First-principles molecular dynamics simulations of UCl_n-MgCl₂ (n = 3, 4) molten salts

Molten chlorides are a preferred choice for fast-spectrum molten salt reactors. Molten MgCl₂ forms eutectic mixtures with NaCl and is considered as a promising dilutant to dissolve fuel salts such as UCl₃ and UCl₄. However, the structure and chemical properties of UCl_n (n = 3, 4) in molten MgCl₂ are not well understood. Here we use first-principles molecular dynamics to investigate the molten salt system UCl_n-MgCl₂ (n = 3, 4) at various concentrations of U³⁺ and U⁴⁺. It is found that the coordination environment of Cl⁻ around U³⁺, especially in the first coordination shell, varies only slightly with the uranium concentration and that both the 7-fold coordinate (UCl₇⁴⁻) and 6-fold coordinate (UCl₆³⁻) structures dominate at ∼40%, leading to an average coordination number of 6.6-6.7. A network or polymeric structure of U³⁺ cations sharing Cl⁻ ions is extensively formed when the mole fraction of UCl₃ is greater than 0.2. In contrast, the average coordination number of Cl⁻ around U⁴⁺ is about 6.4 for a mole fraction of UCl₄, x(UCl₄), of 0.1 but decreases to 6.0 for x(UCl₄) = 0.2 and then stays at about 6.0-6.2 with the uranium concentration. The 6-fold coordinate structure (UCl₆²⁻) is the most populous in UCl₄-MgCl₂, at about 60%. U-Cl network formation becomes dominant (>50%) only when x(UCl₄) > 0.5. Unlike Na⁺, Mg²⁺ forms a network structure with Cl⁻ ions and when x(UCl₃) or x(UCl₄) < 0.5, over 90% of Mg²⁺ ions are part of a network structure, implying the complex influences from Mg²⁺ on the coordination of Cl around U.