Local Structure and Ionic Diffusion in LiF-BeF₂-ThF₄ Molten Salts: Insights from Ab Initio Molecular Dynamics

Understanding the local structure and ionic diffusion in LiF-BeF₂-ThF₄ (FLiBeTh) is critical for designing fourth-generation nuclear reactors. However, high-temperature experiments with these salts are often associated with radioactivity and toxicity. Computational simulations provide a safe and efficient method for exploring these materials. In this study, ab initio molecular dynamics (AIMD) simulations were used to analyze the 2LiF-BeF₂ system with ThF₄ additions ranging from 0 to 18.18 mol % at 973 K. We calculated the radial distribution function (RDF), angular distribution function (ADF), coordination number (CN), mean square displacement (MSD), and anion residence ratio r(t) to investigate the impact of ThF₄ concentration on the local structure and ionic diffusion. We found that increasing concentrations of ThF₄ have little impact on the structure of the cation first coordination shells. The RDF indicates short-range order in the structure of the molten salts. Additionally, the CN and its distribution indicate that Li⁺ is primarily tetra- and penta-coordinated, Be²⁺ is tetra-coordinated, and Th⁴⁺ is octa-coordinated. ADF analysis reveals that while the first coordination shells of Be²⁺ tend toward a regular tetrahedral configuration, Li⁺ exhibits a distorted octahedral structure, often with one or two anionic cavities. In terms of ionic diffusion, the mean square displacement (MSD) indicate that ionic diffusion rates order as follows: Li⁺ > F^- > Be²⁺ > Th⁴⁺. The r(t) indicates that Be-F coordination shell is the most stable, followed by Th-F, with Li-F being the most dynamic. As ThF₄ concentration increases, there is a decrease in the total MSD of the molten salt, suggesting a reduction in overall ionic diffusion. The results from the AIMD simulations align closely with experimental and other simulation data, thereby confirming the reliability of this work. Overall, this study provides insights into the FLiBeTh molten salt, supporting the development of fourth-generation nuclear reactor fuels.