Measurements of Reaction Rates in Zone-Type Cores of Fast Critical Assembly Simulating High Conversion Light Water Reactor

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1 Journal of Nuclear Science and Technology ISSN: (Print) (Online) Journal homepage: Measurements of Reaction Rates in Zone-Type Cores of Fast Critical Assembly Simulating High Conversion Light Water Reactor Makoto ōbu, Tatsuo NEMOTO, Susumu IIJIMA, Takeshi SAKURAI & Yoshihisa TAHARA To cite this article: Makoto ōbu, Tatsuo NEMOTO, Susumu IIJIMA, Takeshi SAKURAI & Yoshihisa TAHARA (1989) Measurements of Reaction Rates in Zone-Type Cores of Fast Critical Assembly Simulating High Conversion Light Water Reactor, Journal of Nuclear Science and Technology, 26:11, , DOI: / To link to this article: Published online: 15 Mar Submit your article to this journal Article views: 65 Citing articles: 10 View citing articles Full Terms & Conditions of access and use can be found at

2 Journal of NucLEAR SciENCE and TECHNOLOGY, 26(11], pp (November 1989). 993 Measurements of Reaction Rates in Zone-Type Cores of Fast Critical Assembly Simulating High Conversion Light Water Reactor Makoto OBU, Tatsuo NEMOTO, Susumu IIJIMA, Takeshi SAKURAI and Yoshihisa T AHARN japan Atomic Energy Research Institute* Received November 10, 1988 Measurements of reaction rates have been performed in three uranium-fueled zone-type cores of the FCA constructed for a series of experiments on a high conversion light water reactor (HCLWR). These cores possess central test zones of different fuel enrichments and moderator to fuel volume ratios. Radial and axial fission rates of U, 239 Pu, 238 U and 237 Np were measured in each test zone by means of the micro-fission counter traverse. A region where the fundamental mode spectrum is established in the test zone were determined by utilizing these fission rate distributions. Central reaction rate ratios relative to the U fission rate were obtained from the measurements by the micro-fission counters and metallic uranium foils to examine changes in the reaction rate ratios among the three cores. The measured data were analyzed by the SRAC code system on the basis of the nuclear data file JENDL-2. The calculated fission rate distributions agree well with the experimental results for the all cases. The results of reaction rate ratios show that the calculations overpredict the experimental values of the 238 U capturej2 35 U fission and 238 U fissionf 35 U fission rate ratios in the three cores. KEYWORDS: high conversion light water reactor, zone-type core, neutron spectra, fundamental mode, reaction rate ratio, reaction rate distribution, uranium, uranium 238, fission rate ratio, Hi U atomic ratio, FCA reactor, SRAC code, JENDL- 2, C!E ratio accuracy I. INTRODUCTION A concept of a high conversion light water reactor (HCLWR) aims at the effective fuel utilization by increasing a conversion ratio of fuel. The HCL WR core is designed with a low moderator to fuel volume ratio and a relatively high fuel enrichment to attain the high conversion ratio. The neutron energy spectrum in this core is harder than that of a conventional thermal reactor. Integral reactor physics experiments have been made to obtain several physics parameters of the HCL WR and to examine the accuracy of neutronics calculations<lj-< J. In these experiments, the reaction rate ratio is an important parameter for interpreting a distinct feature of the neutron energy spectrum in the HCL WR core. A series of experiments on the HCL WR has been performed at the Fast Critical Assembly (FCA) in japan Atomic Energy Research Institute (JAERI). The Phase-1 experiment was carried out to study the basic nuclear characteristics of the HCL WR core by using enriched U fuel<'l<'l. The FCA-HCLWR core is a coupled system composed of a central test zone simulating the HCL WR core and a surrounding annular driver zone. Three kinds of zone-type cores with different fuel enrich- * Tokai-mura, lbaraki-ken t Present address: Visiting scientist from Mitsubishi Atomic Power Ind. Inc., Shibakoen, Minatoku, Tokyo

