Electrochemical Reduction and Mechanistic of U₃O₈ in LiCl-Li₂O Molten Salt

Traditional wet reprocessing technologies, such as the PUREX process, face challenges including high volumes of waste liquid generation and operational complexity. In contrast, dry reprocessing technology has garnered significant attention due to its high efficiency and reduced waste production. The conversion of spent oxide fuel into metallic fuel through dry reprocessing technology is a critical step in advancing the construction of integrated fast reactors in China and realizing a closed nuclear fuel cycle. However, few studies on the electrochemical reduction of oxide spent fuel have been reported in China, with relatively weak theoretical foundations, experimental methods, and associated technologies. Considering that the primary component of oxide spent fuel is UO₂, UO₂ pellets are converted into U₃O₈ powder via oxidation-volatilization techniques during the head-end process. Therefore this work focused on U₃O₈ solid powder, systematically investigating its electrochemical reduction mechanism in LiCl-Li₂O molten salt at 650 ℃ through cyclic voltammetry(CV) and experiments of potentiostatic electrolysis. The phase composition, microstructure, and elemental distribution of electrolytic products were characterized using X-ray diffraction(XRD), scanning electron microscopy(SEM), and energy-dispersion spectroscopy(EDS). CV tests show that during the potential scan(1.50 V to 0.00 V vs. Li⁺/Li), six reduction peaks(c1-c6) and five corresponding oxidation peaks(a1-a5) are observed. The c1 peak(0.00 V) corresponds to Li deposition, while the c2-c6 peaks(0.15-0.84 V) are attributed to the stepwise reduction of U₃O₈ to UO₂ and metallic uranium(U). Potentiostatic electrolysis experiment shows at 1.95 V(vs. Li⁺/Li), there is exclusively generated the UO₂ phase. At 1.20 V, the products contain both UO₂ and Li₂UO₄. However, no metallic U is detected at 0.70-0.10 V. These results indicate that the electrochemical reduction mechanism of U₃O₈ likely proceeds in three steps as following: (1) a redox reaction between U₃O₈ and Li⁺ to form UO₂ and Li₂UO₄; (2) further electrochemical reduction of Li₂UO₄ to UO₂; and (3) stepwise electro-deoxidation of UO₂ to obtain metallic uranium(U). The presence of metallic U and UO₂ in the products of pulsed constant-voltage electrolysis is confirmed by XRD and SEM/EDS analyses, demonstrating that U₃O₈ solid powder can be directly electrochemically reduced to metallic U in the system of LiCl-Li₂O molten salt. Direct oxidation measurements reveal an average reduction rate of 45.47% for U₃O₈ and an average current efficiency of 22.86%. This work reveals the multi-step electrochemical reduction mechanism of U₃O₈ in LiCl-Li₂O molten salt and uncovers the transformation pathways of key intermediates. These findings provide a theoretical basis for optimizing processes parameters in the electrochemical reduction of spent oxide fuels(e.g. potential control and molten salt composition design), offering significant implications for the efficient production of metallic fuels and the sustainable development of nuclear energy.