Application of ¹⁹F NMR for the structure investigation of nuclear-grade fluoride molten salts

BackgroundMolten fluorides (e.g., FLiBe) are widely employed in molten salt reactors as coolants and fuel solvents due to their excellent heat transfer properties and favorable neutronic characteristics.PurposeThis study aims to utilize the unique chemical characteristics of fluoride salts and apply high-resolution 19F Nuclear Magnetic Resonance (NMR) techniques to investigate their microscopic structures and temperature-driven transformation mechanisms.MethodsHigh-resolution 19F NMR was combined with magic-angle spinning (MAS), high-temperature, and variable-temperature experiments to systematically characterize the microstructure and dynamic transformation behavior of molten fluoride samples. Furthermore, the solid-liquid phase transition was investigated, and a quantitative 19F NMR method based on cooled solid samples was established to speculate the distribution and concentration of ionic structures in the molten state.ResultsThe results demonstrate that variations in chemical shifts observed in high-temperature 19F NMR spectra provide key structural information and reveal temperature-dependent transformation mechanisms. In addition, the proposed quantitative solid-state NMR approach successfully enables the analysis of ionic structures of molten salt at high temperature, showing the promising applicability for evaluating the Lewis acidity and basicity of fluoride ions.ConclusionsBy combining multiple 19F NMR analytical strategies proposed in this study provides comprehensive insights into the composition, structural evolution mechanisms of molten fluorides, thereby offering a solid foundation for establishing component-structure–property relationships in molten salt systems.