Thermal Properties of Some Molten Mixtures in System (NaF-KF)_eut–UF₄

The thermophysical properties of molten salts promising for the nuclear industry are crucial, but the available data are limited and contradictory. The thermal diffusivity of the molten mixtures (NaF-KF)_eut–UF₄ containing 30, 40, and 50 mol % UF₄ was measured by the laser flash method. The thermal diffusivity was found to be almost independent of temperature in the range of about 100° (1070 K–1170 K), which makes it possible to extrapolate values to low or high temperatures. The thermal conductivity of the molten mixtures (NaF-KF)_eut–UF₄ was calculated using the thermal diffusivity, density, and heat capacity data. Density was estimated using the molar volume of the molten binary systems NaF-UF₄ and KF-UF₄. The heat capacity of the ternary system NaF-KF-UF₄ was evaluated based on the heat capacity of individual salts, taking into account the additivity law. The thermal conductivity of the molten mixtures 0.7(NaF-KF)_eut–0.3UF₄, 0.6(NaF-KF)_eut–0.4UF₄, and 0.5(NaF-KF)_eut–0.5UF₄ changes slightly with the UF₄ content and temperature; for example, at 1123 K it is 0.48, 0.45, and 0.45 W·m⁻¹·K⁻¹, respectively. The thermal diffusivity of the pure molten UF₄ was estimated using three independent approaches: the concentration, the temperature, and the volume dependences of the thermal diffusivity. The estimated value of the thermal diffusivity for the molten UF₄ was about 0.12 × 10^–6 ± 0.05 × 10^–6 m²·s⁻¹.