NEACRP-A-973 Critical Experiments on Initial Loading Core of Very High Temperature Reactor Critical Assembly(VHTRC) Fujiyoshi Akino, Tsuyoshi Yamane, Hideshi Yasuda and Yoshihiko Kaneko Department of Reactor Engineering, Japan Atomic Energy Research Institute. Tokai-mura.Naka-gun,ibaraki-ken,Japan September 12,1989 Abstract A critical assembly VHTRC(Very High Temperature Reactor Critical Assembly) was constructed by modifying the SHE(Semi-Homogeneous Experiment) assembly in order to verify the nuclear design accuracy of HTTR(High Temperature Engineering Test Reactor). The construction was started on May 1983,and the initial criticality was achieved on May 13 ,1985. The initial core named as the VHTRC-1 core was made by loading fuel rods which contained fuel compacts of the coated particles of the 4% enriched uranium. The following measurements were carried out on the VHTRC-1 core: 1) critical mass, 2) reactivity worths of HTTR mockup control rod and burnable poison rod, 3) neutron flux distribution, 4) temperature coefficient of reactivity, 5) kinetic parameter 8,/,/A. Calculations were performed with the SRAC code system using the nuclear data based on the ENDF/B-IV. The agreements between calcula- tions and experiments were fairly good for most experimental items except the value of kinetic parameter. The results obtained satisfy the accuracy requirements for fundamental nuclear design of HTTR. -I- i518000?
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NEACRP-A-973
Critical Experiments on Initial Loading Core of
Very High Temperature Reactor Critical Assembly(VHTRC)
Fujiyoshi Akino, Tsuyoshi Yamane,
Hideshi Yasuda and Yoshihiko Kaneko
Department of Reactor Engineering,
Japan Atomic Energy Research Institute.
Tokai-mura.Naka-gun,ibaraki-ken,Japan
September 12,1989
Abstract
A critical assembly VHTRC(Very High Temperature Reactor Critical
Assembly) was constructed by modifying the SHE(Semi-Homogeneous
Experiment) assembly in order to verify the nuclear design accuracy
of HTTR(High Temperature Engineering Test Reactor). The construction
was started on May 1983,and the initial criticality was achieved on
May 13 ,1985.
The initial core named as the VHTRC-1 core was made by loading fuel
rods which contained fuel compacts of the coated particles of the 4%
enriched uranium. The following measurements were carried out on
the VHTRC-1 core:
1) critical mass,
2) reactivity worths of HTTR mockup control rod and burnable
poison rod,
3) neutron flux distribution,
4) temperature coefficient of reactivity,
5) kinetic parameter 8,/,/A.
Calculations were performed with the SRAC code system using the
nuclear data based on the ENDF/B-IV. The agreements between calcula-
tions and experiments were fairly good for most experimental items
except the value of kinetic parameter. The results obtained satisfy
the accuracy requirements for fundamental nuclear design of HTTR.
-I- i518000?
I. Introduction
Since 1969,research and development for a High Temperature
Gas-Cooled Reactor(HTGR) have been continued at the Japan Atomic
’ Energy Research Institute(JAER1). In 1987, the Japanese Atomic Energy
Commission issued the revision of the Long-Term Plan for development
and utilization of the nuclear energy. The revision plan specifies early
construction of a High-Temperature Engineering Test Reactor(HTTR)”
proposed by JAERI, aiming to achieve criticality in 1995.
A core of HTTR comprises the prismatic pin-in-block type fuel
blocks using the uranium of 6% enrichment on the average. Such a core
structure is different from both of the pebble-bed type reactors, the
Arbeitsgemeinschaft Versuchsreaktor(AVR) and the thorium high tem-
perature reactor(THTR) in the Federal Republic of Germany(FRG) and
the multihole-type reactor, the Fort St.Vrain(FSV) reactor in the
United States.
All these reactors except the HTTR are based on the highly enriched
uranium and thorium fuel cycle. But, since the International Nuclear
Fuel Cycle Evaluation(INFCE) conferences, the low-enriched uranium
rue 1 cycle has been selected for HTGR, for non-proliferation
consideration. Therefore, such a situation enhanced unique progress
in the reactor physics study related to HTTR
Reconstruction program of the SHE(Semi-Homogeneous Experiment)”
to the VHTRC(Very High Temperature Reactor Critical Assembly) was
approved by Japanese government early in 1981. The object of the pro-
gram is to obtain experimental verification for the nuclear design
accuracy of HTTR with low enriched uranium fuel. The construction
was started in May 1983, and the initial criticality was achieved on
May 13,1985.
