e a s o n s f o r some of t h e s e improvements are sum- marized.

INTRODUCTION

The l a r g e h igh- tempera ture gas-cooled r e a c t o r (HTGR) p r e s e n t l y b e i n g

o f f e r e d commercially by Gulf Genera l Atomic I n c o r p o r a t e d i s t h e r e s u l t

of o v e r a decade of development i n t h e b a s i c technology of gas-cooled

r e a c t o r s and i n t h e d e s i g n and p r o o f - t e s t i n g of t h e components which w i l l

b e used i n t h e p l a n t t o g e n e r a t e h i g h - q u a l i t y steam f o r e lec t r ica l power

g e n e r a t i o n .

(40-MW(e)) p l a n t and from t h e d e s i g n , l i c e n s i n g , and c o n s t r u c t i o n of t h e

The e x p e r i e n c e ga ined from t h e o p e r a t i o n of t h e Peach Bottom

195

196

F o r t S t . V r a i n (330-MW(e)) p l a n t h a s provided t h e guidance f o r systen; and

component d e s i g n f o r t h e l a r g e HTGR n u c l e a r steam supply system.

p r e l i m i n a r y d i s c u s s i o n s have been h e i d w i t h t h e Uni ted States l i c e n s i n g

a u t h o r i t i e s on t h e d e s i g n of t h e p l a n t . T h e i r review h a s been h e l p f u l

i n e s t a b l i s h i n g t h e d e s i g n b a s e s f o r t h e engineered s a f e t y f e a t u r e s and

i n p r o v i d i n g a s s u r a n c e t h a t t h e d e s i g n can b e s u c c e s s f u l l y engineered

and l i c e n s e d and s a f e l y o p e r a t e d .

containment b u i l d i n g and a u x i l i a r y c o o l i n g loops, as a r e s u l t of t h e

d i s c u s s i o n s w i t h t h e l i c e n s i n g a u t h o r i t i e s , i s expected t o b e a major

f a c t o r i n e a s i n g l i c e n s i n g d i f f i c u l t i e s .

Moreover,

The u s e of a c o n v e n t i o n a l secondary

NUCLEAR STEAM SUPPLY DESCRIPTION

The HTGR nuclear s t e a m supply system (NSS) i s p r e s e n t l y des igned t o

produce main s u p e r h e a t e d steam a t 955°F and 2400 p s i g and r e h e a t steam a t

1001°F and 575 p s i g i n s u f f i c i e n t q u a n t i t i e s t o g e n e r a t e 1100 MW(e) i n a

c o n v e n t i o n a l r e h e a t steam t u r b i n e - g e n e r a t o r u n i t . F i g u r e 1 i s a schemat ic

f low diagram f o r t h e p l a n t . Helium a t a p r e s s u r e of 685 p s i g i s c i r c u l a t e d

by means of s team-turb ine-dr iven a x i a l compressors downward through t h e

r e a c t o r c o r e and upward through t h e once-through steam g e n e r a t o r s . S i x

loops are u s e d , each loop having a s i n g l e hel ium c i r c u l a t o r u n i t and a

s i n g l e steam g e n e r a t o r u n i t . The main s u p e r h e a t e d steam p a s s e s t o t h e

h i g h - p r e s s u r e element of t h e t u r b i n e . The h igh-pressure t u r b i n e exhaus t

i s used t o d r i v e t h e hel ium c i r c u l a t o r s b e f o r e p a s s i n g f i r s t t o t h e r e h e a t

s e c t i o n s of t h e steam g e n e r a t o r s and t h e n t o t h e i n t e r m e d i a t e - p r e s s u r e and

low-pressure s e c t i o n s of t h e main t u r b i n e .

The pr imary c o o l a n t he l ium i s h e a t e d from 630" t o 1377°F mixed mean

i n p a s s i n g through t h e r e a c t o r c o r e . The p r e s s u r e drop i n t h e e n t i r e pr imary

c o o l a n t c i r c u i t i s approximate ly 15.25 p s i , about h a l f of which t a k e s p l a c e

through t h e r e a c t o r c o r e .

197

F i g u r e 2 shows t h e g e n e r a l arrangement of t h e major components of t h e

NSS w i t h i n t h e p r e s t r e s s e d c o n c r e t e r e a c t o r vessel (PCRV). The r e a c t o r

c o r e , r e f l e c t o r , c o n t r o l r o d s , and i n t e r n a l s t r u c t u r e s occupy t h e c e n t r a l

c a v i t y , which i s about 37 f t i n d i a m e t e r by 44 f t h i g h . The s i x s e p a r a t e

main c o o l i n g loops occupy 13-f t -diameter cav i t ies which are spaced

symmetr ica l ly around t h e c e n t r a l c a v i t y and connected t o i t by i n s u l a t e d

r a d i a l d u c t s . The PCRV i s a r i g h t c y l i n d e r , 81 f t h igh by 96 f t 6 i n . i n

d i a m e t e r . P r o c e s s and service c o n n e c t i o n s t o t h e steam g e n e r a t o r u n i t s

are made through p e n e t r a t i o n s a t t h e bottom of t h e PCRV, and c o n n e c t i o n s

t o t h e c i r c u l a t o r s a re made a t t h e top . C o n t r o l rod d r i v e a s s e m b l i e s are

mounted i n t h e 73 r e f u e l i n g p e n e t r a t i o n s i n t h e t o p head, where a d d i t i o n a l

p e n e t r a t i o n s a re a l s o provided f o r he l ium p u r i f i c a t i o n system components

and n u c l e a r i n s t r u m e n t s . Three a d d i t i o n a l cav i t ies approximate ly 7 f t

i n d iameter w i t h p e n e t r a t i o n s i n t h e t o p head a re provided f o r t h e a u x i l i a r y

c i r c u l a t o r s and h e a t exchangers which are used t o s a t i s f y t h e emergency

c o o l i n g requi rements f o r t h e r e a c t o r .

0

-

P r e s t r e s s e d Concre te Reac tor Vessel

The PCRV f u n c t i o n s as t h e pr imary c o o l a n t sys-tem boundary. The complete

PCRV c o n s i s t s of (1) a n i n t e r n a l s t e e l l i n e r which acts as a s e a l i n g

membrane, ( 2 ) a thermal b a r r i e r mounted on t h e i n s i d e of t h e l i n e r and a c o o l i n g c o i l system a t t a c h e d t o t h e o u t s i d e of t h e l i n e r whose j o i n t

f u n c t i o n i s t o l i m i t t h e tempera ture of t h e c o n c r e t e , ( 3 ) p e n e t r a t i o n s and

c l o s u r e s which p r o v i d e access t o t h e vessel cav i t i e s , and ( 4 ) h i g h - s t r e n g t h

c o n c r e t e w i t h bonded r e i n f o r c e m e n t s t e e l and a p r e s t r e s s i n g system t o

p r o v i d e s t r e n g t h .

anchored t o t h e f o u n d a t i o n pad.

The e n t i r e assembly i s mounted on a s u p p o r t s t r u c t u r e

The PCRV f o r t h e l a r g e HTGR h a s a d i f f e r e n t c o n f i g u r a t i o n t h a n t h e F o r t

S t . V r a i n vessel . The m u l t i c a v i t y d e s i g n and t h e r a d i a l l o c a t i o n of t h e

steam g e n e r a t o r s and c i r c u l a t o r s are d e s i g n improvements which r e s u l t i n

b e t t e r loop s e p a r a t i o n and a n improved c a p a b i l i t y f o r t h e removal, i f

n e c e s s a r y , of t h e steam g e n e r a t o r s and c i r c u l a t o r s f o r maintenance and

r e p a i r . 0 The m u l t i c a v i t y d e s i g n a l s o r e s u l t s i n a s h o r t e r PCRV, which y i e l d s

198

a n improved c a p a b i l i t y t o w i t h s t a n d a c c e l e r a t i o n s due t o ear thquakes .

The l a r g e HTGR i n c o r p o r a t e s s i n g l e c l o s u r e s on t h e PCRV p e n e t r a t i o n s

w i t h f low r e s t r i c t i o n d e v i c e s i n o r d e r t o l i m i t t h e r a t e of d e p r e s s u r i z a t i o n

of t h e PCRV f o r t h e h y p o t h e t i c a l c l o s u r e f a i l u r e a c c i d e n t . The s t r u c t u r a l l y

s e p a r a t e containment b u i l d i n g p r o v i d e s t h e secondary containment f o r t h e l a r g e

HTGR, whereas t h e containment f o r t h e F o r t S t . V r a i n p l a n t i s provided by

secondary c l o s u r e s on every p e n e t r a t i o n . This p a r t i c u l a r d e s i g n change

r e s u l t e d from d i s c u s s i o n s w i t h t h e l i c e n s i n g a u t h o r i t i e s which l e d t o

i n t h e requi rement t o c o n s i d e r t h e double c l o s u r e f a i l u r e as a d e s i g n b a s i s

a c c i d e n t . It t h u s became a p p a r e n t t h a t t h e c o n t r o l of r a d i o a c t i v i t y

releases t o t h e s i t e boundary under t h e assumed a c c i d e n t c o n d i t i o n s could

b e accomplished more economical ly i f a secondary containment s t r u c t u r e

w a s added.

'

The PCRV i t s e l f i s v e r y s i m i l a r t o t h e H a r t l e p o o l vessel b e i n g c o n s t r u c t e d

by T a y l o r Woodrow C o n s t r u c t i o n i n Great B r i t a i n . L i n e a r tendons are used

f o r v e r t i c a l p r e s t r e s s and wire winding i s used f o r t h e c i r c u m f e r e n t i a l

p r e s t r e s s . Work sponsored by Gulf Genera l Atomic i s under way t o deve lop

and p r o o f - t e s t t h e machinery r e q u i r e d t o perform t h e w i r e winding o p e r a t i o n .

The u s e of w i r e winding r e p r e s e n t s a s i g n i f i c a n t improvement over t h e

l i n e a r c i r c u m f e r e n t i a l p r e s t r e s s i n t h a t c o n s i d e r a b l e r e l i e f i s o b t a i n e d

i n embedment c o n g e s t i o n and i n t h e c o m p l e x i t i e s a s s o c i a t e d w i t h p i l a s t e r s

f o r t h e c i r c u m f e r e n t i a l p r e s t r e s s anchors . Moreover, t h e p r e s e n t d e s i g n

does n o t i n c o r p o r a t e head tendons, s i n c e s u f f i c i e n t p r e s t r e s s t o t h e top

and bottom heads of t h e vessel can b e a p p l i e d by t h e c i r c u m f e r e n t i a l

p r e s t r e s s i n g o p e r a t i o n .

S i n c e b o t h t h e steam g e n e r a t o r and t h e c i r c u l a t o r a re i n s t a l l e d through

t h e same p e n e t r a t i o n , a c o n c r e t e p l u g i s used i n t h e upper end of each

steam g e n e r a t o r c a v i t y as a means of reducing t h e p e n e t r a t i o n d iameter

t o t h a t r e q u i r e d f o r t h e c i r c u l a t o r a f t e r t h e steam g e n e r a t o r h a s been

i n s t a l l e d . These p l u g s are b o l t e d t o t h e top of t h e steam g e n e r a t o r

p e n e t r a t i o n l i n e r s and have a c e n t r a l h o l e through which t h e c i r c u l a t o r

assembly i s i n s t a l l e d . A

199

Steam Generators

The once-through steam gene ra to r s f o r t h e l a r g e HTGR are gene ra l ly

l a r g e r ve r s ions of the For t S t . Vrain modules. Each of t h e s i x u n i t s i s

approximately a f a c t o r of s i x l a r g e r i n weight and hea t ou tput per module.

F igure 3 i s a schematic diagram of an i n s t a l l e d u n i t . Helium e n t e r s t h e

u n i t a t t h e bottom and flows upward through a U-tube r e h e a t e r l oca t ed i n

t h e c e n t e r duc t . The flow i s reversed a t t h e top of t h e u n i t , and t h e

helium then flows downward over t h e h e l i c a l l y c o i l e d superheater-evaporator-

economizer bundles. A t t he bottom of t h e u n i t , t h e flow i s aga in reversed ,

and t h e helium flows upward i n t h e annular reg ion between t h e steam generator

shroud and t h e cover p l a t e s on t h e i n s u l a t i o n f o r t h e steam genera tor c a v i t y

l i n e r . Uphi l l -bo i l ing i n t h e main steam s e c t i o n has been r e t a i n e d i n t h e

design, and t h e d i f f e r e n c e between t h e l a r g e HTGR steam genera tor and t h e

smaller F o r t S t . Vrain module conf igu ra t ions is t h e d i r e c t r e s u l t of t h i s

des ign dec i s ion . Downhill-boiling w a s considered i n earlier conceptual

des igns bu t w a s no t used owing t o concern over flow s t a b i l i t y a t low power.

A s i g n i f i c a n t design s i m p l i f i c a t i o n w a s made i n t h e method of headering

t h e feedwater and steam tubes a t t h e steam genera tor pene t r a t ion . The F o r t

S t . Vrain steam gene ra to r s have a domed primary c losu re .

h e a t e r subheaders pass through t h i s c l o s u r e , a re co i l ed through the i n t e r s p a c e , and then p a s s through t h e secondary c l o s u r e t o r ingheaders mounted outside

of the pene t r a t ion .

p ipe arrangement i n t h e c e n t e r of each c l o s u r e . The l a r g e HTGR uses

tubeshee ts f o r headering t h e feedwater and a l l of t h e steam tubes.

tubeshee ts form p a r t of t h e c:Losure f o r t h e steam genera tor pene t r a t ion .

I n a d d i t i o n t o t h e helium flow r e s t r i c t o r s i n each p e n e t r a t i o n which provide

t h e flow r e s t r i c t i o n necessary t o l i m i t t h e hypo the t i ca l PCRV depressur iza-

t i o n acc iden t , f low r e s t r i c t o r s are a l s o provided t o l i m i t t h e i n g r e s s of

steamlwater i n t o t h e r e a c t o r :€or a h y p o t h e t i c a l tubeshee t f a i l u r e .

Feedwater and super-

Reheat steam i n l e t and o u t l e t are through a concen t r i c

These

200

Direct access f o r t u b e p lugging o p e r a t i o n s is provided by t h i s d e s i g n

s i m p l i f i c a t i o n .

shutdown by removing s e c t i o n s of t h e p i p i n g below t h e t u b e s h e e t s , manually

p lugging t h e l e a k i n g c i r c u i t s , and t h e n r e p l a c i n g t h e p i p e s e c t i o n s .

These o p e r a t i o n s would b e accomplished d u r i n g a p l a n t

The materials used i n t h e steam g e n e r a t o r s range from carbon s t e e l

t o n i c k e l - b a s e a l l o y .

items such as t h e feedwater t u b e s h e e t and t h e feedwater t u b e s . Var ious

grades of low-al loy s t e e l a re used f o r t h e c l o s u r e , t h e co ld r e h e a t t u b e s h e e t ,

p o r t i o n s of t h e shroud, and t h e economizer-evaporator t u b e s and t h e i r s u p p o r t

s t r u c t u r e . N i c k e l iron-chrome a l l o y ( Incoloy 800 o r e q u i v a l e n t ) i s used

f o r t h e s u p e r h e a t e r t u b e s , t h e r e h e a t e r t u b e s , and p o r t i o n s of t h e shroud,

and a h i g h - s t r e n g t h n icke l -base a l l o y ( I n c o n e l 625 o r e q u i v a l e n t ) i s

used f o r the supe rhea te r tubeshee t , t h e h o t reheat tubeshee t , and the s u p p o r t

Carbon s t e e l i s used i n t h e low-design-temperature

s t r u c t u r e s f o r t h e s u p e r h e a t e r and r e h e a t e r .

Main Helium C i r c u l a t o r s

The main hel ium c i r c u l a t o r s (shown i n F i g . 4 ) are g e n e r a l l y l a r g e r

v e r s i o n s of t h e F o r t S t . V r a i n s e r i e s - s t e a m - t u r b i n e d r i v e n a x i a l f l o w

u n i t s , w i t h f l o w and power r e q u i r e m e n t s approximate ly double t h o s e of each

of t h e F o r t S t . Vrain c i r c u l a t o r s . The t h r u s t and j o u r n a l b e a r i n g s a r e

w a t e r - l u b r i c a t e d h y d r o s t a t i c b e a r i n g s w i t h hydrodynamic c a p a b i l i t y a t h i g h

speed , similar t o t h o s e i n t h e F o r t S t . V r a i n u n i t s , Owing t o t h e r e a c t o r

arrangement , t h e u n i t s are i n v e r t e d re la t ive t o t h e F o r t S t . V r a i n c i r c u l a t o r s

and t h e he l ium f l o w through t h e compressor i s r e v e r s e d , b u t t h e s e are minor

d e s i g n m o d i f i c a t i o n s . A s h u t o f f valve i s l o c a t e d a t t h e d i s c h a r g e of each

c i r c u l a t o r t o p r e v e n t backflow of hel ium through a shutdown c i r c u l a t o r .

C o n t r o l Rod Drives

F i g u r e 5 shows a c o n t r o l rod d r i v e u n i t i n s t a l l e d i n a r e f u e l i n g

p e n e t r a t i o n . These u n i t s , which a re v e r y similar t o t h e F o r t S t . V r a i n

u n i t s , are e s s e n t i a l l y e l e c t r i c a l l y powered winches, each of which raises

and lowers a p a i r of c o n t r o l r o d s by means of f ’ l e x i b l e s t e e l c a b l e s . The

201

c o n t r o l r o d s are guided i n t o t h e c o r e by a g u i d e t u b e assembly l o c a t e d

i n t h e upper plenum above t h e r e a c t o r c o r e . Scram o c c u r s by g r a v i t y a c t i o n ,

w i t h t h e v e l o c i t y b e i n g c o n t r o l l e d by r e g e n e r a t i v e b r a k i n g a c t i o n of t h e

motor.

Fuel. Handling Equipment

R e f u e l i n g o p e r a t i o n s w i l l . b e c a r r i e d o u t w i t h t h e p l a n t o f f - l i n e and

w i t h t h e r e a c t o r he l ium i n l e t t empera ture and p r e s s u r e lower t h a n 250°F

and a tmospher ic , r e s p e c t i v e l y . The p r i n c i p a l equipment (shown i n F i g . 6 ) c o n s i s t s of a f u e l h a n d l i n g machine f o r a c t u a l l y removing and r e p l a c i n g f u e l

and r e f l e c t o r e lements i n t h e r e a c t o r ; f u e l t r a n s f e r c a s k s f o r t r a n s f e r r i n g

canned e lements between t h e f u e l h a n d l i n g machine and s t o r a g e ; a n a u x i l i a r y

service c a s k f o r removing and i n s t a l l i n g c o n t r o l rod d r i v e a s s e m b l i e s b e f o r e

and a f t e r r e f u e l i n g ; r e a c t o r i s o l a t i o n valves t o p r o v i d e s h i e l d i n g and s e a l i n g

f o r t h e r e f u e l i n g p e n e t r a t i o n s d u r i n g t h e r e f u e l i n g o p e r a t i o n ; and motorized

v e h i c l e s f o r t r a n s p o r t i n g t h e f u e l t r a n s f e r c a s k s between t h e f u e l h a n d l i n g

machine and s t o r a g e .

