Electrolytic Reduction of UO2 Microspheres Synthesized Via Internal Gelation Method

Abstract

Pyroprocessing is the combination of process steps to extract and recycle actinides from fission products in spent nuclear fuels in high-temperature molten salt media. Pyroprocessing has been studied extensively over the past decades. One of the key sub-processes is the electrolytic reduction of uranium oxides. The electrolytic reduction process originated from the FFC Cambridge process, an approach for electrochemically reducing titanium dioxide into titanium metal in molten CaCl2. In the electrolytic reduction of uranium oxides, the uranium oxide feed is applied as the cathode in LiCl-Li2O molten salt electrolyte at 650°C. LiCl appears more promising than conventional CaCl2 owing to the lower melting point (878 K), and higher decomposition voltage (3.46 V at 923 K). Additionally, Li2O is expected to speed up the reduction via an additional chemical reduction pathway with the participation of Li metal, and prevents the direct dissolution of the Pt anode.This work aims to study the morphology effect of the oxide feed on the reduction kinetics and the incorporation of impurities during the electrolytic reduction process. The electrolytic reduction of UO2 is carried out on sintered UO2 pellets, as well as various UO2 microspheres synthesized via internal gelation. The introduction of uranium oxide microspheres holds several advantages. The spheres provide a higher surface area, which is expected to improve the overall reduction efficiency of uranium oxide during electrolysis. In addition, it is easier to handle the feed during loading and unloading of the cathode basket compared to powder feeds.The potentiostatic and galvanostatic electrolysis of UO2 is performed, and the comparison between the electrolytic reduction of UO2 pellets and microspheres is investigated. The SEM micrographs in figure 1show the morphology change of a UO2 pellet to uranium metals particles. A significant morphology change between the flat surface of the sintered UO2 feed and the reduced U particles is observed at the surface. Further characterization is performed using XRD and TGA to identify the products of the electrolytic reduction process qualitatively and quantitatively. Results show that galvanostatic electrolysis shows a better reduction efficiency compared to potentiostatic electrolysis, applied both below and above the Li formation onset potential. Overall, there is barely un-reacted UO2 visible in the XRD patterns of the reduced feed after galvanostatic electrolysis. U metal and UO are identified in the XRD patterns depending on electrolysis parameters. The presence of UO can be as an intermediate of UO2 reduction during electrolysis, which is a rare compound to be reported in literature. Overall, the reduction of microspheres does improve the overall reduction efficiency compared to the conventional pellet feeds. Figure 1