This paper describes the outline of experimental results on the
initial loading core,VHTRC--1,which is loaded with the coated particles
of 4% enriched uranium. The experimental items are the critical mass,
the reactivity worths of HTTR mockup control rods and burnable poison
rod, the neutron flux distribution, the temperature coefficient of
reactivity and the kinetic parameter. The analyses were performed with
the SRAC code system using the nuclear data based on the ENDFIB-IV data
file.
-2- 1518UoO2
. .
II. Critical assembly
The assembly is a horizontally-placed hexagonal prism in the shape.
made of graphite blocks and can be split into two half assemblies,
one of which is .fixed and the other is movable. The assembly has the
across flat of 2.4m and length of 2.4m. Each graphite block(across
flat:30cm; 1ength:lZOcm) is also the hexagonal prism with 18 holes
for fuel rod insertion and a hole for control or safety rod insertion.
The prismatic blocks are made of isotropic graphite containing very
low impurities of CO.1 ppm equivalent boron content. The whole core
covered with the insulator for heating up to 210°C using the electric
heaters. Outlook of VHTRC3’ is shown in Fig.1.
The driving speed for the movable half is slowed down near the
closing position. The fast-drive speed is 50 cm/min.from 150 cm tb
20 cm separations, and the slow-drive is 1.5 cm/min.from 20 cm
separation to the closed position. The distance between the two halves
is indicated by three position indicators; one is an analogue indicator
and the other two digital indicators. The position of the movable half
at the closed position is found to be settled within about 0.05mm
relative to the fixed one.
Two control and six safety rods driving mechanisms are mounted on
the supporting frames behind the two halves. These control and safety
rods are made of cadmium cylinders covered with stainless steel.
A radium-beryllium neutron source of 5OOmCi is used as a start-up
neutron source and is inserted by the driving mechanism from its coffin
through a guide tube to the radial reflector region in the fixed half.
A fuel rod consists of a stack of 20 fuel compacts packed in a
graphite sheath 732mm long, 5mm thick and 47mm in the outer diameter.
The fuel compact is the hollow cylinder of the BISO-coated particles
(2% and 4% enrichment) or the TRISO-coated particles(6X enrichment)
dispersed in the graphite matrix. The physical constants of the fuel
compact and the’graphite assembly were described in Ref.3.
III. Experiments
1. Critical mass
(1)Initial critical approach at room temperature
The initial core named BS the VHTRC-1 core was made by loading fuel
\ .
rods, each containing 20 fuel compacts of the 4% enriched uranium and
the core had radial and axial reflectors.
The core was loaded with the fuel rods step by step, keeping the
hexagonal symmetry. The neutron multiplication was observed at each
loading step by the source multiplication technique using six neutron
detectors. Four BF3 counters of 8mm diameter were distributed in the
core region. One pair of counters were placed in the fixed half and
the other pair in the movable half and two BF3 counters of 2.54cm
diameter were located outside of the fixed half.
After the step by step loading, the core reached criticality with
282 fuel rods at the ninth step, with all safety rods being fully
withdrawn and control rods being partially inserted. The fuel loading
pattern of this core is shown in Fig.2.
The reactivity worth of control rods was calibrated by the period
method. From the results of calibration, the excess reactivity was
determined to be 28.4*0.6 cents. The reactivity worth of a fuel rod
was measured with the calibrated control rods and found to be
7.11*0.26 cents. Using this fuel rod worth, the core loading of 282
fuel rods was corrected for the excess reactivity and for the reactiv-
ities of inserted materials” in the core, such as the safety and control
rods insertion holes, the gap between the fixed and movable half
assemblies, four in-core BF3 counters, etc., and also the temperature
difference between experiment and calculation. The number of fuel rods
measured at the critical state was obtained to be 269.7+0.6(4%
enriched) which gave the critical mass of 4.520*0.010 kgzz5U at 27’C.
