Heterogeneous Reactivity Effect Analysis of Pu Spots Considering Grain Size Distribution Based on MOC

Abstract

AbstractIn the process of Mixed Oxide (MOX) fuel fabrication, plutonium grains, called plutonium spots (Pu spots), occur in MOX fuel because uranium and plutonium cannot be mixed completely. The previous study showed that the prediction accuracy of criticality is improved by considering the heterogeneity of Pu spots in MOX fuel in the analysis of some critical experiments. The analysis of the heterogeneity has been performed by Monte Carlo calculations, to ensure that core geometry and self-shielding effects are accurately considered in nuclear calculations. However, the results of Monte Carlo calculations are obtained with the statistical errors, thus the small reactivity worth is difficult to be analyzed. Therefore, in the previous study, the heterogeneous model by using a method devised by R. Sanchez was introduced into OpenMOC that is deterministic calculation code. Although there are numerous sizes of Pu spots in MOX fuel, an only single size of the grain could be considered in the modified OpenMOC. Therefore, in this study, the modified OpenMOC code was additionally modified to consider the grain size distribution of Pu spots.The calculation result obtained by the additionally modified OpenMOC shows that the heterogeneous reactivity effect caused by numerous grain sizes of Pu spots is evaluated at −0.190%Δk/kk’ at highly enriched fuels. On the other hand, a heterogeneous reactivity effect by single grain size is evaluated at −0.225%Δk/kk’. The additionally modified OpenMOC makes it possible to study the effect of considering the grain size distribution of Pu spots in MOX fuel.