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

Abstract

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.