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

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 UCl_n (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−UCl_n (n=3, 4) at various concentrations of Mg²⁺ in NaCl−UCl_n 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−UCl_n fuel salts, which will be helpful in further studies and understanding of the thermophysical and transport properties of ternary systems.