Thermodynamic Properties Determination in LiF-CaF2 using Electrochemistry in High Temperature Fused Salts: Application to CeF3

Abstract

Pyrochemical route is a promising option for spent nuclear fuel recycling. The major step of spent fuel reprocessing is the actinides-lanthanides separation performed by electrochemical processes. In this partitioning strategy, the most radiotoxic elements (actinides) are separated from the lowest ones (lanthanides) to decrease the volume of high activity nuclear waste disposal. The knowledge of thermodynamic properties (such as activities, standard potentials…) of actinides and lanthanides in solution is then crucial to predict their electrochemical behaviour in a complex multi-element system. Thermodynamic calculations have been done on cerium fluoride in the LiF-CaF2 eutectics from 1113 to 1233K with FactSage and HSC 6.1 database. From electrochemical measurements (linear sweep voltammetry, equilibrium potential measurements…), the main thermodynamic properties needed for the electrochemical separation evaluation were determined: standard potentials and activity coefficients in different reference states (liquid or solid reference state, and infinite dilution reference state) and for different composition scales (molarity, molality, or molar fraction), and also standard enthalpy and standard entropy of formation in the 1113-1233K temperature range.Finally, constants which interconnected the infinite dilution reference state and the solid and liquid reference states were evaluated.