| Description |
Molten salts have been used as solvents in the pyroprocessing of nuclear martials due to their unique chemical and physical properties. This research focuses on the applications of fundamental electrochemistry in on-line monitoring of the composition of molten chloride salts that are used for nuclear materials processing. The first part of this research is in nuclear waste pyroprocessing in molten LiCl-KCl eutectic at 773 K. MgCl2 (surrogate for PuCl3) and GdCl3 (selected to represent rare earth metals), each mixed with high concentration of UCl3, were studied in molten LiCl-KCl eutectic. The second part is in nuclear material processing in molten CaCl2 at 1123 K. In the UCl3-MgCl2-LiCl-KCl system, normal pulse voltammetry has been applied and optimized to develop correlations between diffusional current and analyte concentrations. A matrix of mixtures were prepared and tested with UCl3 concentration varying between 1 to 9 wt% and MgCl2 concentration varying between 0.3 and 1.5 wt%. The measured diffusional currents correlated well with concentration and matched a model that includes both diffusion and migration from the bulk molten salt to the working electrode surface. This is, thus, a promising method for simultaneous analysis of UCl3 and PuCl3 in molten salt electrolytes used for spent nuclear fuel electrorefining. In the UCl3-GdCl3-LiCl-KCl system, a mixture matrix, which consisted of varying concentrations of UCl3 and GdCl3 from 1 to 10 wt% and from 1 to 3 wt%, respectively, was tested. Optimized normal pulse voltammetry measurements were made on this mixture matrix and used for the concentration correlations. The accuracy of the prediction was improved by including solution resistance compensation and minimizing the working electrode surface area. The presence of GdCl3 showed no effect on the U3+ NPV reduction current, but high U concentrations affected the Gd3+ NPV reduction current. This is speculated to be due to increased migrational current in the molten salt mixture. The average relative measurement errors obtained were 1.6% and 2.7% for UCl3 and GdCl3, respectively. In the CaCl2 based system, the CaCl2 salt produced by the Lawrence Livermore National Laboratory (LLNL) Salt Lab was studied. A robust W electrochemical sensor was developed to detect unwanted impurities in the salt in real-time. A tungsten metal and oxygen ion reaction was identified. The experimental data are consistent with a two-step reaction, W was electrochemically oxidized to WO2 then to WO3, where WO2 is the reaction intermediate, and the first step is kinetically much faster than the second step. This unique reaction can be utilized to determine the CaO concentration in CaCl2. An excellent 6-point linear CaO concentration calibration, R2 of 0.9941, in 200 ppm increments was demonstrated in this research. It was also demonstrated that electrochemical methods have the potential to monitor metal concentrations during CaCl2 based processes |
