Simulation of Voltammetric Measurements of Uranium Electrodeposition in Molten LiCl-KCl Eutectic

The electrochemical behavior of uranium in high-temperature molten salts is central to the development of advanced nuclear reactors (e.g., molten salt reactors) and reprocessing technologies (e.g., pyroprocessing). Numerous studies have been performed on uranium in molten salts, especially LiCl-KCl eutectic. However, many studies rely on models, such as Berzins-Delahay model, which assume mass transfer by diffusion only and a pure deposit (i.e., unit activity for the reduced species). 1 In many situations, these assumptions may not be appropriate for analyzing electrochemical data in molten salts and may introduce significant error in the estimation of properties, such as diffusion coefficients. For example, a foreign substrate, such as tungsten or molybdenum, is often used as the working electrode in electroreduction of uranium ions to metal in molten salts. Hence, nucleation effects and incomplete coverage of the WE can affect measurements, especially at dilute concentrations. Additionally, macro electrodes are often used in molten salts due to a lack of compatible insulating material for construction of microelectrodes. This results in large currents which can introduce significant ohmic losses despite the relatively low ohmic resistance of molten salts. Furthermore, additional effects, such as migration and natural convection, can significantly impact electrochemical measurements in molten salts containing a heavy analyte, such as uranium ions. To address these challenges, a model was developed specifically for electrodeposition in molten salts based on the Nernst-Plank equation, Nernst equation, adsorption isotherms, and activity models. The initial iteration of the model has been designed to account for the combined effects of diffusion, migration, ohmic resistance, deposition onto a foreign substrate, and activity of non-ideal solutions. The significance of each effect and combination of effects has been examined for linear sweep voltammetry at various uranium ion concentrations and scan rates. Furthermore, the model has been validated against uranium electrodeposition measurements on a tungsten WE in molten LiCl-KCl eutectic. 1. T. Berzins and P. Delahay, J. Am. Chem. Soc. , 75 , 555–559 (1953) http://dx.doi.org/10.1021/ja01099a013.