Optimization of loading quantity of minor actinides for elimination of boric acid and burnable absorbers from PWRs

Abstract

The use of burnable absorbers for reactivity control in PWRs comes with added costs, potential residual reactivity and power peaking effects, and demands careful consideration of depletion time and reactivity feedback. Again, boric acid in the coolant further complicates reactor operation by requiring water chemistry adjustments and presenting issues such as corrosion, tritium production (from boron) and a less negative moderator temperature coefficient (MTC). This study investigates the feasibility of using minor actinides (MAs) to eliminate both burnable absorbers and boric acid from PWRs, thereby improving safety and reducing long-term radiotoxicity. Six different models were analyzed – three featuring integral burnable absorbers (IBAs) with MA-based fuels, and three without IBAs. The reference was a VVER-1000 fuel assembly with 1000 ppm boric acid in the coolant. MAs were introduced either as inner or outer coatings on the fuel or as homogeneous mixtures within the fuel matrix. Simulations using OpenMC over a 600 EFPD burnup period showed that all studied cases could suppress excess reactivity equivalently to conventional poisons, though with varying MA requirements. MA addition also led to significant transmutation and improved MTC, albeit with some degradation in fuel cycle performance. Homogeneously loaded cases (3A and 3B) proved to be the most effective in controlling reactivity, achieving higher transmutation rates (TRs), improved reactivity feedback, and reduced penalties in cycle length and discharge burnup. Notably, Case 3B fully compensated for both gadolinia and boric acid, achieved the highest TR of 25.82 %/y for all MAs combined, and exhibited the most negative MTC.