Electrochemical Separation Of Uranium From Lanthanides On Liquid Bismuth Electrode In LiCl-KCL Molten Salt

Nuclear energy as an efficient,stable,clean energy,has become an important part of the world energy structure.In order to ensure the sustainable development of nuclear energy,improving the development of spent nuclear fuel reprocessing technique has become urgent affairs.For the future advanced nuclear energy systems,it is more difficult to dispose the spent nuclear fuel due to higher fuel consumption,radioactivity and fission products content.Based on the excellent resistance to irradiation,pyroprocessing of molten salts electrolysis technology is considered nowadays a promising option for the proposed advanced fuel cycles.Lanthanides are the main fission products,and their chemical properties are highly similar with actinides,which results in extremely difficult to separate between them.There are still some key scientific issues to be solved for this technical route.This dissertation focuses on the study of electrochemical behavior,thermodynamic and kinetic properties of four lanthanides and uranium ions in Li Cl-KCl molten salt.Different electrochemical methods have been used to separate uranium and lanthanide elements on liquid Bi electrodes to provide detailed fundamental parameters and effective separation methods.This dissertation mainly includes three parts of the fundamental research.1.The electrochemical behaviors of four lanthanides（Ln=La,Ce,Pr,Nd）ions and uranium ions were studied in Li Cl-KCl molten salt,respectively.Different electrochemical methods were used to determine the thermodynamic data,and the co-deposition behaviors were investigated on tungsten electrodes in Li Cl-KCl-Pr Cl₃-Bi Cl₃ and Li Cl-KCl-UCl₃-Bi Cl₃ systems.The electro-reduction process of U（Ⅲ）and Ln（Ⅲ）ions were studied on inert tungsten electrodes in Li Cl-KCl molten salt via cyclic voltammetry and square wave voltammetry.The reduction of La（Ⅲ）,Ce（Ⅲ）and Pr（Ⅲ）ions on inert tungsten electrodes was a fast reaction with three electrons exchanged.But the reduction of Nd（Ⅲ）and U（Ⅲ）ions involved two steps:Nd（Ⅲ）+e^-?Nd（II）,Nd（II）+2e^-?Nd and U（IV）+e^-?U（Ⅲ）,U（Ⅲ）+3e^-?U.Galvanostatic voltammetry and open circuit chronopotentiometry were performed to measure the equilibrium electrode potential of Pr/Pr Cl₃ at different temperatures.According to the Nernst equation,the formal electrode potential was determined,and the relationship between electrode potential and temperature was established in the range of 723-813K.The difference of Gibbs free energy of Pr Cl₃ between supercooled state and molten salt state was used to calculate the activity coefficients of Pr（Ⅲ）ions at 723,753,783 and 813K.In addition,the co-deposition behaviors were investigated on tungsten electrodes in Li Cl-KCl-Pr Cl₃-Bi Cl₃ and Li Cl-KCl-UCl₃-Bi Cl₃systems by cyclic voltammetry,square wave voltammetry,chronopotentiometry and open circuit chronopotentiometry.The electrode potential of five intermetallic compounds Bi Pr₂,Bi₃Pr₅,Bi₃Pr₄,Bi Pr,Bi₂Pr and three intermetallic compounds Bi U,Bi₄U₃,Bi₂U were determined.The relationship between Gibbs free energy and temperature for the formation of Bi-Pr and Bi-U intermetallic compounds was calculated and plotted by the electromotive force method.2.The electrochemical behaviors of four lanthanides ions and uranium ions were studied on liquid Bi electrodes in Li Cl-KCl molten salt,respectively.Different electrochemical methods were used to determine the thermodynamic and kinetic data.The alloys of Bi-La,Bi-Ce and Bi-U were prepared by galvanostatic electrolysis.The electro-reduction process of U（Ⅲ）and Ln（Ⅲ）ions were studied on liquid Bi electrodes in Li Cl-KCl molten salt by cyclic voltammetry and glvanostatic intermittent titration technique.The reduction of La（Ⅲ）,Ce（Ⅲ）Pr（Ⅲ）and Nd（Ⅲ）ions on liquid Bi electrodes was a fast reaction with three electrons exchanged.But the reduction of U（Ⅲ）ions involved two steps:U（IV）+e^-?U（Ⅲ）,U（Ⅲ）+3e^-?U.The electromotive force method was performed to calculate the activity coefficients of Ln and U on liquid Bi electrodes at different temperatures.The relationship between activity coefficient and temperature was determined.The solubility of U in liquid Bi was evaluated by glvanostatic intermittent titration technique,and the value was calculated to be 3.6×10^-3 at 753K.Galvanostatic pulse and electrochemical impedance spectroscopy methods were employed to investigate the interfacial processes of Ln（Ⅲ）and U（Ⅲ）on liquid Bi electrodes.The linear relationship between exchange current densities j₀ of Ln（Ⅲ）/Ln and U（Ⅲ）/U and temperature were determined in the range of 723-873K.The results of Nd（Ⅲ）/Nd and U（Ⅲ）/U were relatively close.The Bi-U,Bi-La and Bi-Ce alloys were prepared on liquid Bi electrodes in Li Cl-KCl-UCl₃,Li Cl-KCl-La Cl₃ and Li Cl-KCl-Ce Cl₃systems,respectively.The products of phase composition and microstructure were conducted by XRD and SEM-EDS.The intermetallic compounds Bi₂U,Bi₂La and Bi₂Ce were obtained.Bi₂U showed a layered stacking structure,but Bi₂La and Bi₂Ce showed a spherical structure.The current efficiency was above 90%of Bi-La and Bi-Ce alloys based on the ICP results.However,the current efficiency was about 75%,which was due to the cyclic electrolysis of U（Ⅲ）and U（IV）in molten salt.3.The electrochemical separation of U/Pr,U/Nd and U/（La,Ce,Pr,Nd）were studied using three different electrochemical methods on liquid Bi electrodes in Li Cl-KCl-Pr Cl₃-UCl₃,Li Cl-KCl-Nd Cl₃-UCl₃ and Li Cl-KCl-Ln Cl₃-UCl₃ multi-component molten salt systems.The influence of electrodeposition process and diffusion process on electrochemical separation was evaluated by the change of electrode potential during electrolysis process.The electrochemical behaviors of Li Cl-KCl-Pr Cl₃-UCl₃,Li Cl-KCl-Nd Cl₃-UCl₃ and Li Cl-KCl-Ln Cl₃-UCl₃ multi-component systems were studied on inert tungsten and liquid Bi electrodes by cyclic voltammetry,square wave voltammetry and open circuit chronopotentiometry.According to the potential difference between U and Ln on liquid Bi electrodes,ten groups of electrochemical separation experiments were demonstrated using galvanostatic electrolysis,potentiostatic electrolysis and coulometric titration methods.The results showed that the molar ratio of U/La can reach more than 1000;the molar ratio of U/Ce and U/Pr is about 500;and the molar ratio of U/Nd is relatively low at 350.Among them,the best result was obtained by coulometric titration methods.When the concentration of UCl₃ was greater than 0.5%,the electrochemical separation process was mainly controlled by the diffusion rate of U in liquid Bi.On the contrary,the electrochemical separation process was mainly controlled by the diffusion rate of U（Ⅲ）ions in molten salt.