Electrochemical Behavior Of UO₂F₂ In LiCl-KCl Molten Salt And Effect Of Zr（Ⅳ） On Electrodeposition Of UO₂

To ensure the sustainable development of nuclear energy,uranium（U）and plutonium（Pu）must be separated and recovered from spent fuel.Molten salt electrochemical technology is considered as one of the most promising reprocessing technologies.In order to efficiently recover uranium from spent oxide fuel,it is necessary to explore the electrochemical formation mechanism of UO₂ and the influence of fission products on the recovery of UO₂.Therefore,in this work,the electrochemical behavior of UO₂F₂ was studied in Li Cl-KCl molten salt,the influence of Zr（Ⅳ）on the electrochemical behavior of UO₂F₂ and the process of electrodeposition of UO₂ were investigated,as well as the high-temperature halogenation process of Zr O₂.The main contents are as follows:（1）The thermodynamic feasibility of high-temperature halogenation of Zr O₂ was analyzed using thermodynamic calculations.The high temperature halogenation process（473～873 K）of Zr O₂ was explored by means of XRD and SEM-EDS.It was found that Zr O₂ could react with NH₄HF₂ to form Zr F₄,and the fluorination rate could reach 85.2%.The reaction process was found to be Zr O₂→（NH₄）₂Zr F₆→NH₄Zr F₅→Zr F₄.The optimal temperature for the fluorination reaction was determined to be 673 K.（2）The electrochemical behavior of UO₂F₂ was studied by electrochemical methods in Li Cl-KCl molten salt.The results of cyclic voltammetry,square wave voltammetry,and chronopotentiometry showed that UO₂F₂ was reduced to UO₂ through two steps of one electron transfer.The two-step reduction process of UO₂F₂ at 773 K were found to be reversible and diffusion controlled.The diffusion coefficient of UO₂²⁺in Li Cl-KCl molten salt measured by the three methods were found to be 1.88×10^-5 cm² s^-1,2.97×10^-5 cm² s^-1 and 1.13×10^-5 cm²s^-1,respectively.In addition,the exchange current densities of UO₂⁺/UO₂and U（Ⅲ）/U pairs were measured by polarization curves at various temperatures.It was found that the exchange current densities increased with the increasing temperature,and the reaction activation energy was 32.21 k J mol^-1 and 23.52 k J mol^-1,respectively.UO₂ with a regular octahedral microstructure was prepared by constant potential electrolysis.The disproportionation reaction of UO₂⁺was analyzed by means of electric charge calculation and intermediate potential electrolysis product characterization.The reaction equation was 2UO₂⁺=UO₂+UO₂²⁺.The morphology of UO₂ generated by disproportionation reaction was different from that generated by electric reduction,resulting in different oxidation potentials of the two.The complex oxidation process of UO₂ was analyzed and explained through disproportionation reaction.（3）The influence mechanism of Zr（Ⅳ）on the electrochemical behavior of UO₂F₂ and the process of electrodeposition of UO₂ were studied.First,the E-p O^2-diagrams of U and Zr elements in Li Cl-KCl molten salt were drawn using thermodynamic calculations,which showed that the binding ability of Zr（Ⅳ）with oxygen ions is much higher than that of U（Ⅳ）.The reduction process of Zr（Ⅳ）was studied,and found the reduction potential of Zr（Ⅳ）was less than that of UO₂²⁺,which indicated that Zr（Ⅳ）is electrochemical inert in the process of electrolytic reduction UO₂²⁺.The effect of Zr（Ⅳ）on the electrochemical behavior of UO₂F₂and the process of electrodeposition of UO₂ were studied,and its influence mechanism was determined.The results showed that Zr（Ⅳ）did not directly participate in electrochemical reduction process of UO₂²⁺,however,the displacement reaction between Zr（Ⅳ）and UO₂occurred by the reaction of UO₂+x Zr（Ⅳ）=xZrO₂·（1-х）UO₂+xU（Ⅳ）.