Molten Salt Extraction And Electro-Separation Of Uranium(U) And Lanthanides(La,Ce,Pr,Nd) Aided By Liquid Al-Ga Cathode

Nuclear energy is favored as a safe,clean and efficient green energy.In order to achieve sustainable development of nuclear energy,it is necessary to carry out the spent fuel reprocessing.Dry reprocessing has attracted widespread attentions due to its ability to process spent fuels from different types of nuclear reactors.Molten salt electrorefining is a key step in dry reprocessing,and seeking for electrode materials for the efficient separation of actinides（An）and lanthanides（Ln）is a key issue in the separation of molten salt electrolysis.In order to efficiently separate An/Ln,in this work,using Al-Ga alloy as cathode to explore the feasibility of separating and extracting An/Ln from LiCl-KCl molten salt.The electrochemical behaviors of An and Ln were investigated,and the thermodynamic and kinetic properties of the electrode processes were measured.The specific research contents are as follows:The electrochemical behaviors of Nd（Ⅲ）on inert W and reactive cathodes M（M=Al,Ga,Bi,Zn,Sn,Pb,Cd and Al-Ga alloy）were investigated by electrochemical method.Cyclic voltammetry showed that Nd（Ⅲ）is reduced to metallic Nd in two steps on W electrode,while to Nd（M）solid solution in one-step on the reactive cathodes by underpotential deposition.The appropriate Nyquist plots of Nd（Ⅲ）on solid Al and liquid M cathodes were measured at the temperature of 723-873 K,and the charge transfer resistances were obtained by fitting the equivalent circuit,and the exchange current density were also calculated,to be from 0.0042 to0.012 A cm^-2,and the Nd（Ⅲ）/Nd（M）couples order of exchange current density is Al<Al-Ga<Zn<Pb<Ga<Sn<Bi<Cd at 723 K.The electrochemical behaviors of Ln（Ⅲ）（Ln=La,Ce and Eu）on Al-Ga film electrodes were systematically studied by electrochemical techniques.The underpotential deposition of Ln（Ⅲ）on Al-Ga film electrodes is due to the formation of binary Al-Ln,Ga-Ln and ternary Al-Ga-Ln alloys.The isothermal section diagrams of Al-Ga-Ce and Al-Ga-Eu were calculated using CALPHAD,and the formation of Al₂Ga₂Ce and Al₂Ga₂Eu were predicted.Meanwhile,Ln was extracted on Al-Ga film cathode by potentiostatic electrolysis.The XRD and SEM-EDS results showed the extracted products are Al₂Ga₂La,Al₂Ga₂Ce,Ga₂Ce and Ga₆Ce as well as Ga₄Eu,respectively.The electrochemical behaviors of La（Ⅲ）on W and liquid Al-Ga cathode were studied.La（Ⅲ）on W electrode is one-step three-electron transfer process,and the diffusion coefficient and diffusion activation energy of La（Ⅲ）in LiCl-KCl melts were calculated to be 10^-5 cm²/s and 56.65 kJ/mol,respectively.The reduction mechanism of La（Ⅲ）on the liquid Al-Ga alloy cathode is:La（Ⅲ）+3e^-+Al-Ga→La（Al-Ga）.The Nyquist plots of La（Ⅲ）on solid Al,liquid Ga and Al-Ga cathodes were measured,and the exchange current density of La（Ⅲ）/Al₁₁La₃,La（Ⅲ）/La（Ga）and La（Ⅲ）/La（Al-Ga）couples were calculated,to be from 0.078 A cm^-2-0.42A cm^-2.Moreover,metal La was extracted by potentiostatic electrolysis on Al-Ga cathode.The XRD and SEM-EDS results of the products showed that the extracted products were Al₃La and Ga₆La alloy compounds.The electrochemical behavior of U（Ⅲ）on Al-Ga film electrode was studied by electrochemical method.The underpotential deposition of U（Ⅲ）on Al-Ga film electrode was caused by the formation of Al-U/Ga-U intermetallic compounds.U was extracted by potentiostatic electrolysis from LiCl-KCl-AlCl₃-Ga Cl₃-UCl₃system.XRD result indicated the extracted product was Al₃U and Ga₃U with the same crystal structure.The reduction mechanism of La（Ⅲ）on the liquid Al-Ga cathode is:La（Ⅲ）+3e^-+Al-Ga→La（Al-Ga）.The Nyquist plots of U（Ⅲ）on solid Al and liquid Ga and Al-Ga alloy cathodes were measured,and the charge transfer resistance of U（Ⅲ）/Al₃U,U（Ⅲ）/U（Ga）,U（Ⅲ）/U（Al-Ga）couples were obtained,and the exchange current densities was calculated,to be ranging from 0.00045 to 0.536 A cm^-2,it was found that U（Ⅲ）/U（Al-Ga）couple had the highest exchange current density.Potentiostatic electrolysis extraction of metal U was studied on Al-Ga cathodes with different proportions（Ga-0.38wt.%Al,Ga-1.67wt.%Al（eutectic）,Ga-5.63wt.%Al and Ga-35.0wt.%Al）.The XRD results of the extracted products inidicated the formation of Al₃U and Ga₃U alloy compounds with the same crystal structure.Potentiostatic electrolytic separation of U/Ln was carried out from molten salt containing U（Ⅲ）and Ln（Ⅲ）ions on Al-Ga cathode,U was extracted in the form of Al₃U and Ga₃U from molten salt,and Ln（Ⅲ）remained in the molten salt.The separation factors of U/Ln were calculated,and the values for U/La,U/Ce,U/Pr and U/Nd were 660,550,610 and 400,respectively.