Study on Electrolytic Reduction of Uranium Dioxide in Molten Lithium Chloride Molten Salt

The electrolytic reduction of uranium oxide is recognized as playing a pivotal role in the current mixed oxide fuel (MOX) spent fuel pyroprocessing. This technology is crucial for the entire reprocessing process, as it directly influences the efficiency and effectiveness of the process. In this paper, the reduction potential of uranium dioxide in lithium chloride molten salt was investigated with rigorous methods such as thermodynamic calculations and cyclic voltammetry. During the experimental study, an electrochemical setup was designed. A metal basket loaded with a ten-gram uranium dioxide pellet was utilized as the working electrode, while another metal basket loaded with lithium metal as the counter electrode, and nickel-nickel oxide as the reference electrode. By meticulously controlling either the potential or the current, the intricate effects of various parameters on the electrolytic reduction of uranium dioxide were systematically investigated. These parameters include molten salt impurities, reduction potential, charge ratio, cathode-anode distance, and cathode-anode area ratio. Following preliminary determinations of the optimal process parameters through extensive experimentation, uranium dioxide reduction experiments involving a substantial amount of material, up to 100 g, were conducted. The cathode products and molten salts obtained from these experiments underwent rigorous analysis using advanced techniques such as X-ray powder diffraction, acid-base titration, and metal ion analysis. The experimental results reveal that the reduction products are predominantly composed of uranium metal and uranium carbide, with the characteristic peak area of uranium dioxide accounting for only 6.9%, indicating a high degree of reduction is achieved. Furthermore, the analysis results of the reduced particles show that they contain approximately 11.4% lithium chloride, 5.83% lithium oxide, and 0.11% lithium metal. This work not only validates the feasibility of uranium dioxide reduction but also sheds light on the key influencing factors of the reduction process. The findings obtained provide foundation for subsequent large-scale reduction experiments in the field of MOX spent fuel pyroprocessing.