Balakovo-3 Ex-vessel Exercise: Analysis of Calculation Results Intercomparison and Comparison with Reference Data

Ex-vessel neutron dosimetry at NPP with VVER was used for validation of 3D absolute neutron fluence calculations. As second step of the VVER-1000 Balakovo-3 Ex-vessel Exercise, an analysis of calculational results has been performed. The reference experimental data - End-Of-Irradiation activities AEOI - were used as criteria of validation. The purpose of the research was to perform an intercomparison of independent calculational results and comparison with reference data of the full-scale VVER-1000 ex-vessel experiment. New and improved results of calculations analysed in the paper were run with codes and applied libraries DORT/BUGLE-96, DORT/EURLIB-6, DORT/ABBN-93 (and ABBN-93 modifications), TRAMO/ENDF/B-VI, MCNP/ENDF/B-VI. Calculated pure 3D or 3D synthesis multigroup neutron fluence rates in positions of experimental capsules provided by every calculator were used as basic calculational information. As first step of their comparative analysis a study was done of the used calculation input data prepared on the base of the same reactor information. For example, a difference of neutron source model details may be a reason of some discrepancy of results obtained by similar calculational procedures. Aspects of the influence of different dosimetry cross sections and local power history on the calculation of AEOI from calculated spectra were investigated. The analysed dosimetry cross section libraries were IRDF-90v2, JENDL-D99, RRDF-98 and ENDF/B-VI based BUGLE-96 responses. Final C/E values were based on AEOI. The C/E values for threshold dosimetry reactions 237Np(n,f), 93Nb(n,n'), 238U(n,f), 58Ni(n,p), 54Fe(n,p), 46Ti(n,p) and 63Cu(n,a), which are widely used in reactor pressure vessel dosimetry for fast neutron fluence evaluation, showed on the whole the expected agreement (better 20 %). It was found that the C/E data for the Balakovo-3 exercise in application to the 237Np(n,f) and 93Nb(n,n') reactions may be improved in case of using JENDL-D99 library. Another new aspect of the Balakovo-3 research is an analysis and intercomparison of calculational and experimental results for (n,g) dosimetry reactions. Different type (n,g) detectors were irradiated in cadmium. The obtained reaction rates characterize the distribution of epithermal neutrons. A comparison of the reaction rates, for example, for reactions 93Nb(n,g), 69Co(n,g) and 103Ag(n,g) in case of DORT/BUGLE-96 calculations has shown a coincidence of experiment and calculation data within ± 11 %. For one ex-vessel position where bare (n,g) detectors were irradiated thermal neutron parameters were evaluated and compared.