(418c) Electrochemical Analysis of Bi-Analyte Electrorefiner Salt with High Concentration of UCl3

Authors: 
Zhang, C., University of Utah
Rappleye, D. S., Livermore National Laboratory
Wallace, J., University of Utah
Simpson, M. F., University of Utah
During the spent nuclear fuel electrorefining process, the spent fuel is oxidized into the molten salt mixture followed by selective reduction on a cathode. Consequently, a large inventory of fissile actinides is held up in the salt. Thus, an important requirement for operating the process is to keep track of the actinides in the salt mixture to prevent proliferation. Several recent publications on either low UCl3 concentration or single analytes show that electrochemical senor could potentially monitor salt concentration in real time. However, the UCl3 concentration in the electrorefiner is often between 5-10 wt% during operation. Therefore, the objective of this research is to focus on bi-analyte systems with up to 10 wt% UCl3. Two bi-analyte systems U/Mg and U/Gd were studied. Mg is a surrogate for Pu due to their similar reduction potentials, and Gd is selected to represent the rare earth metals in fission products. In this research, normal pulse voltammetry (NPV) was selected to prevent working electrode surface area growth caused by U deposition. Optimizations were made to the procedure to make NPV compatible and highly accurate for this application. Through the study of U/Mg system, NPV parameters were established, and it was learned that migration currents are comparable in magnitude to diffusional currents. Solution resistance compensation and small working electrode were applied for further optimization during the study of U/Gd system. The final concentration prediction average error values for UCl3 and GdCl3 are 1.6% and 2.70% respectively. This result shows that NPV is a suitable method for monitoring spent nuclear fuel electrorefiner molten salt metal concentrations in real time. It is also a promising approach for monitoring actinide concentrations in molten salt reactors.