Thermal Property Modeling and Assessment of the Physical Properties of FLiNaK

Abstract

The thermodynamic properties of the LiF–NaF–KF system and its subsystems were re-evaluated using the CALPHAD method, focusing on the deviations from ideality in the heat capacity [Cp(T)] of mixtures, temperature-dependence of mixing enthalpies (ΔmixH), vapor pressures of mixtures, and enthalpies of fusion (ΔHfus) using reported values and measurements. The results of this work confirmed a pseudoternary eutectic at 732.9 K and 46.5LiF–11.5NaF–42KF mol % (FLiNaK) with determined Cp(T) = 69.822 + 0.000679T + 2,137,152T–2 J mol–1 K–1 (732.9 < T < 1200 K) and ΔHfus = 18,051 J mol–1 in good agreement to the experimental values. The resulting accurate thermodynamic representation can be used to find relevant excess thermochemical properties in the whole pseudoternary space and provide insights into their composition and temperature dependence. A detailed evaluation of select thermophysical properties was also conducted to determine recommended density, thermal diffusivity, thermal conductivity, vapor pressures, and boiling point with calculated uncertainties over the temperature range of interest for FLiNaK–salt energy applications.