Influence of simultaneous variations in operating parameters on burnup credit criticality analysis for PWR spent fuel assemblies

This study attempted to thoroughly investigate the impact of single and compound effects on burnup credit calculations for PWR spent fuel assemblies originating from various operating parameters. Three operating parameters were considered, including fuel temperature, boron concentration, and axial burnup profile. The single and compound effects defined in this study represent the influence caused by simultaneous variations of one and multiple operating parameters on the curve of effective multiplication factor (k(eff)) versus burnup (B), respectively. All the calculations were performed using SCALE 6.1 computer code together with the ENDF/BVII 238-group data library. In addition, two geometrical models were established based on the Westinghouse 17 x 17 PWR optimized fuel assembly and GBC-32 storage cask. The results revealed that an increase of fuel temperature or boron concentration induces a positive reactivity deviation (or the change of k(eff), Delta k), the opposite is also true when a decrease of fuel temperature or boron concentration is considered. Moreover, the utilization of non-uniform axial burnup profile leads to a positive Delta k. Additionally, in most of the compound effects, Delta k resulting from the compound effects was not a linear summation of the Delta k's resulting from the associated single effects. In particular, such Delta k discrepancy increases as B increases, which could influence the precise assessment of burnup credit to some extent. The mechanisms of both the single and compound effects were studied by the analysis of the spectral distribution of fission. From this analysis, an increase of either fuel temperature or boron concentration leads to a harder neutron spectrum, while a non-uniform axial burnup profile leads to a softer one.