Coupled structural-thermodynamic modelling of the molten salt system NaCl-UCl₃

Abstract

Molten chloride salts are ionic liquids in which the anions and cations exhibit network formation. An attractive salt system for use in molten salt reactors is NaCl-UCl₃, an ionic liquid with complex non-ideal thermodynamic behaviour due to the formation of short-range order. The relationship between local structure and thermodynamic properties is investigated in this work, in which molecular dynamics simulations using the Polarizable Ion Model (PIM) and thermodynamic modelling by means of the CALPHAD method are combined. The system is simulated for a wide range of temperatures and compositions and various properties are derived derived from molecular dynamics data: density/molar volume, thermal expansion, heat capacity and excess properties including excess molar volume, mixing enthalpy and excess heat capacity. Generally, there is good agreement with previously published experimental data. An in depth analysis of the local structure of the liquid is performed for multiple temperatures. This analysis demonstrates the transition from a molecular liquid consisting of primarily Na⁺, Cl⁻, UCl₆^3-/UCl₇^4- at low UCl₃ content to a polymeric liquid at high UCl₃ content, manifesting itself in the formation of species like U₂Cl₁₂^6-, U₃Cl₁₇^8-, U₄Cl₂₂^10- etc. Exceeding 40% UCl₃, the liquid consists of a three-dimensional network of corner or edge-sharing uranium polyhedra. The output of the MD simulations and experimental data are incorporated into a coupled structural-thermodynamic model for the NaCl-UCl₃ system based on the quasi-chemical formalism in the quadruplet approximation, that provides a physical description of the melt and reproduces (in addition to the thermodynamic data) the chemical speciation of uranium polymeric species predicted from the simulations.