Physical Property Measurements and Molecular Dynamics Simulations of LiCl-KCl-CeCl₃ System

During the pyrochemical electrorefining process of spent nuclear fuel, fission product Ce³⁺ continuously accumulate in the LiCl-KCl eutectic molten salt, altering its original physicochemical properties and potentially affecting the overall electrorefining efficiency and system stability. However, systematic research on the physicochemical parameters of the LiCl-KCl-CeCl₃ molten salt system remains limited. In this study, the effects of low-concentration CeCl₃ addition(x(CeCl₃)=5.0%, the same below) on the key engineering properties of LiCl-KCl molten salt were comprehensively investigated within a temperature range of 673-873 K. This work aims to establish an accurate and reliable evaluation framework combining experiments and simulations to provide crucial data and theoretical support for dry reprocessing technologies. Experimentally, three fundamental macroscopic engineering properties were systematically measured under a high-purity argon atmosphere to establish a robust dataset. Specifically, the molten salt viscosity was determined using the rotational method with a high-precision coaxial cylinder viscometer, which effectively captured the fluid resistance. The conductivity( \sigma ) was obtained via the continuous variation conductivity cell constant(CVCC) method employing an advanced LCR bridge to eliminate polarization effects. Furthermore, the initial crystallization temperature, reflecting the solid-liquid phase equilibrium, was accurately recorded through the cooling curve method. To provide deep theoretical insights at the microscopic level, advanced computational simulations were implemented in parallel with the experimental procedures. First-principles molecular dynamics(FPMD) simulations based on density functional theory were performed using the Vienna Ab-initio Simulation Package(VASP) to reveal the quantum-mechanical features. Moreover, a machine learning deep potential molecular dynamics(DPMD) model was trained using the DeePMD-kit package based on the FPMD data, enabling large-scale and long-term simulations to describe the local structures, coordination numbers, and precise self-diffusion coefficients of the multi-component system. The experimental and simulated results provid detailed insights into the macroeconomic variations and microscopic structures. The viscosity of the system decreases nonlinearly as the temperature increases due to accelerated ionic motion, whereas the incorporation of CeCl₃ significantly raises the viscosity; at 673 K, the viscosity of the mixture containing 4.0%CeCl₃ reaches 8.06 mPa•s, approximately twice that of the pure eutectic salt. The electrical conductivity exhibits an upward trend with rising temperature, but decreases slightly with the addition of CeCl₃, with the maximum reduction restricted within 0.2 S/cm due to the larger radius and greater migration resistance of Ce³⁺. Interestingly, both experimental and simulated data manifest a subtle anomalous increase in conductivity at a tiny dosage of 0.5%CeCl₃. Furthermore, as the CeCl₃ content increases, the initial crystallization temperature decreases initially and then rises, reaching a minimum turning point near 2.0%LiCl-KCl-CeCl₃(620 K). Structural analysis derived from DPMD confirms that Ce³⁺ possesses a remarkably strong coordinating capability, with the fractions of 6- and 7-coordinated polyhedral configurations increasing continuously, leading to a locally ordered structure. In conclusion, this study successfully demonstrates that the accumulation of Ce³⁺ leads to increase flow resistance and slightly impeded current efficiency due to its powerful local structural coordination and high diffusion activation energy. The simulated viscosity via the Stokes-Einstein equation shows an excellent agreement with experimental values, with an average prediction error below 10%, validating the reliability of the DPMD potential model. The integrated experimental-computational framework and the validated database established in this work provide essential theoretical guidance and critical engineering parameter optimization for the design and stable operation of high-temperature pyrochemical reprocessing facilities.