Electrochemical Behavior Of UO₂²⁺ In LiCl-KCl Molten Salt And Effect Of F^- On The Electrolytic Extraction Of UO₂

Spent fuel reprocessing is one of the most critical steps in the nuclear fuel cycle.In recent years,dry reprocessing technology based on molten salt electrolysis is considered to be one of the most effective methods for spent fuel reprocessing due to its unique advantages.Uranium is the most abundant element,accounting for 96 wt%in the spent fuel.In order to realize the recovery and reuse of uranium,the spent fuel needs to be reprocessed.Thus,this work intends to explore the recovery of spent oxide fuel and the preparation of UO₂.Using UO₃ as the raw material,the preparation of UO₂F2 and the electrochemical behavior of UO₂²⁺ in LiCl-KCl molten salt and the electrolytic extraction of UO₂ were studied,and the effect of F^-on the electrolytic extraction of UO₂ was discussed.The detailed was as follows:（1）The fluorination reaction of NH₄HF₂ with UO₃ was studied at high temperature.The feasibility of fluorination reaction of UO₃ and NH₄HF₂ to form UO₂F2 was analyzed using thermodynamic data.The fluorination products were characterized by XRD,fluorescence and Raman spectra.It is found that（NH4）3UO₂F5 was firstly formed by the fluorination reaction of UO₃ and NH₄HF₂,then（NH4）3UO₂F5 decomposed to produce NH4（UO₂）2F5,finally,NH4（UO₂）2F5 decomposed to form UO₂F2:UO₃（s）+NH₄HF₂（s）→（NH4）3UO₂F5（s）→NH4（UO₂）2F5（s）→UO₂F2（s）,and the fluorination rate increased with increasing temperature.The morphology changes were observed using scanning electron microscope（SEM）during the fluorination process.According to the mass changes before and after the fluorination reaction,the fluorination rate of UO₃ was calculated to be 96.16%and 90.12%at 573 K and 673 K,respectively.（2）According to the thermodynamic data,the E-pO2-diagram of UO₂²⁺in LiCl-KCl molten salt was drawn.The electrochemical behavior of UO₂²⁺in LiCl-KCl molten salt was studied by a series of electrochemical methods.It was confirmed that UO₂²⁺was reduced to UO₂ by two-step single electron transfer.Therein,the reduction of UO₂²⁺to UO₂+ was found to be a reversible and diffusion controlled process of soluble/soluble system;and the reduction of UO₂+ to UO₂ to be a quasi-reversible and diffusion controlled reaction of the soluble/insoluble system.The diffusion coefficient of UO₂²⁺in LiCl-KCl molten salt at 773 K was estimated to be 6.22×10-5 cm2s-1,and the average charge transfer coefficient of UO₂+reduction to UO₂ to be 0.614.The disproportionation reaction of UO₂+ was explored.The results showed that the oxidation peak related to the formation of monolayer UO₂ by disproportionation reaction of UO₂ was observed.Potentiostatic electrolysis was performed at-0.8 V,and the characterized results of the products indicated the formation of octahedral UO₂.The final extraction rate of UO₂ could reach 98.53%after 8 h of electrolysis.（3）The effects of F^-on the electrochemical behavior of UO₂²⁺and the extraction of UO₂ by electrolysis were studied.The electrochemical behavior of U02+was investigated in different concentration ratio of[UO₂²⁺]/[F^-]by cyclic voltammetry.According to the relationship between the reduction peak of UO₂/UO₂ and pF^-,it was found that UO₂ existed in the form of UO₂FCl33-in LiCl-KCl molten salt.The diffusion coefficients of UO₂²⁺with different F^-concentrations were calculated by square wave voltammetry curves determined at different frequencies.It showed that the diffusion coefficient of UO₂²⁺decreased slightly with the increase of F^-concentration.The effect of different concentration ratios of[UO₂²⁺]/[F^-]on the extraction of UO₂ was studied by potentiostatic electrolysis.It was found that the extraction rate of UO₂ decreased with the increase of F^-concentration.However,the increase of Fconcentration could help to reduce the formation of UO₂ dendrite by the analysis of SEM-EDS.