Electrodeposition And Monitoring Of UO₂²⁺ Ions In Chloride Molten Salt

In the electrodeposition process of spent oxide fuel,the core step is the electrodeposition and recovery of uranium.In order to efficiently recover UO₂ from molten salt,the electrochemical behavior of UO₂²⁺ions and electrodeposition mechanism of UO₂ were investigated in different molten salts（LiCl-KCl and LiCl-Na Cl）,respectively.The remaining UO₂²⁺ion concentration in the salt and the end point of UO₂ deposition were monitored.The detailsare as follows:1.The electrodeposition process of UO₂²⁺ions was studied in LiCl-KCl-1.50wt.%UO₂Cl₂molten salt at 673 K using cyclic voltammetry and square wave voltammetry to study.The results show that UO₂²⁺/UO₂⁺is a reversible process controlled by diffusion,and UO₂⁺/UO₂ is a quasi-reversible process controlled by diffusion,the charge transfer coefficient a is 0.25.The diffusion coefficient of UO₂²⁺ions was calculated to be D=（2.64±0.30）×10^-5 cm² s^-1.The nucleation mechanism of UO₂²⁺under different overpotentials was explored using chronoamperometry and potentiostatic electrolysis.The results showed that the quantity of electricity Q was found to be the linear relationship with the square root of time t,and the electrocrystallization process of UO₂ was consistent with the continuous nucleation mechanism.The dynamics of UO₂⁺/UO₂ was by linear polarization and Tafel methods,which showed that the exchange current density j₀ increases with the increase of temperature,and the transfer charge resistance decreases with the increase of temperature.The calculated activation energy results are 21.31 k J mol^-1 and 25.94 k J mol^-1,respectively.2.The endpoint determination of UO₂ deposition and the online monitoring in LiCl-KCl-1.50wt.%UO₂Cl₂ molten salt was carried out at different temperatures by square wave voltammetry.The feasibility using working curve to monitor was verified by ICP-AES.The results show that the peak current has a linear relationship with 1/T in a certain temperature range,and the peak current has a linear relationship with a certain concentration range of UO₂²⁺ion.The diffusion coefficient D of UO₂²⁺ion were calculated at different temperatures,and diffusion activation energy E_a was estimated.The product was characterized by XRD and SEM-EDS,and found to be UO₂ with octahedral structure.3.The electrodeposition process of UO₂²⁺ions in LiCl-Na Cl-1.50wt.%UO₂Cl₂ molten salt was studied at 923 K by cyclic voltammetry.The results showed that UO₂²⁺/UO₂⁺was found to be a reversible process controlled by diffusion,the diffusion coefficient of UO₂²⁺ions was estimated to be D=（3.14±0.87）×10^-5 cm² s^-1.The rduction of UO₂⁺/UO₂ was found to be a quasi-reversible process controlled by diffusion,and the charge transfer coefficient a is 0.66.Square wave voltammetry result showed that the reduction reaction was affected by the existence of electric double layer currenta.The nucleation of UO₂ under different overpotentials by chronoamperometry and potentiostatic electrolysis which showed that the electrocrystallization process of UO₂ was consistent with the continuous nucleation mechanism.