First-Principles Molecular Dynamics Simulations of UCln-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 UCln (n = 3, 4) in molten NaCl at various mole fractions. We find that 7-fold- and 8-fold-coordinate structures of CI 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 solution 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.