The i n - c o r e o p e r a t i o n s are i d e n t i c a l t o t h o s e f o r t h e F o r t S t . Vrain

r e a c t o r ; i . e . , a complete s e v e n column r e f u e l i n g r e g i o n i s r e f u e l e d a t any

one t i m e . However, t h e F o r t S t . V r a i n r e f u e l i n g machine i t s e l f t e m p o r a r i l y

s t o r e s t h e f u e l and r e f l e c t o r e lements and i s moved to the location of the

f u e l s t o r a g e t o d e p o s i t t h e s p e n t f u e l and r e l o a d w i t h new f u e l . This t y p e

of o p e r a t i o n w a s p r o h i b i t e d i n t h e c o n v e n t i o n a l l y c o n t a i n e d p l a n t u n l e s s a

v e r y l a r g e access door w a s provided between t h e containment and t h e r e a c t o r

service b u i l d i n g . Moreover, it was found t h a t t h e r e f u e l i n g t i m e could b e

s h o r t e n e d c o n s i d e r a b l y i f t h e r e f u e l i n g machine w a s n o t moved and f u e l w a s

t r a n s p o r t e d by t h e t r a n s f e r c a s k s and motor ized v e h i c l e s .

The r e f u e l i n g c y c l e i s based on a 4-yr l i f e ; i . e . , one-quar te r of t h e

c o r e i s r e p l a c e d each y e a r by new o r r e c y c l e d f u e l .

time r e q u i r e d f o r t h e r e f u e l i n g o p e r a t i o n i s e s t i m a t e d t o b e a b o u t 1 4 d a y s ,

i n c l u d i n g t h e t i m e r e q u i r e d f o r pump-down and cool-down of t h e r e a c t o r

The r e a c t o r shutdown

0 and t h e t i m e r e q u i r e d f o r pump-up and r e a c t o r s t a r t u p .

202

R e a c t o r Core

The r e a c t o r c o r e (shown i n F i g . 7 ) c o n s i s t s of approximate ly 3800

hexagonal g r a p h i t e b l o c k f u e l elements of t h e same b a s i c t y p e as w i l l b e

used i n t h e F o r t S t . V r a i n p l a n t ; i . e . , e lement o v e r a l l dimensions are

approximate ly 31 i n . l o n g and 1 4 i n . a c r o s s t h e f l a t s . The g r a p h i t e f u e l

e lements are a r r a n g e d i n columns e i g h t b l o c k s h i g h and are surrounded by a

g r a p h i t e r e f l e c t o r approximate ly 4 f t t h i c k . The c o r e i t s e l f i s d i v i d e d

i n t o 73 r e g i o n s , each c o n s i s t i n g of a c e n t r a l f u e l element column surrounded

by up t o s i x columns. Edch c e n t r a l f u e l e lement column c o n t a i n s t h e ve r t i ca l

h o l e s f o r t h e a s s o c i a t e d c o n t r o l r o d p a i r and a h o l e f o r t h e i n s e r t i o n of

t h e reserve shutdown material .

Convent iona l nuc lear -grade g r a p h i t e w i l l b e used throughout t h e c o r e .

The f u e l materials i n t h e i n i t i a l c o r e w i l l b e 93% e n r i c h e d uranium and

f e r t i l e thorium i n c a r b i d e form. I n t h e l a te r r e c y c l e mode, U-233 w i l l b e

i n c l u d e d as a f e e d material , r e p l a c i n g much of t h e U-235. The uranium and

thorium c a r b i d e p a r t i c l e s a re c o a t e d w i t h l a y e r s of p y r o l y t i c carbon and

bonded i n t o r o d s b e f o r e b e i n g loaded i n t o t h e g r a p h i t e e lements .

Apar t from d i f f e r e n c e s i n l o a d i n g s , t h e major d i f f e r e n c e between t h e

l a r g e HTGR f u e l and F o r t S t . V r a i n f u e l i s i n t h e c o a t i n g s . I n o r d e r t o

minimize t h e accumulat ion of Sr-90 as p l a t e o u t i n t h e c i r c u i t , which

would o t h e r w i s e b e p o t e n t i a l l y r e l e a s a b l e i n t h e h y p o t h e t i c a l depres-

s u r i z a t i o n a c c i d e n t , a l l t h e F o r t S t . Vra in f u e l p a r t i c l e s have an

added l a y e r of s i l i c o n c a r b i d e . The a d d i t i o n of c o n v e n t i o n a l containment

on the l a r g e HTGR p l a n t h a s r e l i e v e d t h i s release problem. However, i n t h e

l a r g e HTGR c e r t a i n U-235 f u e l p a r t i c l e s a l s o have a s i l i c o n c a r b i d e c o a t i n g

t o f a c i l i t a t e s e p a r a t i o n d u r i n g r e p r o c e s s i n g o p e r a t i o n s .

A u x i l i a r y Cooling Loops

Three a u x i l i a r y c o o l i n g l o o p s have been provided i n t h e NSS f o r t h e

l a r g e HTGR. Each loop c o n s i s t s of a hel ium c i r c u l a t o r , a water-cooled

203

o p e r a t i n g c o n d i t i o n s , i n c l u d i n g a

f u l l i n v e n t o r y p r e s s u r e and

h e a t exchanger , and t h e i r a s s o c i a t e d service equipment. F i g u r e 8 shows one

of t h e l o o p s i n s t a l l e d i n t h e PCRV c a v i t y .

i s t o p r o v i d e emergency c o r e c o o l i n g and decay h e a t removal i n case of

f a i l u r e of t h e main loops . Any two loops can c o o l t h e c o r e .

The pr imary f u n c t i o n of t h e loops

c o r e c a v i t y p r e s s u r e anywhere between t h e

a tmospher ic . O i l - l u b r i c a t e d b a l l b e a r i n g s are

The requi rement f o r t h e s e a u x i l i a r y c o o l i n g l o o p s was a consequence of

d i s c u s s i o n s w i t h t h e l i c e n s i n g a u t h o r i t i e s and t e c h n i c a l d i f f i c u l t i e s i n

p r o v i d i n g a d e q u a t e s t a t i c c o o l i n g f o r t h e l a r g e HTGR c o r e i n a manner s i m i l a r

t o t h a t p rovided f o r t h e F o r t S t . V r a i n c o r e . F a i l u r e of b o t h t h e steam

d r i v e s and t h e water t u r b i n e d r i v e s on a l l t h e F o r t S t . V r a i n c i r c u l a t o r s

r e s u l t s i n t h e h y p o t h e t i c a l l o s s of f o r c e d c i r c u l a t i o n a c c i d e n t w i t h t h e

decay h e a t b e i n g removed by r a d i a t i o n and conduct ion t o t h e l i n e r c o o l i n g

system. Although h i g h tempera tures are reached i n t h e c o r e of t h e F o r t S t .

V r a i n r e a c t o r d u r i n g t h e c o u r s e of t h i s h y p o t h e t i c a l e v e n t , t h e r e a c t o r can

r i d e through t h e long term h e a t u p and cooldown w i t h o u t v i o l a t i n g t h e containment

provided by t h e PCRV.

system of t h e l a r g e HTGR i s more d i f f i c u l t s imply because of t h e l o n g e r

r a d i a t i o n and conduct ion p a t h s . Higher tempera tures would b e a t t a i n e d i f a

s imilar l o s s of a l l f o r c e d c i r c u l a t i o n a c c i d e n t i s h y p o t h e s i z e d . Attempts

t o a s s u r e s a f e t y by some means of s t a t i c c o r e c o o l i n g d i d n o t prove sa t i s -

f a c t o r y , and as a r e s u l t a n a d d i t i o n a l method of p r o v i d i n g f o r c e d c o o l i n g

w a s adopted.

However, removal of a f t e r h e a t by t h e l i n e r c o o l i n g

c i r c u l a t o r s are o p e r a t i n g .

204

ENGINEERED SAFEGUARDS AND SAFETY PHILOSOPHY

The e v o l u t i o n of t h e l a r g e HTGR d e s i g n h a s r e s u l t e d i n two major changes

i n t h e engineered s a f e g u a r d s systems:

1. The a d d i t i o n of a s e p a r a t e c o n v e n t i o n a l containment b u i l d i n g t o

p r o v i d e g r e a t e r p h y s i c a l s e p a r a t i o n between t h e pr imary and

secondary conta inments .

2. The i n c o r p o r a t i o n of a u x i l i a r y c o o l i n g loops t o p r o v i d e emergency

c o r e c o o l i n g i n case of f a i l u r e of t h e main c i r c u l a t o r / h e a t removal

system.

A s noted p r e v i o u s l y , t h e acceptance of a hypo the t i ca l d e p r e s s u r i z a t i o n

of t h e PCRV as a d e s i g n b a s i s was t h e pr imary r e a s o n f o r t h e a d d i t i o n of t h e s e p a r a t e containment b u i l d i n g .

l i m i t f o r t h e r a t e of d e p r e s s u r i z a t i o n , t h e h y p o t h e t i c a l a c c i d e n t i s assumed

t o occur through a p e n e t r a t i o n i n t h e PCRV a f t e r f a i l u r e of t h e c l o s u r e ,

w i t h t h e subsequent d e p r e s s u r i z a t i o n r a t e b e i n g determined by t h e f r e e - f l o w

area around a f l o w r e s t r i c t o r i n t h e p e n e t r a t i o n .

r e s t r i c t o r s are engineered s a f e t y f e a t u r e s and have t h e c a p a b i l i t y of

I n o r d e r t o p r o v i d e a n a c c e p t a b l e upper

The p e n e t r a t i o n f low

L l i m i t i n g t h e e f f e c t i v e f ree- f low area t o a maximum of 100 i n . F i g u r e 9 shows t h e tempera ture and p r e s s u r e p u l s e i n t h e containment which would occur

d u r i n g t h i s e v e n t . Since t h e energy c o n t e n t of t h e s ing le-phase hel ium

c o o l a n t i s s o s m a l l , a s e p a r a t e h e a t removal sys tem i n t h e containment i s

n o t n e c e s s a r y ; t h e h e a t c o n t e n t of t h e hel ium i s removed by t r a n s f e r t o

t h e c o o l e r s u r f a c e s of t h e containment and i t s c o n t e n t s .

The f u n c t i o n of t h e a u x i l i a r y c o o l i n g l o o p s , which a re a l s o engineered

s a f e t y f e a t u r e s , i s t o p r o v i d e a d e q u a t e removal of t h e decay h e a t from t h e

c o r e f o l l o w i n g t h e d e s i g n b a s i s d e p r e s s u r i z a t i o n a c c i d e n t . The main c o o l i n g

loops are n o t c o n s i d e r e d t o b e engineered s a f e t y f e a t u r e s ; however, t h e y a l s o

have t h e c a p a b i l i t y of removing t h e decay h e a t and a d e q u a t e l y c o o l i n g t h e

r e a c t o r c o r e .

205

The e v o l u t i o n of t h e s a f e t y phi losophy f o r t h e NSS h a s a l s o l e d t o

c o n s i d e r a t i o n of t h e p o s s i b i l i t y of some a d d i t i o n a l i n p u t t o t h e d e s i g n

b a s i s a c c i d e n t from small l e a k s i n t h e secondary c o o l a n t system. It i s

becoming common p r a c t i c e t o c o n s i d e r s u i t a b l e d e s i g n margins f o r such a n

e v e n t i n t h e l i c e n s i n g of power r e a c t o r s . For t h e HTGR, t h e p o s s i b i l i t y of

steam/water i n g r e s s i n t o t h e r e a c t o r c o i n c i d e n t w i t h o r subsequent t o t h e

d e p r e s s u r i z a t i o n a c c i d e n t does r e q u i r e t h e a d d i t i o n a l engineered s a f e t y

f e a t u r e s . U n t i l a l l s e c t i o n s of t h e r e a c t o r c o r e are cooled t o below t h e

t h r e s h o l d f o r t h e s team-graphi te chemica l r e a c t i o n , some hydrogen and carbon

monoxide w i l l b e g e n e r a t e d . I t i s n e c e s s a r y t o p r o v i d e a d e q u a t e moni tor ing

f o r m o i s t u r e i n t h e system and s u i t a b l e i s o l a t i o n d e v i c e s t o l i m i t t h e amount

of steam/water i n g r e s s t o less t h a n t h a t which could l e a d t o p o t e n t i a l l y

flammable m i x t u r e s of hydrogen and carbon monoxide i n t h e a i r - f i l l e d c o n t a i n -

ment. The m o i s t u r e moni tor ing d e v i c e s and t h e i s o l a t i o n systems f o r s o u r c e s

of l a r g e steam/water l e a k s have t h e r e f o r e a l s o been c l a s s e d as engineered

s a f e t y f e a t u r e s f o r t h e l a r g e HTGR.

SUMMARY

T h i s paper h a s covered only t h e h i g h l i g h t s of t h e NSS f o r t h e l a r g e HTGR.

S u f f i c i e n t d e s i g n and development work h a s been done on t h e d e s i g n f o r Gulf

General Atomic t o now o f f e r t h e n u c l e a r steam supply f o r t h e g e n e r a t i o n of

base load electrical power in 1100-MW(e) units on a commercial basis. The

HTGR's p o t e n t i a l f o r improved performance i n terms of h i g h e r p l a n t e f f i c i e n c y ,

reduced r a d i o a c t i v e and thermal e f f l u e n t , and less s t r i n g e n t f u e l r e q u i r e -

ments i s r a p i d l y approaching r e a l i z a t i o n i n t h e 1100-MW(e) d e s i g n .

F i g u r e 10 shows a n o v e r a l l view of t h e 1100-MW(e) HTGR power p l a n t .

Design c r i t e r i a have been e s t a b l i s h e d f o r t h e major sys tems and components

i n t h e p l a n t , and work on t h e d e t a i l e d d e s i g n i s proceeding .

containment s t r u c t u r e s u r r o u n d i n g t h e PCRV i s a c a r b o n - s t e e l - l i n e d conven-

t i o n a l l y r e i n f o r c e d c o n c r e t e s t r u c t u r e w i t h c y l i n d r i c a l walls, a f l a t b a s e ,

and a h e m i s p h e r i c a l dome.

d i a m e t e r about 126 f t .

The secondary

Its o v e r a l l h e i g h t i s about 230 f t and i t s i n s i d e

A minimum c l e a r a n c e of approximate ly 8 f t i s main ta ined

206

around t h e PCRV t o permi t p o s t - t e n s i o n i n g of t h e c i r c u m f e r e n t i a l w i r e wrappi@

Access t o t h e containment b u i l d i n g i s provided by a 7-f t -diameter p e r s o n n e l

h a t c h and a 22-f t -diameter equipment h a t c h .

The r e a c t o r service b u i l d i n g c o n t a i n s t h e new and used f u e l s t o r a g e

f a c i l i t i e s and t h e r e a c t o r a u x i l i a r y systems and i s approximate ly 90 f t

wide, 130 f t long , and 172 f t h i g h . The t h r e e - s t o r y t u r b i n e b u i l d i n g i s

approximate ly 136 f t wide, 300 f t l o n g , and 102 f t h i g h .

207

CIRCUMFERENTIAL PRESTRESS CHANNELS-

Fig. 1. Schemat ic Flow Diagram of NSS for Large HTGR.

-REFUELING PENETRATION nous CONTROL I NG ROO

MECHANISM

-CIRCULATOR

N J X l L IARY CIRCULATOR'

CORE AUYIL I ARY HEAT EXCHANGERc

PRESTRESSED' CONCRETE PRESSURE VESSEL-

\

F i g . 2 . General Arrangement of Large HTGR.

- V E R T I C A L PRESTRESS 1 END 0 N S

* S T E M GENERATOR

COLD H E L l

E C O N O M I Z E R - EVAPORATOR SUPERHEATER BUNDLE

HOT H E L l

' FEEDWATER

COLD REHEAT S T E A M

M A I N S T E A M

REHEATER B U N D L E

, PCRV

PR l MARY CLOSURE

Fig. 4. Helium Circulator Installation Assembly.

Fig' Q Steam Generator Installation. ' HOT R E H E A T S T E A M

Fig. 5. Cont ro l Rod Drive and Reserve Shutdown Assembly.

Fig. 6. Fuel Handling Equipment.

A U X I L I A R Y C I R C U L A T O R SHUTOFF V A L V E

PCRV\

C O R E + /ue COOLANT FLOW REMOVABLE P E R M A N E N T REFLECTOR REFLECTOR. - -- --- 1 . - - - - "%a-*t- -_-- -.- ..-"-- *___- --- "

Fig. 7 . Core Arrangement.

. A U X I L I A R Y C I R C U L A T O R

.D

. I UM

CORE A U X l L l ARY H E A T EXCHANGER

I UM

Lng Lo p s .

211

-

PHERE PRESSURE E I E N T S FROM A PCRV

- BLOWDOWN AREA OF 100 SQ. I N . POST-SCRAM C O O L I N G BY TWO A U X I L I A R Y LOOPS

h

U - a 30

CONTAINMENT VOLUME OF 1 . 7 X I O 6 CU F T - ln -.

v

-

-

I I 1

500

400 0 v

W CT 3

300 5 cc W a E k

200 U U cc W >

100

0

Fig . 9. Containment Pressure and Temperature Tran- sients during Design Basis PCRV Accident in 1100-MW(e) H E R with Concurrent Steam Leak.

212

DISCUSSION

D. Tytgat: What l e d you t o t h e choice of a secondary containment v e s s e l ? Was it t h e u n r e l i a b i l i t y of the secondary closures of the l a r g e penetrations o r t h e general s a f e t y philosophy adopted i n the USA f o r l a r g e water reac tors ?

A. J. Goodjohn: After severa l discussions with l icensing authori- ties, it became apparent that a rapid depressurization of the PCRV could be considered as a design bas is accident f o r the concept. Moreover, it a l s o became apparent t h a t the general public would r a i s e several concerns over a plant without the secondary containment independent of any safe ty arguments on the very low probabi l i ty of PCRV failure. adopted secondary containment by means of a conventional containment bui lding i n order t o satisfy the general s a f e t y philosophy adopted i n the

United S ta tes and not because o f any concern over the r e l i a b i l i t y of the

secondary closures on the l a r g e penetrations,

Therefore, GGA

G. Meijer: One of the major differences between the GGA design f o r a 1100 Mwe HTGR and t h e "PG HTGR design i s t h a t you a r e designing f o r off-load re fue l ing and TNPG f o r on-load refuel ing. Therefore, there ap-

pears t o be no p a r t i c u l a r reason f o r having a downflow i n the GGA reac- t o r . It appears t h a t w i t h an upflow i n the core the steam generator de-

s ign would have been simplified, i .e . , cheaper - and a l s o the core sup- port would have been cheaper. Therefore, I am in te res ted t o l e a r n what

made you decide on a downward cooled core; i s t h i s because of pressure drop or because of o ther reasons?

A. J. Goodjohn: We decided t o s t a y w i t h downflow through the core f o r the following reasons: A. The p o s s i b i l i t y of going t o on-line re- fuel ing on a fu ture p lan t improvement, B. Fort St . Vrain has downflow and t h e developnent of t h e control rod dr ives and the thermal insu la t ion have been based on t h i s arrangement. We would have t o develop a control rod dr ive f o r ho t hel iun (cable problem) and redo much of our insu la t ion

work.

