Effects of Sodium Fluoride Additives on Reactivity and Heat Transfer in a Molten Salt Fast Reactor

This study investigates how adding sodium fluoride (NaF) to NaCl–ThCl4–UCl3 molten salts affects both the neutronic and thermal behavior of a Molten Salt Fast Reactor (MSFR). Simulations carried out using the Monte Carlo N-Particle (MCNP6) code provide insight into changes in neutron flux, effective multiplication factor (keff), and key safety-related coefficients such as Doppler and density reactivity feedback. The impact of NaF is also evaluated through an empirical thermal conductivity model to determine how heat transfer within the reactor core is affected. As NaF concentration increases from 0% to 10%, keff decreases slightly (from 1.029 to 1.017), due to small shifts in neutron energy distribution. Simultaneously, both the Doppler and density coefficients become more negative, pointing to improved passive safety. Although the breeding ratio drops from 0.953 to 0.900 with higher NaF levels, this is considered manageable, especially when factoring in online reprocessing. On the thermal side, NaF proves beneficial: it boosts thermal conductivity by roughly 28%, helping to even out temperature gradients and reduce thermal stress within the core. Altogether, these results suggest that NaF can be a valuable additive for balancing safety, efficiency, and sustainability in chloride-based MSFR designs.