Ex‐core transition to thorium cycle in a small modular heavy‐water moderated molten salt reactor with unchanged concentration of heavy metal nuclides in the fuel salt.

Summary: Ex‐core transition to thorium fuel cycle is an effective way to address the issue of the lacking of the 233U in nature. It consists to extract online the thorium bred 233Pa from the core and store it outside. The obtained 233Pa stockpile will decay into the 233U (T1/2 = 27 days), and when enough 233U is cumulated, a new 233U‐based reactor can be loaded. However, this approach will inevitably provoke the heavy nuclides (HN) concentration in the core during the extraction operation, which would affect the chemical stability of fuel salt and subsequently endanger the safety of the core operation. In this study, an improved ex‐core transition with unchanged HN concentration is proposed for a 500‐MWth small modular heavy‐water moderated molten salt reactor (SM‐HWMSR). To achieve this, two operating stages are needed. In the first stage, transuranic (TRU) from the spent fuel of LWRs is used to feed the low enrichment uranium (LEU, 19.75 wt% 235U/U) started core with 300 operating days to ensure negative temperature reactivity coefficient (TRC). In the second stage, the thorium bred 233U from the first stage is mixed with TRU to refuel the core. The proposed ex‐core transition scheme aims to burn TRU, meanwhile breed 233U to start new MSRs. The fuel cycle performance over 60‐years' operation is analyzed. The results demonstrate that it is possible to realize ex‐core transition with unchanged concentration of HN in the fuel salt. As the fraction of 233U in the mixed fuel is set to be 15 mol%, it only takes 3 years to start a new reactor with the stored 233U. The radiotoxicity of TRU‐based fuel is decreased after 60 years' burning in the SM‐HWMSR core. Novelty Statement: "Ex‐core transition to thorium cycle in an SM‐HWMSR with unchanged concentration of heavy metal nuclides in the fuel salt" was investigated by Yapeng Zhang, Jianhui Wu, Jingen Chen, and Xiangzhou Cai. The results demonstrate that it is possible to realize ex‐core transition with unchanged concentration of HN in the fuel salt. As the fraction of 233U in the mixed fuel is set to be 15 mol%, it only takes 3 years to start a new reactor by the stored 233U. [ABSTRACT FROM AUTHOR]