213

D. Pa t te rson: You assume a 100 sq in . opening f o r t he design b a s i c

acc ident . What determined t h e s i z e of t h i s assumed fai lure?

A. J. Goodjohn: The 100 sq in . opening i n a pene t r a t ion as a design

b a s i s f o r t h e hypoteht ica l dep res su r i za t ion acc ident i s g r e a t e r than t h e

s i ze of opening which one obta ins by design of flow r e s t r i c t o r devices

(approximately 30 t o 50 sq i n . f o r l a r g e pene t r a t ions ) and smaller than

t h e s i z e of t h e opening which could result i n secondary e f f e c t s i n the

primary c i r c u i t ; i . e . , fa i lure of i n t e r n a l seals or duct work. The li- censing a u t h o r i t i e s agreed with t h i s design b a s i s f o r For t S t . Vrain.

T. A. Jaeger : My ques t ion refers t o the design of t h e concrete pres-

sure vesse l . You s t a t e d t h a t t h e c i rcumferent ia l w i r e winding p r e s t r e s s -

i n g provides adequate p r e s t r e s s i n g of t h e v e s s e l cap. I n t h i s way t h e

cap i s t ak ing t h e load e n t i r e l y by a rch ing a c t i o n which may imply a sudden

mode of fa i lure . f o r t h e design of t he cap a s compared with t h e c y l i n d r i c a l wall?

What i s your philosophy with regard t o s a f e t y margins

A. J. Goodjohn: By applying s u f f i c i e n t c i rcumferent ia l p r e s t r e s s t o

t h e head one can a s su re s u f f i c i e n t s t r e n g t h i n the h e a d , t o y i e l d as g r e a t

a s a f e t y margin a g a i n s t overpressure blowout of t h e head as aga ins t f l e x -

ural fa i lure of t he s i d e w a l l . One, i n f a c t , designs t h e p r e s t r e s s s o t h a t ultimate failure mode a t very high pressures i s p red ic t ab le and i s

not head fa i lure or s i d e w a l l fa i lure but fa i lure a t t h e haunch between the head and the s ide w a l l . The model tes ts have proven t h i s concept.

R. E. Helms: Most large f o s s i l p l a n t s i n t h e U.S. use t h e super-

c r i t i c a l steam cycle 3500 psia/1000"F/1000"F.

s e l e c t i n g the s u b c r i t i c a l steam cycle 2400 psia/1000"F/1000"F?

o f f were made, i f any?

What was GGA's basis f o r

What t r ade -

A. J. Goodjohn: GGA d id eva lua te the s u p e r c r i t i c a l steam cycle f o r

t h e l a r g e HTGR and decided on an economic b a s i s p r imar i ly t o remain wi th

t h e s u b c r i t i c a l s 2400 psia/1000 "F/1000"F cycle.

Paper 5/115

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I n t h e F e d e r a l R e p u b l i c of Germany a development program f o r a 6 0 0 MWe High Tempera tu re Reactor Power P l a n t w i t h c l o s e d g a s t u r b i n e c y c l e h a s been i n i t i a t e d . The p a p e r describes t h e p r e s e n t s t a t e of the c o n c e p t u a l d e s i g n s t u d i e s .

The a c t i v i t i e s are c o n c e n t r a t e d on t h e l a y o u t of t h e c o m p l e t e p l a n t , deve lopment of f u e l e l e m e n t s f o r h i g h t e m p e r a t u r e s w i t h l o w f i s s i o n p r o d u c t release ra tes , problems of d e p o s i t i o n of f i s s i o n p r o d u c t s , a n d development and t e s t i n g of s p e c i a l components .

The d e c i s i o n c o n c e r n i n g t h e l a y o u t of t h e main components w i l l be made i n 1 9 7 0 , a p r e l i m i n a r y d e c i s i o n o n t h e f u e l e l e m e n t t y p e i n 1 9 7 2 . By 1975 c o m p l e t e c o n s t r u c t i o n documents w i l l be a v a i l a b l e .

.-

+ ) The work h a s been c a r r i e d o u t w i t h i n a c o o p e r a t i o n c o n t r a c t between t h e c o r p o r a t i o n s Brown Bover i / Krupp R e a k t o r b a u G m b H , Brown,Boveri & C i e A G , F r i e d . Krupp GmbH, G u t e h o f f n u n g s h t i t t e S t e r k r a d e A G , M a s c h i n e n f a b r i k Augsburg-Ntirnberg A G , N U K E M , Nuklear-Chemie und -Metallurgic Gmbh and Kern fo r - s c h u n g s a n l a g e J u l i c h GmbH , r e g a r d i n g t h e p r e p a r a t i o n of t h e c o m p l e t e c o n s t r u c t i o n documents f o r a 600 M W e power p l a n t w i t h HTR and h e l i u m t u r b i n e , u n d e r t h e s p o n s o r s h i p of t h e F e d e r a l M i n i s t r y f o r S c i e n t i f i c R e s e a r c h ( I n v . Reactor 6 5 1 .

214

215

I N T R O D U C T I O N

The r a p i d p r o g r e s s i n t h e development o f f u e l e l e m e n t s for h i g h t e m p e r a t u r e r e a c t o r s i n r e c e n t y e a r s h a s p rov ided t h e t e c h n i c a l basis f o r t h e e x p l o i t a t i o n o f t h e h i g h e f f i c i e n c y of t h i s r e a c t o r t y p e , by c o u p l i n g it d i r e c t l y w i t h a he l ium t u r b i n e . Other a d v a n t a g e s a s s o c i a t e d w i t h d i r e c t c o u p l i n g , such as lower s p e c i f i c c a p i t a l c o s t , lower consumption of c o o l a n t water and h i g h e f f i c i e n c y even i n p a r t i a l load o p e r a t i o n , h e l p t o e n s u r e t h a t t h e h i g h t e m p e r a t u r e r e a c t o r w i t h he l ium t u r b i n e w i l l , i n t h e l o n g r u n , t a k e up a f a v o u r a b l e p o s i t i o n a l o n g s i d e fas t b r e e d e r s i n t h e ene rgy p r o d u c t i o n i n d u s t r y .

I n t h e F e d e r a l Repub l i c o f Germany a program has been i n i t i a t e d for t h e development o f a 600 M W e p l a n t . The program is b e i n g c a r r i e d o u t by s e v e r a l companies i n co- o p e r a t i o n w i t h t h e Kernforschungsanlage J u l i c h Gmbh ( K F A ) . For e x p e r i m e n t a t i o n and development up t o t h e s t a g e a t which a n of fe r c a n be p r e s e n t e d , a p p r o x i m a t e l y 1 3 0 Mio DM are a v a i l a b l e . The a c t i v i t i e s a re c o n c e n t r a t e d on t h e l a y o u t of t h e comple te p l a n t , development of f u e l e l e m e n t s f o r h i g h t e m p e r a t u r e s with l o w f i s s i o n p r o d u c t release rates, problems of d e p o s i t i o n o f f i s s i o n p r o d u c t s , and development and t e s t i n g of s p e c i a l components. U s e o f s p h e r i c a l and rod- t y p e f u e l e l e m e n t s are c u r r e n t l y b e i n g i n v e s t i g a t e d i n p a r a l l e l ; however, f i n a l d e s i g n f o r c o n s t r u c t i o n i s ' t o be carried o u t f o r one f u e l e l emen t t y p e o n l y .

D E S I G N A S P E C T S

The a i m of t h e d e s i g n work is t o e s t a b l i s h a n optimum between h i g h e f f i c i e n c y and low c a p i t a l c o s t , fo r a p l a n t t o be b u i l t w i t h i n a few y e a r s .

216

I n F i g . 1 l i s t e d , showing a se t of d a t a A f o r t h e minimum o p e r a t i n g cost and a set B f o r t h e minimum c o s t .

t h e b a s i c data f o r t h e p l a n t d e s i g n a re

- The maximum p r o c e s s t e m p e r a t u r e ( g a s t u r b i n e i n l e t ) o f 85OoC w a s selected as a r ea l i s t i c v a l u e w i t h r e g a r d t o t h e p e r m i s s i b l e f u e l e l emen t l o a d , release of f i s s i o n p r o d u c t s and u s e of a v a i l a b l e materials. A t e m p e r a t u r e i n c r e a s e c a n be env i saged r e s u l t i n g i n a h i g h development p o t e n t i a l s i n c e it c a n be d i r e c t l y e x p l o i t e d w i t h t h e a p p l i c a t i o n of g a s t u r b i n e s . A t e m p e r a t u r e i n c r e a s e by 5OoC r e s u l t s for i n s t a n c e i n a n i n c r e a s e of t o t a l e f f i c i e n c y by a p p r o x i m a t e l y 2 p o i n t s and w i l l p robab ly a l l o w a n e s t i m a t e d r e d u c t i o n i n cap i ta l cos t by 2 0 DM/kW i n s t a l l e d power.

- To a c h i e v e a maximum p o s s i b l e s i t e independence a i r c o o l i n g h a s been s e l e c t e d . Under t h e a s sumpt ion t h a t a n i n t e r m e d i a t e c o o l i n g c i r c u i t i s r e q u i r e d for s a f e t y r e a s o n s t h e minimum p r o c e s s t e m p e r a t u r e h a s been chosen t o 35OC. A p o s s i b l e l o w e r i n g of t h i s t e m p e r a t u r e down t o a p p r o x i m a t e l y 2 2 O C by u s i n g r i v e r water f o r r e c o o l i n g would r e s u l t i n a n i n c r e a s e o f t o t a l e f f i c i e n c y by a p p r o x i m a t e l y 2 p o i n t s , which c o r r e s p o n d s t o a n i n c r e a s e of t u r b i n e i n l e t t e m p e r a t u r e by 5OoC.

- A s i m p l e i n t e r c o o l i n g w a s f avoured t o r e d u c e t h e b u i l d i n g volume, and machinery costs . Although doub le i n t e r m e d i a t e c o o l i n g r e s u l t s i n a n i n c r e a s e i n e f f i c i e n c y by a p p r o x i - ma te ly 1 1 / 2 p o i n t s , t h e o v e r a l l economics of t h e power p l a n t however, becomes less f a v o u r a b l e because of t h e h i g h e r c a p i t a l cos t .

- F o r t h e c h o i c e of maximum p r e s s u r e l e v e l - i n t h i s case a p p r o x i m a t e l y 6 0 bar - a compromise had t o be r eached between p l a n t s i z e and costs on t h e one hand and e f f i c i e n c y of t h e t u r b o machinery on t h e o t h e r hand.

0 - The i n v e s t i g a t i o n s of t h i s p r o j e c t are based on t h e u s e of a s i n g l e - s h a f t h o r i z o n t a l l y - a r r a n g e d g a s t u r b i n e p l a n t . A m u l t i - s h a f t a r r angemen t has been exc luded due t o t h e f a c t t h a t it r e s u l t s i n h i g h e r c a p i t a l c o s t s , h a s a more c o m p l i c a t e d l a y o u t and r e q u i r e s a l a r g e r c o n t r o l sys tem. Thus fo r example t u r b i n e compressor u n i t s w i t h v e r t i c a l sha f t c o u l d be deve loped o n l y a t c o n s i d e r a b l e a d d i t i o n a l e x p e n s e s .

- The n e t e f f i c i e n c y of t h e power p l a n t o f 4 2 % w a s de t e rmined on the basLs of these basic d a t a by o p t i m i z a t i o n of t h e t u r b i n e expans ion r a t i o o v e r t h e r e a c t o r i n l e t t e m p e r a t u r e , t h e p r e s s u r e l o s s e s , and t h e h e a t exchanger t e m p e r a t u r e d i f f e r e n t i a l . With d o u b l e i n t e r c o o l i n g and r e c o o l i n g by r i v e r water, p l a n t e f f i c i e n c y i s i n c r e a s e d t o 45,s %. The o v e r a l l economics, however remain a p p r o x i m a t e l y unchanged because o f t h e i n c r e a s e of c a p i t a l c o s t .

SAFETY ASPECTS

The e s s e n t i a l s a f e t y r e q u i r e m e n t s have been de te rmined unde r t h e f o l l o w i n g c o n s i d e r a t i o n s :

- possibility to s h u t down t h e reactor

- removal of decay h e a t - r a d i a t i o n dose i n normal o p e r a t i o n and i n

f a u l t c o n d i t i o n s .

The most s e r i o u s problem r e g a r d i n g shut-down of t h e reactor i s t h e " lo s s of c o o l a n t " a c c i d e n t , caused by r u p t u r e of a p i p e i n t h e main c i r c u i t . Due t o t h e r e l a t i v e l y l a r g e p i p e d i a m e t e r s i n t h e main g a s c i r c u i t and t h e h i g h v e l o c i t y of sound i n he l ium t h e r u p t u r e of a p i e e r e s u l t s i n a v e r y r a p i d d e p r e s s u r i z a t i o n of t h e c i r c u i t and t h e c o r e . The same a p p l i e s , though t o a lesser e x t e n t , t o a n e q u a l i z a t i o n o f p r e s s u r e w i t h i n t h e sys t em, caused by a n a c c i d e n t , w i t h o u t release of c o o l a n t g a s t o t h e envi ronment . Because of t h e

218

p o s s i b i l i t y t h a t a r a p i d d e p r e s s u r i z a t i o n may o c c u r , s p e c i a l d e s i g n and l a y o u t a re n e c e s s a r y for t h e c o r e s t r u c t u r e t o p r e v e n t damage t o t h e c o r e and t h e re la ted s t r u c t u r e so t h a t r e a c t o r shut-down i s n o t i m p a i r e d . The same a p p l i e s t o t h e removal o f decay h e a t .

The decay hea t removal sys t em must o p e r a t e under v a r i o u s p r e s s u r e c o n d i t i o n s , between f u l l d e s i g n p r e s s u r e and approximate ambient p r e s s u r e . T h e r e f o r e s t r i n g e n t r e q u i r e - ments r e s u l t f o r t h e d e s i g n c F t h i s sys t em and f o r t h e c o n t r o l of t h e b lower power. S p e c i a l a t t e n t i o n must be a t t a c h e d t o t h e problem o f t he rma l shocks .

The r a d i a t i o n I exg_osure -- of t h e environment and w i t h i n t h e power p l a n t d u r i n g normal o p e r a t i o n and under f a u l t c o n d i t i o n s d o e s n o t create any in su rmoun tab le problems for t h e e q u i l i b r i u m c o o l a n t g a s a c t i v i t y e x p e c t e d .

-

Based on e x p e r i e n c e , t h e p e r m i s s i b l e normal l e a k a g e w i l l n o t be exceeded , even w i t h a s h a f t p e n e t r a t i o n th rough t h e PCPV and a g r e a t number of f l a n g e s t o be sealed.

- The release of t h e t o t a l c o o l a n t i s h a n d l e d , f rom t h e s a f e t y s t a n d p o i n t , by a c o n t r o l l e d re lease t h r o u g h a v e n t i l a t i o n s t a c k .

- There are some u n c e r t a i n t i e s i n m a i n t a i n i n g s u f f i c i e n t l y low r a d i a t i o n d o s e s d u r i n g i n s p e c t i o n and r e p a i r work w i t h i n t h e area o f t h e main c i r c u i t , s i n c e a t t h e p r e s e n t d a t e i n s u f f i c i e n t i n f o r m a t i o n on c o n t a m i n a t i o n mechanisms i s a v a i l a b l e . Favourab le e x p e r i e n c e w i t h t h e DRAGON and t h e AVR r e a c t o r i n d i c a t e , however, t h a t a l s o f o r t h i s problem a c c e p t a b l e s o l u t i o n s c a n be e x p e c t e d .

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The c i r c u i t components a r r a n g e d c o n c r e t e p r e s s u r e v e s s e l : compressors

LAYOUTS

o u t s i d e of t h e p r e s t r e s s e d and t u r b o s e t , h e a t

I n the development phase a g r e a t number of d i f f e r e n t d - s i g n s and l a y o u t s are p o s s i b l e which a p p e a r e q u i v a l e n t , T h e r e f o r e a s y s t e m a t i c e v a l u a t i o n i s r e q u i r e d . According t o r e c e n t c o n s i d e r a t i o n , t h i s e v a l u a t i o n c a n be reduced t o a f e w p r i n c i p l e f e a t u r e s . It becomes e v i d e n t t h a t t h e d i f f e r e n t p o s s i b i l i t i e s of l a y o u t are c h a r a c t e r i z e d by t h e a r rangement of t h e main c i r c u i t components i n s i d e or o u t s i d e t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l , by t h e i r i n t e r c o n n e c t i o n s and by t h e d e s i g n o f t h e t u r b o s e t . These e s s e n t i a l l a y o u t s are c a t e g o r i z e d as n o n - i n t e g r a t e d (N), f u l l y - i n t e g r a t e d (J) and p a r t l y - i n t e g r a t e d ( T I sys tems and t h e i r d i f f e r e n t t y p e s of l a y o u t are compared i n t h e d i f f e r e n t columns of F i g . 2 ,

Going from l e f t t o r i g h t t h e p o s s i b i l i t i e s o f l o c a t i o n of components are compared, s t a r t i n g f rom i n s t a l l a t i o n of t h e heat exchange r s i n a n a n n u l a r s p a c e around t h e core and end ing w i t h t h e pod t y p e l a y o u t .

Based on t h e a n a l y s i s o f t h e p o s s i b l e p l a n t l a y o u t s , t h o s e o f f e r i n g good p r o s p e c t s were i n v e s t i g a t e d r e g a r d i n g t h e i r t e c h n i c a l f e a s i b i l i t y , c a p a b i l i t y of s a f e t y r e q u i r e m e n t s , a v a i l a b i l i t y , and economics,

I n t h e n o n - i n t e g r a t e d sys t em, o n l y t h e r e a c t o r i s l o c a t e d i n t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l ; t h e p r e s s u r e v e s s e l and t h e o t h e r corn\ponents of t h e c i r c u i t are connected t o t h e r e a c t o r v i a ductis ( c f . F i g . 3 ) .

220

The g a s , coming from 4 v e r t i c a l l y a r r a n g e d r e c u p e r a t i v e h e a t exchange r s , e n t e r s t h e upper p a r t o f t h e r e a c t o r by 4 r a d i a l openings i n t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l , and e x i t s t o t h e t u r b i n e by a c e n t r a l p i p e i n t h e bottom of t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l . The g a s e x i t f rom t h e r e a c t o r c o n t a i n s a s h i e l d i n g body t o p r o t e c t t h e t u r b i n e a g a i n s t e x c e s s i v e r a d i a t i o n . T h i s body s i m u l t a n e o u s l y s e r v e s a s a c r o s s s e c t i o n l i m i t a t i o n .

All h i g h p r e s s u r e g a s l i n e s ending i n t h e r e a c t o r are manufactured as doub le w a l l p i p e s . For s a f e t y c o n s i d e r a t i o n s even t h e i n n e r p i p e is des igned f o r t h e f u l l p r e s s u r e d i f f e r e n c e a g a i n s t ambient p r e s s u r e .