3 994 ]. Nucl. Sci. Techno/., ments and moderator to fuel volume ratios were constructed in the Phase-1 experiment. Reaction rate ratios of 239 Pu and 238 U fissions relative to the 286 U fission were measured with micro-fission counters at the center of 286 each test zone. The mu capture to U fission rate ratio was measured by U foils, as a parameter related to the conversion ratio of fuel. The objectives of these measurements are to examine changes in the neutron spectrum among the three cores by observing reaction rate ratios and to evaluate the accuracy of the neutronics calculation employed for the investigation of the HCL WR core characteristics. In the foil measurement, a neutron self-shielding effect of the 238 U capture reaction was corrected experimentally for the thick fuel plate in the cell. Radial and axial fission rate distributions of U, 239 Pu, 238 U and 237 Np were measured by the micro-fission counter traverse in each core to check the effect of the driver region on the neutron spectrum in the test zone. The formation of a fundamental mode neutron spectrum in the part of test zone was confirmed by using the measured fission rate distributions. The measured data were analyzed by the SRAC code system (JAERI thermal reactor analysis code)' 7 > on the basis of the nuclear data file JENDL-2' 8 >. In the present paper, the experimental procedures and the measured results of reaction rates and reaction rate ratios are described. Comparisons between the calculated and experimental results, moreover, are discussed for the three cores. ll. DESCRIPTION OF FCA-HCLWR CORES Three FCA-HCLWR cores XIV-1, XIV-1 (45V) and XIV-2 were constructed in the Phase-1 experiment. The test zone which simulates the neutron spectrum in the HCL WR core is radially surrounded by a buffer zone of stainless steel, driver zones fueled with enriched U and Pu, and a blanket zone of depleted U metal. The cross-sectional view of the FCA XIV -1 core is shown in Fig. 1. Dimensions of the -16- test zone are about 50x50 em in rectangular base and 90 em in height. For the FCA XIV-1 (45V) core, dimensions of the test zone are identical with those of the FCA XIV -1 core, whereas the FCA XIV-2 test zone (about 50x 50x70 em) is smaller than those of the other two cores. II : Safety/control rod : Enriched uranium driver : Plutonium driver Fig. 1 Cross-sectional view of FCA XIV-1 core The plate configuration in each test zone cell of the FCA XIV -1 and XIV -2 cores is shown in Fig. 2. The FCA XIV-1 core was constructed as a reference core in the Phase-1 experiment. Polystyrene plates are used in the test zone cell to simulate H 2 0 moderator. The plate configuration of the FCA XIV-1(45V) core is identical with that of the FCA XIV-1, except for the use of foamy polystyrene plates (45% void) instead of normal ones (0% void). Namely, the test zone cell in the FCA XIV-1 (45V) core simulates a moderator voidage state of 45%. For the FCA XIV-2, normal polystyrene plates are used, but the number of the enriched U and polystyrene plates differs from the above two cores. Main cell parameters in the test zone of FCA-HCLWR cores are given in Table 1. Specifications of the material plates and the nuclide number densities in the cell are reported in Ref. (6).

