Optimization of temperature coefficient and breeding ratio for a graphite-moderated molten salt reactor

•The temperature feedback coefficient with different moderation ratios for TMSR in thermal neutron region is optimized.•The breeding ratio and doubling time of a thermal TMSR with three different reprocessing schemes are analyzed.•The smaller hexagon size and larger salt fraction with more negative feedback coefficient can better satisfy the safety demands.•A shorter reprocessing time can achieve a better breeding ratio in a thermal TMSR.•The graphite moderator lifespan is compared with other MSRs and discussed.

Molten salt reactor (MSR) has fascinating features: inherent safety, no fuel fabrication, online fuel reprocessing, etc. However, the graphite moderated MSR may present positive feedback coefficient which has severe implications for the transient behavior during operation. In this paper, the feedback coefficient and the breeding ratio are optimized based on the fuel-to-graphite ratio variation for a thorium based MSR (TMSR). A certain thermal core with negative feedback coefficient and relative high initial breeding ratio is chosen for the reprocessing scheme analysis. The breeding performances for the TMSR under different online fuel reprocessing efficiencies and frequencies are evaluated and compared with other MSR concepts. The results indicate that the thermal TMSR can get a breeding ratio greater than 1.0 with appropriate reprocessing scheme. The low fissile inventory in thermal TMSR leads to a short doubling time and low transuranic (TRU) inventory. The lifetime of graphite used for the TMSR is also discussed.