Development and Applications of Polarizable Force Fields for LiCl-KCl-UCl₃ Molten Salt System

The LiCl-KCl-UCl₃ molten salt systems play a crucial role in the electrorefining process of spent nuclear fuel pyro-processing, where their microstructure and physicochemical properties directly affect the efficiency of uranium recovery. Experimental studies on actinide-containing molten salts are often challenging due to the unavoidable extreme conditions such as high temperatures, corrosiveness and strong radioactivity. Molecular dynamics (MD) simulations serve as a powerful alternative tool for investigating the microscopic structure and thermophysical properties of such systems. In particular, classical MD based on the polarizable ion model (PIM) potential has been widely used to relatively accurately describe the microstructure, thermodynamics, and transport properties of high-temperature molten salts. In this work, a new set of PIM force field parameters were developed via a force-matching approach, incorporating the polarization effects of U³⁺, K⁺, and Cl⁻ ions. Using this force field, MD simulations were performed to systematically study the microstructure, coordination chemistry, ionic correlations, and dynamic properties of the LiCl-KCl-UCl₃ system. The results demonstrate that the newly developed force field fairly accurately reproduces U-Cl coordination structures, bridging coordination modes, and molten salt density, agreeing well with experimental data and ab initio molecular dynamics (AIMD) simulations. The MD simulation results reveal that the U-Cl coordination shell exhibits relatively high stability, with the coordination number of uranium decreasing as temperature increases. Moreover, KCl acts as a “spacer salt”, leading to KCl-separated correlations between complexes and networks at intermediate length scales. Additionally, the self-diffusion coefficients of the ionic species and the viscosity of the melt were computed, both of which follow Arrhenius-type behavior. This study provides a computational simulation framework for understanding the structures and properties of actinide molten salt systems. The developed simulation method offers atomic-scale insights into the microstructural and dynamic behavior of uranium-containing melts, providing theoretical guidance for the separation and extraction of actinide elements.