A Review of Electrochemical and Non-Electrochemical Approaches to Determining Oxide Concentration in Molten Fluoride Salts

Abstract

The design of reactor core and heat transfer systems for molten salt reactors (MSRs) and fluoride-salt cooled high temperatures reactors (FHR) necessitates a precise understanding of the parameters affecting thermophysical and chemical properties of the molten salt medium. Oxides constitute a significant component of the impurities in alkali metal halide melts commonly implemented in MSRs. They are present initially in the raw materials due to the manufacturing process. If present in high enough concentrations, oxides can affect molten salt chemistry and thermodynamics, potentially affecting its thermophysical properties and corrosivity to structural materials. The dissolved oxide content in the melt can vary during reactor operation due to transmutation or migration to/from the environment. Therefore, it is important to develop capabilities to quantify the oxide concentration at reactor conditions. The benefit is threefold: firstly, measuring oxide concentration precisely and accurately will allow us to correlate oxide concentration with variation in thermophysical and chemical properties; secondly, knowing these correlations can facilitate the formulation of more incisive experiments to more accurately determine the activity of oxide in molten fluorides ; thirdly, such an oxide measurement system can be deployed online in a reactor as a sensor to add to the suite of reactor diagnostic instrumentation. These goals in concert will help us gain a better understanding of the basic science of a molten salt solvent environment. In context of the ongoing effort to design an oxide sensor for molten salt systems, a review of the literature of electrochemical and non-electrochemical approaches to determining oxide concentration in molten fluoride salts was conducted and an evaluation of their merits is presented here. This evaluation served as the basis on which we selected square wave voltammetry as the most promising technique with which to develop an oxide sensor.