4 Vol. 26, No. 11 (Nov. 1989) Bmm Calculated neutron energy spectra in test zone cells of the FCA XIV-1, XIV-1(45V) and XIV-2 cores are shown in Fig. 3. It is seen that the neutron spectrum changes from hard to soft with increasing the H/U atomic ratio. FCA XIV-1 cell 1 20 % enriched U metal ( t = rrrn l Atz03 8 ( t =1.5875mm l n Polystyrene t= 3.175rrrn l U ( FCA XIV-2 cell ( t : thickness of plate l Fig. 2 Plate configuration in each test zone cell of FCA XIV-1 and XIV-2 cores c: "' :u Cl. N c e.. :; z trf 10 1 Energy Core nome FCA XIV-1145Vl ---- FCA XIV-I - FCA XIV-2 I ev I Fig. 3 Calculated neutron energy spectra in FCA XIV-1, XIV-1(45V) and XIV-2 cores Table 1 Main cell parameters in test zones of FCA-HCLWR cores Cell parameter FCA XIV-1(45V) FCA XIV-1 FCA XIV-2 Polystyrene state 45% void 0% void 0% void Fuel enrichment (% 285 U/U) Moderator /Fuel volume ratio 1.0 H/U atomic ratio m. EXPERIMENTAL AND ANALYTICAL METHOD 1. Fission Rate Distributions Radial and axial fission rate distributions of 236 U, 289 Pu, 288 U and 237 Np were measured in test zones of the FCA XIV-1 and XIV-1(45V) cores. Micro-fission counters used for the measurements were of 6 mm in outer diameter and of 32 mm in effective length with a 0.5 mm thick Al wall. An experimental hole of 20 mm in diameter was provided through the entire core by arranging plates of the fuel and the structural materials with holes. The micro-fission counter mounted in a long Al guide tube was inserted into the experimental hole and was moved by a counter drive mechanism. The counter traverse measurements were made over the test zone of the core. Fission pulses from the counter were counted by an electronic system coupled with a micro-computer. The measurements were made at ,10 W reactor power level by taking into account of the counter sensitivity. During the measurement, a minute fluctuation of the power level was monitored by two medium-size fission counters installed in the blanket region. 2. Reaction Rate Ratios Reaction rate ratios to the 23 'U fission rate were measured at the center of each test zone in the three cores. The ratios of the 239 Pu and 238 U fission reactions to the U fission were measured by the micro-fission counters -17-

5 996 ]. Nucl. Sci. Technol., described above, while that of the 238 U capture to the U fission was measured by thin metallic foils of depleted and enriched U. The micro-fission counter, which contains a calibrated number of effective atoms, was located radially at the center cell by adjusting its position in the experimental hole. Pulses from the counter were accumulated for a certain live time to achieve sufficient counts in a conventional multi-channel analyzer. A correction of "extrapolation-to-zero" < 9 l was applied to the measured pulse height distribution for separating fission origin counts from the a-background. Fission counting events due to isotopic impurities contained in the counter material were corrected to deduce the fission rate of the objective isotope. The later correction, in particular, is necessary for a threshold fission detector, since the magnitude of correction becomes significant in a soft neutron spectrum. The depleted U foils containing 0.04% of U were used for the 238 U capture rate measurement. The foils have a disk shape of 12.7 mm in diameter, mm in thickness and 290 mg in nominal weight. The 93% enriched U foils for the U fission rate measurement have the same diameter as the depleted ones, whereas the foil thickness is mm and the nominal weight is 58 mg. The depleted and enriched U foils of the disk shape were mounted on the depleted uo2 plate at the center cell for the irradiation, as shown in Fig. 4(a). The enriched U foils were placed at the intermediate position and the both surfaces of the depleted uo2 plate to estimate the plate averaged U fission rate. To measure the 238 U capture rate, a neutron self-shielding effect inside the depleted U0 2 plate should be taken into account. This effect is caused by the 238 U resonance absorption in the depleted uo2 plate, and consequently, the neutron flux depression occurs inside the plate. To correct this