Evaluating the benefit of thorium in a two-zone thermal molten salt reactor using open fuel cycle

Thorium is a promising alternative fuel to uranium. Unlike uranium, thorium can attain breeding condition in thermal neutron spectrum. This benefit can be achieved especially by using molten salt reactor (MSR), especially with its online reprocessing capability. In an open fuel cycle, however, this benefit is less clear. This study evaluates the utilisation of thorium in an MSR using open fuel cycle using OpenMC code and ENDF/B-VII.1 library. The evaluated model was based on two-zone, single fluid MSR commonly used in thermal breeder MSR. Four fuel scenarios were simulated, namely LEU-U8, LEU-Th, RGPu-U8, and RGPu-Th, based on different configurations of fissile and fertile materials. Among the analysed parameters are reactor effective full power days (EFPD), conversion ratio (CR), fuel utilisation efficiency, and minor actinide (MA) production. The LEU-U8 configuration exhibits high initial reactivity but a large reactivity swing, which shortens its EFPD. In contrast, LEU-Th demonstrates slower reactivity depletion due to the efficient conversion of ²³²Th to ²³³U. RGPu-based fuels, particularly RGPu-U8, have shorter cycles due to the large neutron absorption by ²⁴⁰Pu. LEU-Th and RGPu-Th attain the highest CR, indicating more efficient use of fertile fuel than their uranium counterparts. Using thorium generates lower MA waste, although the difference is small in RGPu-based fuel. Therefore, in a two-zone MSR using open fuel cycle, thorium exhibits some advantages over uranium both using LEU or RGPu as the fissile driver.