Electro-reduction processes of U3O8 to metallic U bulk in LiCl molten salt

Abstract

U3O8, the most stable state of uranium oxides, is a key intermediate compound for pyroprocessing of used nuclear fuel. However, compared to UO2, the electro-deoxidation mechanism of U3O8 is still cursory due to the complicated U-O intermediate phases. In this paper, the electro-deoxidation processes of U3O8 were investigated via diverse methods. A molybdenum metallic cavity electrode loaded with U3O8 powder was adopted to investigate the electrochemical behavior via cyclic voltammetry and square wave voltammetry. Sintered U3O8 pellets were used as cathodes to obtain electrolytic products with varying applied voltages or duration time. The composition and microstructure of electrolytic products were characterized by Raman spectroscopy, X-ray diffraction, scanning electron microscope and energy dispersive spectrometer. Based on above analysis results, electro-reduction processes of porous U3O8 pellets were proposed. First, U3O8 undergoes a rapid transition to U4O9 (U4O9-y). Then, the intermediate compound was gradually reduced to UO2 from the outer layer to the center of the bulk due to the limited migration rate of O2−. Before the transformation completed, metallic U generated on the surface of the bulk through direct electro-deoxidation and Li metal thermal reduction. With the increase of porosity and the propagation of metal-oxide-electrolyte three-phase interline, the U3O8 pellet was finally reduced to metallic U bulk.