effect, an in-plate measurement was additionally made using rectangular-shaped foils (plate averaging foils) having about 1.57 mm by 12.7 mm in rectangular size and mm in thickness. Four rectangular-shaped foils were successively arranged on a cut end of the half-size depleted uo2 plate and the other half-size one was put on it, as shown in Fig. 4(b). Center line Enriched uranium foil Depleted uranium foil (a) Disk-shaped foil Depleted uo. plate 6.35 mm Depleted uranium foils (b) Plate averaging foil Fig. 4(a),(b) Foil location in depleted U0 2 plate in test zone cell Foil activities were determined by using a r-ray spectroscopy system including coaxial type pure Ge-detector (43 mm dia. x 38 mm length). The fission rate of U was measured by counting r activities from fission products; 143 Ce (293.2 kev), (529.8 kev) and 97 Zr (743.4 kev). On the other hand, the capture rate of 238 U was measured by counting the r activity from 239 Np (277.6 kev). The Ge-detector efficiency for each activity was calibrated<'oj<lll by the thermal neutron irradiation of the enriched and depleted U foils at the standard thermal neutron field in the heavy water facility of the Kyoto University Reactor (KUR)*. For the in-plate measurement, the 238 U capture rates corresponding to four regions in the plate were measured. The plate averaging factor to convert the measured 238 U capture rate at the intermediate position into the plate averaged 238 U capture rate was derived by averaging the four measured values. To comtirm a reliability of the measured results, the cell averaging factor was calculated by the continuous energy Monte Carlo code VIM(l 2 ) using the JENDL-2 cross section library. A one-dimensional infinite slab model was applied to the calculation (see APPENDIX 1). * This work was carried out under the Visiting Researcher's Program of the Research Reactor Institute, Kyoto University. -18-

6 Vol. 26, No. 11 (Nov. 1989) 997 The measured region-wise 238 U capture rate distribution is shown in Fig. 5, together with the result of the calculation. The calculated value of the plate averaging factor is somewhat higher than the measured one. A further study would be necessary to clarify the reason of the difference between the measured and calculated values "' :::> u ::::::> :8 N FCA XIV-I (45Vl Plate averaging factor Measured : 1.05 tt.t% VIM : :!:1.3% L - I. VIM. fcl.,.,.. j Measured r _j Cole..s I Depleted I.5 I I I (Region) Fig. 5 Region-wise capture rate distribution in depleted 00 2 plate On the contrary, the U fission rate distribution was approximately flat inside the depleted U0 2 plate, and therefore, the plate averaged U fission rate was estimated from the average value of the three enriched U foils shown in Fig. 4(a). 3. Analytical Method Reaction rate ratios and fission rate distributions were analyzed by the SRAC code system. Cell calculations were executed by a one-dimensional infinite slab model using the group constant set of 86-energy group cross sections which was generated from the JENDL-2 cross section library for the SRAC system. Resonance cross sections were generated with the table-look-up method based on the narrow resonance approximation. The reaction rate ratios were obtained through cell calculations by assuming that the fundamental mode spectrum was established in the central part of the test zone where the measurements were made. Fission rate distributions were calculated by a three-dimensional diffusion model with 10-energy group cross sections which were generated by collapsing the 86-energy group cross sections. Transport correction factors, which were obtained through a comparison with the result of a one-dimensional Sn calculation code ANISN (see APPENDIX 2), were applied to these distributions. W. RESULTS AND DISCUSSION 1. Fission Rate Distributions Radial and axial fission rate distributions measured in the test zone of FCA XIV -1 are shown in Figs. 6(a), (b) and 7, together with the results of SRAC calculations. The radial distributions are normalized to unity at the core center, while the axial distributions at the position of 7.62 em apart from the center in consideration of the existence of stainless steel at the midplane of the FCA-HCL WR core, which may cause a distortion in the fission distribution (there exist 0.8 mm thick 1.0 FCA XIV FCAXIV-1 "'0.8 0 c:: IJ o Exp. 231J x Exp. 23 -Cole. 23 \J f--test zone '---J._J..--L..---'-:--'--'-.I...J---'--'--'-"' -:A> Dis lance from core center (em l (a) Fig. 6(a), (b) 0 and 239 Pu fissions 08 c:: 06 0 c;:: o 4 o Exp. U A Exp. 238 U 0 2 Calc Test zone : '---J.-"'--'::----'-:---'--'-.I...J---'--'---'-"' Distance from core center (em) (b) 0 and 238 U fissions Radial fission rate distributions in test zone of FCA XIV-1 core -19-

