IAEA-126 BASIC STRUCTURAL DESIGN PHILOSOPHY, CRITERIA AND SAFETY OF CONCRETE REACTOR PRESSURE VESSELS REPORT OF A PANEL SPONSORED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY AND HELD IN VIENNA, 9-13 FEBRUARY 1970 A TECHNICAL REPORT PUBLISHED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1970
BASIC STRUCTURAL DESIGN PHILOSOPHY, CRITERIA AND SAFETY OF CONCRETE REACTOR PRESSURE VESSELS
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BASIC STRUCTURAL DESIGN PHILOSOPHY, CRITERIA AND SAFETY OF CONCRETE REACTOR PRESSURE VESSELSOF CONCRETE REACTOR PRESSURE VESSELS
REPORT OF A PANEL SPONSORED BY THE
INTERNATIONAL ATOMIC ENERGY AGENCY AND HELD IN VIENNA, 9-13 FEBRUARY 1970
A TECHNICAL REPORT PUBLISHED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1970
The IAEA does not maintain stocks of reports in this series. However, microfiche copies of these reports can be obtained from
INIS Microfiche Clearinghouse International Atomic Energy Agency Kdrntner Ring 11 P.O. Box 590 A- 1011 Vienna, Austria
on prepayment of US $0.65 or against one IAEA microfiche service coupon.
PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT
WERE ORIGINALLY BLANK
FOFW0HD
A panel on"Baeic Structural Design Philosophy Criteria and Safety of Concrets Reactor Pressure Vessels" was held by the International Atomic Energy Agency on 9 to 13 February 19?0 at Agency Headquarters. A total of 34 specialists representing 14 countries and two international organizations participated in the discussions.
Since the first two prestrsssed concrete reactor pressure vessels at Mar-coule, Prance, were built in 195^» the technology of concrete pressure vessels for nuclear power reactor application has-developed rapidly and there are now 15 vessels of this type in operation or under construction» In addition, there are eight concrete reactor pressure vessels known to be in the planning stage in the United Kingdom, France, U.S.A. and Federal Republic
è '
of Germany. It is also known that several other countries have already started very extensive studies and research for using prestreesed concrete reactor pressure vessels.
Although the problem of vessel availability is not yet critical for the PWH and BWH systems, it is clear that the potential for continued growth in unit rating will ultimately be limited "by shop-fabricated vessels and that, at that time, either field fabrication of steel vessels or prestreesed concrete vessels will bs required. Of these two alternatives, "based on experience in the United Kingdom end France, and taken into considera- tion, integrated nuclear power station's concepts, prestreesed concrete vessels now appear to offer excellent possibilities.
In view of the growing significance of this type of vessel in relation to the future of nuclear pov/er plants, the Agency- has organized a Panel on this subject with the"purpose of reviewing the latest pertinent deveJ opme'nts, ' to facilitate the exchange of informations and, mainly, to discuss the various aspacts of the design philosophy criteria,'economics and safety of prestressed concrete pressure vessels as '#ell as to formulate general guidelines concerning the -objectives.
It is hoped that this collection of nine papers, together with the conclusions and recommendations which were worked out by the Panel Meeting, will be of interest to reactor designers and to the authorities concerned with the safe working of nuclear pressure vessels. The texts of these papers have been supplied by the Panel members and no editing has "been done by the Agency.
The Agency is grateful to the authors of papers, to all the participants of the Panel for their contributions to the discussions and, it would particularly like to ejtpress its thanks to -the Chairman of the Panel, Mr. I. Bavidson, of the United Kingdom Atomic Energy Authority, for his guidance of the discussions in the most productive manner.
CONTENTS
. > General ;. Analysis for Servies Load Conditions » Limit Design . Ul t i mat e Be s i gn , Subjects of Particular Interest for Further Research or
Dcvelopmewt Study
". . Structural Design Philosophy and Criteria for Concrete Heaetor Vessels ~ U.S. Practice 17 W. Rcckenhauser Contribution from the United Kingdom 53 I. Davidson
. > Work on Reactor Pressure Vessels of Prestressed Concrete in Yugoslavia (>1 B. Petrovic Problems and Perspectives of Prestressed Concrete Pressui'e Vessels - Franch Experiance up to 1970 o9 D. Costes
;3, Teat Stand for the Prestressed Concrete Vessels Containing the Keliusi Loop 85 L.K. Komoli
•J. Ultimate Design, Experience fronn Small Dimension Models Testing 99 P. Scotto
V. Report on the Starting of a Coordinated Work Programme for Prostresssecl Concrete Reactor Preesxire Vessels in the Federal Republic of Germany 115 T , Jae ger
•'•', Design Philosophy and Criteria of Safety of Prestressed Concrete Pressure Vessels ~ Practice in Czechoslovakia 179 M. IDavid Swedish Development Work on Prestreased Concrete Pressure Vessels for Water Reactors 213 S.
