LiCl-Li2O熔盐中U3O8的电化学还原及机理研究OA

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.