7 998 ]. Nucl. Sci. Techno!., stainless steel walls at both surfaces of the FCA half assemblies<' 3 >). In Fig. 6(b), the U fission distribution is plotted again for a comparison with that of 238 U. Experimental errors are within ± 1.5% for the radial distributions, and ±2% for the axial ones, respectively. Agreements between the experimental and the calculated results are very good for the all cases. 08 c: 06 LL. o Exp. U 04 " Exp. 238 U -Cole U Cole. 238 U ---Test zone FCA XIV-I,. 0 0 o Distance from core center (em) Fig. 7 Axial fission rate distributions in test zone of FCA XIV-1 core The measured fission rate distributions were utilized to clarify the fundamental mode region in the test zone. The distributions of various type reactions with different energy sensitivities must have a unique shape in a region where the fundamental mode of neutron flux is established. In the radial direction, the 239 Pu fission rate distribution agrees well with that of U in the test zone. However, comparing the U fission (low energy sensitive reaction) distribution with that of 238 U (high energy sensitive threshold reaction), the agreement is limited in a small range, since the effect of the outer drivers is not negligible outside that region. It is found that the two distributions agree within 2% in the range of ±8 em from the core center. This limit of agreement can be accepted, when the experimental errors for both U ( ± 1.5%) and 238 U ( ± 1.5%) distributions are taken into account. A similar distribution was observed for the 237 Np fission. These results indicate that the fundamental mode in the radial direction is established, at least, in the range ±8 em from the core center. In the axial direction, the fission rate distributions of four type reactions, i.e. the U, 239 Pu, 237 Np and 238 U fissions, agree well with each other in the most part of the test zone. It is found that the fundamental mode in the axial direction is widely established (from 0 to 38 em) in the test zone. In the same manner as the above, the aspect of fission rate distributions was examined for the FCA XN -1( 45V) test zone which had a smaller geometrical buckling. The typical results of fission rate distributions are shown in Figs. 8(a), (b) and 9. The FCA XN-1(45V) core shows flat distributions of the reaction rates in the test zone. It is found that the FCA XIV-1(45V) core possesses the similar fundamental mode region in the radial and axial directions to that of the FCA XN-1 core e c:: 0.6.<:?., 04 FCA XIV-I (45,V) r I \ : : 1 o Exp. u 1 \ 'I -Cole. I x Exp. 23Th ---Calc. 0.2!----Test zone --j 0 0 L...-J._.J..._..J._!_l._J_..J._!_l._J_l_l. J Fig. 8(a), (b) Distance from core center (em) (a) U and 239 Pu fissions e c: 0., LL. 12 FCA XIV- f (45V) l/' ' '). I 10 r_o::=s- a "%.] 08 \, I 1\ 06 I 10 o Exp. mu 04 -Cole. Exp. 23eu Cole. Test zone----! Distance from core center (em) (b) U and 238 U fissions Radial fission rate distributions in test zone of FCA XIV-1 (45V) core -20-

