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.