Chemical Footprints of Thorium and Uranium in Molten LiF–BeF₂ Explored by First-Principles Molecular Dynamics Simulations

The chemical forms and transport properties of thorium and uranium ions in molten LiF-BeF₂ are systemically studied via first-principles molecular dynamics simulations. The densities, diffusion coefficients, densities of electronic states, and ionic pair and cluster structures of molten LiF-BeF₂-ThF₄ (FLiBeTh), LiF-BeF₂-UF₄ (FLiBeU), and LiF-BeF₂-ThF₄-UF₄ (FLiBeThU) are analyzed in detail. Studies have shown that the density and thermal expansion coefficient of molten FLiBeTh are higher than those of FLiBeU in the temperature range of 873-1073 K. Besides, FLiBeTh has higher electron energy, and the active Th electrons contribute to the diversification of its coordination structure and the improvement of diffusion property. Furthermore, the concept of Be-F tetrahedron stress index (SIT) is proposed in molten FLiBe, and the higher SIT of molten FLiBeU is one of the structural factors leading to slow diffusion coefficients of Be and F ions. Overall, the understanding and characterization of fuel salt structure and property are underscored, and the relationships between microstructure and diffusivity performance are preliminarily established.