Two of t h e f o u r decay heat c o o l e r s l o c a t e d above t h e

p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l , s e r v e as r e s e r v e u n i t s . The decay h e a t c o o l e r s a re connec ted t o t h e r e a c t o r by a doub le w a l l p i p e and are l o c a t e d i n a s a f e t y c o n t a i n e r .

A l l g a s l i n e s t o and from t h e t u r b o s e t a re connec ted t o t h e upper p a r t of t h e c a s i n g , Apar t f rom t h e t u r b i n e i n l e t , f o r a l l g a s l i n e s t w o p a r a l l e l l i n e s are provj-ded. For assembly , i n s p e c t i o n and r e p a i r pu rposes t h e lower p a r t of t h e c a s i n g of t h e t u r b o s e t w i t h t h e r o t o r can be lowered by a s p e c i a l h y d r a u l i c or mechanic d e v i c e ; t h e uppe r p a r t of t h e t u r b o - se t c a s i n g and t h e g e n e r a t o r remain on t h e f o u n d a t i o n .

I n t h i s l a y o u t f o u r p r e c o o l e r s and f o u r i n t e r c o o l e r s a re p rov ided . The p r e c o o l e r s a r e d i r e c t l y connec ted t o t h e r e c u p e r a t i v e hea t exchange r s . A l l h e a t exchange r s o p e r a t e i n coun te r f low and are manufactured as p i p e h e a t exchangers .

I n t h e p a r t l y i n t e g r a t e d sys t em, t h e reactor i s l o c a t e d i n t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l , t h e t u r b i n e i s i n s t a l l e d i n a machine c a v i t y , and t h e h e a t exchange r s and

221

@ p r e c o o l e r s are l o c a t e d i n pods ( F i g . 4 ) .A second p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l c o n t a i n s t h e compresso r s , i n t e r - c o o l e r s , and t h e he l ium p u r i f i c a t i o n sys t ems . T h i s v e s s e l i s des igned f o r a p r e s s u r e of 8 b a r which r e p r e s e n t s t h e e q u i l i b r i u m p r e s s u r e a f t e r t h e maximum loss o f c o o l a n t a c c i d e n t . Both p r e s s u r e v e s s e l s are connected by a common f o u n d a t i o n p l a t f o r m . T h i s l a y o u t shows e s s e n t i a l c h a r a c t e r i s t i c s o f t h e i n t e g r a t e d sys tem.

The he l ium f low from t h e r e a c t o r e n t e r s t h e t u r b i n e v i a a s h u t t e r t u b e i n t h e h i g h p r e s s u r e p r e s t r e s s e d c o n c r e t e v e s s e l . (The t u r b i n e i s f l a n g e d d i r e c t l y t o t h e s h u t t e r t u b e and i s p r o t e c t e d by s h i e l d i n g s t r u c t u r e s a g a i n s t e x c e s s i v e r a d i a t i o n . ) From t h e t u r b i n e t h e he l ium f l o w s th rough t h e f r ee p a r t of t h e machine c a v i t y i n t o t h e h e a t exchangers and i n t o t h e p r e c o o l e r s i n s t a l l e d above . Then t h e coo led g a s f l o w s downwards th rough t h e c h a n n e l s i n t h e c o n c r e t e and i n h o r i z o n t a l d i r e c t i o n th rough t h e f o u n d a t i o n p l a t e t o t h e compressors w i t h a n i n t e r m e d i a t e c o o l i n g s t a g e . The h i g h p r e s s u r e g a s i s a l s o d i r e c t e d th rough t h e f o u n d a t i o n p l a t e t o t h e h e a t exchange r s . I t f l o w s a l o n g t h e t u b e bund les of the h e a t exchangers i n cross c o u n t e r flow and e n t e r s t h e

reactor l a t e r a l l y a t t h e h e i g h t of t h e lower r e f l e c t o r , c o o l i n g t h e metal s t r u c t u r e s b e f o r e b e i n g r e h e a t e d i n t h e c o r e . Thus, all h a n d l i n g and measuring equipment f o r f u e l e l emen t s are l o c a t e d i n t h e r e g i o n of low g a s t e m p e r a t u r e s . Otherwise t h e r e a c t o r c o r r e s p o n d s t o t h o s e used w i t h steam t u r b i n e p l a n t s .

The main c i r c u i t i s i n t e g r a t e d up t o t h e p r e c o o l e r o u t l e t . T h i s e n s u r e s s u f f i c i e n t l y l o w p r e s s u r e change ra tes i n t h e i n t e g r a t e d p o r t i o n of t h e main c i r c u i t , i n t h e e v e n t of r u p t u r e of a g a s p i p e , because of t h e t h r o t t l i n g e f f e c t of t h e t u r b i n e , t h e l a r g e g a s volumes o f t h e heat exchange r s

@

222

and p r e c o o l e r s , and t h e p a r a l l e l p i p e s l e a v i n g t h e pre- Q s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l . The O 2 c o n t e n t of t h e g a s r e s u l t i n g from t h e m i x t u r e of a i r and he l ium does n o t c o n s t i t u t e a danger t o t h e reactor.

Removal of decay h e a t i s e f f e c t e d by a r edundan t sys tem c o n s i s t i n g of a p r e c o o l e r , a b lower , and a h e a t exchange r , l o c a t e d i n t w o a d d i t i o n a l c a v i t i e s i n t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l .

I n t h e i n t e g r a t e d sys tem a l l components are l o c a t e d i n s i d e t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l ( F i g . 5). P e n e t r a t i o n s th rough t h e p r e s s u r e v e s s e l are r e q u i r e d only f o r t h e s h a f t c o n n e c t i n g t h e t u r b i n e w i t h t h e a l t e r n a t o r , and f o r assembly and d i sa s sembly of t h e components l o c a t e d w i t h i n t h e v e s s e l .

A two-room p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l i s env i saged ( w i t h o u t p r e s s u r e s e p a r a t i o n ) . Heat exchange r s and coolers are a r r a n g e d i n t h e a n n u a l s p a c e around t h e r e a c t o r and t h e compressor t u r b i n e u n i t , r e s p e c t i v e l y . Compressors and t u r b i n e are l o c a t e d i n a s e p a r a t e room below t h e r e a c t o r and are p r o t e c t e d by s h i e l d i n g s t r u c t u r e s . I n t h i s room also t h e c o o l e r s are l o c a t e d which o p e r a t e i n a pu re he l ium envi ronment , a s t h e compressor t u r b i n e u n i t .

The c o o l a n t gas flows from t h e core th rough t h e s h i e l d i n g and th rough a s h o r t l i n e , c o n t a i n i n g a s h u t - o f f v a l v e , i n t o t h e t u r b i n e . From t h e t u r b i n e i t f l o w s upwards t o t h e h e a t exchangers and downwards t o t h e c o o l e r s and compressors .

T h i s d e s i g n i s c h a r a c t e r i z e d by t h e f e a t u r e s t h a t a l l components are i n t e r c o n n e c t e d by d u c t s , no d u c t of t h e main c i r c u i t , however, l e a v e s t h e p r e s t r e s s e d c o n c r e t e v e s s e l , and t h a t t h e room c o n t a i n i n g t h e compressor t u r b i n e u n i t

- - - - _- - - - - . . . . . . . . . . . . . . . . . . . . . . . - . . . . . . . .

223

6d and t h e c o o l e r s must be k e p t a t t h e maximum he l ium p r e s s u r e a n d , u s i n g i n v e n t o r y c o n t r o l , t h e p r e s s u r e l e v e l w i t h i n t h i s room w i l l v a r y w i t h t h e p r e s s u r e l e v e l of t he whole sys t em.

For emergency c o o l i n g t w o emergency c o o l i n g b lowers w i t h v a r i a b l e speed and emergency c o o l e r s are p r o v i d e d .

T h i s sys t em is a l s o c h a r a c t e r i z e d by a two-room p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l w i t h o u t p r e s s u r e s e p a r a t i o n , t h e heat t r a n s f e r u n i t s a r e , l o c a t e d i n pods ( F i g . 6). The o t h e r c h a r a c t e r i s t i c s c o r r e s p o n d t o t h o s e o f t h e l a y o u t shown i n F i g . 5 .

I n t h e i n t e g r a t e d sys t em i n F i g . 7 t h e compressor t u r b i n e is l o c a t e d i n a machine c a v i t y . The i n t e r c o o l e r s a re a r r a n g e d i n h o r i z o n t a l open ings i n t h e lower p a r t of t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l . I n a d d i t i o n , t h i s l a y o u t is c h a r a c t e r i z e d by t h e f e a t u r e t h a t a l l g a s d u c t s are formed by c h a n n e l s i n t h e c o n c r e t e of t h e p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l and i n t h e s l a b s e p a r a t i n g c o r e and machine c a v i t y .

u n i t

T h e v a r i o u s l a y o u t c o n c e p t s d e s c r i b e d show a g r e a t number o f common and s p e c i f i c problems. The i n v e s t i g a t i o n of t h e s e problems i s b e i n g c a r r i e d o u t w i t h i n a l a r g e sys t ems a n a l y s i s . To d e c i d e which d e s i g n c o n c e p t w i l l be f u r t h e r deve loped a n e v a l u a t i o n of t h e i n d i v i d u a l c o n c e p t s is b e i n g carried o u t w i t h i n 1 9 7 0 . Applying common s a f e t y s t a n d a r d s t h e

e v a l u a t i o n i s c a r r i e d o u t a c c o r d i n g t o t h e f o l l o w i n g c r i t e r i a :

- f e a s i b i l i t y - a v a i l a b i l i t y and - c o s t s .

224

The main i t e m s which a re be ing i n v e s t i g a t e d , a r e :

- assembly , d i s a s s e m b l y , p i p e c o n n e c t i o n s , a c c e s s i b i l i t y , f o u n d a t i o n l a y o u t o f t h e t u r b o s e t

- l a y o u t o f t h e g a s p i p e s o f t h e main c i r c u i t

- f l o w behav iour a f t e r p i p e r u p t u r e s and t h e i r effects

- d e s i g n and l o c a t i o n of s h u t - o f f v a l v e s o f t h e main c i r c u i t

- h a n d l i n g he l ium volume d u r i n g c o n t r o l procedures and r e p a i r

- open ings and p e n e t r a t i o n s of t h e

p r e s t r e s s e d c o n c r e t e p r e s s u r e v e s s e l .

The c o n c e p t s d e s c r i b e d are t h e r e s u l t of t h e f i r s t d e s i g n phase ( sys t ems a n a l y s i s ) . The d e s i g n w i l l be deve loped i n s u f f i c i e n t d e t a i l so t h a t i n 1970 a d e c i s i o n c a n b e made on t h e g e n e r a l l a y o u t . The d e c i s i o n r e g a r d i n g t h e t y p e o f f u e l e l e m e n t s i s e x p e c t e d i n 1 9 7 2 . Complete c o n s t r u c t i o n documents w i l l be a v a i l a b l e i n 1 9 7 5 .

Components w i l l be t e s t e d i n t h r e e he l ium t e s t l o o p s which are under c o n s t r u c t i o n . By e x t e n s i v e f u e l e l emen t development work, i n c l u d i n g a n i n v e s t i g a t i o n of t h e d e p o s i t i o n mechanisms of f i s s i o n p r o d u c t s , a c c o u n t w i l l be t a k e n of t h e s p e c i a l c o n d i t i o n s of t h e s i n g l e - c y c l e p l a n t . The e x p e r i e n c e g a i n e d w i t h t h e o p e r a t i n g h i g h t e m p e r a t u r e t e s t r e a c t o r s and t h e p r o t o t y p e power p l a n t s unde r con- s t r u c t i o n o r t o be c o n s t r u c t e d i n t h e n e a r f u t u r e , w i l l c o n s i d e r a b l y c o n t r i b u t e t o t h e s u c c e s s f u l e x e c u t i o n of t h e pro j ec t .

225

max. processtemperat ure min. processtemperature number of intercoolers max. processpressure thermal reactor power electrical output net efficiency

F i g . 1.

PARTLY INTEGRATED(

MACHINERY ROOM I

mffi 515EI i INTEGRATED LAYOUT

P, = e

A 850 O C

22 O C

2 60 bar 1333MW 600 MW 45%

B 850 O C

35 O C

1 60 bar

1428MW 600 MW

42 Yo

i iominal 3 e s i g n Data

I I

F i g . 2 ., V a r i o u s l a y o u t s c h e m a t i c s 63

Teilintegrierte Anlage

Fig. 4. Partly integrated layout

. . . , .. ----- Fig. 3. Non-integrated layout

F i g . 6 . I n t e g r a t e d l a y o u t 2

l a y o u t

228

DISCUSSION

S. I. Kaplan: How would you provide emergency c o o l i n g ?

H. Oehme: A l l designs descr ibed have two a u x i l i a r y loops for emer-

gency cool ing which can ope ra t e a t a l l pressures from fu l l pressure t o

atmospheric . J. D. Thorn: To what e x t e n t i s your s e l e c t i o n of p l a n t layout i n -

f luenced by a preference f o r a h o r i z o n t a l s h a f t and a s i n g l e machine?

H. Oehme: I n the beginning of OUT a n a l y s i s our t u r b i n e des igners

looked for t h e most d e s i r a b l e s o l u t i o n s f o r t h e i r po in t of view. They

proposed t h e s i n g l e s h a f t h o r i z o n t a l machine, s o we made it a b a s i s of

our l ayou t a n a l y s i s . The reasons f o r t h a t a r e descr ibed i n t h e paper. I may mention one po in t - having a s i n g l e s h a f t h o r i z o n t a l machine o r v e r -

t i c a l l y arranged compressor t u r b i n e units wi th a h o r i z o n t a l power t u r b i n e

as i n some B r i t i s h proposals , one needs i n both cases approximately t h e

same lower p a r t of diameters i n t h e lower p a r t of t h e PCRV f o r p l ac ing or

r e p l a c i n g t h e power tu rb ine .

L. J. Connery: W i l l your helium t u r b i n e [discharge i t s waste h e a t t o

a i r r a t h e r than a cool ing water s o u r c e ?

H. Oehme: All t h e l ayou t s d i scussed i n t h e paper i nco rpora t e a n i n -

te rmedia te water cool ing c i r c u i t f o r s a f e t y reasons and a f i n a l d i s p o s i -

t i o n of h e a t t o t h e a i r t o acqu i r e s i t e independence. Because of t h e

r e l a t i v e l y h igh of f -gas temperature i n t h e HHT t h i s s o l u t i o n i s t echn i -

c a l l y f e a s i b l e and burdened wi th a reasonable decrease i n e f f i c i e n c y only.

G. Meijer: Regarding t h e problem of fast c l o s i n g i s o l a t i o n va lves

i n connection wi th nonin tegra ted and p a r t i a l l y i n t e g r a t e d layout , I wodd

l i k e t o make t h e fol lowing remarks: I n s p i t e of decay h e a t removal loops

be ing capable t o remove the decay h e a t a f t e r a fas t dep res su r i za t ion , I am a f r a i d t h a t one s t i l l w i l l need fas t operati-ng i s o l a t i o n va lves f o r

nonin tegra ted designs. Based on the present knowledge, I must emphasize

t h i s , t h e reasons are:

even wi th a v e n t u r i i n t h e duc ts , ( 2 ) h igh temperatures of t h e a i r -he l ium

mixture i n t h e secondary containment dur ing and a f t e r dep res su r i za t ion

(1) D i f f i c u l t core l e v a t i o n problems poss ib ly

229

0 through a main coolant duc t .

have t o opera te i n a high temperature environment f o r t he nonin tegra ted

des igns .

development per iod and cons iderable cos t w i l l be involved and even i f

t e c h n i c a l l y f e a s i b l e a :Licensing problem must be a n t i c i p a t e d . I n a par-

t i a l l y i n t e g r a t e d design, one may or may not need fas t c los ing i s o l a t i o n

va lves . This has not been determined y e t . However, i f w e would need them

i n t h i s type of layout , t h e va lves would be loca ted i n t h e cold duc ts .

They, t h e r e f o r e , a r e more f e a s i b l e from a t e c h n i c a l s tandpoin t and e a s i e r

t o l i c e n s e . Because of t h e above reasons, I a m of t h e opinion t h a t non-

i n t e g r a t e d designs do not seem t o be as f e a s i b l e and t h a t one should con-

c e n t r a t e on p a r t i a l l y i n t e g r a t e d and f u l l y i n t e g r a t e d des igns .

t h i s t h e requirements f o r nonin tegra ted des igns w i l l au tomat i ca l ly emerge.

I f one needs fas t c los ing va lves , t hey w i l l

Although fas t c l o s i n g va lves may be t e c h n i c a l l y f e a s i b l e , a long

I n doing

H. Oehme: I f u l l y agree . I n f a c t , I may add two remarks wi th r e -

s p e c t t o t h e nonin tegra ted des igns .

t h a t even i f t h e f e a s i b i l i t y of a n emergency c los ing va lve i s a problem

it may not be accepted by t h e l i c e n s i n g committee, even i f you have two

va lves i n ser ies . For t h e reasons mentioned by you and for o t h e r reasons,

w e , i n f a c t , t end t o f avor one of t h e i n t e g r a t e d des igns .

We have a l r e a d y had t h e experience,

L. A. Lys: You ha-ve a d d i t i o n a l emergency cool ing loops i n your de-

Does it mean tha- t you must r e l y upon a fast c los ing va lve t o i s o - s ign . l a t e the main loop in order t o avoid bypassing of the core in the case of

emergencies ?

W. Twardziok: We (don’t need quick c l o s i n g va lves t o ope ra t e t h e

emergency cool ing system, because w e have t i m e enough before s t a r t i n g t h e

emergency cool ing. We ag ree t h a t w e have t o i s o l a t e t h e main loop t o de-

c rease t h e mass flow f o r t h e emergency cooling, b u t t h a t can be done

slowly.

L. A. Lys: I n your i n t e g r a t e d des ign I have not no t i ced space f o r

helium which m u s t be removed from t h e loop i n t h e case when t h e p re s su re

l e v e l c o n t r o l system i s used,

p re s su re v e s s e l pipes , pene t r a t ing t h e w a l l of t h e v e s s e l i s , of course,

If the helium i s s t o r e d ou t s ide of t h e

necessary. This , however, w i l l c o n f l i c t wi th t h e p r i n c i p l e s of t h e i n -

@ t e g r a t e d design.

230

W. Twardziok: For a l l HTR gas tu rb ine designs ( in t eg ra t ed or non- i n t e g r a t e d ) w e proposed t o have a combination of bypass and pressure l e v e l cont ro l . Quick changes w i l l be done w i t h the bypass cont ro l . For t he

pressure l e v e l c o n t r o l there w i l l be more time, t h a t means only small d i -

ameter pipes for the connection of the primary c i r c u i t and the helium

s to rage system, arranged outs ide of the pressure ves se l . Loss of coolant

m u s t be taken i n t o account a l s o for t h e in t eg ra t ed systems, b u t it means only small dep res su r i za t ion rates which leaves enough t i m e for valve ac- t i o n .

