Thermodynamic modeling of KCl-PrCl3 and KCl-LiCl-PrCl3 systems

Molten salt electrolysis can recover the actinides from spent nuclear fuels, and it involves a eutectic LiCl-KCl in molten form as an electrolyte. During reprocessing, the concentration of fission products such as La, Nd, Pr, etc., increases and affects the recovery efficiency of the electrolyte. In this work, thermodynamic modeling of KCl-PrCl3 and KCl-LiCl-PrCl3 systems was carried out using the CALPHAD (Calculation of Phase Diagrams) approach for the first time. The thermodynamic functions for the pure salts were taken from the SGTE (Scientific Group Thermodata Europe) Substances (SSUB) database. The experimental thermochemical and phase equilibria data available in the literature were used as input for the assessment of KCl-PrCl3 and KCl-LiCl-PrCl3 systems. The model parameters for the KCl-LiCl system were adjusted to include the new Gibbs energy descriptions for the pure salts. In addition, the sublattice model for the liquid phase in the LiCl-PrCl3 system was modified and reassessed to ensure the model compatibility for higher-order extrapolation. There is a good agreement between the experimental and calculated thermochemical and phase diagram data for all the optimized constituent bi-naries and the ternary system. This work will be beneficial for determining the solubility limit of PrCl3 in molten LiCl-KCl electrolytes and their thermodynamic properties for improving the efficiency of the pyrochemical process.

Automated summary

Molten salt electrolysis is used to recover actinides from spent nuclear fuels, using a molten LiCl-KCl electrolyte. This study employed the CALPHAD approach to thermodynamically model the KCl-PrCl3 and KCl-LiCl-PrCl3 systems for the first time. Experimental data and thermodynamic functions were utilized for the assessment of these systems, and good agreement was observed between the experimental and calculated results. This work contributes to determining the solubility limit of PrCl3 in molten LiCl-KCl electrolytes and improving the efficiency of the pyrochemical process.