Influence of ZrF4 additive on the local structures and thermophysical properties of molten NaF-BeF2

Molten NaF-BeF2 (FNaBe) eutectic salt containing a small amount of ZrF4 additive has attracted extensive attention in the application of fuel salt carriers for the Gen IV nuclear reactor. Herein, the local structures of radial and angular distribution functions (RDFs and ADFs) as well as the thermophysical properties of densities, thermal conductivities, and viscosities for molten FNaBe-xZrF4 (x = 2.3 to 12.5 mol%) are investigated through first-principles molecular dynamics simulations. Compared with pure FNaBe, the additive hardly changes the nearest neighbor structures between cations and F anions but has a noticeable impact on the RDFs of F-F and Na-Na pairs. Besides, the interaction potential barriers between Zr-F and Be-F ionic pairs are quite similar, indicating that the ability of Zr and Be to capture F ions is equivalent. From ADFs, the quadrilateral Na-F-Na-F rings, corner-shared tetrahedral Be-F series, and corner-shared octahedral Zr-F cluster configurations are clearly identified. Overall, the density, thermal conductivity, and viscosity of molten FNaBe are prominently increased by the addition of ZrF4. Eventually, it is predicted that the optimal additive concentration of ZrF4 is 8.7 mol% considering the comprehensive physicochemical performance of the molten mixture.