The field of molten salts has experienced a recent resurgence due to potential applications in the energy sector. In particular, molten salt reactors are among the leading candidates for next-generation nuclear reactor designs. In support of these efforts, we are focused on two items: (1) evaluating the performance of classical molecular models of molten salts across a range of thermodynamic and transport properties, and (2) developing and applying molecular simulation methods to compute critical macroscopic thermodynamic properties such as solid-liquid coexistence, excess properties, and solid solubilities.<br/><br/>In our previous work, we demonstrated that the polarizable ion model (PIM) has mixed performance for predicting the structural, dynamic, and thermodynamic properties of the alkali chlorides. Here, we maintain our focus on the chloride salts but begin investigating mixtures and divalent salts: KCl-NaCl, KCl-LiCl, and KCl-MgCl<sub>2</sub>. We first evaluate the excess properties upon mixing and then proceed to compute binary phase diagrams with a direct coexistence method and a method based upon the equality of the chemical potentials in the solid and liquid phases. We utilize charge neutral alchemical transformations and the Gibbs-Helmholtz equation to map the excess Gibbs energy, enthalpy, entropy, and volume of the liquid mixtures as a function of temperature and composition. We find that the PIM does a reasonable job of predicting the excess properties of the mixtures. Though the PIM does poorly at reproducing the experimental binary phase diagrams, we show that the large discrepancies between the PIM and experiment can be primarily attributed to the failure of the PIM to accurately predict the pure component solid-liquid equilibrium. Our calculations totalled multiple microseconds of simulations with a polarizable model and the techniques we applied demonstrate rigorous methods of evaluating the ability of molecular models of molten salts to predict the types of macroscopic thermodynamic and dynamic properties that are relevant to molten salt reactor applications.

Computing excess properties and binary phase diagrams of molten salts from molecular dynamics

ACS Fall 2021

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