J-''22 _»i* Fro^r'i:ftne 243 'L3I;2S~JZ> List of P":.ot:i cip'inhs £47
Sumaaary Report ^and Recoamendatiojgg of. the Panel Meeting on/"Basic.Structural fiesiign PhirlQsophy_ Criteria and Safety of Concrete Heaotor Pressure _Vgssels"
I. Genejral
1.1. Following the discussions of tra Panel which met in Vienna from 9 to 13 February 1970 the present, report was prepared' with a view to setting out the general requirements and principles which appear to be applicable in this field. Attention is' drawn to the advantage of farther work in certain areas, and recommendations are made regarding further international collaboration. Because this is a rapidly developing field of study,, the present report must "be regarded as provisional.
1.2. Reactor vessels perfora the function of containing the nuolear reactor, the primary coolant, which is normally under pressure, and various othsr cbsponants and equipment essential to the operation of the reactor. The vessel must perform this duty for
* its design life under all noriaal and foreseeable abnormal conditions of operation, with a degree of reliability such as to preclude any -unacceptable risk to the public.
1.3. The vessel is loaded by the primary coolant pressure and by the effect of temperature-induced strains in the various structural components. The basic principle of a prestressed concrets vessel is that, for a range of predetermined loads, including normal operation, the concrete is maintained by the tendons in a state of net compression across any section of the vessel, The admissible state of stress and strain in the concrete may be influenced by passive reinforcement.
1.4. The prestressed concrete structure may be furnished with features such as penetrations and associated closures, an impermeable liner, insulation, a cooling system, passive reinforcement and means for pressure relief.
1.5» This application of prestressed concrete is novel in many respects} consequently, existing standards are of limited application, Progress is, however, being made in the preparation of national and international standards for PCRPVa.
1.6. Early development in this field was associated with gas-cooled reactors (OCRs). The potential, of the concspt f c .- other types, such as liquid cooled and/or moderated reactors, is now appreciated. It is not expected that the basic principles of PCRPV design will be influenced by the type of reactor contained. Variations in engineering applications must be expected.
1,7» Sfce vessel must be capable of performing its function under certain predetermined conditions of interaction between the contained reactor and the vessel. It follows that, not only must the vessel provide the necessary standard or' containment under predetermined reactor conditions, but there. must also be an acceptably low probability that vessel behaviour will of itself induce an unsafe state in the reactor.
1.8. One incentive for adopting PCRPVs is the economic advantage to be expected thereby. This advantage arises, for example, from: (a) the ability to contain large reactors or reactor systems
with the acceptance of high pressures? (b) simplification of a plant layout} (c) the fact that a highly developed steel fabrication industry
is not necessary,
1.9» Attractive features from the safet,y point of view include: (a) physical isolation of the stseJ prestressing tendons and
reinforcement from sources of heat and radiation and from the primary coolant;
(b) the high degree of redundancy in the preetressing system;
(c) the possibility in many cases that tendon loads may ba measured and reset;
(d) the possibility of removing tendons during operation for inspection or replacement;
(e) inherent ability to withstand seismic shock.
1.10. 1-1 some cases the conventional practice of grouting tendons is adopted in order to provide sotae protection against corrosion and an alternative anchorage,, These .advantages must be set against the inability to test, inspect and replace individual tendons.
1.11.. Where a penetration as required bo provide access for services, equipment etc , it is customary to provide a purpose-built removable closure. The design objective in such cases is that the standard of integrity of the closure and its attachment should be at least as good as that of the main structure? the presence of a penetration should not prejudice the necessary integrity of the main structure,
1.12. Zt is necessary to provide some raeans of limiting ths effect on the structure of an excessive rise in internal operating pressure. The aiagnitude and rate of a postulated pressure rise can be determined only by reference to the characteristics of the particular reactor contained. It is common practice to provide automatic venting devices, such as safety valves, for this purpose. An alternative is to design the vessel so that it is self-venting by partial structura] faiïure. It is generally recognized that this alternative cannot be relied upon at present*
1.13. It is customary and advantageous to make provision for verification of .the state of the vessel. This may be done fey installed instrumenta- tion and/or by periodic in-service inspection» Such measures, which are taken in a manner appropriate to the particular situation, serve to verify the vessel integrity ana to confirm the design criteria.