8 Vol. 26, No. 11 (Nov. 1989) Test zone 0. 0 L-L._L_-'--:-.L...L...L_-'-::-'---'-----:":-'--=' Distance from core center (em I Fig. 9 Axial fission rate distributions in test zone of FCA XIV-1(45V) core 2. Reaction Rate Ratios Central reaction rate ratios in the FCA XIV cores are summarize::! in Table 2, where symbols F 2 S, F'", F 28 and C 28 denote the U fission, 23 'Pu fission, 238 U fission and 238 U capture rates, respectively. In the foil measurement, three kinds of U fission rates were obtained independently from the measurements of the r-activities of 143 Ce, and 97 Zr. The F 25 value measured by the foil, then, was obtained as an average value of them. Main errors in the fission counter measurements were introduced through the calibration for the effective number of atoms (±2%) contained in the counter, the counting statistics ( ±2% or less) and the impurity correction. The P 8 value, in particular, includes a large uncertainty in the impurity correction factor for the U fission events, since the natural u counter was used. The errors in the foil measurements were estimated from the calibration error of the Ge-detector efficiency, the counting statistics and the uncertainty in the decay correction of the induced activity. For the 238 U capture, the calibration error was ±0.9%, the counting statistics ±0.7% and the uncertainty in the decay correction ±0.6%. On the other hand, the error for the U fission measurement was larger than that of the 238 U capture measurement. The errors produced by the activity measurement of the single fission product were the calibration error of ± 1.8%, the counting statistics of ±3% and the uncertainty in the decay correction of ± 1.3%, at most. The total error was estimated by taking a mean error of the measurements for 143 Ce, and 97 Zr. Table 2 Central reaction rate ratios and C/E ratios in FCA-HCLWR cores Core Reaction rate ratio Experimental Calculated SRAC C/E FCA XIV-1 (45V) p ; p2s r ±3.0% ps; p2sr ±5. 0% c2s; p2s rr ±2. 2% FCA XIV-1 F 49 / F 2 5t ±3.1% (reference) p2s; p2s r ±6. 0% c2s; p2s rr ±2. 8% FCA XlV-2 F' ; p2s r ±3.1% p2s; p2s r ±9.1% c2s; ps rr ±2. 4% t Measured by a micro fission counter tt Measured by foils located in the depleted U0 2 plate The neutron spectrum in the test zone becomes softer in the order, the FCA XIVl (45V), XIV-1 and XIV-2 cores, depending on the H/U atomic ratio, as mentioned previously. The neutron spectral change is shown evidently in the experimental results tabulated in Table 2. The C 28 j P 5 values increases largely with hardening the neutron spectrum in the three cores. Similarly, the psi P 5 value also increases with hardening the neutron spectrum. On the contrary, the F'" I F 25 value is rather insensitive to the test zone spectrum. The calculated cell-averaged reaction rate ratios are compared with the results of the micro-fission counter measurements. On the -21-

9 1000 ]. Nucl. Sci. Techno!., other hand, the calculated average reaction rate ratios in the depleted uo2 plate are compared with the results of the foil measurements. The calculated to experimental (C/ E) ratios are also given in Table 2. The calculated es 1 ps values are considerably higher than those of the experimental ones, for all of the three cores; the calculations overestimate the C 28 1 F 25 values by 8 to 10%. For the low energy fissions, the calculations of F 49 1 F 2 " values lead to underestimations, especially for the FCA XIV-1(45V) core with a hard neutron spectrum. However, a certain dependency on the neutron spectrum is observed in the Cl E ratios, which indicates that the detailed check of the calculation is necessary. For the high energy range, the experimental F 28 values have relatively large errors, since the fraction of high energy neutrons is very small in the cores. The calculations overestimate the experimental values of ps 1 F 25 by about 10% in the cases of the FCA XIV-1(45V) and XIV-1 cores, and by around 20% in the XIV -2 core. V. CONCLUDING REMARKS Experimental and calculated results are presented and discussed for the reaction rates and reaction rate ratios in the Phase-1 experiment of the zone-type FCA-HCL WR cores. Several conclusions are drawn as follows: (1) It is found from the measurements that the fundamental mode spectrum is established within the region of about 8 em in radius in the test zone and in the most part of the test zone in the axial direction, except for the neighborhood of the axial test zone boundary. (2) Results of the calculated fission rate distributions agree well with the measured ones in the test zone. (3) The reaction rate ratios of C 28 I F 25 and P 8 1 P 5 are sensitive to the HIU atomic ratio in the core. The