Parametric study of the influence of 135Xe build-up on required excess reactivity for load-following operations

Abstract

An analytical model was used to study the effect of 135Xe build-up after reactor shutdown on reactor reactivity, with implications on possible restrictions for reactor operation in the load-following mode. The parameters of the analytical model were estimated using a 2D Serpent Monte Carlo unit-cell model of a typical pressurised water reactor and ENDF/B-VII.1 nuclear data library. Parametric studies to fuel composition, boron concentration in the moderator, and moderator-to-fuel volume ratio were performed. Mixed-oxide fuel has a smaller impact on reactivity decrease than UO2 fuel due to 135Xe after reactor shutdown and is therefore more suitable for load-following operations. For UO2, higher initial 235U enrichment is beneficial for the same reason. Similarly, higher boron concentration and less moderated neutron spectrum lead to a less pronounced 135Xe effect on reactivity. The methodology is general and may in principle be expanded to other operational parameters, fuel and reactor types.