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

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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 UCl63− and UCl74− 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.