comparison between the XIV-1 reference core and the XIV-1(45V) core shows that the C 28 l F 25 value increases 40% with decreasing the HIU ratio by 80%. (4) Agreements between the results of SRAC calculations and the experimental values are not very good for the reaction rate ratios. For the parameter C 28 l F 25 closely related to the conversion ratio of fuel, the SRAC calculations overestimate the experimental values by 8 to 10%. ACKNOWLEDGMENT The authors would like to thank Mr. H. Yoshida and Dr. M. Nakano for their encouragement and valuable discussions in the course of this study. Thanks are also due to Prof. I. Kimura (at present, Department of Nuclear Engineering, Kyoto University) and Mr. Keiji Kobayashi of Research Reactor Institute, Kyoto University for their supports on the foil calibration experiments at KUR. -REFERENCES- (1) CHAWLA, R., et al.: Nucl. Techno!., 61, 360 (1984). (2) CHAWLA, R., et at.: ibid., 13, 296 (1986). (3) SEILER, R., et at.: ibid., 80, 311 (1988). (4) CHAWLA, R., BoHME, R.: LWHCR physics experiments and their interpretation, Proc. Mtg. Advances in Reactor Physics and Safety, Saratoga Springs, New York, Sep , (1986). (5) OsuGI, T., et at.: ]AERI-M87-126, 3.6, 57 (1987). (6) OsuGI, T., et al.: ]. Nuct. Sci. Techno!., 26 (5], 477 (1989). (7) TsucHIHASHI, K., et al.: ]AERI 1302, (1986). (8) NAKAGAWA, T., (Ed.): ]AERI-M84-103, (1984). (9) STEVENSON, J.M., BROOMFIELD, A.M.: AEEW- 526, (1967). M BRUMBACH, s. B., MADDISON, D. w.: ANL-82-38, (1982). 1) GMUR, K. : El R-Berichet Nr. 529, (1984). M MIL TON, L.J.: "VIM User's Guide", ANL, (1980). (1$ HIROTA, J.: ]AER/ 1289, (in Japanese), (1984). [APPENDIX] 1. Procedure of VIM Calculation in Depleted U02 Plate The 238 U capture rate distribution in the depleted uo2 plate was calculated by the continuous energy Monte Carlo code VIM using the JENDL-2 data file. A cell model was considered with a one-dimensional slab geometry arranging stainless steel plates at both sides of the test zone cell, where these plates simulated a structural material of a unit matrix tube and a fuel drawer of the FCA -22-

10 Vol. 26, No. 11 (Nov. 1989) 1001 (Refs. (6) and M) surrounding each test zone cell. The unit cell model was devided into 34 regions; 2 stainless steel regions with a width of 0.22 em at both sides of the unit cell and 32 cell regions with an equal width of em. A periodic boundary condition was applied to the cell model by setting the boundary at the outer side of the stainless steel plate. The VIM calculation was carried out with the number of histories of 10 5 The 238 U capture rates were obtained with standard deviations of 0.9'"'-'1.4% in the depleted uo2 regions, 0.6""'1.0% in the other fuel regions and 5.0""' 8.8% in the polystyrene regions. To derive the plate averaging factor, the data of the 238 U capture rates were averaged over the depleted U0 2 plate regions. The standard deviation of the plate averaging factor was estimated to be 1.3% on the basis of the error propagation. 2. Description for Correction of Transport Effect Core models of a one-dimensional slab ge- ometry for x (perpendicular to cell plate) and z (axial) directions were used to calculate the transport correction factors. The correction factors ft(x) and ft(z) for x and z directions are expressed as ft(x)=t r(x)j D 11(x) and ft(z) =T r(z)/ DJJ(Z), respectively, where T r(x) and T r(z) are the reaction rate distributions obtained by the transport calculations, and DJJ(X) and Dff(z) are those by the diffusion calculations for the designate directions. The transport calculations were executed by the one-dimensional transport code ANISN, whereas the diffusion calculations by the multidimensional diffusion code CITATION. In the x-model transport calculation, neutron leakages for y and z directions were evaluated using the measured buckling Bt and B (see Ref. (6)), whereas a buckling search to criticality was adopted in the z-model calculation. The radial and axial fission rate distributions obtained from the three-dimensional diffusion calculation as the base calculation, thus, were corrected by using the ft(x) and the ft(z), respectively. -23-