Electrochemical Behaviors Of UO₂²⁺ And Ag⁺ In LiCl-KCl Molten Salt And Effect Of Ag⁺ On Electrodeposition Of UO₂

Spent fuel reprocessing is considered to be a key step of nuclear fuel cycle.In recent years,spent fuel dry reprocessing has been vigorously developed in spent fuel of fast reactor,wherein molten salt electrolysis is regards as one of the most effective methods.The presence of silver in spent fuel seriously hinders the recovery of uranium.In order to realize the full utilization of uranium resources,it is very important to study the electrochemical properties of uranium and silver in chloride molten salt.Therefore,in this work,UO₃ was used as the raw material for the preparation of UO₂Cl₂.The electrochemical behaviors of UO₂²⁺and Ag⁺in Li Cl-KCl molten salt were studied,and the effect of Ag⁺on the electrochemical behavior of UO₂²⁺and electrodeposition of UO₂ were also discussed.（1）The reaction feasibility of UO₃ with NH₄Cl to prepare UO₂Cl₂ was analysis by thermodynamic calculation.The chlorination of UO₃ was carried out and the chlorination product was analyzed by Raman spectroscopy.The result showed that the product was UO₂Cl₂,the chlorination rate of UO₃ was 96.90%at 673 K.The electrochemical behaviors of UO₂²⁺on W electrode in Li Cl-KCl molten salt were studied by electrochemical methods.The reduction of UO₂²⁺to UO₂⁺was a reversible process controlled by diffusion,and the reduction of UO₂⁺to UO₂was a quasi-reversible process controlled by diffusion.The diffusion coefficient of UO₂²⁺in molten salt calculated by cyclic voltammetry and square wave voltammetry at 773 K,was of the same order of magnitude.The average charge transfer coefficientαof the reduction from UO₂⁺to UO₂ was calculated to be 0.71.Potentiostatic electrolysis in UO₂Cl₂-Li Cl-KCl molten salt was carried out at the potential of-0.8 V.The result showed that the product prepared was UO₂ with micron polyhedron structure.（2）The electrochemical behavior of Ag⁺on W electrode in Li Cl-KCl molten salt was studied by various electrochemical methods.The results showed that Ag⁺was reduced to metal Ag by one-step single electron transfer on W electrode,and the reduction process was a reversible process controlled by diffusion.The diffusion coefficient of Ag⁺calculated by cyclic voltammetry and chronopotentiometry was slightly different,but the order of magnitude was the same.The exchange current density,charge transfer resistance and reaction activation energy of Ag⁺/Ag were calculated by using the linear polarization curve method in the temperature range of 723～798 K.Meanwhile the thermodynamic datas of Ag Cl were determined by open-circuit chronopotentiometry,the apparent formation Gibbs free energy,enthalpy and entropy of Ag Cl and the activity coefficient of Ag⁺were calculated.The potentiostatic electrolysis results obtainedin Ag Cl-Li Cl-KCl system at the potential of-0.40 V showed that dendritic Ag was prepared.（3）The electrochemical behavior of UO₂²⁺in Ag Cl-Li Cl-KCl molten salt was studied by various electrochemical methods.The results showed that the reduction of UO₂²⁺to UO₂ was a two-step single electron transfer process,and the reduction of UO₂²⁺to UO₂⁺was a reversible process controlled by diffusion.The diffusion coefficient of UO₂²⁺was calculated by square wave voltammetry with different frequencies.The electrochemical behavior of UO₂²⁺in Li Cl-KCl melt with different Ag⁺concentration was also studied.It was found that Ag⁺concentration had no effect on the electrochemical reduction mechanism of UO₂²⁺.However,the reduction potential of UO₂⁺and Ag⁺overlapped and the UO₂ and Ag might be deposited simultaneously.Potentiostatic electrolysis in UO₂Cl₂-Ag Cl-Li Cl-KCl molten salt was carried out at the potential of-0.8 V,and the products was comprised of UO₂ and metal Ag.XRD result showed that the addition of Ag⁺decreased the cell parameters of UO₂.The result of SEM-EDS showed that UO₂ with polyhedral and dendrite structures was generated.Compared with UO₂obtained by potentiostatic electrolysis in UO₂Cl₂-Li Cl-KCl molten salt,the grain size of UO₂decreased,which was related to the decrease of UO₂ cell parameters and the uneven local current density on the electrode surface.