Solubility of oxides and other substances in fluoride molten salts with a primary focus on FLiNaK (eutectic): overview and experimental challenges

Molten fluoride salts are of growing interest for advanced energy technologies, particularly as heat-transfer fluids and potential coolants in next-generation nuclear reactors. Among them, the eutectic mixture FLiNaK (46.5 mol% LiF–11.5 mol% NaF–42 mol% KF) is widely used as a model of and complement to FLiBe due to its low cost and the absence of toxic beryllium. A critical issue for reliable deployment of such salts is the solubility of oxides and other compounds, which strongly affects both material compatibility and the interpretation of corrosion studies. This review provides a systematic overview of solubility data for oxides, fluorides, semimetals, and selected transition-metal compounds in FLiNaK. Reported solubility of oxides and other substances in fluoride melts differ by orders of magnitude, from μg/kg to g/kg, depending on the oxide-fluoride combination. Studies on transition-metal oxides reveal complex interactions with alloying elements, highlighting the importance of redox chemistry and impurity control. Attention is drawn to the formation of oxofluoride species, the role of microstructural degradation products, and the limited information on semimetals, borides, and refractory ceramics. Available data remain fragmentary and often inconsistent due to challenges such as oxygen contamination, reference stability, and uncertainties in quantification. Addressing these gaps through improved solubility measurements and a deeper understanding of chemical equilibria in FLiNaK is essential for assessing long-term material behaviour and for advancing molten-salt technologies in future energy systems. • Systematic review of oxide and compound solubility in molten FLiNaK. • Solubility spans μg/kg to g/kg due to thermodynamics and complex formation. • Identifies the most suitable solubility methods: ICP, CV/SWV, Raman, XRD, EIS. • Key gaps remain in complex ions, oxo-fluorides, and refractory oxides. • Proposes a workflow: purification, speciation control, solubility test, validation.