On the potential use of F2Be-molten-salt for hybrid reactors

Breeder reactors are considered a unique tool for fully exploiting natural nuclear resources. In current Light Water Reactors (LWR), only 0.5% of the primary energy contained in the nuclei removed from a mine is converted into useful heat. The rest remains in the depleted uranium or spent fuel. This evident need to improve resource-efficiency has stimulated interest in Fast-reactors, and with it, boosted the need to answer many of the remaining safety issues attached to such systems (i.e. coolant positive void coefficients). Among the existing candidates to overcome this fundamental drawback, the F2Be-molten-salt, has proved to feature outstanding neutronic properties. In previous studies, in an analysis that took into account requirements for criticality, for breeding, and for safety, it was demonstrated that a design window could be found in the definition of an F2Be-cooled system, where the safety requirement was met for a critical breeder reactor. In this paper we give a deeper insight on the neutronic features of F2Be coolant by comparing it with the moderator par excellence; light water. Its potential use on a thorium-fuel based system will be completed by a resilience analysis of the design window along the burn-up cycle. The ultimate goal is to give a sound answer to how a F2Be-cooled system could contribute to improve the resource-efficiency of nuclear systems without overlooking the main safety aspects. This is one of the main pending questions in the quest to sustainability.