Electrochemical Behavior And Mutual Separation Of Gd,Sm And Eu On Bi Cathode In LiCl-KCl Molten Salt

Nuclear energy is a green,environmentally friendly and sustainable energy.It is a vital part of the world’s energy supply and plays a significant role in energy supply.However,with the development of nuclear power,the continuous accumulation of spent fuel has brought difficulties to the expand of nuclear power.With the aim of the nuclear energy sustainable development,it is imminent to upgrade the reprocessing technology of spent fuel.Dry reprocessing based on molten salt electrolysis has become the most promising method for spent fuel reprocessing in the future because of its radiation resistance,economic feasibility and high energy efficiency.But dry reprocessing has not yet been scaled up to commercial scale,the most important reason being the lack of basic data on the main elements in spent fuel.Lanthanide elements are neutron agents in spent fuel,and have similar electrochemical properties with actinide elements.Therefore,three adjacent lanthanide elements,Gd,Sm and Eu,are selected as research objects to explore their electrochemical behavior in LiCl-KCl molten salt and measure relevant thermodynamic and kinetic datum.Moreover,the extraction lanthanide elements and separation of Gd/Sm and Gd/Eu on liquid bismuth electrode are carried out by potentiostatic or galvanostatic means in order to provide detailed basic parameters and effective separation methods.The specific contents of this experiment are as follows.1.Cyclic voltammetry was used to study the electrochemical reduction process of Gd(Ⅲ)ion on W and liquid Bi electrodes,and it was found that the reduction process of Gd(Ⅲ)ion on W and Bi electrodes was a one-step transfer three-electron process.The equilibrium electrode potential was measured by open-circuit chrono potential,and the variation of formal electrode potential in the range of 723～798K was obtained by Nernst equation.The diffusion coefficients of Gd(Ⅲ)ions in the range of 723～823 K were tested by cyclic voltammetry and semidifferential method.The diffusion coefficient increased from 6.183×10-7 cm2 s-1 to 1.915×10-5 cm2 s-1 with elevating temperature by cyclic voltammetry.The diffusion coefficient increased from 1.205 ×10-6 cm2 s-1 to 1.3 84×10-5 cm2 s-1 with increasing temperature by semi-differential method.The exchange current density of GdCl3 on W at different temperatures was measured by Tafel and LP means respectively.The exchange current density was positively correlated with temperature.The exchange current density and activity coefficient of Gd on liquid Bi electrode were measured by constant current batch titration technique,and the relationship between activity coefficient and temperature was established.The co-reduction behavior of Gd(Ⅲ)and Bi(Ⅲ)ions in molten salts and the thermodynamic data of different Bi-Gd intermetallic compounds were investigated by various electrochemical methods.Bi-Gd alloy was prepared by constant potential,and the obtained intermetallic compounds were BiGd and Bi3Gd5 by XRD and SEM-EDS analysis.2.The electrochemical reduction process of variable elements Ln(Sm,Eu)in molten salt W cathode was researched by cyclic voltammetry and square wave voltammetry.The reduction of Ln(Ⅲ)ion on W electrode involves two electrochemical steps:Ln(Ⅲ)+e-→Ln(Ⅱ);Ln(II)+2e-→Ln(0).Because the reduction potential of the second electrochemical step exceeded that of lithium metal,no second electrochemical reduction signal was observed.The peak potential of Ln(Ⅲ)was measured by cyclic voltammetry,and the link between formal electrode potential and temperature was established by Nernst equation.The diffusion coefficient of LnCl3 in the range of 723～823 K was studied by semi-differential method,and it was discovered that the diffusion coefficient increased with the rising of temperature.In addition,the co-reduction behavior of Ln(Ⅲ)and Bi(Ⅲ)ion in molten salt W electrode was investigated by different electrochemical methods.Five Bi-Sm intermetallic compounds and four Bi-Eu intermetallic compounds were noticed.Bi-Ln alloy of LiCl-KCl-LnCl3 system was prepared on liquid Bi electrode by constant potential.The electrolytic products were analyzed by XRD and SEM-EDS,and SmBi and Eu4Bi3,EuBi2 and Eu5Bi3 intermetallic compounds were obtained.3.The co-reduction behavior of LiCl-KCl-BiCl3-SmCl3-GdCl3 and LiCl-KCl-BiCl3EuCl3-GdCl3 multicomponent systems on W electrode was discussed by different electrochemical methods.It was noticed that the potential difference of the most positive intermetallic compounds between Bi-Gd and Bi-Sm and Bi-Eu is 0.19 V and 0.21 V separately.Potentiostatic electrolytic separation was carried out on liquid Bi electrode in LiCl-KClSmCl3-GdCl3 and LiCl-KCl-EuCl3-GdCl3 melts.The electrolytic products were mainly GdBi and GdsBi3.After electrolysis in LiCl-KCl-SmCl3-GdCl3 molten salt system,the extraction efficiencies of Gd and Sm were 98.5%and 20.01%,respectively,and the separation factor of Gd/Sm was 427.The extraction efficiency of Gd and Eu in LiCl-KCl-EuCl3-GdCl3 system was 98.4%and 18.81%,and the separation factor of Gd/Eu was 456.