At four loading steps in the initial critical approach, the effective
multipilication factor,k,,,. was measured by the pulsed neutron source
technique(PNS) using the four in-core BF3 counters. The data were
analyzed by means of the simple method based on integral version of
the SjGstrand’s area-method. This method d t e ermines the static reac-
tivity from one-point measurement by using 8 correction factor for
the value of area-ratio observed at the measuring position.”
The target of PNS(KAMAN A-801) was attached to the center of the
back-surface of the fixed half. Four. BF3 counters of 8mm diameter were
distributed in the core region. The data were collected using a mul-
tichannel analyzer(ORTEC 7010/7000-37) with the four independent input
in the multichannel scaling mode. The area.s of prompt and delayed
neutrons were obtained by fitting the decay curves using the ALPHA-D
code6’. This code determines prompt neutron decay constant taking
account of the decay of delayed neutrons.
The measured values of k,,, agree fairly well with those predicted
by a three dimensional diffusion calculation, as shown in Fig.3. This
result confirms that at early loading steps the neutron mulitiplication
would increase as expected.
(2)The heated core at 200°C
After the initial critical approach at room temperature, thk whole
core including the reflector was heated up to ZOO’C by using 30kw
electric heaters located in the radial reflector. The 14 K-type ther-
mocouples were placed in the core for measurement of the temperature
distribution. The average core temperature reached ZOO’C by about 50
hour continuous heating. and the spatial standard deviation converged
within 1.3’C.
At the temperature, about ZOO’C, the number of fuel rods was
increased stepwise to approach the criticality. The 2% and 4% enriched
fuel rods were added to make critical again. Then, the heated core at
200.8”C reached criticality with 453 fuel rods(4% enriched:309, 2%
enriched:144). The fuel loading pattern of this core is shown in Fig.4.
After correcting for the reactivities of a.11 inserted materials ‘)
including the reactivity of the heaters in ~the core. the number of the
fuel rods measured at the critical state was obtained to be
432.3*1.2(4% enriched:288.3*1.2, 2% enriched:144) which gave the
critical xnass of 6.043*0.019kg235U at 200.8”C.
2. Reactivity worth of HTTR mockup control rods
Reactivity worths of the HTTR mockup control rods(CRs) were
measured in the VHTRC-1 core with the pulsed neutron method.
A CR consists of two semi-rods, each having 46 neutron absorber
Fig.2 Critica fuel rod loading pattern of the VHTRC-1 core at 17.7’C
lU8(3015
. .
1.0
0
/I
I/
/ 6 Measured
_ / ---. Calculated
/ I
4 I
I
I I I
I I &--- e-+--lf - *OH- .-- ;T
Critical paint
I I I I
0 100 200 ; L Number ol fuel rods
0 Fuel rod(4iE
0 Fuel rad(Z%EU)
A Safety rod
(Fixed half)
Fig.4 Critical fuel rod loading pattern of the VHTRC-1 core at ZOO’C
)O
(Fixed hail)
Fig.5 Arrangement of the CR in the VHTRC-1 core (1C core pattern)
*(Fixed ha1 I)
Fig.6 Arranaement.ol the CRs in the VHTRC-1 core (ZC core pattern)
~Contrbl rod 0 CR
(Fixed half)
Fig.7 Arrangement of the CRs in the VHTRC-1 core (2R core pattern)
(Fixed half)
Fig.8 Arrangement of the CRs in the VHTRC-1 core (1ZR core pattern)
15180046
o”b”able half) (unit in I) (Fixed half)
The rsspective reaclivity value without or with bracket show that on the 15cm or 40cm distant plane from the midplans between the fixed and movable half.
Fig.9 Dependence of negative reactivity raluer on the detector position in the VHTRC-1 coce, measured by the araa:type pulsed neutron
method.
A Safety rod
l Fuel rod(4!4EU),m ,
n Control rod
_ v
*(Fixed half)
Fig.10 Arrangement of activation samples in the VHTRC-I core at the room Lemperature
151813019
@ Fuel rod( ZMEU)
q Control rod
A .
(Fixed half)
Fig.11 Arrangement of activation samples in the VHTRC-1 core at 200°C
. .
0 I, I I ) I I I,, I,, , I I 1 I , 1
l : Expt:
-I- : c a .I c
noon ,tenperature (Normalized point)
O-
Fig.14 Reactivity change in raising the core temperature