Molecular Dynamics Simulation Of Diffusion And Viscosity In Liquid Fluorides

At present, the energy problem is increasingly serious, the research and development of the nuclear energy with high energy density, low carbon emissions and the potential of sustainable development has caused widespread concern. The nuclear energy includes nuclear fusion energy and nuclear fission energy. Molten salt reactor is a kind of nuclear fission reactors, in which liquid fluoride salt mixtures as the coolant, solvent and carrier. As the coolant and solvent, the liquid fluoride salt mixtures make the fuel can be dissolved, simplify the structure of the reactor, and also make the fuel consumption equably. As the carrier, the liquid fluoride salt mixtures can provide the molten carrier, and improve the physical and chemical properties of the eutectic, improve the energy efficiency of molten salt reactor, and reduce the risk of accident s, thus make a safer environment for the reactor. As a result, their diffusion properties and flowabilit y, i.e., the diffusion coefficients and viscosit y has caused much attentio n from people. In this paper, we study the diffusion coefficients and viscosity of Li F, Na F, and KF these three fluorides in liquid state, and analyze the influence of temperature and pressure on the diffusion coefficient, the influence of temperature on the viscosity by molecular dynamics simulation method with Born-Mayer potential.At first, the RDF curves of Li F, Na F, and KF are calculated at various temperatures, and then we forecast the respective melt ing point though the results of the RDF curves, and then verify the prediction by calculating the lattice energy along with the change of temperature. The melt ing points of the three kinds of fluorides are less than the experimental values, and this is mainly due to the potential functions. In addition, the melt ing points of the three fluorides are getting bigger after applying pressure to the system, this is because that the distance between the ions will become smaller, and the combination between ions will become very strong after applying pressure, and then lead to the result of the melting point of ionic crystal becoming bigger because of the pressure. In the present work, we study the diffusion properties of Li F, Na F, and KF these three fluorides in liquid state as the key tasks. The diffusion coefficients of anions and cations in all three fluorides are increasing with the increase of the temperature when the system is not coupled with the pressure. Our calculated values are in consistent with the experimental data and other molecular dynamics valves. After combined with the pressure, the diffusion coefficients also increase with the rising of temperature, but for the same temperature, the diffusion coefficients decrease with the increasing pressure, while the diffusion activation energy increases. These results are in agreement with the empirical rules. Contrasting the diffusion coefficients of these three fluorides, we find out that the diffusion coefficients of the ions which have the lighter qualit y are bigger than the ones of ions with the heavier qualit y, and the ions with lighter qualit y are easier to diffuse. In addit ion, this article also calculates the relationship between temperature and viscosity of liquid Li F, Na F, and KF. The viscosity of the three fluorides decreases with the increase of temperature in this work, and has some slightly difference with the experimental results, which is mainly caused by the melt ing points lowering than the experimental values of this work. In this work, we also calculate the activation energy of viscosit y for the three fluorides, and the calculated results are in consistent with the experimental results. The rule of viscosit y in the present paper is in agreement with the experience rule, the higher the temperature, the greater diffusion coefficients, the better of liquid properties of material, and the smaller the viscosity would have been.The research of this work will provide the basis for the analysis of the mechanism of the diffusion properties and flowabilit y in liquid fluorides.