First-Principles Molecular Dynamics Simulations of UCl_n-NaCl (n = 3, 4) Molten Salts

Molten chloride salts are one of the two main choices to dissolve nuclear fuels for fast-spectrum molten-salt reactors, as a generation IV reactor design. Both U³⁺ and U⁴⁺ can stably exist in molten chlorides, but the differences in their structure and chemical properties in molten NaCl are not well understood. In this work we use first-principles molecular dynamics simulations based on density functional theory to investigate the structure of UCl_n (n = 3, 4) in molten NaCl at various mole fractions. We find that 7-fold- and 8-fold-coordinate structures of Cl around U become more populous in UCl₃-NaCl when UCl₃ concentration is higher than 25 mol %, while in UCl₄-NaCl the 6-fold-coordinate structure is the leading species when the UCl₄ concentration is higher than 32 mol %. Moreover, we find that the first solvation shell of Cl around U is more dynamic in UCl₃-NaCl than UCl₄-NaCl, evidenced by weaker U-Cl bonds and faster U-Cl dissociation in the former. Further analysis reveals that a network structure with sharing Cl^- between U cations starts forming at the low U³⁺ concentrations, whereas the U⁴⁺-Cl^--U⁴⁺ linkages become noticeable only after UCl₄ concentration is larger than 32 mol %. This work sheds new light on the difference between U³⁺ and U⁴⁺ in molten NaCl.