A Methodology for Determining the Transmutation Efficiency of Minor Actinides

The long-lived minor actinides (MA); americium, neptunium, and curium are main contributors to the long-term radiotoxicity of used fuel. Thus, the transmutation of these MAs is considered as an alternative to direct burial. Until now, no unambiguous internationally recognized quantitative criterion for the effectiveness of MA transmutation has been developed, although this would be highly desirable. The absolute and relative decrease in the total mass of MA is completely inadequate, since they ignore the accumulation of higher radiotoxic MA from the transmuted nuclide. In this paper, we propose a new criterion for the efficiency of MA transmutation in nuclear reactors and demonstrate its efficiency when comparing two molten salt reactors; Single-fluid Double-zone Thorium-based Molten Salt Reactor (SD-TMSR) and Small Molten Salt Fast Reactor (SMSFR). In addition, the proposed criterion takes into account the mass of all useful MA, short-lived MA, and short-lived fission products (FPs). We present a new approach to loading MA in SD-TMSR and SMSFR. The total change in the mass of actinides and FPs during irradiation was calculated using SERPENT-2 Monte Carlo code. The results show that the transmutation efficiency of 241 Am (a major candidate for transmutation) in SD-TMSR is much higher than in SMSFR. After 1500 days of irradiation, the transmutation efficiency reaches 82.6% for SD-TMSR, but for SMSFR it reaches 52.5%.