1.14. Design philosophy is based on the recognition of two or more phases in -vessel response to increasing pressure. Wie design objective is to ensure that a particular response to the imposed load can b© achieved in each pbase and that this behaviour is consistent with the appropriate operational and predetermined fault conditions.
1.15. Over a range of pressure and temperature, including- normal operating conditions, the vessel will respond to short-term variations in pressure in an elastic nurimîer, This facilitates machine analysis of stress and strain .in the structure, Longe r-ter-m stress and strain are affected by shrinkage and creep of the concrete, re- laxation of tendons and possibly fatigue, ïn -fois range of response the affects of short- and long-ierifi behaviour can be combined to demonstrate that stresses and strains are limited to acceptable values.
1.16. Beyond the elastic range the response becomes increasingly inelastic and non-linear. Tho vessel would not be expected to enter this phase except under the most severe overpressure fault conditions. In this phase tho structure is stable but may experience permanent damage. It is in this phase of vessel response that some limit states occur,
1.17. The ultimate load condition, in which the vessel is incapable of sustaining- any further Increase of internal pressure, is a further limit state, Evaluation of tM ultimate load provides a measure of the factor of safety above design conditions.
1.18» The methods of design verification which are referred to in paragraphs 1.14 to 1.17 are described more fully below* it should be noted that it is the usual practice to use two different methods of design verification simultaneously.
II. Analyses foi*, Service Load_ Conditio.ns_ 2.1.Loading conditions
Stresses, strains and deflections in the vessel structure îîhould be analysed for all relevant combinations of mechanical and thermal loads which can arise under normal service conditions throughout its life.
The tsndon forces adopted in each analysis should include allowances for the most severe effects of friction and loss of prestress. Account should, be taken of all significant loadings applied to the structure, including stresses arising as a result of construction procedures and normal operating transients. Any significant effects of penetrations and -liner on the vessel structure should be taker, into considération,
2.2.J Analysis
The analysis method selected for each loading condition should take appropriate account of time and temperature dependent characteristics of the concrete and bsve clue regard for the complexity of the design and loading conditions and the accuracy required.
Each analysis should provide adequate details of the stresses induced in the concrete, in the passive reinforcement and strains in the liner to enable the acceptability of the design to be assessed.
For the purpose of the analyses covering the prestressing forces and dead loads at completion of construction, arid under test pressure, the concrete may be assumed to be a linear elastic material»
For all other service load conditions, including start-up and shut-dowi both initially and at the end of the vessel life, the stress-strain characteristics used for the concrete should take account of the age, temperature and time under load.
2_,j_._ Minimum design près tress Notwithstanding the acceptability of stresses in the concrete under normal service load conditions a net cornpressive force should be maintained across any section of the vessel under a pressure which exceeds the maximum normal service load pressure by a suitable margin.
2.4» Tendon anchorage zone design Concrete supporting tendon anchorages should be reinforced in accordance with applicable codes v^here existing, 'The safety of the vessel structure is specially dependent upon the integrity of the prestressing tendon system. Suitable tests should be carried out on representative tendons and anchorages in combination under support conditions representative of those obtained in the pressure vessel.
2.3. Cracking It is considered that lire: ted cracking may be accepted provided due regard is paid to any significant redistribution of the stresses which may arise and the integrity and leak tightness of the liner are not impaired.
2.6. Concrete gtrèse concentra.tiens Where local concentrations of stress occur, due to the presence of embedments or other discontinuities in the vessel geometry, these should be assessed individually. Where such stresses are
very local and can be shown to "be self-limiting, they may be disregarded» Where they are more extensive, due regard should be paid to the effects of increased creep rates or tensiJe cracking on the distribution of stresses in the vessel concrete and the influence which they taay have on the strains in the vessel liner and the stress distribution arising in the vessel under conditions of shut-down.
Comment on gialti-a.xj.a.j jîompresslye stresses in concrete
2.7. A certain amount of evidence is available to demonstrate that for short-term loading, under multi-axial corepressive stresses, concrete can safely withstand higher compressave stresses than are generally accepted under uniazial loading. It is generally accepted, however, that if one of the stresses approaches zero (or becomes marginal I?/ tensile) the effect can be nullified or reversed. There is a lack of knowledge regarding long-term loading unuer multi-axial compressive stresses.
