Assessment of 232Th Nuclear Data through Analysis of Thorium-loaded Critical Experiments in Thermal-Neutron Systems using the Kyoto University Critical Assembly.

Abstract

The main purpose of this study is to make an assessment of 232Th nuclear data through the analysis of thorium- loaded critical experiments in thermal neutron systems using the Kyoto University Critical Assembly (KUCA). The impact of the difference among the nuclear data libraries of232Th was examined through neutronics calculations. Using JENDL-3.2, k eff for thorium-loaded KUCA cores were overestimated by 0.9% to 1.2%. Although approximately 0.7% of the overestimation is due to the 235U cross section used in the driver region, the overestimation for thorium-loaded cores is apparently larger than those for cores free of thorium by about 0.2% to 0.5%. This overestimation depends on the neutron spectrum, and becomes larger for hard-spectrum cores. The use of 232The cross sections from ENDF/B-VI or JEF2.2 also lead to overestimation of k eff. The overall trend of the overestimation by ENDF/B-VI and JEF2.2 is the opposite of that by JENDL-3.2, and becomes smaller in hard-spectrum cores. The reactivity difference caused by different 232Th evaluations reaches up to -0.5% in hard-spectrum cores. Those significant discrepancies in reactivity of thorium-loaded thermal systems, caused by the difference of 232Th cross sections compiled in different nuclear data libraries, are mainly due to the difference in the capture cross section at thermal region, lower resonance and unresolved resonance regions, and also due to the difference in scattering cross section at MeV region. It became clear that none of the current 232Th evaluations can accurately simulate the criticality of the thorium-loaded KUCA experiments. These facts indicate that there remains a certain ambiguity in the evaluated 232Th cross sections, which is desirable to be eliminated for further design studies of thorium-based fuel cycle.