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

Abstract

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 UCln (n = 3, 4) in molten MgCl2 are not well understood. Here we use first-principles molecular dynamics to investigate the molten salt system UCln-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. The present work reveals the impact of MgCl2 as a solvent for UCln (n = 3, 4) on the U-Cl coordination and structure, and motivates further studies of their transport properties and the tertiary systems containing MgCl2-UCln.