Design-Related Studies for the Preliminary Safety Assessment of the Molten Salt Fast Reactor

Molten salt reactors are liquid fuel reactors so that they are flexible in operation, but they are very different from solid fuel reactors in the approach to safety. This study concentrates on the specific concept named Molten Salt Fast Reactor (MSFR). Since this new nuclear technology is in development, safety is an essential point to be considered all along the research and development studies. After a short description of the MSFR systems, necessary to device accidental scenarios, this paper will focus on the decay heat evaluation of such a reactor. Among different contributions, the decay heat of fission products in the MSFR is evaluated to be low (3% of nominal power), mainly due to the reprocessing during the reactor operation. As a result, the contribution of the actinides is significant (0.5% of nominal power). However, the decay heat of the fission products is important, and among the different uncertainty sources, the fission yield uncertainties are pointed out. The unprotected loss of heat sink transients are studied in this paper. It appears that slow transients are favorable (>1 min) to minimize the temperature increase of the fuel salt. This work will be the basis of further safety studies as well as an essential parameter for the design of the draining system.