Influence of Thorium Utilization on the Safety of a Small Modular Molten Salt Reactor

Abstract Molten salt reactors (MSRs) are one type of reactor system that are characterized by their application of liquid fuel and thorium utilization. Up till now, the performance of thorium-uranium fuel cycle has been extensively studied for MSRs, while the influence of thorium utilization on the MSR core safety is rarely investigated. In this study, a systematic research on the impact of thorium utilization on the core safety for a small modular MSR with a thermal power of 500 WM is conducted. The influence of thorium utilization on the decay heat and radioactivity of spent fuel salt is firstly investigated by comparing the performance of cores fueled with low enriched uranium (LEU) and LEU+Th mixed fuel. It reveals that thorium utilization would raise the decay heat and radioactivity of spent fuel salt due to the yield increase of Pa-233, Sr-93, La-142, La-140, La-143 and Cs-141. Then, the influence of thorium utilization on negative temperature feedback and core delayed neutron (DN) fraction is analyzed, and the results demonstrated that thorium addition to the core will weaken the negative temperature coefficient at the beginning of fuel cycle (BOC) and decrease the core DN fraction, but the negative temperature feedback turns to be better than the core fueled with LEU at the end of fuel cycle (EOC). Finally, the influence of thorium utilization on the response of core to six typical transients, including overcooling/overheating of the inlet fuel salt, increase/decrease of the fuel salt flow rate, reactivity insertion, are studied. The results show that thorium utilization would deteriorate the core response to the transient at BOC, while improve the safety of core at EOC. Most importantly, the safety of the core varies less through the fuel cycle than that of the core fueled with LEU, which would benefit for the design of safety control system.