2.8. In view of the above, it is recommended that for tne time being, if any advantage is to be taken of multi-axial compressive stress conditions? - The loading condition must be short-term only.
The minimum stress value irust ne suown to oe sulstantially compressive beyond all reasonable doubt. Careful attention must be paid to the effects of any increased strain on the stress distribution un<5er subsequent reduced or reversed load conditions.
Ill,r Limit Design 3.1. Five limit states are recognized for the present:
- Limit of instantaneous linear elastic response. Defines the upper end of the range through which the overall response of the vessel to short-term loads remains essentially linear and reversible. Minor, localized clacking of the concrete may occur.
- Limit of instantaneous, reversible overall structural response. Similar to item 1. Defines the upper end of the range over which the vessel response remains reversible although no longer linear.
- Limit of permissible deformation (short-term and/or long-term), Represents the largest oeformation under wbjch the contained reactor system wil] still function properly. The limit usually applies to penetrations und other such parts of the vease"1 where relatively close tolerances must "be preserved,
- Limit of" liner defect stability. Defines the upper end of the deflection regime in which liner integrity can be reasonably assured. For liner deformations beyond this range, such defects as may be present in the liner, may propagate through the wall. Siraila^jy, highly stressed areas may lead to local liner faj'iure. This could result in substantial leakage into the vessel concrete, and consequently, to crack pressurisation,
- Ultimate strengtn limit. , As defjne in more detail in Part IV "Ultimate Design", tftis represents the ultimate load carrying capability of the vssaeT struct ire. It is evaluated wjtnout regard to the credibility or manner in which this load might come about.
3*2. The limit of linear elastic response and the limit of permissible permanent deformations are considered significant in terras of evaluating normal and upset loading conditions. An upset condition is defined as a transient operating state resulting from a single operator or equipment malfunction. Similarly, the limit of reversibility is related to the concept of (potentially repetitive) emergency loading conditions» For reactor systems where major leakage from the vessel con- stitutes the design basis accident, that is, where liner failure represents the faulted condition, the lirait of liner defect stability 'becomes the significant consideration. Thus, the reactor systems design and vessel design 'interact directly in the assessment of limit states three and four.
3.3. The designer, in evaluating the various limit states of the •vessel should provide suitable margins of safety with respect to the loading conditions and their probability of occurrence, against which the limits are assessed.
3.4. At present, quasi-elastic and visso-elasti.c analysis methods, as well as structural model tests are used to estimate these limits. Because of the inherent uncertain ties, considerable design conservatism results, much of which may be unnecessary.
3.5» Finite element techniques are under development to predict vessel behaviour in the aneiastic range. The methods consider cracking of the structure, but so far only on an axisymuzetrical basis, Chess finite element techniques should be expanded to three dimensions. In addition, meaningful failure criteria for concrete under multiaxial loading, and defect stability criteria for liner ftaiberiais, should be formulated. Furthermore, additional materials characterisations work, should be performed for concrete in the elevated-temperature visco-elastic regime.
IV. Ultimate Design
,3 . Outline
The ultimate design is a. simple method of ensuring that there is an adequate margin of safety between the design pressure and the pressure at which the vessel will fail. In calculating the failure pressure of the Tr3S^el it is assumed that the liner will remain intact up to the failure pressure and that the over- load pressure is applied tu a new and cold vessel. The safety factor chosen should allow for the degree of predictability and the gradualness of the failure mode and should he adequate to cover any possible time-dependent deterioration of the vessel materials.
4+J?*, _ Re 1 e van oe
•fbte ultimate load calculations are not in^nded to represent a realistic condition. The calculations do not take account of possible gross liner rupture at a pressure somewhat lower than the vessel failure pressure wftich could, result in premature vessel failure due to pressure acting on the cracked concrete. Keither does the approach consider a possible weakening of the vessel due to time dependent material degradation. The approach doea not calculate the precise margin available against a realistic condition. It merely gives an indication of the margin available against a hypothetical over-preesuriaation under the oondit ': ons assumed aho ve .
4.3» Method of Gal cu] ation
The method of calculation depends on a knowledge of the mechanisms of failure of the vessel. This information is normally obtained from appropriate model tests to destruction.
It is essential that the mechanism of failure is fully understood and is shown to be progressive.
