Electrochemical measurements of LiF-CaF2-ThF4 melt and activity coefficient of ThF4 in LiF-CaF2 eutectic melt

Thermodynamic and electrochemical properties of actinides in molten fluoride salts are one of the key data required for design and safety assessment of any molten salt reactor concept using liquid fluoride based fuel. In the case of the Molten Salt Fast Reactor (MSFR), special attention has to be paid to thorium fluoride, which is a direct fuel carrier salt constituent. This work presents experimental measurement of ThF4 activity coefficient in eutectic LiF-CaF2 melt by electrochemical techniques. The proposed approach is overcoming problems related to the lack of a thermodynamic reference electrode for high-temperature molten fluoride media. The method can be used to compare stability of fissile material and fission products in a given molten fluoride solvent by estimating their activity coefficients using the described electrochemical measurements. In addition, electrochemical methods were successfully used to evaluate and confirm purity of the used materials, with a special regard to oxygen content in the ThF4 input material synthesised during the previous work from ThO2. (C) 2021 The Authors. Published by Elsevier Ltd.