Towards Electroanalytical Measurements in the Elemental Soup of Molten Salts Bearing Nuclear Fuel
Authors: Rappleye Devin , Torrie Jason , Williams Tyler , Fuller Ranon G 
Published: November 2022

Molten salt mixtures containing nuclear fuel present a complex and aggressive environment for electroanalytical measurements. These types of molten salt mixtures may be found within molten salt nuclear reactors (MSRs) or in used nuclear fuel (UNF) reprocessing units, such as an electrorefiner. These molten salts contain many elements from the periodic table at vastly varying concentrations creating a proverbial alphabet soup of elements. Adding to the challenge is that the ‘utensils’ available to probe this ‘soup’ are limited because of severe restrictions on suitable materials created by high temperatures, radioactivity, and harsh chemical environments.
Electrochemical probes are a natural fit for real-time and in situ measurements in the ionic environment of molten salts due to their robust and simple construction. Responses from electrochemical probes can provide valuable information for characterizing and monitoring important species in nuclear processes. Monitoring the quantity of nuclear material in a process is required within a certain degree of accuracy and timeliness to deter the diversion of special nuclear material (SNM), such as uranium or plutonium, for nonpeaceful purposes. The degree of accuracy and timeliness is determined by domestic regulators, such as the United States Nuclear Regulatory Commission (NRC), and/or by the International Atomic Energy Agency (IAEA). Furthermore, the development and optimization of molten salt processes benefit from real-time feedback on the chemical composition and properties. However, due to the complex and aggressive environments, further refinement of sensor design and data analysis is required to achieve the needed accuracy and to apply electrochemical measurements over a greater range of conditions.
Recent advancements in sensor design include more precise measurement and control of the working electrode area. However, additional issues still exist, such as the lack of standard reference electrodes and an inability to measure high concentrations of ions. Reference electrodes in molten salts are custom made resulting in a high degree of variation in construction and, therefore, reference potential. Recent results on a highly repeatable (±3 mV) and stable reference electrode for molten chloride salts will be presented. Furthermore, efforts to develop a thin layer sensor to perform bulk electrolysis techniques in molten salts containing analytes at high concentrations will be presented.
Analyzing electrochemical measurements in nuclear fuel laden molten salts is complicated due to a high degree of signal overlap, as shown in Figure 1, and is limited due to a lack of advanced relations developed for electrodeposition (soluble-insoluble) reactions, which constitute most electrochemical reactions in molten salts. For example, semi-differentiation has been a powerful tool for improving peak separation for soluble-soluble reactions in potential sweep methods. However, relations for peak shapes and characteristic features (i.e., height and width) of semi-differentiated peaks for soluble-insoluble reactions are not well-established in the literature. Progress on the development of these relations through numerical methods will be presented and compared to experimental data. Additionally, a simple model developed for determining the number of electrons exchanged in a metal electrodeposition onto a foreign substrate will be presented. Lastly, the role and preliminary results of advance analytical techniques, such as partial least squares and principal component analysis, in analyzing the electrochemical responses in complex mixtures with a high degree of signal overlap will be discussed.

Figure 1