Oxide Sensor Development By Square Wave Voltammetry for Molten Fluoride Salts
Authors: Williams Haley , Scarlat Raluca Olga 
Published: December 2020

A fundamental understanding of oxide concentration is critical for operation of advanced nuclear reactors which use molten salts. Molten fluoride salts have been proposed as fuel solvents, coolants, and tritium breeding blankets in fission and fusion nuclear reactors due to their high temperature operation and favorable neutron-interaction properties. An electrochemical sensor which can measure the real-time concentration of oxide anions in the salt melt could be used to identify air ingress to the system. Additionally, similar electrochemical analyses used in the development of a sensor could be used to study the fundamental properties and behavior of oxides in molten fluoride salts. These properties include the activity coefficient, diffusion coefficient, and solubility of oxides. Understanding of these properties could, in turn, inform sensor development.
A further understanding salt chemistry can also help predict and control the effects of oxide concentration on salt corrosion. Material integrity under exposure to high-temperature molten salts is an impactful technical challenge for the design of nuclear reactors that employ molten salts. Oxide anion concentration and complexation of cations and anions in the melt can influence corrosion. Therefore, electrochemical oxide sensors for molten fluoride salts would be instrumental in corrosion studies. Fundamental understanding of oxide chemistry in fluorides would also advance understanding of corrosion mechanisms in fluoride salts.
Cyclic voltammetry, square wave voltammetry, and chronopotentiometry have been previously used to study the electrochemical behavior of oxides in fluoride salt melts. Square wave voltammetry (SWV) is particularly useful in this application because it limits the noise caused by oxygen bubbling from the oxidation of O2- anions, as found by Massot et al.. Titrations of known additions of oxide to the melt have previously been used to show that the intensity of the SWV oxide oxidation peak is linear with oxide concentration.
Oxides have yet to be studied electrochemically in 2LiF-BeF2 (FLiBe) or LiF-NaF-KF (FLiNaK) molten salts, which are of interest for nuclear reactors. FLiBe is a likely coolant and solvent candidate for molten salt nuclear reactors. FLiNaK has been studied as a surrogate salt with similar thermo-physical properties to FLiBe. It is easier to handle experimentally since it presents no beryllium health hazard, but its chemical and neutronic behavior are different. FLiNaK is often used in salt loops for reactor design, safety analysis, component testing, and corrosion testing. Therefore, availability of oxide electrochemical sensors for FLiBe and FLiNaK would be instrumental in reactor operation as well as reactor development.
This talk will summarize the application of SWV to quantification of oxides in FLiBe and FLiNaK melts. It will also discuss the available chemical and thermodynamic information for oxides in FLiBe and FLiNaK. Ultimately, an understanding of oxide behavior and a quantification of oxide concentration in molten fluoride salts will aide in the development of an electrochemical sensor for detecting leaks in molten salt reactors.