Methods for Determining the Working Electrode Interfacial Area for Electroanalytical Measurements of Metal Ions in Molten LiCl-KCl

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Voltammetry is a promising approach to determining the concentration of actinides and rare earths in molten chloride salts for real time or near real time monitoring of nuclear fuel electrorefining. In this process, actinides and rare earths accumulate in eutectic LiCl-KCl from reaction of UCl3 with spent nuclear fuel. Monitoring the salt composition is key in order to maintain optimal process conditions, to determine when to treat or discard salt, and to safeguard the process against nuclear material diversion. Voltammetry methods such as cyclic voltammetry (CV), chronopotentiometry (CP), chronoamperometry (CA), normal pulse voltammetry (NPV), and square wave voltammetry (SWV) have been extensively reported in the literature for application to molten LiCl-KCl typically containing one rare earth or actinide chloride. Our studies have expanded the salt matrices to include quaternary salts with two electrochemically active metal chlorides. We believe this to be the first step in determining if these basic electrochemical analysis methods can be applied to salts containing many components with similar standard reduction potentials. In the simpler ternary salt systems, classic diffusion limited rate equations such as those derived by Randles-Sevcik and Berzins-Delahay have been applied to correlate CV peak height to concentration. Application of these equations requires knowledge of both the diffusion coefficient of the salt compound in the LiCl-KCl and the interfacial area between the working electrode and salt. In our experimental investigations, we have determined that both of these quantities can be difficult to establish a priori. Diffusion coefficients for metal chlorides in molten LiCl-KCl appear to vary with concentration. And the high temperature electrochemical cells located in insulated furnaces make it challenging to precisely determine the electrode-salt interfacial area. In this paper, we focus on empirical evaluation of methods for determining this area—ranging from the simplest approach of dipping a cold electrode into the molten salt to measure the film height to more elaborate approaches such as using a vertical electrode actuator, coating the electrodes with glass, and integrating a pre-peak which may represent formation of a monolayer of metal on the electrode. Salt mixtures investigated include LiCl-KCl-UCl3, LiCl-KCl-UCl3-MgCl2, LiCl-KCl-ThCl4-LaCl3, LiCl-KCl-LaCl3-GdCl3, and LiCl-KCl-LaCl3-MgCl2. It will be shown that with accurate determination of the electrode-salt interfacial area that concentration correlations can be developed with error as low as 2-3%. Both using differential insertion depth with the vertical electrode translator and using glass-coated working electrodes are promising approaches—the latter of which is believed to be more easily implemented in a production operation scenario.