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

Rotational viscometry with the FRS 1600 (Anton Paar, Austria) high-temperature rheometer was used to obtain temperature dependences of the dynamic viscosity of molten lithium and beryllium fluoride salts considered as candidate fuel and coolant compositions for the molten salt reactor (MSR) for burning long-lived actinides from the spent nuclear fuel of the PWR 1000/1200 pressurized water reactor. 0.66LiF–0.34BeF2 and (0.73LiF–0.27BeF2)+UF4 molten salt mixtures containing 1 and 2 mol.% UF4 were investigated with regard to the MSR intermediate and fuel circuits. Salt mixtures were prepared by the direct melting of components and certified using X-ray phase and elemental analysis. The «shear rate» parameter was selected according to the viscosity curves obtained in the studied melts at 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 shear rate was 11 s–1. Viscosity values of LiF–BeF2–UF4 melts obtained from experiments in the temperature range from liquidus to 800 °C are described by the linear equation logη = a + b/t, but their temperature coefficients differ evidently, which indicates a significant dependence of the viscosity of these melts on composition and temperature. Viscosity values obtained for the 0.66LiF–0.34BeF2 melt agree with the available literature data within 7–10 % in the temperature range of 650–750 °C. With an increase in the LiF content, melt viscosity decreases: it is lower by 20 % in the 0.73LiF–0.27BeF2 melt at t = 650 °C. However, when 2 mol.% UF4 is added, the 0.73LiF–0.27BeF2+UF4 fuel salt viscosity increases by 10 % at the same temperature.