Structure and transport properties of LiF–BeF2 mixtures: Comparison of rigid and polarizable ion potentials#

Abstract

Molecular dynamics simulations are performed to study the structure and dynamics of the LiF-BeF 2 system over a range of compositions using the transferable rigid-ion model (TRIM). The densities obtained with the TRIM potential are approximately 17-20% lower than the experimental values while polarizable ion models (PIM) give densities within 5% of the experimental value. The TRIM and PIM potentials give essentially identical radial distribution functions (RDFs) for Li-F and Be-F ion pairs though the Be-Be pair correlations differ significantly and reflect the corresponding density differences. The variation in the radial distribution functions with concentration, particularly the anion-anion pair correlation function, reflects the reorganization of the fluoride ions as the addition of BeF 2 in the mixture promotes the formation of the tetrahedral fluoroberyllate network. Along the 67 mol% LiF isopleth, diffusivities and Nernst-Einstein ionic conductivities from simulations using the PIM and TRIM potentials are in good agreement for temperatures up to 925 K. The viscosity data using the PIM model is also found to be in good agreement with the TRIM results presented here along the 873 K isotherm for compositions ranging from 0 to 50 mol% BeF 2. The main conclusion from this study is that the non-polarizable, TRIM provides reasonable results for the structural correlations and transport properties of the LiF-BeF 2 system in comparison with first-principles-based, PIM.

Thermodynamic and Transport Properties of LiF and FLiBe Molten Salts with Deep Learning Potentials

“…This is a critical feature for general modeling of molten salts, considering the complex and diverse local complexes formed in melts such as BeF 4 2− tetrahedral and polymer-like chains in denser compositions. 21 In addition, we validate the results of the training by evaluation of transport properties as functions of temperature. It is well known that the transport properties of molten salts are the most difficult to evaluate both experimentally and theoret-ically, 10,22−25 and from our knowledge, no other works have evaluated the transport properties of molten salts with MLPs.…”

The investigation of thermal neutron scattering data for molten salt Flibe

“…Mixture Flibe (67%LiF-33BeF 2 ) [18] is given in Figure 2. From Table 2 and Figure 2, it is easy to conclude that diffusion coefficient of LiF is an order of magnitude higher than that of BeF 2 .…”

Temperature-Dependent Properties of Molten Li2BeF4 Salt Using Ab Initio Molecular Dynamics

“…Among the leading candidate is the molten salt reactor (MSR) where molten fluoride salts are being considered both as a primary coolant and as a solvent for nuclear fuel. Lithium tetrafluoroberyllate (FLiBe) is one of the fluoride salts that have been extensively used in the MSR experiment(s) as a coolant and solvent for fissile material due to its advantageous thermophysical properties. ,,− Properties such as small absorption cross section for thermal neutrons, high thermal efficiency, low spent fuel per unit energy, high degree of passive safety, high heat capacity, atmospheric pressure operation, chemical stability at high temperature, low melting point, and high boiling point make FLiBe a primary candidate for application in MSRs. ,,− FLiBe has a melting and boiling point of ∼732 and ∼1703 K, respectively . The liquid salt is a good solvent for nuclear fuel as it can dissolve high concentrations of Th + U fuel .…”

“…Although the experimental efforts have been accumulating ,− in parallel with simulation, ,− the lack of fundamental understanding at the atomistic level has been an issue in the required application of FLiBe for the MSR. Recently available specialized facilities and instrumentation have enhanced the state-of-the-art experimental research on molten salts. ,− However, an understanding of the connectivity between atomistic properties with experimental characterization results is crucially important especially given that FLiBe is toxic and corrosive at high temperatures which precludes cost-effective experimental research over a wide range of temperatures .…”

“…There are relatively few such studies on FLiBe and they are limited to a structure at a particular or a narrow range of temperatures. Even where classical and first-principles molecular dynamics (MD) have been used to some extent to study crystalline, solid, and liquid FLiBe ,,,− , using different methods and potentials, they are mostly focused on the structure, dynamical features, thermodynamic and vibrational properties, thermal neutron scattering cross section data, and so forth, whereas information on electronic, optical, and mechanical properties of FLiBe over a wide range of temperatures is still not covered.…”