Density Measurements of Molten LiF-BeF₂ and LiF-BeF₂-LaF₃ Salt Mixtures by Neutron Radiography

The densities of eutectic (LiF)₂-BeF₂ and mixtures of this salt (FLiBe) with LaF₃ were measured by dilatometry and by neutron attenuation from 673 K to 1,073 K. Because LaF₃ has a limited solubility in FLiBe, it was necessary to determine the amount of LaF₃ in solution before the density could be determined. The FLiBe density determination was favorably benchmarked against the literature data. A simple comparison was not available for the LaF₃-FLiBe mixtures, so extrapolation of published data was necessary based on analysis using the Molten Salt Thermal Properties Database-Thermochemistry, or MSTDB-TC, developed by the US Department of Energy. Solubilities for LaF₃ in FLiBe ranged from 1 to 4 mol % over 673 to 1,073 K. The salt system was heated and cooled over 24 h to evaluate potential changes in composition and hysteresis during the measurement. Changes in the meniscus were observed, and these were included in the correction for density determinations. Salt surface tension may have led to supersaturation of LaF₃ in the salt because the solubility curve was nonlinear with respect to the inverse temperature, as would be expected for an ideal system. Surface tension measurements are currently underway to test this hypothesis.