Fluoride Cooled High-Temperature Reactors (FHRs) are Generation IV Nuclear Reactor designs which will use molten fluoride salts as coolants. FHRs operate at a much higher temperature than the water-cooled reactors and have lower operating pressure. This greatly improves their thermal efficiency and decreases the likelihood of severe accidents. Fluoride salts such as FLiNaK (46.5-11.5-42 mol% LiF-NaF-KF) are considered for use as coolants in FHRs.

One of the major concerns for FHR designs is the significant corrosion of structural alloys in the FLiNaK. The corrosion in the form of selective dissolution of active alloying elements in these molten salts depends on the redox potential of the salt. This redox potential can be controlled by various chemical and electrochemical techniques and thereby corrosion of structural alloys in the salt can be controlled. However, the lack of reliable and reproducible reference electrodes for application in molten fluoride salts limits the ability to study redox control in these salts. Most tested reference electrodes in molten FLiNaK salt do not give stable potential due to a combination of the high electronegativity of fluorine and the high temperature of the medium. As such, the development of a reliable and stable reference electrode for these salts is paramount.

In this study, various kinds of reference electrodes that could be used for molten FLiNaK salt such as Dynamic Reference Electrodes (DRE), Quasi Reference Electrodes (QRE) etc. have been studied. In addition, in this study, corrosion behaviour of selected Ni and Fe based alloys were studied in molten FLiNaK at 700 ^oC. The effect of various chemical and electrochemical redox control techniques such as addition of reducing agents to the salt, purification of the salt, and the use of applied potential against a QRE were studied to understand their ability to mitigate corrosion of selected alloys in FLiNaK. This presentation will discuss our results on different reference electrode designs as well as the efficacy of each tested redox control method in controlling the corrosion of candidate alloys in molten FLiNaK.