10
IK the case of flft>ru.ra'i fa i lure , it- is posai r/e to calculate,
with smf f~ cie>i t accuracy, the r>ressure at v,*hlcV tho vesseJ will finally f a l ? ,
If a shear modo of fa i lure ayr l iea • tr» a s 'aY; , the xredic4 '!.bâ !t i ty oT this may be sufTioier.tly u r -ne r tu i r j to require a higher load f .actor.
The moâel shou'H ix su.*Ti c i s…
REPORT OF A PANEL SPONSORED BY THE
INTERNATIONAL ATOMIC ENERGY AGENCY AND HELD IN VIENNA, 9-13 FEBRUARY 1970
A TECHNICAL REPORT PUBLISHED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1970
The IAEA does not maintain stocks of reports in this series. However, microfiche copies of these reports can be obtained from
INIS Microfiche Clearinghouse International Atomic Energy Agency Kdrntner Ring 11 P.O. Box 590 A- 1011 Vienna, Austria
on prepayment of US $0.65 or against one IAEA microfiche service coupon.
PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT
WERE ORIGINALLY BLANK
FOFW0HD
A panel on"Baeic Structural Design Philosophy Criteria and Safety of Concrets Reactor Pressure Vessels" was held by the International Atomic Energy Agency on 9 to 13 February 19?0 at Agency Headquarters. A total of 34 specialists representing 14 countries and two international organizations participated in the discussions.
Since the first two prestrsssed concrete reactor pressure vessels at Mar-coule, Prance, were built in 195^» the technology of concrete pressure vessels for nuclear power reactor application has-developed rapidly and there are now 15 vessels of this type in operation or under construction» In addition, there are eight concrete reactor pressure vessels known to be in the planning stage in the United Kingdom, France, U.S.A. and Federal Republic
è '
of Germany. It is also known that several other countries have already started very extensive studies and research for using prestreesed concrete reactor pressure vessels.
Although the problem of vessel availability is not yet critical for the PWH and BWH systems, it is clear that the potential for continued growth in unit rating will ultimately be limited "by shop-fabricated vessels and that, at that time, either field fabrication of steel vessels or prestreesed concrete vessels will bs required. Of these two alternatives, "based on experience in the United Kingdom end France, and taken into considera- tion, integrated nuclear power station's concepts, prestreesed concrete vessels now appear to offer excellent possibilities.
In view of the growing significance of this type of vessel in relation to the future of nuclear pov/er plants, the Agency- has organized a Panel on this subject with the"purpose of reviewing the latest pertinent deveJ opme'nts, ' to facilitate the exchange of informations and, mainly, to discuss the various aspacts of the design philosophy criteria,'economics and safety of prestressed concrete pressure vessels as '#ell as to formulate general guidelines concerning the -objectives.
It is hoped that this collection of nine papers, together with the conclusions and recommendations which were worked out by the Panel Meeting, will be of interest to reactor designers and to the authorities concerned with the safe working of nuclear pressure vessels. The texts of these papers have been supplied by the Panel members and no editing has "been done by the Agency.
The Agency is grateful to the authors of papers, to all the participants of the Panel for their contributions to the discussions and, it would particularly like to ejtpress its thanks to -the Chairman of the Panel, Mr. I. Bavidson, of the United Kingdom Atomic Energy Authority, for his guidance of the discussions in the most productive manner.
CONTENTS
. > General ;. Analysis for Servies Load Conditions » Limit Design . Ul t i mat e Be s i gn , Subjects of Particular Interest for Further Research or
Dcvelopmewt Study
". . Structural Design Philosophy and Criteria for Concrete Heaetor Vessels ~ U.S. Practice 17 W. Rcckenhauser Contribution from the United Kingdom 53 I. Davidson
. > Work on Reactor Pressure Vessels of Prestressed Concrete in Yugoslavia (>1 B. Petrovic Problems and Perspectives of Prestressed Concrete Pressui'e Vessels - Franch Experiance up to 1970 o9 D. Costes
;3, Teat Stand for the Prestressed Concrete Vessels Containing the Keliusi Loop 85 L.K. Komoli
•J. Ultimate Design, Experience fronn Small Dimension Models Testing 99 P. Scotto
V. Report on the Starting of a Coordinated Work Programme for Prostresssecl Concrete Reactor Preesxire Vessels in the Federal Republic of Germany 115 T , Jae ger
•'•', Design Philosophy and Criteria of Safety of Prestressed Concrete Pressure Vessels ~ Practice in Czechoslovakia 179 M. IDavid Swedish Development Work on Prestreased Concrete Pressure Vessels for Water Reactors 213 S.
