Molten salts have the potential to support low-carbon emission technologies, including nuclear reactors, fusion power, advanced concentrating solar power, and thermal energy storage. Thermophysical properties of salt mixtures are needed to understand and predict salt performance and behavior. Density is fundamental as it determines salt volume and underlies many of the other thermophysical properties of salt systems. Neutron radiography at the High-Flux Isotope Reactor (HFIR), Oak Ridge National Laboratory, was used to measure the density of molten chloride and molten fluoride salts as a function of temperature. Analysis of the radiographs to obtain densities was done either by measurement of changes in the height of the column of molten salt as it was heated or cooled (dilatometry) or by calculating the change in neutron attenuation. Resulting densities as a function of temperature for different molten chloride salts were compared with calculated values using Redlich-Kister (RK) correlations and showed good agreement (within 0.1~7%) except for KCl(63)-MgCl₂(37) and KCl-UCl₃-NaCl salt mixtures. Deviations were attributed to bubbles or voids in the salts that were especially problematic when the salt was heated through the melting point. The results improved if data were taken during cooling from the maximum temperature, 800°C for these measurements. Radiography also permitted the measurement of contact angle and surface tension.

Neutron attenuation coefficient comparison in 2D contour graph between FLiBe and 20% LaF<sub>3</sub>+FLiBe. The top portion above the crystal formed from 20% LaF<sub>3</sub> mixture (right) showed 9.1% of LaF<sub>3</sub> dissolved in the FLiBe salt. The neutron attenuation coefficient difference between FliBe and the bottom portion in the right image matched with ~100% LaF<sub>3</sub> crystal as an insoluble phase.

Neutron attenuation coefficient comparison in 2D contour graph between FLiBe and 20% LaF₃+FLiBe. The top portion above the crystal formed from 20% LaF₃ mixture (right) showed 9.1% of LaF₃ dissolved in the FLiBe salt. The neutron attenuation coefficient difference between FliBe and the bottom portion in the right image matched with ~100% LaF₃ crystal as an insoluble phase.

Hunter Andrews

Oak Ridge National Laboratory

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