Electrochemical Methods for Determination of Activity Coefficients of Lanthanides in Molten Salts
Authors: Bagri Prashant , Bastos Tarcisio , Simpson Michael Forrest 
Published: February 2020

The electrochemical behavior of lanthanides and actinides in molten salts is an important area of research from the standpoint of development of Generation IV nuclear reactors and management of spent fuel. Electrochemical separation of these elements in molten salts is a key technology within nuclear fuel pyroprocessing, which can be an effective approach to managing spent nuclear fuel waste for a closed nuclear fuel cycle. In electrorefiners, for example, metallic spent fuel is placed in a steel basket, which serves as the anode for the cell. These electrorefiners use molten LiCl-KCl eutectic salt as the electrolyte. By controlling the cathode potential, uranium or grouped actinides can be selectively reduced onto the cathode while rare earths and other fission products remain in the molten salt. For such separations to be efficient and have low levels of rare earth contamination, it is imperative that we understand and are able to predict key electrochemical properties (apparent reduction potential and activity coefficients) of these elements accurately under conditions of interest. Models based on thermodynamic and kinetic relationships coupled with knowledge of such properties will help in design, development and operation of optimized electrorefiners.
For this paper, the apparent reduction potentials and activity coefficients of two lanthanides, lanthanum chloride and neodymium chloride, were determined experimentally using a two electrode electrochemical cell. Equilibrium potential was measured as the open circuit potential for the La/La(III) and Nd/Nd(III) redox couple versus a reference electrode. For the lanthanum system, the reference electrode was 5 mole % Ag/AgCl in LiCl-KCl salt. For the neodymium system, the reference electrode was Nd/NdCl3(saturated) in LiCl-KCl salt. Using this set-up, the activity coefficients can be calculated using the Nernst equation. However, the difference in reference electrodes in the two systems represents fundamentally different approaches to measuring activity of a species in a molten salt mixture. The former approach with lanthanum chloride requires calculation of the standard potential (E0), which in this case is equivalent to the Gibbs free energy of pure supercooled liquid. But the latter approach with neodymium chloride allows us to directly measure of activity, since the standard potential terms drop out when using a reference electrode based on the standard state. However, the measurement of the activity coefficient of neodymium chloride was complicated by the presence of two oxidation states of neodymium ions present in the molten salt (+2 and +3). For determining these activity coefficients, it was necessary to be able to determine the speciation of the two ions present.
This study has revealed interesting concentration dependence of the activity coefficients. For example, the activity coefficient of LaCl3 systematically varies between 2 x 10-3 to 1.2 x 10-2 at 773 K and goes through a maximum at a mole fraction of approximately 0.55 mole % La. Temperature dependence of the activity coefficients was measured as well as the effect of the presence of other components (CsCl) in the salt mixture.