Paper 6/li7

D EV EL 0 PME N T WO R K F 0 R L _ ~ E ; = b S $ J d J f 3 ~ ~ C , L - ~ A_R_._HFL-IJJM T U R B I N E PLANTS \\'ITH WIG1I-TEMPERATURE REACTORS -**-Emu--

% E. B6hm and W . Twardziok - GHN-Oberhausen 3? 2- H. Oehme - BBC/Krupp Mannheirn iq/ 0 !-z H. Ke i skopf - BBC Mannheim

ABSTFtACT

The main p a r t o f t h e deve lopmen t programme f o r h i g h - t e m p e r a t u r e r e a c t o r s w i t h h e l i u m t u r b i n e i s p roposed t o be s p e n t on t e s t f a c i l i t i e s and t e s t work. Three t e s t r i g s f o r t h e n o n - n u c l e a r t e s t s on h e l i u m components and s y s t e m s a r e d e s c r i b e d . The r i g s a r e p r o p o s e d t o be e r e c t e d j o i n t l y w i t h t h e a u x i l i a r y i n s t a l l a t i o n s i n a common t e s t f i e l d on t h e p r e m i s e o f KFA i n J u l i c h . The pro- b lems o f t e s t work a re b r i e f l y d e s c r i b e d by t h e two examples , h o t g a s v a l v e s and i n s u l a t i o n .

231

232

1. Introduction

Within the framework of the development programme for high-temperature reactors with helium turbine of high

output (HHT-programme), an extensive test programme will be performed parallel to the projecting work. The importance of this part o f the programme can be seen from a cost comparison, which shows that about 80 $ of the total cost is proposed to be spent on test work and 10 % each on the projecting work and on an accom- panying programme for basic research.

The nuclear test programme requires about 25 $I o f the total cost, it comprises mainly the development and testing of fuel elements and materials. For the non-

nuclear test programme about 55 % of the total c o s t

is envisaged; here the task is to develop and test reactor and helium turbine Components and systems. The overall idea is to prove, with the aid of the test programme, the technical feasibility, the safety and the economy of components and systems for large HTR- plants with a direct cycle.

The following is dealing only with the non-nuclear test programme, which can be divided into two phases. The first phase comprises non-concept-bound tests on

smaller test rigs and the erection o f rigs for testing full-scale components of the 600 MWe-plant. The second phase covers the concept-bound tests and the testing

of prototypes.

2. Test rigs

For carrying out the test and development work, exis- ting helium test rigs will be concentrated in one

place and completed by loops in which the parameters

o f the coolant are to be realized which are envisaged

233

for the nuclear power plant being projected. I n all installations helium is circulated at high temperature

and pressure, with reactor impurity levels being aimed

at.

The arrangement and main data of the three helium test

rigs are shown in Fig. 1. The helium test loop (HVK)

consists o f the circulator ( a ) , the heat exchanger (b), the test section (b'), the heater (c), the mixer (d),

the cooler (e) and the motor (f). At a mass flow of

1 kg/sec in the test section a temperature o f 8 0 0 ° C

and a pressure o f 20 bar is achieved. The main com- ponents of the helium test turbine rig (HVT) are the LP compressor (a), the intercooler (b), the HP com- pressor (c), the heat exchanger (d), the natural-gas-

fired heater (e), the turbine (f), the pre-cooler (g),

the by-pass cooler (h), the test section (i), the

gearbox ( k ) and the starting motor (1). The mass-flow

in the test section is about 6 kg/sec at a tempera-

ture of approximately 700 C and a pressure of 22 bar.

The high-temperature helium test plant (HHV), with a mass flow of about 220 kg/sec at a maximum temperature of 1 0 0 0 ° C and a maximum pressure of 50 bar, is pro- posed to be used for prototype testing. Its main components are the hot-gas compressor (a), the test section ( b ) , the turbine ( c ) , the mixer (d), the by-

pass cooler (e), the gas circulator (f) and the

driving motor for the turbo set. The thermal energy

for the loop is supplied by the motor driver of the

turbo set.

0

._

The helium test loop (HVK) is proposed to go into operation in the autumn of 1970. A view o f the HVK model is shown in Fig. 2. In the foreground is the vertical test section, on the right is the heater

234

and i n t h e background i s t h e v e r t i c a l l y a r r a n g e d h e a t e x c h a n g e r .

The h e l i u m t u r b o s e t o f t h e h e l i u m t e s t t u r b i n e r i g i s shown i n F i g . ? d u r i n g a s s e m b l i n g . From l e f t t o r i g h t one c a n s e e t h e LP c o m p r e s s o r , t h e IIP compresso r and t h e h e l i u m t u r b i n e w i t h a x i a l o u t l e t , s t i l l e q u i p p e d w i t h n o z z l e r i n g s . T h i s f i r s t h e l i u m t u r b i n e s e t o f t h e

w o r l d w a s o r i g i n a l l y u s e d u n d e r t h e name o f L a F l e u r i n Phoen ix , A r i z o n a ( U S A ) , i n a n i t r o g e n l i q u e f y i n g p r o c e s s . The machine s e t w a s b u i l t by E s c h e r Wyss, Z u r i c h . I t i s u n d e r m o d i f i c a t i o n by GHH t o become a t e s t i n g i n s t a l l a t i o n . F i g . 4 , wh ich shows t h e h e l i u m h e a t e r of a b o u t 15 m h e i g h t , g i v e s a n i m p r e s s i o n o f t h e s i z e of t h i s t e s t r i g . The h e a t e r i s f i r e d w i t h

n a t u r a l g a s ; t h e t h e r m a l o u t p u t i s a b o u t 7 M W t h and t h e g a s consumpt ion about 1300 Nm 3 /h . Between t h e

h e a t e r and t h e t u r b i n e t h e r e i s a n a u s t e n i t i c p i p e l i n e o n t o wh ich t h e t e s t s e c t i o n i s t o be c o n n e c t e d .

A c r o s s s e c t i o n o f t h e t u r b o s e t o f t h e BBC-planned h i g h - t e m p e r a t u r e h e l i u m t e s t p l a n t i s shown i n F i g . 5 , w i t h 1 b e i n g the machine c a s i n g , 2 t h e h o t - g a s i n l e t , 3 t h e h o t - g a s o u t l e t , 4 t h e by-pass i n l e t , 5 t h e by- p a s s o u t l e t , 6 t h e t u r b i n e b l a d i n g , 7 t h e r o t o r , 8 t h e c o m p r e s s o r b l a d i n g and 9 t h e o i l - l u b r i c a t e d r a d i a l b e a r i n g . The two-s tage t u r b i n e has an o u t p u t o f a b o u t 50 MW, w h i l e t h e IO-s tage c o m p r e s s o r h a s a power r e - q u i r e m e n t o f a b o u t 90 MW. The a d d i t i o n a l power r e - q u i r e d i s s u p p l i e d by a s y n c h r o n o u s moto r w i t h s t a r t i n g equ ipmen t . F o r c o o l i n g t h e f e r r i t i c - d i s c r o t o r and f o r s e a l i n g t h e r e i s a s e p a r a t e b lower c i r c u i t . The r o t o r o f t h e t u r b o s e t h a s a b o u t t h e same d i m e n s i o n s as t h a t o f a 300 MWe-helium t u r b i n e s e t .

T h e t e s t r i g s d e a l t w i t h i n t h e f o r e g o i n g a re p roposed

235

t o be e r e c t e d j o i n t l y w i t h t h e a u x i l i a r y i n s t a l l a t i o n s i n a common t e s t f i e l d o n t h e p r e m i s e s o f K F A , t h e

n u c l e a r research c e n t r e i n J i i l i c h . The ground p l a n on t h e h e l i u m t e s t f i e l d a t p r e s e n t i n t h e p l a n n i n g s t a g e i s shown i n F i g . 6. The s m a l l e r i n s t a l l a t i o n s , t h e h e l i u m t e s t l o o p ( W K ) and t h e h e l i u m t e s t t u r b i n e r i g (I-IVT) a r e accommodated i n t h e h a l l of a b o u t 1 2 m h e i g h t on t h e r i g h t and t h e h i g h - t e m p e r a t u r e h e l i u m t e s t p l a n t (HIIV) i n t h e h a l l o f a b o u t 20 m h e i g h t on t h e l e f t . I n a l o w e r c o n n e c t i n g bay a r e t h e common h e l i u m s u p p l y s y s t e m w i t h t h e s t o r a g e t a n k s and t h e common h e l i u m p u r i f i c a t i o n p l a n t . Above t h e s e t h e c o n t r o l s t a t i o n s f o r t h e r i g s and t h e data c o l l e c t i o n s y s t e m a r e a r r a n g e d . The t e s t h a l l s a r e p r e c e d e d by t h e shop , l a b o r a t o r y and o f f i c e b u i l d i n g . On t h e l e f t and r i g h t b e s i d e t h e h a l l s a r e t h e power f e e d i n g and d i s t r i b u t i n g s t a t i o n s s o t h a t t h e r e i s o n l y a s h o r t way t o t h e consumers . The e r e c t i o n of t h i s b u i l d i n g i s s c h e d u l e d t o be comple t ed by t h e m i d d l e o f 1971 t o t h e e x t e n t t h a t o p e r a t i o n s c a n be s t a r t e d . The t e s t o p e r a t i o n s are p roposed t o be ca r r i ed o u t by t h e g r o u p of BBC, GHH and F K i n c o o p e r a t i o n w i t h KFA.

A p a r t f rom t h e a f o r e - d e s c r i b e d i n s t a l l a t i o n s , a con- v e n t i o n a l l y f i r e d 50 MWe-helium t u r b i n e power s t a t i o n i s p roposed t o be e r e c t e d u n d e r t h e HHT programme, i n o r d e r t h a t a l l components o f a h e l i u m t u r b i n e i n s t a l l a t i o n c a n be combined and t e s t e d i n c o n t i n u o u s o p e r a t i o n i n a r e a l power s t a t i o n c i r c u i t . The main data o f t h e p roposed p l a n t a r e shown i n F i g . 7 . The t u r b i n e i n l e t t e m p e r a t u r e o f 7 3 O o C a p p l i e s t o con- t i n u o u s o p e r a t i o n . F o r t e s t i n g p u r p o s e s i t i s p o s s i b l e t o a c h i e v e h i g h e r t e m p e r a t u r e s f o r a s h o r t t ime . With a maximum p r e s s u r e o f 40 bar, a n e x p a n s i o n r a t i o o f 2 .5 , c o m p r e s s o r i n l e t t e m p e r a t u r e s o f 25 C , s i n g l e - 0

236

s t a g e i n t e r m e d i a t e c o o l i n g and a t e m p e r a t u r e a p p r o a c h i n t h e h e a t e x c h a n g e r o f 3 O o C , a mass f l o w o f 86 k g / s e c i s a c h i e v e d . The n e t o u t p u t i s 50 MWe and r e s u l t s , t o g e t h e r w i t h t h e t h e r m a l o u t p u t o f t h e h e a t e r o f 156 MWth, a t a n e t e f f i c i e n c y o f a b o u t 32 9. I n t h e 50 MWe-plant, i t i s i n t e n d e d t o t e s t s y s t e m s and components o f l a r g e d i m e n s i o n s f o r r e l i a b i l i t y and s a f e t y i n c o n t i n u o u s o p e r a t i o n and t o a s c e r t a i n t h e dynamic b e h a v i o u r and t h e j o i n t o p e r a t i o n o f t h e components o f t h e h e l i u m t u r b i n e p l a n t .

3. T e s t programme

During the f i r s t p r o j e c t phase, i n which the des ign

c o n c e p t o f t h e 600 MWe-plant i s t o be f o u n d , o n l y s u c h t e s t s c a n be c a r r i e d o u t which a re o f a bas i c , i , e . non-concept-bound c h a r a c t e r . F o r t h e s e t e s t s , which s e r v e t o s u p p o r t t h e judgemen t and c h o i c e o f t h e f i r s t p l a n t c o n c e p t , t h e two small t e s t r i g s HVK and HVT a r e s u i t a b l e . F o r c o s t r e a s o n s t h e com- p o n e n t s o f o r i g i n a l d i m e n s i o n s can o n l y be b u i l t and t e s t e d a f t e r c o m p l e t i o n o f t h e r e f e r e n c e d e s i g n o f t h e 600 MWe-plant. They a re i n t e n d e d t o s e r v e as p r o t o t y p e s f o r d i r e c t p r e p a r a t i o n o f f i n a l d r a w i n g s a n d data f o r t h e 600 W e - p l a n t .

3 . 1 S u r v e y

The t i m e s c h e d u l e f o r t h e most i m p o r t a n t t e s t s i s shown i n F i g . 8. A t t h e t o p t h e t h r e e t e s t r i g s a re g i v e n , s t a r t i n g f rom commencement o f t h e t e s t o p e r a t i o n s . The g a s d u c t s w i t h t h e h o t - g a s i n s u - l a t i o n s and e x p a n s i o n b e l l o w s a re f i r s t t o be examined by p r e l i m i n a r y t e s t s on t h e HVK, t h e r e - a f t e r , w i t h more c o m p l i c a t e d g e o m e t r i e s , on t h e IIVT

237

and finally, with the dimensions resulting from the

reference design, on the HHV rig. The same proce- dure will be adopted for the hot-gas valves, which,

depending on the concept, may be equipped with or

without trip gear. The flow distribution, thermal-

shock behaviour, tightness and dynamic behaviour

of the heat-exchanging units will also b e investi-

gated step by step on the three test rigs. The hot testing of the reactor internals i s proposed to be carried out on the HVT and IIHV rigs. While the tests aiming at design solutions t o safety problems are carried out on all of the three rigs, the dy-

namic behaviour o f a helium turbo set and the other components can only be studied on the HVT. However,

the main results will have to come from the 50 MWe- plant.

Finally the shaft seals are shown, which will be

developed at a separate test installation and tested during operation o f the plants. The HHV rig is pro- posed to be equipped with a shaft seal whose di-

mensions correspond to those o f a 600 MWe-turbine seal.

The specifications for the various tests are under

preparation. The problems are briefly described

in the following by two examples.

3.2 Hot-gas valves

HTR plants with helium turbine require shut-off and control valves o f large diameters for operation in helium of high purity at high temperature and

pressure. Especially in plants which are not fully

integrated, these valves must have a high inner and outer tightness and a short closing time. Fig. 9 shows a design of a non-conventional hot-gas valve

238

with small dimensions. Such a valve with small di-

mensions i s proposed to be examined by preliminary tests before a large unit is tested.

The pneumatic control principle i s briefly described in the following (see Fig. 9 ) . O n normal opening and closing operations the valves V 5 and V 6 are actuated. On trip-closing the servo valve 1 is operated, with

a sufficient quantity o f gas being caused, by actua- tion o f control valve V I , t o flow into the lower cylinder space from the pressure accumulator 5 through the charging line 6. Before reaching its upper end position, the valve plate is braked by the damping

pin 4. The blocking p i n 7 fixes the plate in its upper end position; it is operated via the valves

V 3 and V 4 . During every velve motion the load on the two sealing plates 8 i s removed to the LP tank through

the pipes 2 . The control system is fed from the auxi-

liary supply system 9. The entirc valve casing and

the valve plate have to be kept at a lower tempera- ture by means of the cooling gas 3.

The most essential difference as compared to con-

ventional designs is that the valve plate, which is

guided in needle bearings, has also the function of

the control slide. This design avoids penetrations

and is particularly compact, which is advantageous

especially for installation in the prestressed- concrete vessel. The most important problems to be

examined are :

cold welding o f parts i n motion and at rest, material abrasion,

wear o f needle-bearing guide, tightness at seats,

control-gas requirement and closing forces,

239

cooling-gas supply and requirement, and efficiency o f damping.

The purpose of the investigations is to prove the functional reliability and safe availability with respect to application o f such valves in the safety concept of large HTR-plants with helium turbine.

3.3 Hot-gas insulations

The special problems o f hot-gas insulation in HTR- plants with helium turbine result from

the system-inherent high-pressure-chan- ging rates during control operations and

accidents,

the high gas flow velocities in the gas ducts due to the large volumes and the

correspondingly high-pressure drops and dynamic forces , and the high gas temperature and pressure with helium of reactor impurity.

Fig. 10 shows the most important insulation geo- metries to be examined. These are the straight

insulation section (a), the bend ( b ) , the T- branching (c) and the straight vessel wall (d). In addition to these geometries there is the insu- lation at the turbine inlet, which may have par- ticularly complicated geometries.

With regard to the nuclear purity conditions, the insulations used in reactor building are either all-metallic foil or mat insulations or ceramic

fiber insulations with tight metallic liners.

Fig. 10 shows as an example the connection o f two sections of a foil insulation (e) and a pipe branching ( f ) having a fibrous insulation with a

2 40

liner. In designing these insulations it is necessary to provide the clearances or openings required for thermal

expansion and depressurization in such a way as to en- sure that the dynamic behaviour is not badly affected

and hot spots are avoided.

The problems t o be investigated can be divided into three main groups :

(a) the thermodynamic behaviour with regard

to heat conductivity,

jb) the mechanical behaviour with regard to

thermal txpansion and depressurization,

and

( c ) the vibration behaviour with regard to

alternating stress from gas and sound

forces and mechanical vibrations.

The testing is proposed to be performed step by step. At the beginning there will be preliminary tests on

the helium test loop concerning the thermodynamic

behaviour of a pipe insulation in helium at high

temperature, pressure and flow velocities.

For these tests a special test section has been de-

signed and built, which is shown in Fig. ll. The

inner pipe (a) is provided with a metal foil insu- lation (b) which carries the displacement body (c).

At the displacement body the static pressure

tappings (d) are fixed. A joint of the insulation

sections (e) is shown in the lower part and the

bellows to absorb thermal expansion in the upper

part (f). The hot gas enters at OOO°C at point 1

and leaves at point 2, the cooling gas at 350°C flows in counter-current from 3 t o 4. The test sec- tion is being provided with the instrumentation and

241

the tests are proposed to start at the end o f this year.

With the displacement body it is possible to achieve

gas flow velocities of up to about 100 m/sec, so that

pressure drops per insulation section can be achieved

and even exceeded as will occur in the future large

unit. Fig. 12 shows a comparison of the test sec- tion data of the three loops with the data of the

600 We-plant. A s can be seen, it is necessary to provide the insulation test sections o f both the HVK and HVT with a displacement body to achieve the gas flow velocities required for ascertaining the

thermodynamic behaviour. The integral behaviour of

the insulation can only be examined with the origi- nal dimensions o f the pipe on the HHV rig as in this installation comparable dynamic gas forces

are expected.

Parallel to the thermodynamic tests described, the

insulations, especially with helium turbine plants,

have to be tested with regard to their behaviour

on rapid pressure changes. Other than in HTR two-

operation, to master pressure changing rates which amount to about 10 bar/sec and are thus higher by about the factor 10 to 20. Depending on the arrange- ment of the plant as a whole (integrated or non-

integrated), rates o f up to 100 bar/sec may occur

in an accident such as pipe rupture.

cycle plants, it is necessary even i n normal control

A depressurization vessel in which these high changing rates are achieved is shown in Fig. 13. In the tank (a) a pipe section with a test insulation (b) is fitted. At the outlet nozzle in the upper

head it is possible t o set different cross sections

242

by means o f an orifice, which before conmencement of a test is covered by two bursting discs. These discs are caused to burst during the test by admitting

pressure gas to the space between them. The pressule

djfferentials between the pipe interior and the interior of the insulation or behind the insulation

are measured during depressurization as well as the

i deformation o f the inner liner at the joints.

