Investigating the possible advantages of using different concentrations of transuranic elements with thorium-uranium dioxide as a fuel for PBMR-400

Reactor reactivity control materials play a crucial role in managing the stability and efficiency of nuclear reactors by regulating neutron flux and maintaining the desired reactivity levels throughout the reactor's operational cycle. This study explores the feasibility of using transuranic dioxide (TRUO₂) as reactivity control materials in pebble bed modular reactor 400 (PBMR-400) with thorium-based fuel. The TRU elements (Np, Pu, Am, and Cm) were extracted from spent uranium dioxide (UO₂) with a discharge burnup of 45 MWD/kgHM, following 30 years of cooling. The investigation covered four Th233UO2/TRUO2 mixtures, with ThO2 concentrations ranging from 75% to 95% and TRUO2 from 5% to 25%. This aims to determine the optimal composition that maximizes the TRUO2 concentration and minimizes ThO2 while preserving reactor performance in order to achieve the longest fuel cycle length with lower keff at the beginning of the fuel cycle (BOC) to avoid excess reactivity issues. Comprehensive neutronic analyses were conducted on these fuel mixtures, including burn-up, safety parameters, and flux and power distributions. The findings showed significant improvements in the PBMR-400's neutronic performance with the proposed fuel materials. From a safety, and economic standpoint, the optimal configuration was found to be 85% ThO2 and 15% TRUO2, as it provided the longest fuel cycle length with less excess reactivity at BOC and lower PPF.