Mapping relationships between cation-F bonds and the heat capacity, thermal conductivity, viscosity of molten NaF-BeF2

Abstract

Molten NaF-BeF2 has been proposed as one of the potential solvents and coolants for the Generation IV nuclear reactor, due to the lower price and satisfactory neutron capture cross-section. To gain comprehensive information on the structures and thermophysical properties of molten NaF-BeF2, FPMD simulations are applied to study its bond lengths, bond angles, and polyhedrons as well as the densities, specific heat capacity, thermal conductivities, and viscosities in the temperature range of 673–1073 K. These results show that 4- and 6-fold coordination structures of Be-F and Na-F are dominant in molten NaF-BeF2, respectively, and the diversity of Na-F polyhedral clusters originates from the inferior strength and stability of Na-F bonds. Besides, the Be-F bond is 6 times more stable than Na-F, whose strong stability leads to the large specific heat capacity (>1.5 J g−1 K−1) of NaF-BeF2, thereby the high ability of heat storage. Furthermore, the mapping relationship between the viscosity and structure of molten NaF-BeF2 are revealed, where the Na-F bond plays a key role on reducing viscosity (<5 cP above 873 K) that makes its use as an industrial coolant possible.