Gibbs energy modeling of LiCl-KCl-LnCl3 (Ln: U, Pr) systems facilitating spent nuclear fuel reprocessing using molten salt electrolysis

Pyrochemical reprocessing based on molten salt electrolysis is a viable method to recover the actinides from spent nuclear fuels. The reprocessing scheme involves an electrometallurgical method with eutectic LiCl-KCl as electrolyte at 500^oC. As the spent fuel is reprocessed, the concentration of the fission products such as lanthanides (La, Nd, Pr, etc.) that are more stable than UCl₃ increases. These fission products alter the thermodynamic properties and affect the recovery efficiency of the molten LiCl-KCl electrolyte. Hence, the knowledge of the solubility limit of fission product chlorides in molten LiCl-KCl and their thermodynamic properties will be beneficial for improving the efficiency of the pyrochemical process. In this work, Gibbs energy modeling of LiCl-KCl-UCl₃ and LiCl-KCl-PrCl₃ systems has been performed using the CALPHAD (Calculation of Phase Diagrams) method using experimental and ab initio data. These ternary systems form a part of the thermodynamic database for multicomponent chloride salts to be developed for improving the efficiency of the molten salt electrolysis process. The thermodynamic descriptions for the pure salts were taken from the SGTE Substance database (SSUB6) since it was found to correlate well with the experimental thermodynamic properties in comparison with other commercial databases. Constituent binary systems such as LiCl-PrCl₃ and KCl-UCl₃ were re-assessed using new experimental data available in the literature. Besides, the KCl-PrCl₃ system was assessed for the first time in this work. The enthalpy of formation of the intermediate compounds such as K₂PrCl₅ and K₃PrCl₆ were computed using ab initio calculations. With reported phase equilibria and thermochemical data and ab initio calculations in this work, the Gibbs energy parameters were optimized. There was good agreement between the experimental and calculated thermochemical and phase diagram data. Similarly, the thermodynamic parameters for LiCl-KCl-UCl₃ and LiCl-KCl-PrCl_{3 ­}systems will be optimized with experimental phase equilibria data available in the literature as input.