Adding MgCl2 to Molten NaCl-UCln (n=3, 4): Insights from First-Principles Molecular Dynamics.

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Molten chlorides are proposed for fast-spectrum molten salt reactors. Molten MgCl2 with NaCl forms eutectic mixtures and is considered as a promising dilutant to dissolve fuel salts such as UCl3 and UCl4 . A previous study suggests the formation of a U-Cl network at the U : Na=1 : 1 binary salt. However, it is unclear how the structure of UCln (n=3, 4) in NaCl will change after adding MgCl2 in the salt. Here, we use first-principles molecular dynamics to investigate the molten ternary salts NaCl-MgCl2 -UCln (n=3, 4) at various concentrations of Mg2+ in NaCl-UCln with a fixed ratio of Na : U at 1 : 1. It is found that the addition of Mg2+ to NaCl-UCl3 leads to a higher coordination number (from 6.5 to 6.7) of Cl around U while the U-Cl network structure slightly decreases with the Mg concentration. Adding MgCl2 to NaCl-UCl4 , however, breaks down the U-Cl network more completely. We attribute the different behavior of adding Mg2+ into NaCl-UCl3 and NaCl-UCl4 to the difference between U(III) and U(IV) in attracting Cl- ions to form the first coordination shell. The present work reveals the impact of MgCl2 as a dilutant solvent for NaCl-UCln fuel salts, which will be helpful in further studies and understanding of the thermophysical and transport properties of ternary systems.