J-''22 _»i* Fro^r'i:ftne 243 'L3I;2S~JZ> List of P":.ot:i cip'inhs £47
Sumaaary Report ^and Recoamendatiojgg of. the Panel Meeting on/"Basic.Structural fiesiign PhirlQsophy_ Criteria and Safety of Concrete Heaotor Pressure _Vgssels"
I. Genejral
1.1. Following the discussions of tra Panel which met in Vienna from 9 to 13 February 1970 the present, report was prepared' with a view to setting out the general requirements and principles which appear to be applicable in this field. Attention is' drawn to the advantage of farther work in certain areas, and recommendations are made regarding further international collaboration. Because this is a rapidly developing field of study,, the present report must "be regarded as provisional.
1.2. Reactor vessels perfora the function of containing the nuolear reactor, the primary coolant, which is normally under pressure, and various othsr cbsponants and equipment essential to the operation of the reactor. The vessel must perform this duty for
* its design life under all noriaal and foreseeable abnormal conditions of operation, with a degree of reliability such as to preclude any -unacceptable risk to the public.
1.3. The vessel is loaded by the primary coolant pressure and by the effect of temperature-induced strains in the various structural components. The basic principle of a prestressed concrets vessel is that, for a range of predetermined loads, including normal operation, the concrete is maintained by the tendons in a state of net compression across any section of the vessel, The admissible state of stress and strain in the concrete may be influenced by passive reinforcement.
1.4. The prestressed concrete structure may be furnished with features such as penetrations and associated closures, an impermeable liner, insulation, a cooling system, passive reinforcement and means for pressure relief.
1.5» This application of prestressed concrete is novel in many respects} consequently, existing standards are of limited application, Progress is, however, being made in the preparation of national and international standards for PCRPVa.
1.6. Early development in this field was associated with gas-cooled reactors (OCRs). The potential, of the concspt f c .- other types, such as liquid cooled and/or moderated reactors, is now appreciated. It is not expected that the basic principles of PCRPV design will be influenced by the type of reactor contained. Variations in engineering applications must be expected.
1,7» Sfce vessel must be capable of performing its function under certain predetermined conditions of interaction between the contained reactor and the vessel. It follows that, not only must the vessel provide the necessary standard or' containment under predetermined reactor conditions, but there. must also be an acceptably low probability that vessel behaviour will of itself induce an unsafe state in the reactor.
1.8. One incentive for adopting PCRPVs is the economic advantage to be expected thereby. This advantage arises, for example, from: (a) the ability to contain large reactors or reactor systems
with the acceptance of high pressures? (b) simplification of a plant layout} (c) the fact that a highly developed steel fabrication industry
is not necessary,
1.9» Attractive features from the safet,y point of view include: (a) physical isolation of the stseJ prestressing tendons and
reinforcement from sources of heat and radiation and from the primary coolant;
(b) the high degree of redundancy in the preetressing system;
(c) the possibility in many cases that tendon loads may ba measured and reset;
(d) the possibility of removing tendons during operation for inspection or replacement;
(e) inherent ability to withstand seismic shock.
1.10. 1-1 some cases the conventional practice of grouting tendons is adopted in order to provide sotae protection against corrosion and an alternative anchorage,, These .advantages must be set against the inability to test, inspect and replace individual tendons.
1.11.. Where a penetration as required bo provide access for services, equipment etc , it is customary to provide a purpose-built removable closure. The design objective in such cases is that the standard of integrity of the closure and its attachment should be at least as good as that of the main structure? the presence of a penetration should not prejudice the necessary integrity of the main structure,
1.12. Zt is necessary to provide some raeans of limiting ths effect on the structure of an excessive rise in internal operating pressure. The aiagnitude and rate of a postulated pressure rise can be determined only by reference to the characteristics of the particular reactor contained. It is common practice to provide automatic venting devices, such as safety valves, for this purpose. An alternative is to design the vessel so that it is self-venting by partial structura] faiïure. It is generally recognized that this alternative cannot be relied upon at present*
1.13. It is customary and advantageous to make provision for verification of .the state of the vessel. This may be done fey installed instrumenta- tion and/or by periodic in-service inspection» Such measures, which are taken in a manner appropriate to the particular situation, serve to verify the vessel integrity ana to confirm the design criteria.
