Evaluation of Neutronic and Thermophysical Characteristics of Molten Salts Specialized for Long-Lived Fission Products Transmutation in a Fusion Reactor^∗)

This study proposes new molten salts, which are specialized for transmuting long-lived fission products (LLFP) using a helical fusion reactor FFHR-d1, as a neutron source. Molten salts are binary systems consisting of BeF₂ and LLFP fluorides, such as BeF₂–ZrF₂, BeF₂–PdF₄, or BeF₂–CsF. This study evaluates the effect of molten salts on the transmutation performance, and the amount of heat generated was evaluated by Monte Carlo-based neutron transport and burnup calculations. Therefore, when the transmutation area was fixed at 4% of the blanket system volume, the higher molar ratio of LLFP fluorides leads to higher transmutation performance. Also, the viscosity, specific heat, and thermal conductivity of BeF₂–CsF were evaluated by molecular dynamics (MD) simulations. The heat transfer characteristics were evaluated by calculating the Prandtl number from MD simulation results. The Prandtl number of 50% CsF, which produced the lowest melting point (475°C), was evaluated to be 29.23, which was 60% larger compared to the Prandtl number of Flibe (LiF–BeF₂) when the BeF₂ molar ratio was 50%. This result suggested that the heat transfer performance was inferior to that of Flibe.