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 thoroughly 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 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 33 mol%. Moreover, we find that the first solvation 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 U4+ concentration is larger than 33 mol%. This work sheds new light on the difference between U3+ and U4+ in molten NaCl.

First principles molecular dynamics simulations of UCln–NaCl (n=3, 4) molten salts

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.