Identification, measurement, and mitigation of key impurities in LiCl-Li2O used for direct electrolytic reduction of UO2

Direct electrolytic reduction (DER) of UO2 utilizes a molten LiCl-Li2O electrolyte operating at 650 °C.Productive processes at the cathode are UO2 and Li2O reduction (followed by metallothermic reduction of UO2 by Li metal). Non-productive reduction current has been observed that could be attributed to impurities produced from interaction of the salt with water. Cyclic voltammetry (CV) scans of molten mixtures of LiCl containing Li2O, LiOH, and/or Li2O2 indicate that LiOH is the primary cause of reduction current preceding UO2 and Li2O reduction. LiOH reduction appears as a broad soluble-soluble transition and forms a pre-reduction plateau characteristic of micro-electrode reactions. Electrochemical response to LiOH in the salt is reduced with addition of Li0, providing a practical approach to eliminating LiOH from LiCl-Li2O salt before the DER process. Generation of H2 gas was detected while heating Li0 with LiCl-Li2O-LiOH, consistent with a reaction mechanism where LiOH reduction causes formation of Li2O and H2.