Development and Optimization of Normal Pulse Voltammetry for Real Time Analysis of Electrorefiner Salt with High Concentrations of MgCl₂ and UCl₃

Electro-refiners can accumulate large holdup of fissile actinides from used nuclear fuel including plutonium and will need to be well safeguarded in a commercial facility. Idaho National Laboratory has operated the Fuel Conditioning Facility, an engineering scale demonstration facility, with two electro-refiners for two decades using mass tracking models supplemented with destructive analysis for the requirements of domestic safeguards. This approach is known to be insufficient of meet international safeguards timeliness requirements. Since there are non-nuclear weapons states that are interested in commercial electro-refiners, there is a need to develop a real time concentration measurement technique. In addition to being useful for safeguards, such a real time measurement would be functional for maintaining optimal process control. Normal pulse voltammetry (NPV) is a powerful electrochemical analysis method that has been identified to have great potential for measuring concentrations of various ions in a given salt mixture. While cyclic voltammetry (CV), chronoamperometry (CA), and chronopotentiometry (CP) result in progressive growth of the working electrode surface area, NPV potentially can circumvent this problem via periodically stripping deposited metals from the electrode (WE) surface. With each successive pulse, the potential is dropped to a more negative level. The corresponding current is a summation of partial currents from each of the reducing species. The ultimate objective of this work is to develop correlations between limiting currents measured via NPV with known concentrations. This is potentially a more accurate method than CV and other methods, because it should keep the WE surface area from growing significantly. Previous work on using NPV for actinide concentration determination in eutectic LiCl-KCl has been reported by M. Iizuka et al. However, only low uranium concentrations were studied in that paper. The maximum concentration of uranium tested was 2.48 wt%. Limiting currents from NPV were found to be proportional to their concentrations up to about 1.7 wt%. They were unable to maintain double plateaus (U and Pu) at uranium concentrations above 1.69 w%. This issue was resolved during our high concentration study. This result represents a significant breakthrough for optimizing voltammetry for analysis of actinides in molten salt