Machine Learning-assisted Molecular Dynamics Simulation of LiCl-KCl-PuCl₃ Molten Salt System

In the molten salt electrorefining process of spent nuclear fuel, plutonium (Pu) is one of the key elements requiring extraction and separation. Therefore, it is necessary to conduct in-depth studies on the coordination chemistry of Pu³⁺ ions in molten salts and their potential impacts on macroscopic properties of molten salts. Based on high-precision ab initio molecular dynamics (AIMD) simulation data, a machine learning-based deep potential (DP) model was developed via deep learning training for the LiCl-KCl-PuCl₃ system. Subsequently, large-scale deep potential molecular dynamics (DPMD) simulations were performed based on the trained DP model to investigate the LiCl-KCl-PuCl₃ system under high-temperature conditions. The predicted density of the LiCl-KCl-PuCl₃ system decreases gradually as the temperature increases from 773 K to 1 073 K. The results of the radial distribution function (RDF) reveal the presence of locally ordered coordination structures within the mixed molten salt. From the first peaks of the RDF curves, the predicted Pu-Cl bond length ranges from 2.66 to 2.68 Å over the studied temperature range, and the Cl-Pu-Cl bond angle is predominantly centered at approximately 49°, indicating that temperature exerts a negligible effect on the geometric structure of Pu complexes. With increasing temperature, the chloride coordination numbers of Pu³⁺ ions decrease gradually ranging from an average of 6.62 to 6.11, with the co-existence of PuCl₅²⁻, PuCl₆³⁻, PuCl₇⁴⁻, PuCl₈⁵⁻. At 773 K, Pu³⁺ ions primarily form seven-coordinate complexes in the LiCl-KCl system, and elevated temperatures gradually drive the structural transformation of Pu³⁺ complexes from seven-coordinate to six-coordinate configurations. In addition, increasing temperature leads to a significant reduction in molten salt viscosity while correspondingly increases the diffusion coefficient of Pu³⁺ ions. The calculated activation energy of Pu³⁺ ions is 32.4 kJ/mol, which is in excellent agreement with the reported experimental value of 32.1 kJ/mol. This work elucidates the microscopic coordination structures and property evolution mechanisms of the LiCl-KCl-PuCl₃ system, providing a solid theoretical foundation for the future extraction and separation of actinide elements as well as the rational design of advanced molten salt systems.