1.14. Design philosophy is based on the recognition of two or more phases in -vessel response to increasing pressure. Wie design objective is to ensure that a particular response to the imposed load can b© achieved in each pbase and that this behaviour is consistent with the appropriate operational and predetermined fault conditions.
1.15. Over a range of pressure and temperature, including- normal operating conditions, the vessel will respond to short-term variations in pressure in an elastic nurimîer, This facilitates machine analysis of stress and strain .in the structure, Longe r-ter-m stress and strain are affected by shrinkage and creep of the concrete, re- laxation of tendons and possibly fatigue, ïn -fois range of response the affects of short- and long-ierifi behaviour can be combined to demonstrate that stresses and strains are limited to acceptable values.
1.16. Beyond the elastic range the response becomes increasingly inelastic and non-linear. Tho vessel would not be expected to enter this phase except under the most severe overpressure fault conditions. In this phase tho structure is stable but may experience permanent damage. It is in this phase of vessel response that some limit states occur,
1.17. The ultimate load condition, in which the vessel is incapable of sustaining- any further Increase of internal pressure, is a further limit state, Evaluation of tM ultimate load provides a measure of the factor of safety above design conditions.
1.18» The methods of design verification which are referred to in paragraphs 1.14 to 1.17 are described more fully below* it should be noted that it is the usual practice to use two different methods of design verification simultaneously.
II. Analyses foi*, Service Load_ Conditio.ns_ 2.1.Loading conditions
Stresses, strains and deflections in the vessel structure îîhould be analysed for all relevant combinations of mechanical and thermal loads which can arise under normal service conditions throughout its life.
The tsndon forces adopted in each analysis should include allowances for the most severe effects of friction and loss of prestress. Account should, be taken of all significant loadings applied to the structure, including stresses arising as a result of construction procedures and normal operating transients. Any significant effects of penetrations and -liner on the vessel structure should be taker, into considération,
2.2.J Analysis
The analysis method selected for each loading condition should take appropriate account of time and temperature dependent characteristics of the concrete and bsve clue regard for the complexity of the design and loading conditions and the accuracy required.
Each analysis should provide adequate details of the stresses induced in the concrete, in the passive reinforcement and strains in the liner to enable the acceptability of the design to be assessed.
For the purpose of the analyses covering the prestressing forces and dead loads at completion of construction, arid under test pressure, the concrete may be assumed to be a linear elastic material»
For all other service load conditions, including start-up and shut-dowi both initially and at the end of the vessel life, the stress-strain characteristics used for the concrete should take account of the age, temperature and time under load.
2_,j_._ Minimum design près tress Notwithstanding the acceptability of stresses in the concrete under normal service load conditions a net cornpressive force should be maintained across any section of the vessel under a pressure which exceeds the maximum normal service load pressure by a suitable margin.
2.4» Tendon anchorage zone design Concrete supporting tendon anchorages should be reinforced in accordance with applicable codes v^here existing, 'The safety of the vessel structure is specially dependent upon the integrity of the prestressing tendon system. Suitable tests should be carried out on representative tendons and anchorages in combination under support conditions representative of those obtained in the pressure vessel.
2.3. Cracking It is considered that lire: ted cracking may be accepted provided due regard is paid to any significant redistribution of the stresses which may arise and the integrity and leak tightness of the liner are not impaired.
2.6. Concrete gtrèse concentra.tiens Where local concentrations of stress occur, due to the presence of embedments or other discontinuities in the vessel geometry, these should be assessed individually. Where such stresses are
very local and can be shown to "be self-limiting, they may be disregarded» Where they are more extensive, due regard should be paid to the effects of increased creep rates or tensiJe cracking on the distribution of stresses in the vessel concrete and the influence which they taay have on the strains in the vessel liner and the stress distribution arising in the vessel under conditions of shut-down.
Comment on gialti-a.xj.a.j jîompresslye stresses in concrete
2.7. A certain amount of evidence is available to demonstrate that for short-term loading, under multi-axial corepressive stresses, concrete can safely withstand higher compressave stresses than are generally accepted under uniazial loading. It is generally accepted, however, that if one of the stresses approaches zero (or becomes marginal I?/ tensile) the effect can be nullified or reversed. There is a lack of knowledge regarding long-term loading unuer multi-axial compressive stresses.
2.8. In view of the above, it is recommended that for tne time being, if any advantage is to be taken of multi-axial compressive stress conditions? - The loading condition must be short-term only.