The first preliminary tests on a foil insulation with cold gas have been started at EURATOM in Ispra.

At present a larger test vessel is designed by GHH, in which insulation geometries o f original dimen- sions can be tested also at high temperatures. The

flow tests and depressurization tests will be

carried out on insulations with the same design and

geometry.

243

Helium Test Loop

HVK

Ressure : 20 bar

Temperature : 800 OC

Mass Flow : 1 kg/s

Helium Test Turbine

HVT

u

2 2 bar

700 "C

6 kg/s

High Temperature Helium Test Plant

H HV

5 0 bar

1000 O C

220 kg/s

F i g . 1 Arrangement and Main Data o f t h e Hel ium T e s t R i g s

- HVK ( h e l i u m t e s t l o o p ) - HVT ( h e l i u m t e s t t u r b i n e )

a c i r c u l a t o r a b h e a t e x c h a n g e r b b' t e s t s e c t i o n C

c h e a t e r d d m i x e r e e c o o l e r f f mo to r g

h HHV ( h i g h - p r e s s u r e i

h e l i u m t e s t r i g ) k 1 a c o m p r e s s o r

b t e s t s e c t i o n c t u r b i n e d mixer e by-pass c o o l e r f c i r c u l a t o r g d r i v i n g moto r

-

LP compresso r i n t e r c o o l e r €IP compressor h e a t e x c h a n g e r h e a t e r t u r b i n e p r e - c o o l e r by-pass c o o l e r t e s t s e c t i o n g e a r b o x s t a r t i n g motor

244

F i g . 2 Model of a H e l i u m T e s t Loop (HVK)

F i g . 3 Turbo S e t o f the H e l i u m T e s t Turbine ( H V T )

24 5

F i g . 4 Gas-Fired Heater o f the Helium Test Turbine(HVT)

F i g . 5 Turbo S e t of the High-Temperature Helium Test Plant (HHV)

1 machine c a s i n g 6 turbine b lading 2 ho t-gas i n l e t 7 r o t o r 3 hot-gas o u t l e t 8 compressor b lading 4 by-pass i n l e t 9 o i l - l u b r i c a t e d r a d i a l bearings 5 by-pass o u t l e t

246

Fig. 6 Helium Test Field for HEIT Development

in Julich

Turbine inlet temperature Turbine inlet pressure Pressure ratio Compressor inlet temperature Number of intercooling Hea t exchanger temp era t ur e difference Mass flow Net power Heater thermal power Net efficiency

C bar

0

C 0

deg C

MWe k d s

MW th %

730 40

2 9 5 25 1

Fig. 7 Preliminary Layout Data of the Conventionally

Fired 50 MWe-Helium Turbine Power Station

247

'est Facilities

Helium Test Loop (HVK)

1 Helium Test Turbine (HVT)

I Hlgh Temperature Helium Test Plant (HHV)

1 Ducts, Compensation. Insulation

2 Valves

3 Heat Exchangers

L Reactor lnternals

5 Safety Aspects

6 Dynamic Behaviour

7 Shaft Seals

1970

1

Fig. 8 HHT Test Programme

vz

. ~~

Fig. 9 Hot-Gas Valve for Helium Turbine Plants

1 servo valve 6 charging line 2 to LP tank 7 blocking pin 3 cooling gas 8 sealing plate 4 damping pin 9 auxiliary supply system 5 pressure accumulator VI to V 6 control valves

248

b

f

Fig. 10 Problems with Hot-Gas Insulations

a str~ight insulation section b 180 bend c pipe branching d vessel wall e foil insulation f fibrous insulation at a pipe branching

C

‘ 3 Fig. 11 Test Section for Hot-Gas Insulations

a inner pipe f bellows b foil insulation 1 ho t-gas inlet c displacement body 2 hot-gas outlet d static pressure tappings 3 cooling-gas inlet e joint of insulation packs 4 cooling-gas outlet

249

4elium Test Loop

HVK

lelium Turbine Test Loop

HVT

lelium High Temperature

e s t Plant HHV

600 MWe-HHT-Plant

with displacement bod

ressure Temperature Mass- Diameter of Velocity of Gas Frow Hot Gas Duct

bar O C kg/s mm m/s

20 800 1 216l’ 100

22 7 00 6 400 76

50 1000 220 1020 142

60 850 700 1570 140

Fig. 12 Main Data of the Helium Test Rigs

I: 25 bar Pressure Temperature 5 5OOOC

Depressurisation S 1OOborA

Fig. I3 Depressurization Vessel a vessel

b test insulation

250

DISCUSSION

B. N. Furber: I would be i n t e r e s t e d i n f u r t h e r d e t a i l s of the metal-

l i c i n s u l a t i o n proposed f o r t h e duc t i n s u l a t i o n , p a r t i c u l a r l y f o r t h e 180"

bend.

s t a t i c pressure f i e l d make t h e i n s u l a t i o n problems ( p a r t i c u l a r l y l e a k i n g )

very severe .

Here t h e d i f f i c u l t i e s of i n s u l a t i o n combined wi th a very complex

W. Twardziok: I f u l l y ag ree t h a t t h e r e a r e s p e c i a l problems concern- i ng t h e duc t i n s u l a t i o n f o r bends.

t h e s e geometries wi th f o i l i n s u l a t i o n as w e l l as wi th f i b e r i n s u l a t i o n .

I n both cases w e have t o t ake ca re f o r t h e s e a l i n g wi th r e spec t t o t h e

forced convect ion on t h e one s i d e and for t h e openings wi th r e spec t t o

t h e dep res su r i za t ion ra te on t h e o t h e r s i d e .

t i o n w e propose to take a m e t a l l i c foil i n s u l a t i o n w h i c h i s d iv ided i n t o

s e v e r a l s e c t i o n s and weld i t t o t h e p ipe circumference on one end.

Therefore , we propose t o do tes ts on

For our f i r s t bend t e s t sec-

Paper 7/127

THE PROGRAM OF PLATE-OUT I INVESTIGATIONS IN THE G A S TURBINE PROJECT

3 I . , ~ - . o p 1 c r r ; r ; l ~ ~ ~ ~ ~ ~ ~ - ---4-M--

0 C.B. von der Decken &/$-3*3e

The HHT-project, the project for the development of a HTR- system with direct cycle gas turbine has been described extensively in the foregoing papers. In this paper I want to speak about a special subject from this project, the plate-out of fission products.

Due to the development of the coated particles the fission products are retained very well inside the fuel elements. One need not speak at present of a contaminated cooling gas system in HTR's. The level of activity in the primary system of HTR's is comparable to the level in the primary system of reactors with metallic cladded fuel elements, at least in practical reactor operation taking into account problems of the production failures, etc. This has been demonstrated by a large number of in-pile experiments, but especially by the operation experience in three reactors, -Dragon, Peach Bottom and AVR.

Nevertheless, the low-level contamination of the cooling gas and the components is still large enough to play an important role in the safety philosophy and possible accidents. The lack of detailed knowledge of the behavior of fission products leads to a pessimistic view in the safety philosophy and with this to an over-emphasis of the safety provisions in the lay- out of the plant which could be an important factor in the economics of the HTR's.

This is a problem f o r all HTR-systems but specially for a system with a direct cycle gas turbine because of the relative- ly large conduit cross-section which could be opened by a practical or hypothetical accident. In such a case the depressurisation of the plant would be much quicker and higher gas velocities would occur in many parts of the system. In a large loss of coolant accident therefore the release of fission products out of the primary system could be much higher compared to steam cycle systems. Furthermore the distribution of fission products over the surface of the system is important for maintenance and repair because the gas turbine itself represents a large piece of equipment and the necessary precautions of shielding and safety for the maintenance operations will have an important influence on the design, the costs and the availability. For these reasons in the frame work of bhe HHT-project an extensive program has been developed to study the problems of the plate- out of fission products. I will try to give an overview of this program. 6i3

251

252

The discussion of such a program shoulq start with a summary of what is known up to now. Over the previous years important and good work has been done specially at Batelle, Oak Ridge and Gulf General Atomic. But I believe I can avoid giving such a summary today because this will be the subject of a special paper later in this conference,

@

The plate-out program in the HHT-project consists of the following main items.

1) Calculation models and computer programs 2) Measurement of the adsorption and desorption rates 3 ) In-pile experiment "Saphir" 4) Out-of-pile experiment "Smoc" (Saphir - model - out-of-pile 5) Adsorption of fission products on graphite dust 6) Investigations in the Am-reactor

- c ircui tT

Due to limited time I will be able only to discuss the main ideas of the different items and to give a short description of the experiment "Saphir" ,

253

6lJ CALCULATION MODELS AND COMPUTER PROGRAMS

To understand the behavior of fission products in complex system like primary circuits in HTR's a number of calculation models have to be worked out. The ultimate hope is that it might be possible to describe the complete process in the cooling system with a large computer program.

The mass transport in the gaseous phase can be described by the known laws of mass transport and heat transfer under flow conditions. Immediately near the wall there exists a boundary layer out of which the adsorption or desorption takes place. To work out a calculation model one can assume as a first step that the adsorption equilibrium with a boundary layer takes place spontaneously. With this assumption we are now preparing a computer program "fish-trade No. 1" (fission - product transport and deposition) which describes the tem- poral and local behavior of the fission products in any given complex tube system. Tube sections in parallel and in series, different temperatures, tube proflles, Reynolds numbers and adsorption constants can be taken into account. The program is based on the description of the adsorption by the of Henry and Langmuir. It will be ready soon.

The second step will be the program "fish-trade X" which corresponds to the program "fish-trade No. l", but experi- mentally determined adsorption isotopes will be used which will have been measured for a number of fission products and materials. The next step will be the program "fish-trade Y" which corresponds to the Predep-program from Kress not using the assumption of spontaneous equilibrium in the boundary layer. In this program also, experimentally determined ad- sorption and desorption rates will be used. O n e m i g h t better use in this case the expression, reaction rates, because in addition to the adsorption effects, chemical reactions of the fission products with structural materials will play an important role.

laws

MEASUREPENT OF THE ADSORPTION AND DESORPTION RATES

To use the computer programs alarge number of characteristic basic data have to be measured; for example, the adsorption and desorption rates for the most important fission products on the structural reactor materials. These basic data depend on a large number of parameters.

The fission products of main interest are Sr, I and Cs. Bay Ce, Ru and Te are of minor importance. The adsorption materials are all materials which are in contact with the primary cooling gas. This means ferritic and austenitic steels with different surface properties and graphites with @

2 54

different structure and surface areas and other ceramic materials . For the fission products Sr, I and Cs the already known measurements of the adsorption isotherms under static or laminar conditions will be repeated and extended. In the literature a large number cf such measurements have been described. Specially the publications of Zumwalt and co- workers and those from ORNL have to be mentioned. These measurements are already giving a rough picture of the circumstances, but many of the detailed results are still contradictiony and they cannot be extrapolated to other materials and conditions. During these experiments in our program the following view points will be taken into account particularly.

3) We will select only materials as an adsorber which will be important for the technical lay-out of a plant. The surface properties and conditions of these materials will be defined as clearly as possible. The surface conditions of some of the materials will change during reactor opera- tion and we will specially look in this problem. The influence of oxidic layers on steel surfaces is an example.

b) In the work already published the adsorption of the diffe- rent isotopes has been measured f o r single isotopes only. The influence of any co-adsorption has been neglected. Probably the influence of the co-adsorption is only important near the saturation. Adsorption isotherms had to be measured up to saturation and repeated in the presence of other fission products, for example: Cs/Sr.

Furthermore, an important parameter will be the level of impurities in the cooling gas. This influences the condi- tion of the adsorption surfaces and the fission products themselves. Concentrations of only a few Natm of water vapor or carbon-dioxide in the cooling gas represents some orders of magnitude more atoms per cm3 compared with the concentrations of' fission products. This means that, f o r example, Cs, which might be created in the atomic form would have a good chance to react with the impurities of the cooling gas before reaction with any surface takes place .

Furthermore, the measurements of the adsorptioc isotherms will be extended to turbulent flow conditions. Up to the present such measurements have been done only up to 20 m/sec max. gas velocity.

. . . . . - - . . . . - . . . - . .. . . . . . . . . .

255

Since there will be much higher gas velocities in the reactor system we want to measure up to 100 m/sec gas velocity in maximum. This is because it has not been clearly establiehed today whether the velocity of turbulent motion immediately next to the wall is comparable to the mean velocity of the molecules. It might be sufficient to make experiments for a few typical cases only.

A next group of important data are the desorption rates especially for safety predictions in case of reactor accident with loss of coolant. The measurements of desorption rates have to be done under static or laminar and under turbulent flow conditions.

In addition to adsorption, chemical reaction of the fission products with the structure materials will be important. In some cases the mass transport coefficient must be considered, while in other cases the velocity cnnstant of the surface reactions will be the more essential. The problem has not been examinated systematicly up to now. As the mass transport coefficient for the most common flow conditions and geometries is well known we will measure in the next series of experi- ments the velocity constants for a number of important reactions and the time dependence. If the methods prove to work well, we will also study the temperature dependence. Based on the results of these experiments it should be possible to determine whether the quantity of adsorbed fission products for various materials and conditions depends mainly upon chemical kinetics of surface reactions or upon the convective mass transfer coefficient.

The experiments to measure the adsorption and desorption rates under static or laminar conditions will be done in an apparatus which is shown schematicly in Fig. 1. A quartz tube contains the adsorption material and the source fission product. Independent heating provides for temperature regula- tion and for establishing given partial pressure. For de- sorption measurements the system can be evacuated.

Part of the experiments with iodine will be done with inactive iodine. We hope to develope the detection technique for iodine to a high level of sensitivity. The advantage of using an inactive apparatus is, of course, a simpler technique, and greater flexibility. We hope also for better accuracy to define the different parameters such as partial pressure, etc. A schematic drawing of the apparatus is shown in Fig. 2. The experiment will be done in standard atmospheric conditions. Clean air will be mixed with iodine vapor. The partial pressure of the iodine can be regulated by the temperature of the iodine source. The apparatus will be started by by-passing the test section until the partial pressure is constant. Then the gas

256

stream will be switched over to the test section. By this method conditions during the start-up of the experiment will be avoided. The same apparatus can be used for desorption measurements.

The experiment Smoc which will be described later will be used for all the other experiments under turbulent flow conditions.

IN-PILE EXPERImNT "SAPHIR"

In the reactor Phgase in Cadarache we are preparing an irra- diation experiment in the Loop Saphir, which we consider as an integral test with respect to the several out-of-pile experiments mentioned above. This experiment will last about two years. We will measure the deposition of solid fission products released from spherical elements under operation conditions of a gas cooled high temperature reactor. We will find out the dependence on Reynolds number, temperature, different materials and surface conditions of the materials. The gaseous impurities of the helium coolant will be moni- tored and controlled continuously.

Description of the Loop "Saphir"

The loop consists of the main circuit with the test assembly in an in-pile position of the reactor and an auxiliary circuit for purification and analysis of the coolant. The auxiliary circuit is connected to a bypass of the main circuit in an out-of-pile position.

The main circuit

Fig. 3 shows the scheme of the main circuit. The helium coolant flows from the blower through an electrical pre- heater (110 kW) and a Venturi nozzle measuring the total helium-flow to the test assembly. Here the helium divides into two streams. About 90 $ of the helium flows through an outer annulus of the test assembly back to the blower. Depending on the test conditions one can lead a part of the helium through a heat exchanger, which is placed in a bypass in front of the blower. The other part of the helium flows through the test assembly to the auxiliary circuit and then back to the main circuit.

The main circuit is mounted in a frame which is lowered to the bottom of the reactor tank. It then is moved to the core on rails.

257

Fig. 4 shows the test assembly. In a graphite sleeve 14 sphere elements each with a diameter of 60 mm are arranged in line. The sleeve is surrounded with Silicagel-wool in- sulation. Between the insulation and the outer tube of the test assembly is the annulus for the external helium-flow.

bove the fuel elements the interchangeable test tube is mounted. It is 2 m in length with a diameter of 30 mm, followed by a tube with a length of about 1 m and an inner diameter of 20 mm. The last one fits through the upper plug of the main circuit. A 180 - bend connects this tube with an absolute filter for solid fission products, both mounted out of the pressure tube of the main circuit in the reactor tank.

The absolute filter has a volume of 2 ltr. It contains some layers of a special glass fibre, and silver plated molecular sieve, type 13 X.

The inner mass flow of the helium (10 $ of the total flow) flows inside of the insulation over the fuel elements. Depending on the test conditions it is heated up from 250 - 400 OC entrance temperature to 800 - 850 OC at the exit of the sphere arrangement. Then the helium passes the test tube. The helium stream which flows outside along the insulation and the test tube cools the hot helium in the inner side of the tube to 500 - 650 OC. The inner helium flows through the bend and the absolute filter which is cooled by water of' the reactor tank. It the1 flows through a control filter filled with charcoal and a mass flow meter to the auxiliary circuit.

A part of the released solid fission products deposits on the inner side of the test tube and the bend depending on temperature and flow conditions. The rest will be adsorbed quantitatively in the filter,

The main circuit is laid out for an operation pressure of 60 ata and a maximum temperature of 550 OC. In a preliminary test the mean neutron flux in the fuel has been determined to be 6 1013 cm'* s-1.

The auxiliary circuit

Fig. 5 shows a scheme of the auxiliary circuit. Its operatio pressure is 60 at, the gas temperature is about 40 OC. The helium coming from the main circuit is driven through the aux'liary circuit by a compressor with a maximum output of

second one it will be purified. In addition it is possible 9 m 3 /h at 40 at. In a bypass the gas will be analysed; in a

0

258

to inject defined amounts of impurities into the helium.

The gaseous impurities in the helium are measured with a hygrometer and a gas chromatograph. The H2 will be determined with a Cu 0 - filter followed by a hygrometer. To measure the gaseous fission products a part of the inner mass flow (about 50 mg/sec) will be bypassed before the absolute filter and led through a mass flow meter into a filter f o r short-lived or another filter for long-lived fission products. The short-lived isotopes are determined by their solid daughter products.

The total activity in the auxiliary circuit is controlled by a monitor.

The purification facility consists of three elements. A filter with BTS-catalysator oxidizes H 2 and CO. The operation temperature is 200 OC. Behind the filter the helium is cooled to 10 oc.

Then the helium passes two molecular sieve filters working at room temperabure. In the first one H20 is adsorbed in the second one C02.

Then a part of the helium flows through a charcoal trap cooled by liquid air. Its volume is 2,5 ltr. In this filter N2, 02, Kr and Xe is adsorbed.

To control the effectivity of the purification system it is possible to connect a gas chromatograph or a hygrometer behind the filters.

The purification system is laid out in duplicate to avoid interruptions in operation due t o defects o r the regeneration of the filters.

Experimental procedure

The test tubes together with the bends and the absolute filter are changed five times a year and analysed quanti- tatively with respect to solid fission products. O f special interest are the isotopes I 131, S r 89, Sr 90, Cs 137, Te 132 and Ba 140. The concentration profile of the deposited solid fission products is measured in both longitudinal and radial directions of the test tube.

