First-principles molecular dynamics simulations of UCl n -MgCl 2 ( n = 3, 4) molten salts.

Molten chlorides are a preferred choice for fast-spectrum molten salt reactors. Molten MgCl 2 forms eutectic mixtures with NaCl and is considered as a promising dilutant to dissolve fuel salts such as UCl 3 and UCl 4 . However, the structure and chemical properties of UCl n ( n = 3, 4) in molten MgCl 2 are not well understood. Here we use first-principles molecular dynamics to investigate the molten salt system UCl n -MgCl 2 ( n = 3, 4) at various concentrations of U 3+ and U 4+ . It is found that the coordination environment of Cl - around U 3+ , especially in the first coordination shell, varies only slightly with the uranium concentration and that both the 7-fold coordinate (UCl 7 4- ) and 6-fold coordinate (UCl 6 3- ) structures dominate at ∼40%, leading to an average coordination number of 6.6-6.7. A network or polymeric structure of U 3+ cations sharing Cl - ions is extensively formed when the mole fraction of UCl 3 is greater than 0.2. In contrast, the average coordination number of Cl - around U 4+ is about 6.4 for a mole fraction of UCl 4 , x (UCl 4 ), of 0.1 but decreases to 6.0 for x (UCl 4 ) = 0.2 and then stays at about 6.0-6.2 with the uranium concentration. The 6-fold coordinate structure (UCl 6 2- ) is the most populous in UCl 4 -MgCl 2 , at about 60%. U-Cl network formation becomes dominant (>50%) only when x (UCl 4 ) > 0.5. Unlike Na + , Mg 2+ forms a network structure with Cl - ions and when x (UCl 3 ) or x (UCl 4 ) < 0.5, over 90% of Mg 2+ ions are part of a network structure, implying the complex influences from Mg 2+ on the coordination of Cl around U. The present work reveals the impact of MgCl 2 as a solvent for UCl n ( n = 3, 4) on the U-Cl coordination and structure, and motivates further studies of their transport properties and the tertiary systems containing MgCl 2 -UCl n .