The minimum stress value irust ne suown to oe sulstantially compressive beyond all reasonable doubt. Careful attention must be paid to the effects of any increased strain on the stress distribution un<5er subsequent reduced or reversed load conditions.
Ill,r Limit Design 3.1. Five limit states are recognized for the present:
- Limit of instantaneous linear elastic response. Defines the upper end of the range through which the overall response of the vessel to short-term loads remains essentially linear and reversible. Minor, localized clacking of the concrete may occur.
- Limit of instantaneous, reversible overall structural response. Similar to item 1. Defines the upper end of the range over which the vessel response remains reversible although no longer linear.
- Limit of permissible deformation (short-term and/or long-term), Represents the largest oeformation under wbjch the contained reactor system wil] still function properly. The limit usually applies to penetrations und other such parts of the vease"1 where relatively close tolerances must "be preserved,
- Limit of" liner defect stability. Defines the upper end of the deflection regime in which liner integrity can be reasonably assured. For liner deformations beyond this range, such defects as may be present in the liner, may propagate through the wall. Siraila^jy, highly stressed areas may lead to local liner faj'iure. This could result in substantial leakage into the vessel concrete, and consequently, to crack pressurisation,
- Ultimate strengtn limit. , As defjne in more detail in Part IV "Ultimate Design", tftis represents the ultimate load carrying capability of the vssaeT struct ire. It is evaluated wjtnout regard to the credibility or manner in which this load might come about.
3*2. The limit of linear elastic response and the limit of permissible permanent deformations are considered significant in terras of evaluating normal and upset loading conditions. An upset condition is defined as a transient operating state resulting from a single operator or equipment malfunction. Similarly, the limit of reversibility is related to the concept of (potentially repetitive) emergency loading conditions» For reactor systems where major leakage from the vessel con- stitutes the design basis accident, that is, where liner failure represents the faulted condition, the lirait of liner defect stability 'becomes the significant consideration. Thus, the reactor systems design and vessel design 'interact directly in the assessment of limit states three and four.
3.3. The designer, in evaluating the various limit states of the •vessel should provide suitable margins of safety with respect to the loading conditions and their probability of occurrence, against which the limits are assessed.
3.4. At present, quasi-elastic and visso-elasti.c analysis methods, as well as structural model tests are used to estimate these limits. Because of the inherent uncertain ties, considerable design conservatism results, much of which may be unnecessary.
3.5» Finite element techniques are under development to predict vessel behaviour in the aneiastic range. The methods consider cracking of the structure, but so far only on an axisymuzetrical basis, Chess finite element techniques should be expanded to three dimensions. In addition, meaningful failure criteria for concrete under multiaxial loading, and defect stability criteria for liner ftaiberiais, should be formulated. Furthermore, additional materials characterisations work, should be performed for concrete in the elevated-temperature visco-elastic regime.
IV. Ultimate Design
,3 . Outline
The ultimate design is a. simple method of ensuring that there is an adequate margin of safety between the design pressure and the pressure at which the vessel will fail. In calculating the failure pressure of the Tr3S^el it is assumed that the liner will remain intact up to the failure pressure and that the over- load pressure is applied tu a new and cold vessel. The safety factor chosen should allow for the degree of predictability and the gradualness of the failure mode and should he adequate to cover any possible time-dependent deterioration of the vessel materials.
4+J?*, _ Re 1 e van oe
•fbte ultimate load calculations are not in^nded to represent a realistic condition. The calculations do not take account of possible gross liner rupture at a pressure somewhat lower than the vessel failure pressure wftich could, result in premature vessel failure due to pressure acting on the cracked concrete. Keither does the approach consider a possible weakening of the vessel due to time dependent material degradation. The approach doea not calculate the precise margin available against a realistic condition. It merely gives an indication of the margin available against a hypothetical over-preesuriaation under the oondit ': ons assumed aho ve .
4.3» Method of Gal cu] ation
The method of calculation depends on a knowledge of the mechanisms of failure of the vessel. This information is normally obtained from appropriate model tests to destruction.
It is essential that the mechanism of failure is fully understood and is shown to be progressive.
10
IK the case of flft>ru.ra'i fa i lure , it- is posai r/e to calculate,
with smf f~ cie>i t accuracy, the r>ressure at v,*hlcV tho vesseJ will finally f a l ? ,
If a shear modo of fa i lure ayr l iea • tr» a s 'aY; , the xredic4 '!.bâ !t i ty oT this may be sufTioier.tly u r -ne r tu i r j to require a higher load f .actor.
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