In case the activation by neutrons of the test tube is neglegible a scan is performed in longitudinal direction in the hot cells. Then the tube is cut into equal segments each of which is leached and analysed cy -spectrornetrically,

2 59

6$ As test material several kinds of steel and graphite tubes such as may be used in a HT-reactor with different diameters are provided. The gas inlet and outlet temperature and thereby the surface temperature of the test tube is varied by changing the inner mass flow. The power output of the fuel elements is kept constant. Between two changes the experimental conditions are not varied.

Table 1 shows the operation data which are attained in the

Spherical fuel elements of the HTR type are to be used. To get a sufficient output of solid fission products the coated particles are fabricated with a thinner coating than usual. The thickness of the sealing and outer coating is about 6 0 , ~ . The kernal consists of U02 and Th 02 with a ratio of 1 : 5. The lay-out of the elements allows a constant power generation of 3 kW per sphere for one year. To compensate for the fuel consumption the loop is moved nearer to the core during the test time. The reference temperatures are those of the fuel elements and the inlet gas temperature of the test tube.

loop.

In case the released rate of solid fission products is too small to produce measurable effects at the test tube the fuel elements are exchanged by a second charge. This charge containes coated particles with a buffer coating of 60 - 7 0 , ~ but no sealing and outer coating.

In a preliminary test which will probably start at the beginning of July this year the commissioning tests of the loops are to be carried out with normal THTR fuel spheres. If this test is satisfactory the first radiations will start at the beginning of August this year.

260

Table 1 Data of operation

$as temperature [OC]

entrance fuel outlet fuel entrance test tube outlet test tube

250 - 400 800 - 850 800 - 850 500 - 650

gas pressure [at) 40 - 60

pas flow [ g / SI total flow main circuit test tube auxiliary circuit

purification system

mean Reynolds number fuel

Reynolds number test tube

fuel element mean power / sphere number of spheres maximum surface temperature loading of U 235 / sphere

mean thermal neutron flux

100 - 250

15 - 24 15 - 24

261

c3 OUT-OF-PILE EXPERIMENT "SMOC"

A large reactor experiment like "Saphir" is very costly in operation and not very flexible. Therefore the possibilities of varying the different parameters such as material, surface conditions, geometries and temperatures etc. are very limited. Each experiment with a given set of parameters wosld require two to four months for completion. F o r this reason we are planing an out-of-pile experiment "Smoc" (Saphir model out- of-pile circuit) in which we can vary all Eecessa'Fy para- meters. An important consideration for the lay-out of Smoc is to simulate the operating conditions of Saphir as closely as possible in order to correlate both experiments. It is hoped that the results which c a n be achieved with Smoc can be translated easily to in-pile conditions.

In addition to this important comparison of the two experi- ments, Saphir and Smoc, the main purpose of the Smoc experi- ment is to measure the adsorption and desorption rates under turbulent flow conditions. Different steel and graphite test samples up to 50 mm in diameter and 4 m in length can be used and may be examined at temperatures up to 800 OC. Gas pressures up to 80 at, gas temperatures up to 800 O C , gas velocities up to 50 m/sec and Reynolds numbers up to lo5 can be achieved. The gaseous impurities will be monitored and controlled. More complex test sections will also be used to test different computer programs.

The experiment Smoc is still under design and I am not able to give you more detailed information today.

ADSORPTION OF FISSION PRODUCTS ON GRAPHITE DUST

In HTR's with prismatic fuel elements only minor quantities of graphite dust are present in the primary system. The quantity of graphite dust in pebble-bed reactors is somewhat larger. We are trying to determine the total amount graphite dust present in the AVR reactor.

Estimates have shown that the quantity of graphite dust in the AVR reactor is much less than had been assumed for the lay-out of the reactor. Nevertheless, even small quantities of graphite dust could be important in case of a large accident with loss of coolant if a large amount of fission products would be adsorbed on the dust. In such a case the dust could be blown out immediately during loss of coolant without any desorption process. Besides the adsorption rates the quantity of fission products on the dust depends on the surface area and this depends on the particle size distri- bution of the dust. @

2 62

Experiments at Brown Boverie / Krupp which had been done in connection with the problems of errosion have shown that the graphite dust at high gas velocities in the cooling system is broken into small pieces very quickly. The dust particles under operation conditions are very small and have a large specific surface area. A typical particle size distribution of graphite dust which had been blown around. in a circuit with a velocity of l r ) m/sec is shown in Fig. 6. Curve 1 represents a particle size spectrum of the graphite dust which had been inserted before the start oi' the experiment.

Curve 2 shows the particle size distribution after 30 minutes of operation of the circuit. If the graphite dust is blown around in the circuits for a longer time the particle size distribution does not change very much. As one can see from the diagram the particle size is about a few ,u.

In the investigation program we will make experiments which should define the graphite dust and will show how to handle this very fine dust. Later on we will insert dust in the experiments Saphir and Smoc to measure the amount of fission products which will be adsorbed on this dust. In addition we will try to find out how the dust will plate-out on surfaces and will be removed by a gas stream from the surface.

IWVESTIGATIONS I N THE AVR-REACTOR

In model experiments it is very difficult to simulate the conditions of particle reactor operation well enough to extrapolate the results of these model experiments with sufficient certainty to actual reactor conditions. 'This is because in case of plate-out of fission products a larger number of' parameters are involved and the corresponding data f'or the reactor conditions are not known. Measurements of' fission product distribution in reactors in operation now could in principal give important results. Unfortunately it turns out to De impossible to test the calculation models on reactor systems in operation at present because of the lack of knowledge or' all necessary data. In addition the number of possible investigations on reactor systems under operation is very limited due to time and design reasons.

in the first place tne main of objection of' the AVR operation, for example, is to the testing of' the whoie system, the fuel elements and the components. Major interference in the system to measure the plate-out fission products on surfaces of the system would interrupt the reactor operation too much. In addition the AVR reactor is designed as a power station and this means that the possibilities for making detailled experiments are very limited. 63

~ - - - - - . . - - .. . . . . . -. . . - . . . . . . . . . . . . . . . . . . . . . . . .

263

Besides the routine measurement of the rare gas isotopes, the moderator spheres which contain only graphite are being investigated to measure the fission products on the surface of these spheres, Furthermore the surface of the primary system which has to be dismantled for maintenance and repair will be investigated for plate-out of fission products. Up to now, fortunately for the reactor, but unfortunately for these experiments, only components out of the secondary circuits had to be dismantled, for example, the components of the fuel handling system. In principle only the blowers and the butterfly valves for the bypass can be dismantled from the primary system. These components worked without failures up to now and dismantling is for inspection purposes later on. Also the access to the dust filter is very difficult. For this we plant to design a small bypass with a dust filter which can be changed easily. We hope that it might be possible to measure the quantity of dust which is present in the primary circuit under operation and measure the particle size and the contamination of this dust.

In closing, I would very much like to emphasize that this program is a co-operative one that has been worked out by all the partners of the HHT-project. The theoretical and experimental work is being done and will be done by the different partners. I would like to thank all the people who have contributed to this program,

264

A Vakuum pump Ouartz t u b Heater lup lo 1000°C) Thermostat Source Shielding

7 Test specimen 8 p-measurement

Fig . 1. M e a s u r e m e n t of a d s o r p t i o n - i s o t h e r m s ( C s , I)

sorptions-and desorptions- test -section

circulotor /

I analytical Y I U

Fig . 2. A p p a r a t u s f o r s o r p t i o n s - and deso rp t ions e x p e r i m e n t s

265

f-

auxiliary circuit

r - - I I I I I I I I I core

I I I

n n nozzle \ I 1 1 1 1 1 I

1 1 - 0 0 I -

Fig . 3 . Main c i r c u i t of Loop Saph i r

f&40*c absolute filter

_-ca 500'C

1

W

Fig . 4 . T e s t a s s e m b l y of Loop S a p h i r

filter

- 7

I I

I I I I I I I I I I

I I

266

compressor

Fig . 5 . A u x i l i a r y c i r c u i t of L o o p Saph i r

fraction of particles up to diameter D co/o]

'tides diam.

Fig . 6 . P a r t i c l e s i z e d i s t r ibu t ion of g raph i t e d u s t

267

DISCUSSION

L. R. Z m w a l t : 1. Do you a t p re sen t have d a t a on t h e f i s s i o n prod-

uc t conten t of g raph i t e d u s t from t h e AVR? 2.

l a b o r a t o r y experiments f o r determining t h e abso rp t ion isotherms of mixed

f i s s i o n products on g raph i t e d u s t t y p i c a l of HHT ( A V R ) ?

Have you contemplated

C . B. von de r Decken: 1. Unfortunately w e do not have access t o

t h e d u s t f i l t e r i n t h e AVR. Dismantling t h i s f i l t e r would i n t e r f e r e wi th

t h e r e a c t o r ope ra t ion g r e a t l y and it i s not contemplated i n t h e near fu-

t u r e . We have designed a s m a l l bypass wi th an e a s i l y removable f i l t e r t o

do s t u d i e s on t h i s problem i n t h i s d i r e c t i o n . 2. We w i l l t r y t o do t h i s

f i r s t s i n c e one has t o de f ine what w i l l be p re sen t i n t h e HTR.

F. H. N e i l l : What i s t h e source of f i s s i o n products i n your out-of-

p i l e l o o p ?

C. B. von de r Decken: The des ign of t h e ou t -o f -p i l e loop has not y e t

been completed.

We w i l l des ign t h e loop f o r us ing d i f f e r e n t sources .

s i o n a t a l a t e r da t e and hope t h a t w e can p r o f i t from t h e experience of

o t h e r groups.

We have not decided on t h e kind of source w e w i l l use.

We w i l l make a dec i -

A. P. Fraas : Could you do d i r e c t con tac t maintenance on AVR compo-

nents ?

G. Ivens : I an sorry t h a t I cannot g ive you exac t f i g u r e s concern-

i n g the a c t i v i t y l eve l on t h e AVR components. I can s ta te , however, t h a t

maintenance work on components of t h e f u e l handl ing f a c i l i t y which were

i n con tac t w i th f u e l elements and t h e primary c i r c u i t gas could be c a r r i e d

out without any s p e c i a l s h i e l d i n g precaut ions .

C . B. von der Decken: An examination of t h e g raph i t e spheres (gra-

p h i t e on ly ) i nd ica t ed t h a t t h e h ighes t level of a c t i v i t y r e s u l t e d from

impur i t i e s i n t h e g raph i t e .

found i n t h e spheres . I a m s o r r y t h a t I do not have any d a t a wi th m e on t h i s sub jec t .

A very low l e v e l of f i s s i o n products was

Paper 8/102

THE HT8.- _DIREC.T-GY.C.U2 : ENGINEERING PO~SIBILITIES AND MATERIAL REQUIREYENTS

G. E. Lockett R. A. U. Huddle OECD Dragon P ro jec t

ABSTRACT

I n t e r e s t i n direct cyc le Gas Turbine Power P lan t s l inked t o High Temperature Gas Cooled Reactors, stems from the p o t e n t i a l savings i n e l e c t r i c i t y generat ing cos ts . An expected reduct ion i n c a p i t a l c o s t s toge ther with a somewhat higher thermal e f f i c i ency appears s u f f i c i e n t t o j u s t i f y se r ious study; wh i l s t , i n the longer t e r m , t he gradual r a i s i n g of temperature and e f f i c i ency can provide a cont inuing opportuni ty f o r improvement.

A s a f i r s t s t e p towards eva lua t ing t h e p r a c t i c a l p o s s i b i l i t i e s , r ecen t Dragon Project w o r k has selected a plant layout arrangement and produced a re ference design. It has been used a s a b a s i s f o r s i z i n g components, i d e n t i f y i n g problem a reas and assess ing the effects of hea t exchanger performance, pressure drops, design conf igura t ions and gas temperature l i m i t s .

When opera t ing with helium a t high core o u t l e t temperatures and a t t h e low and r ep resen ta t ive impurity levels , expected i n a c losed cyc le system, the re i s a need t o show t h a t t he behaviour of any chosen high temperature ma te r i a l i s completely sa t i s f ac to ry . regarding the choice of ma te r i a l s and an i n d i c a t i o n from exploratory test r e s u l t s i s given.

A philosophy

The paper g ives a view on some of t h e problems foreseen and some of t h e proposed general so lu t ions . b e no insuperable problem b u t f u r t h e r study could lead t o u s e f u l improvements.

It concludes t h a t t h e r e appears t o

INTRODUCTION

The growing i n t e r e s t i n a d i r e c t cycle gas turb ine power p l an t

l inked with a high temperature helium cooled r e a c t o r stems from t h e

p o t e n t i a l savings i n e l e c t r i c i t y generat ing costs .

t h e i n d i r e c t steam cycle system the savings appear s u f f i c i e n t t o

j u s t i f y se r ious study.

When compared with

The expected savings r e s u l t from the o v e r a l l

268

2 69

p l a n t and bui ld ing s i ze , coupled with t h e somewhat higher thermal

e f f i c i ency of power generat ion which can be achieved with present ly

acceptable operat ing temperatures. Furthermore, i n the longer term,

the gradual r a i s i n g of temperature and e f f i c i ency can b e seen a s

providing a continuing opportuni ty f o r improvement.

The l i m i t i n g temperatures are d i c t a t e d on t h e one hand by the

turbo-machinery design which must opera te continuously and r e l i a b l y ;

and on t h e o ther hand by t h e r e a c t o r core i n which f u e l and moderator

must opera te with a high degree of i n t e g r i t y and with a low f i s s i o n

product emission.

c o n t r o l l i n g t h e gas f low between them i s not without i t s problems,

b u t i n a l l a r eas the re appears t o be a so lu t ion ava i l ab le and no

immediately r e s t r i c t i v e b a r r i e r t o achieving very adequate working

temperatures i s evident.

The supplementary requirements of duct ing and

FUEL AND GAS TEMPERATURES

For a f i r s t s tage exploratory study, a peak nominal f u e l tempera- 0 ture of 1300 C i s regarded a s a s u i t a b l e l i m i t and i n present s t u d i e s

corresponding o u t l e t gas temperatures from peak channels can reach

lloooc.

The mixed gas temperature from a l l core channels, toge ther with

con t ro l rod and r e f l e c t o r cooling streams, l e a d s t o a t u r b i n e i n l e t

temperature of about 967'C (124O0K) and t h i s i s t h e o r e t i c a l l y capable

of producing a thermal e f f i c i ency of power generat ion of over 47%.

Although the above mixed o u t l e t gas temperature i s between 100°C 0 and 200 C higher than cu r ren t ly conceived steam cycle designs t h e core

i tself appears v i r t u a l l y i d e n t i c a l .

more than 50 C higher and t h i s i s l a rge ly achieved by l i m i t i n g core

power dens i ty t o about 5.8 MW/m . even before taking i n t o account t h e expected improvements i n f u e l

spec i f i ca t ions and performance which could occur during t h e next

4-5 years.

Peak f u e l temperatures a r e no 0

3 These are predic tab ly safe values

270

Regarding the r e a c t o r core , down-flow cool ing i s now usual i n

H T R ' s and it i s even more appropr ia te f o r the direct cycle system

because acc identa l pressure t r a n s i e n t s i n the d i r e c t i o n of flow must

not l e v i t a t e t he core (e.g., loss of tu rb ine b lad ing) . A coarse

f l a t t e n i n g of channel gas o u t l e t temperatures by s h i f t i n g gagged top

r e f l e c t o r blocks a t r e f u e l l i n g i n t e r v a l s corresponds t o a devia t ion

of approximately 240 C, about a mean value when c a r r i e d out a t l i f e

i n t e r v a l s .

considered necessary i n HTR cores where t h e f u e l can t o l e r a t e t he

temper a t u r e swing s occurring.

0

The complication of continuously va r i ab le gagging i s not

The core i n l e t temperature i n t h e d i r e c t cycle system b r ings i t s own p a r t i c u l a r problems. With good recupera t ive hea t exchangers, core

i n l e t gas temperatures will be l i k e l y t o exceed 500 C, thus , on-load

f u e l changing must be designed t o s u i t t h i s temperature i n t h e upper

plenum and a l s o around the core support framework, which, toge ther with

t h e thermal neutron sh ie ld ing , must e i ther opera te a t t h i s temperature

o r be cooled by co lder by-pass streams. The l a t t e r so lu t ion has been

adopted i n t h e present study and the cool ing streams are used

subsequently t o remove r a d i a l r e f l e c t o r hea t before en te r ing t h e under

core plenum a t a temperature which does not degrade t h e core o u t l e t

gas temperature unduly.

0

The main penal ty arises from the f a c t t h a t cool by-pass streams

do no t pass through the recupera tor and hence the h e a t exchanger

e f f i c i ency i s lowered.

I n the d i r e c t cyc le system considered here , f u e l element surface 0 temperatures can reach about 1170 C compared with values of about

1000uC i n t he steam cycle case, b u t s ince the re i s no steam inleakage

problem i n t h e direct cyc le , g raph i t e corrosion effects can be

discounted and the change i n dimensional s t a b i l i t y of t he graphi te due

t o shrinkage behaviour, although worse, i s not seen a s r e s t r i c t i v e .

271

Control rod cooling streams which pass khhrough the core i n

p a r a l l e l with the f u e l channels w i l l be l a rge ly heated by t h e

swrounding core graphi te , thus con t ro l rods must be designed to

opera te a t a re la t i .vely high temperature, kypical ly 700-800 C. 0

CHOICE OF LAYOUT

Whilst previous cornments o u t l i n e a general view of t he d i r e c t

cyc le which can be backed by thermodynamic eva lua t ion over a range of

cases, i t becomes necessary t o a s ses s the p r a c t i c a b i l i t y of the

concept.

design which i s followed through i n t o p r a c t i c a l d e t a i l t o expose the

s p e c i f i c a reas where design and ma te r i a l problems exist. This s tage

i s preceded by some eva:luation of major components and explora t ion of

layout p o s s i b i l i t i e s . As a r e s u l t of such work a s u i t a b l e design

philosophy tends t o emerge.

It i s the re fo re necessary t o choose and develop a re ference

A nwnber of b a s i c opt ions a r e ava i l ab le t o t h e designer; he can

choose f u l l y in t eg ra t ed , semi-integrated o r non-integrated p l a n t

arrangements, and may s e l e c t e i t h e r a f ixed speed turbo-machinery

design with a l l compressors and tu rb ines on a s ing le s h a f t , o r a s p l i t

s h a f t design i n which the power tu rb ine opera tes a t f i xed speed while

t he turbo-compressor set may run a t va r i ab le speed. A f u r t h e r freedom

of choice app l i e s i n r e spec t of s i n g l e o r mul t ip le turbo-compressor loop arrangements.

I n the re ference design a s p l i t s h a f t turbo-machinery design was

selected f o r severa l reasons. F i r s t l y i t lends i tself t o a va r i e ty of

p l an t part-load con t ro l methods, un l ike t h e s i n g l e s h a f t arrangement,

which, by v i r t u e of t he r e l a t i v e l y narrow range of flow v a r i a t i o n

achievable with f ixed speed compressors, i s made l a rge ly dependent on

helium inventory control . Secondly the turbo-compressor u n i t can

r e a d i l y be mounted v e r t i c a l l y t o occupy a reac tor -vesse l w a l l cavi ty .

