Coupling Dependence of Multiple Operating Parameters on Burnup Credit Calculations for BWR Spent Fuel Assemblies

This study aims to investigate the coupling dependence resulting from three and four operating parameters for burnup credit calculations in boiling water reactor (BWR) spent fuel assemblies. Four operating parameters are under investigation, including fuel temperature, axial burnup profile, axial moderator density profile and control blade usage. In this study, the effects of variation on the curve of effective multiplication factor (keff) versus burnup (B) resulting from one and multiple operating parameters were defined as the single and compound effects, respectively. Particularly, the compound effects adopt more practical operating parameters than single effects does and thus affect the precise assessment to some extent. In our previous study, the compound effects resulting from two operating parameters were investigated in depth. However, the influence of compound effects resulting from three and four operating parameters on burnup credit calculation is still unknown. Therefore, this constitutes the purpose of this study. All the calculations were performed using SCALE 6.1 computer code together with the ENDF/B-VII 238 energy group neutron data library. Two geometrical models were established to represent the typical GE14 10 × 10 BWR fuel assembly and the GBC-68 storage cask. The results revealed that the reactivity deviation (or changes of keff, Δk) resulting from the compound effects was not a summation of the Δk’s resulting from the associated single effects. Moreover, such Δk discrepancies increase as B increases. In this study, the curves of keff versus B due to single and compound effects were approximated by a second degree polynomial of B. A general formula was thus proposed to express these curves.