Development and Optimization of Voltammetry for Real Time Analysis of Multi-Component Electrorefiner Salt
Authors: Zhang Chao , Rappleye Devin , Simpson Michael Forrest 
Published: February 2020

There is interest by several countries in pursuing pyroprocessing of spent nuclear fuel –including both nuclear weapons states and non-nuclear weapons states. This technology features electrorefiners that can be operated to recover fissile actinides from spent nuclear fuel. This separation capability can be viewed as a proliferation risk factor, which necessitates the implementation of nuclear safeguards. For non-nuclear weapons states, safeguarding would be the responsibility of the International Atomic Energy Agency. Unlike nuclear facilities in which items can be discretely tracked, inspecting and safeguarding pyroprocessing facilities requires complicated monitoring systems and material control and accountability approaches to support the safeguards system. Therefore, there needs to be developed high precision monitoring technology for electrorefining and other key unit operations in pyroprocessing that can be implemented in real or near real time. Currently in research and development facilities such as the Fuel Conditioning Facility at Idaho National Laboratory, they have been employing mass tracking simulation supplemented with destructive analysis for the last two decades. This approach meets neither the need for timeliness or accuracy needed to detect significant quantities of material diversion in commercial facilities. Since much of the inventory of actinides in electrorefiners are held up in the molten salt, electrochemical sensors would be an ideal solution to accurately monitor actinide concentrations in real time.
Electrochemical analysis methods such as cyclic voltammetry (CV), chronoamperometry (CA), and chronopotentiometry (CP), and normal pulse voltammetry (NPV) can be used to develop correlations between electric signals measured and concentrations of various ions in the molten salt. In salts with multiple components and high overall concentration of active solute, one major difficulty in applying these electroanalytical methods is to prevent area growth of the working electrode (WE)—which is largely unpredictable and affects the observable current density. Practically, all of the aforementioned voltammetry methods can encounter this problem. However, during an NPV scan, the metal deposited onto the WE is periodically oxidized back into the salt, which should effectively minimize working electrode area growth. NPV was, thus, the main focus of our study. Pulse time, relaxation time, and relaxation potential were all varied in an attempt to isolate the diffusion-limited partial current values for the different active ions in the salt. CV and CA were also applied to supplement and validate the NPV results. CV was employed to determine the potentials at which different ions reduce as well as the relaxation potential used to clean off deposited metals from the WE. CA was utilized to validate if the pulse time selected was still within the time period that the change in current was described by the Cottrell equation. Therefore, a method that was a combination of NPV, CV, and CA was applied to develop and optimize real time analysis of electrorefiner salt.
Two salt mixtures were investigated in this study, LiCl-KCl-UCl3-MgCl2 and LiCl-KCl-UCl3-GdCl3, where Mg was the surrogate for Pu and Gd represented the rare earth elements in the electrorefiner salt based on the similarity in standard reduction potentials. While previously, correlations between limiting current and concentration in molten salt had been reported only up to about 1.7 wt% UCl3, this study included limiting current and concentration correlations for binary systems with up to 9 wt% UCl3. The highest MgCl2 concentration tested was 1.5 wt%, which represented 5.5% PuCl3 because the limiting current is proportional to molar concentration rather than weight percentage. When comparing limiting current density with MgCl2 concentration at fixed 1 wt% UCl3, the results did not show a good linear fit, as the slope increased with each concentration increment. However, R2 of 0.9996 was obtained when the data was fitted with a second order polynomial. The nonlinear results show that there was possibly a slight WE surface area increase. Also it is possible that the diffusion coefficient increased significantly with high concentration, resulting in higher current density. Further investigation with shorter pulse time is required to validate these hypothesizes. Nevertheless, the results show that electrochemical sensors would be an ideal solution to accurately monitor electorefiner salt concentrations in real time.