Thirdly a separa te turbo-compressor u n i t can be designed t o r u n a t

speeds s u b s t a n t i a l l y higher than t h e generator synchronous speed.

This r e s u l t s i n more compact machines and smaller w a l l c a v i t i e s .

272

With regard t o p l a n t i n t e g r a t i o n , t h e r e can be no doubt t h a t from

sa fe ty cons idera t ions preference must l i e w i t h a f u l l y in t eg ra t ed

layout , i.e., one i n which t h e r e a c t o r and gas turb ine p lan t a r e

housed within a s ing le , enclosing pres t ressed concrete vessel . On

the o the r hand, t he complexity of t he gas c i r c u i t tends t o pose

d i f f i c u l t problems f o r t h e ves se l designer which make it d i f f i c u l t ,

i f not impossible, t o achieve a x i a l symmetry i n a f u l l y in t eg ra t ed

design. The tendency i s f o r ves se l conf igura t ions t o emerge where

the tendon arrangements f o r p re s t r e s s ing are unsa t i s f ac to ry and the

ana lys i s w i l l r equ i r e t h e de r iva t ion of very complex programmes.

The so lu t ion adopted f o r t he re ference design houses the r e a c t o r ,

turbo-compressor u n i t s and a11 hea t exchangers and coolers i n an

e s s e n t i a l l y axi-symmetric concrete vesse l , very similar i n design to

t h e pod-vessel developed f o r t h e steam cycle HTR. The depar ture from

complete i n t e g r a t i o n concerns t h e power tu rb ines which are located i n

separa te ho r i zon ta l p res t ressed concrete ves se l s posi t ioned underneath

t h e main vessel . By ensuring t h a t t h e in te rconnec t ions a r e arranged

t o s a t i s f y t h e same i n t e g r i t y r u l e s a s would apply t o a s i n g l e vesse l ,

t he arrangement o f f e r s t h e advantage of a fu:lly in t eg ra t ed design.

The arrangement i s i l l u s t r a t e d i n Figs. 1 and 2 where it can be

seen tha t four turbo-compressor loops a r e used.

each loop forms one half of a back-to-back p a i r so t h a t only two

genera tors are required.

The power turb ine i n

FEATURES

Whilst the safe ty aspec ts of a pres t ressed concrete ves se l are

wel l known, some add i t iona l po in t s are worthy of mention s ince they

con t r ibu te t o easy maintenance and s impl i f i ca t ion of the enclosed

components . F i r s t l y t h e water cooled l i n e r can be called upon i n many areas

t o apply a use fu l temperature control .

elastomer s e a l s may be used a t pene t ra t ion plugs giving both

f l e x i b i l i t y and easy removal.

One obvious example i s t h a t

. ~ _ - . - ............... . .....

273

Secondly t h e ves se l l i n e r r e q u i r e s no expansion f e a t u r e s and many

of t h e interconnect ing duc ts r e q u i r e no in su la t ion . Where higher

temperature duc t s are requi red separa te in su la t ed l i n e r s can be

arranged.

removable assemblies.

I n the reference design these have been designed a s

Penet ra t ion diameters need only be l a r g e enough t o pass t h e

contained assemblies, gas plenums and flow pa ths may be made much

l a r g e r by i n t e r n a l l i n e r design.

I n t h e re ference design pre-coolers and i n t e r c o o l e r s are each

b u i l t a s one of t h e pene t ra t ion plugs and no demountable water j o i n t s

are i n s i d e the vessel .

assemblies, mounting may be arranged so t h a t p re s t r e s s ing d i s t o r t i o n s

do not react on machine un i t s .

By arranging machine u n i t s i n self-complete

T h e general philosophy on maintenance i s t o withdraw and r ep lace

Such an exe rc i se should be much f a s t e r than the a u n i t i f necessary,

t a s k of opening up the sealed pressure envelope of a non-integrated

set.

Regarding the machine frames, they only have t o be designed t o

s u i t t he l o c a l p r e s s u r e d i f f e rences and s ince t h e gas plenums

genera l ly l i e outs ide the frame diameters i n t h e ves se l w a l l s , t h e

s i z e and handling weight of each assembly i s s u b s t a n t i a l l y reduced.

The foregoing remarks cover some of the main advantages b u t most i n t e re s t w i l l be centred on t h e problems and one of t h e most important

of these i s t h e achievement of adequate sea l ing between c i r c u i t zones

of s i g n i f i c a n t pressure d i f fe rence . P i s ton r i n g type s e a l s have been

used i n t h e re ference design study b u t it i s not y e t known i f sonic

ve loc i ty leakages may a r i s e and cause e ros ion; some R. & D. work here

i s c l e a r l y necessary.

I n the case of p o t e n t i a l l y hof seals such as those around t h e

H.P. Turbine She l l an Iexcess pressure co ld feed i s appl ied between

mul t ip le r i n g s after bl?ing used t o cool t h e s h e l l and s h e l l hangars.

Leaks towards the tu rb ine i n l e t w i l l degrade the temperature s l i g h t l y

274

and can be made good by a small i nc rease i n r e a c t o r o u t l e t temperature,

b u t leakages downstream towards t h e L.P. tu rb ine involve an e f f i c i ency

loss.

space between ho t face duct i n s u l a t i o n and cold ves se l l i n e r i n su la t ion ,

thus permit t ing a higher h e a t f l u x through t h e h o t f a c e i n s u l a t i o n than

t h a t l o s t t o hea t r e j e c t i o n v i a the vesse l l i n e r .

Where poss ib le , use i s made of t hese flows t o sweep an i n t e r -

S i m i l a r p r i n c i p l e s a r e used f o r the L.P. t u rb ines bu t i f e f f i c i ency

lo s ses a r e t o be minimised any cool ing stream must be l imi ted t o j u s t

s a t i s f y i t s required hea t load and unwanted leakages must a l s o be

m i nimi sed . T h e Table , in Fig. 3 l i s t s t h e effect of an eva lua t ion between a

t h e o r e t i c a l case and t h e p r a c t i c a l case of t h e re ference design where

the lower e f f i c i ency of t h e l a t t e r i s almost en t . i re ly due t o by-pass

cool ing streams and Leakages.

The use of higher gas temperatures has necess i ta ted a new look a t

t h e problem of high temperature i n s u l a t i o n , and i n p a r t i c u l a r f o r t he

under-core plenum which w i l l experience t h e h ighes t temperatures i n t h e

c i r c u i t and which, furthermore, must support -the core load. A s u i t a b l e

so lu t ion which depends on t h e use of s i l i c o n n i t r i d e ( S i N 1 i s

i l l u s t r a t e d i n Fig. 4. Each column of f u e l l e d blocks i n the core r e s t s

on i t s own g raph i t e support column which i s headed by end r e f l e c t o r

blocks and a l l of these members can be removed i f required.

columns a r e seated on slabs of foamed s i l i c o n n i t r i d e which are

mounted i n t u r n on a series of thermal neutron sh ie ld p l a t e s .

reasonably good conductive path f o r hea t f low i s f i n a l l y made by a

l aye r of lead and steel shot compressed t o give a s u f f i c i e n t l y f l a t

upper face f o r near contac t with t h e lowest sh i e ld p la te . If

necessary the s i l i c o n n i t r i d e can a l s o be removed after a graphi te top

l aye r has been cleared.

3 4

The

A

Although t h e thermal conduct ivi ty o f S i N i s not a s low a s one

would l i k e and a knowledge of i t s conduct ivi ty i n the foamed form i s

s t i l l lacking, it appears t o have very a t t r a c t i v e design p rope r t i e s

such a s a very low expansion c o e f f i c i e n t (about ha l f t h a t of g raph i t e ) ,

3 4

275

c.$ a s t r eng th much higher than graphi te and a high thermal shock

r e s i s t ance , I t s behaviour under i r r a d i a t i o n from the l i m i t e d da t a

ava i l ab le t o da t e , looks q u i t e s a t i s f a c t o r y f o r t h i s appl ica t ion . I t s

s t a b i l i t y i n the r eac to r coolant i s t h e o r e t i c a l l y good up t o a t l e a s t

1000°C, b u t f u r t h e r work must be undertaken t o prove i t s s u i t a b i l i t y

on a l l counts.

If t h e r e a r e no problems with the use of s i l i c o n n i t r i d e it will be very s u i t a b l e f o r use a s a high temperature i n s u l a t i o n i n s i d e metal

duct l i n e r s . Should i t be f e l t necessary t o provide a metal covering

t o t h e sur face then methods of a t t ach ing a sheet layer , such a s

molybdenum, can be arranged.

CONTROL

Three b a s i c methods are ava i l ab le f o r load cont ro l :

1.

2.

3 ,

Inventory Control: Where t h e system pressure i s var ied t o s u i t

the load required. T h i s i s the most e f f i c i e n t method b u t t he

speed of response i s dependent on the t i m e required t o change

p res sure.

By-pass Control:

where a by-pass can be used f o r con t ro l purposes.

can be r ap id b u t t he thermal e f f i c i ency of the p l a n t a t lower loads i s poor.

There are var ious pos i t i ons i n t h e c i r c u i t

The response

Temperature Control: By moving r e a c t o r con t ro l rods t h i s

method can be r e l a t i v e l y f a s t i f high r a t e s of r e a c t i v i t y change

are accepted.

previous cases.

The e f f i c i ency a t part-load l i e s between the two

For m a x i m u m f l e x i b i l i t y some combination w i l l very l i k e l y be

used. I f , f o r example, p ressure l e v e l i s set f o r t h e peak load

expected then l imi ted v a r i a t i o n s i n demand can be handled by tempera-

ture con t ro l with l i t t l e loss of e f f ic iency . If quicker response i s

required than can be (achieved by con t ro l rod speed then by-pass

con t ro l i s needed, bu t t h e e f f i c i e n c y loss w i l l be grea te r .

276

Loss of generator load i s a more exac t ing problem and i n t h i s

event a quick ac t ing tu rb ine by-pzss, toge ther with a blocking of the

normal f low path through t h e tu rb ine i s Reeded. A t y p i c a l overspeed

r a t e i s lG% per second so t h a t f u l l d ive r s ion of f low i n about ha l f

a second i s required. A so lu t ion t o t h i s problem can be achieved by

t h e use of mul t ip le poppet valves each held shut by an overpressure

Sellows-sealed ac tua tor through an appropr ia te linkage.

ac tua to r pressure t h e l inkage co l l apses and system pressure blows each

valve open. Twenty-four 10 c m diameter va lves b u i l t i n t o each L.P.

t u rb ine s t a t o r casing provide t h e d ive r s ion i n t h e re ference design

case. The c los ing of t h e path through t h e turb ine i s achieved by

f l a p s which are l inked t o the r e spec t ive ac tua tor and which f o l d i n

from t h e s h e l l t o expose t h e gas path through each corresponding

poppet valve. The gas temperature a t L.P. tu rb ine en t ry i s about

760 C and t o l i m i t t h e temperature of t h e d ive r t ed gas, a g raph i t e

hea t s ink i s interposed which a l s o provides a s u i t a b l e pressure drop

t o r n i n i m i s e pressure t r a n s i e n t s and t h e consequent tendency t o speed

up the H.P. turbine.

By r e l e a s i n g

0

Other by-pass valves do no t seem t o involve any real d i f f i c u l t y .

A clear s p e c i f i c a t i o n f o r t h e i r duty i s dependent on the completion of

con t ro l s t u d i e s b u t they may be t y p i c a l l y as fol lows:

(i) Small by-pass con t ro l valves between H.P. compressors and

H.P. t u rb ine t o give temperature con t ro l of H.P. t u rb ine i n l e t .

One need f o r t h i s valve would b e i n an e a r l y shut down condi t ion

when H.P. tu rb ine load would be small and a low tu rb ine i n l e t

temperature would be needed t o prevent t h e o u t l e t temperature

from being too high.

It can a l s o be used f o r quick load reduct ion.

(ii) A by-pass valve from t h e cold H.P. s i de of t he recupera tor

(13OoC) t o the h o t L.P. s i d e t o produce a l imi ted flow d ive r s ion

w i l l e f f e c t i v e l y cool t h e gas en te r ing t h e recupera tor from t h e

L.P. t u rb ine exhaust.

tend t o co l lapse t h e pressure d i f f e rences between H.P. and L.P.

If t h i s i s opened on loss of load it w i l l

- . - - - - - - - - - . . - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -

277

( i v )

s i d e s of t he loops w h i l s t a t t h e same t i m e reduce any tempera-

t u r e t r a n s i e n t t.o t h e recupera tor tubes.

about 20 c m would serve two loops.

A valve diameter of

A con t ro l by-pass across t h e L.P. tu rb ine may be necessary

f o r s ta r t -up purposes. It may a l s o be u s e f u l f o r load con t ro l

trimming, b u t s ince t h e i n l e t temperature would be about 760 C

i t s duty would be less arduous i f t he l a t t e r purpose w e r e

unnecessary.

arrangement so t h a t any s e n s i t i v e ac tua tor features are kept

clear of t h e high temperature gas stream.

0

The only design problems seen are those of

C i r c u i t Stop Valves are necessary i f loops are t o be closed

down while t he o t h e r s are working.

p a i r of loops must be shu t down toge ther s ince they a r e t i e d

t o one generator , thus t h e p l an t can be run a t 50% load on t h e

remaining two loops.

I n t h e re ference design a

The Stop Valves i n the re ference design are placed between

t h e L.P. compressor o u t l e t and i n t e r c o o l e r i n l e t . The tempera-

ture w i l l be about 130 C when open, b u t when closed any leakage

gas w i l l be a t about 4OoC. The pressure d i f f e rences across t h e

closed valve w i l l be equiva len t t o the core Ap which i s about

half an atmosphere. Elastomer seals are poss ib le f o r these valves i n view of t h e low temperature, it should be noted

however t h a t under closed loop condi t ions the pressure throughout

w i l l reach r e a c t o r plenum values so t h a t a l l pressure ves se l s

must be designed f o r f u l l system pressure.

0

MATERIALS

It could be simp1.y s t a t e d t h a t t h e only requirement concerning

materials i s t h a t they should r e t a i n t h e i r s t r u c t u r a l i n t e g r i t y during

t h e whole of t h e i r se rv ice l i f e . This i n t e g r i t y i s dependent on both

mechanical and chemical factors. I n t h e d i r e c t cycle HTR it i s the

higher coolant temperatures of both core i n l e t ( 500-550°C) and o u t l e t

(1000-1050 C ) a s compared t o the steam cycle HTR t h a t dominates t h e 0

choice of mater ia ls . At ten t ion must, t he re fo re , be d i r ec t ed t o the

problems of duc ts , l i n e r s and i n s u l a t i o n , p a r t i c u l a r l y those t h a t a r e

b u i l t permanently i n t o t h e p l an t and must therefore have a se rv i ce l i f e

of 250,000 hours. The f i n a l so lu t ion becomes a compromise between

design philosophy, ma te r i a l s s e l ec t ion , f a b r i c a t i o n inf luences and cost .

Experience i n the opera t ion of nuclear i n s t a l l a t i o n s has

emphasised the importance of component replacement and p l an t mainten-

ance. The design porposals have the re fo re involved the p r inc ip l e ,

where p rac t i cab le , of component replacement, inc luding members which

are normally permanent such as duc t s and l i n e r s , p a r t i c u l a r l y those

opera t ing i n t h e high temperature a reas of t he plant . I n a l l such

designs dimensional s t a b i l i t y i s of prime importance, and g rea t care

must be taken i n the choice of metals and alloys t o ensure freedom

from d i s t o r t i o n under a c t u a l s e rv i ce condi t ions. Such dimensional

s t a b i l i t y demands meta l lurg ica l s t a b i l i t y , and f o r temperatures above

500 C t he choice becomes limited, e spec ia l ly i f s i g n i f i c a n t

stresses are involved.

0

Most conventional engineer ing metals used a t temperatures i n t h e

range 500-750 C are based on a l l o y s of i ron , n icke l and chromium. A t

these temperatures atomic re-arrangement ( d i f f u s i o n ) can take place

r e s u l t i n g i n t h e formation of a new phase, f o r example, t h e sigma

phase i n c e r t a i n a u s t e n i t i c h e a t r e s i s t i n g steels. These phases

( p r e c i p i t a t e s ) are formed i n i t i a l l y a t t h e g ra in boundary, they can

a l s o form wi th in t h e grain.

it i s genera l ly accompanied by a change of volume which produces l o c a l

s t r a i n s .

condi t ions - a condi t ion r a r e l y a t t a i n e d i n serv ice - t he phase change

does not t ake p lace homogeneously, s i g n i f i c a n t s t r a i n s a r e set up, and

d i s t o r t i o n i s inev i t ab le .

o the r components, where the maintenance of c i r c u l a r geometry i s v i t a l ,

opera t ing a t a temperature i n excess of t h a t where atomic re-arrangement

can take place.

0

Whatever the na ture of the phase change

Unless the whole component i s operat ing under t r u l y isothermal

This i s e spec ia l ly t r u e of f l anges and

- ._ - - - . . . . . . - . . . . . . . . . . . . . . - - . . - - . . . . - . - . . . . . . . .

279

The use of a thermodynamically s t a b l e a l loy would of course

e l imina te t h i s phenomenon completely. However, t he r e l a t i v e l y poor

mechanical p rope r t i e s of such a l l o y s i n h i b i t s t he designer t o such an

ex ten t t h a t some degree of me ta l lu rg ica l i n s t a b i l i t y must be accepted

i n order t o ob ta in adequate s t r eng th a t opera t ing temperatures.

At ten t ion must also be directed t o t h e choice of a l loy ing add i t ions

and t o t h e na ture of t he p r e c i p i t a t e s t h a t are formed. Very long

per iods a t temperatures approaching the use fu l l i m i t of the a l loy

(from t h e mechanical p rope r t i e s po in t of view) can lead t o continuous

g r a i n boundary p rec ip i t a t ion .

e n t i r e l y d i f f e r e n t corrosion c h a r a c t e r i s t i c s t o t h a t of the parent

metal, and i f f o r example t h e p r e c i p i t a t e i s a t tacked premature

f a i l u r e w i l l occur.

These p r e c i p i t a t e s may w e l l have

A cons idera t ion of t h e above f a c t o r s suggests t h a t t he s t a b i l i s e d

a u s t e n i t i c s ta inless steels (provided they a r e completely f r e e from

sigma phase formation) t h e Inconels , t he Incoloys, I N 102, and f o r

temperatures i n excess of 800 C, molybdenum should b e appropriate . 0

The use of helium avoids any gross cor ros ion problem. However,

t he effect of t he r e s i d u a l impur i t i e s are such tha t they must no t be

overlooked.

t h a t , under opera t ing condi t ions impurity concentrat ions i n the range 1/10 t o 1 vpm t o t a l impur i t i e s can be expected i n a direct cyc le

plant . Although these o r i g i n a t e pr imar i ly from air and moisture

adsorbed on t o the core and t h e r e f l e c t o r g raph i t e - only a small

f r a c t i o n desorbes from metals - they soon e q u i l i b r a t e with t h e l a rge

volume of ho t carbon and graphi te i n t h e core.