Safety Aspects of Thorium Fuel in Sodium-Cooled Fast Reactors

While detrimental to the breeding performance, use of thorium (Th) in Fast Reactors (FRs) has certain positive aspects: in principle, it has better thermal properties and irradiation performance than uranium (U)-based fuels, and it increases the potential burning rate of legacy transuranic waste for a burner design. The impact of thorium on the top-level operational and safety characteristics of a sodium-cooled transuranic-burner FR design is the focus of this paper. Different fuel cycle burning schemes are considered, including the development of a breakeven design to cover the long-term transition to a self-sufficient cycle. For completeness, a comparison with the counterpart U-based cores is also given. Reactivity feedback coefficients and other parameters are employed to predict the impact on the reactor passive-safety features of the various fuel cycle options. Reactivity decomposition techniques are employed to show the key contributors to the reactivity effects of the different fuels. The results show significant safety improvements fostered by implementing thorium fuel, with notable reductions in the reactivity insertion in case of core voiding, ~2$ for the burner designs and ~6$ for the breakeven design (which shows overall negative voids), while preserving a relatively simple core configuration. (author)