Molecular dynamics simulations of structural and transport properties of molten NaCl-UCl₃ using the polarizable-ion model

Molten chlorides are a good choice for fast-spectrum molten salt reactors, but many of their thermophysical and transport properties are not available. In this work we use classical molecular dynamics simulations based on the polarizable-ion model to investigate the structural and transport properties of NaCl-UCl₃ at various U³⁺ mole fractions. Molar volume, coordination structure and number, network structure, heat capacity, diffusivity, and ion conductivity have been simulated. We find strong variation of the second coordination shells (U-U and Cl-Cl) with U³⁺ mole fraction. This is consistent with the increasing network formation or oligomerization of UCl₆ ^{3 −} and UCl₇ ^{4 −} units via shared Cl⁻ ions. The movement of Cl⁻ is strongly coupled with U³⁺ as the U-Cl network structures become dominant in the mixture when the mole fraction of UCl₃ is >0.25. Evaluation of a heat-transfer figure-of-merit suggests that 0.20 to 0.30 is an optimal mole fraction of UCl₃ to achieve both lower operating temperatures and better heat transfer.