The Viscosity of Molten Salts Based on the LiF–BeF2 System

Abstract

The temperature dependences of the dynamic viscosity of molten lithium and beryllium fluorides, which are considered as candidate compositions for fuel and coolant of the molten salt reactor (MSR) burner of long-lived actinides from spent nuclear fuel of a VVER 1000/1200 (pressurized water reactor), were obtained by rotational viscometry using an FRS 1600 high-temperature rheometer (Anton Paar, Austria). Molten salt mixtures 0.66LiF–0.34BeF₂ and (0.73LiF–0.27BeF₂) + UF₄ containing 1 and 2 mol % UF₄ were studied with regard to the intermediate and fuel circuits of the MSR. The salt mixtures were prepared by direct melting of components and certified using X-ray phase and elemental analyses. The shear rate parameter was selected according to the viscosity curves obtained in the melts at a temperature of 700°C. It was found that the viscosity does not depend on the shear rate in the range of γ = 6–20 s^–1. When measuring the temperature dependence of viscosity, the γ value was 11 s^–1. The experimentally obtained values of the viscosity of the LiF–BeF₂–UF₄ melts in the temperature range from liquidus to 800°C are described by the linear equation log η = a + b/t, but their temperature coefficients differ noticeably, which indicates a significant dependence of the viscosity of these melts on composition and temperature. The obtained viscosity values for the 0.66LiF–0.34BeF₂ melt agree with the available published data within 7–10% in the temperature range of 650–750°C. With an increase in the LiF content, the viscosity of the melt decreases: it is 20% lower in the 0.73LiF–0.27BeF₂ melt at t = 650°C. However, when 2 mol % UF₄ is added, the viscosity of the fuel salt 0.73LiF–0.27BeF₂ + UF₄ increases by 10% at the same temperature.