Kinetics Of Rare Earth Molten Salt Electrolysis Process And Regularity Of Rare Earth Deposition Potentials At Cathodes

As a stable and reliable low-carbon clean energy,nuclear energy has become a vital part of the energy structure in China.In order to ensure the sustainable and high-quality development of nuclear energy,the research and development of safe and effective spent fuel reprocessing technology has become an urgent matter.Pyroprocessing technology which based on molten salt electrolysis can withstand high radiation doses and effectively reduce criticality safety risks.Therefore,it can be applied to the Generation Ⅳ nuclear reactor system and is considered to be the most promising spent fuel reprocessing technology in the future.To date,pyroprocessing technology has not been expanded to commercial scale.One of the important reasons affecting its further development is the lack of accurate basic parameter data of the main elements in the spent fuel,which hinders the development of a series of key technologies such as online monitoring,computer simulation and corrosion protection.Lanthanides(Ln)are the main fission products in spent fuel,and their chemical properties are highly similar to actinides.Therefore,the lanthanide elements were selected as the research objects in this dissertation.The basic kinetic parameters of four lanthanides(Pr,Sm,Eu,and Ce)in LiCl-KCl molten salts,and the regularity of the deposition potentials of fourteen lanthanides except promethium on different active electrodes(Zn,Al,and Cu)were investigated.The specific research contents are as follows:Cyclic voltammetry(CV),semi-integral method(SI)and square wave voltammetry(SWV)were employed to investigate the electrochemical reduction processes of Pr(Ⅲ)on the tungsten(W)electrodes and liquid zinc(Zn)electrodes in LiCl-KCl-PrCl3 molten salts at 773 K.The diffusion coefficients of Pr(Ⅲ)in LiCl-KCl molten salts were measured and calculated through CV and SI at 773 K by using W wires and liquid Zn as working electrodes,respectively.When W wires were used as the working electrodes,a more accurate electrode area can be obtained by the standard area addition method.The diffusion coefficients of Pr(Ⅲ)in LiCl-KCl molten salts at 773 K calculated by the standard area addition method are:DPr(Ⅲ)=3.59×10-5 cm2·s-1(CV),DPr(Ⅲ)=1.62×10-5 cm2·s-1(SD).The electrochemical reduction processes of Sm(Ⅲ)and Eu(Ⅲ)on the W electrodes at different temperatures were studied by CV in LiCl-KCl-SmCl3 and LiCl-KCl-EuCl3 molten salts,respectively.Since the different valence ions of the variable element Sm and Eu can coexist in the LiCl-KCl molten salts at the same time,some traditional electrochemical calculation methods of diffusion coefficient have certain limitations.In order to solve this problem,the diffusion coefficients of Sm(Ⅲ)and Eu(Ⅲ)in LiCl-KCl molten salts at different temperatures were calculated by non-simplified CV,SI and semi-differential method(SD),which are applicable to this molten salt system.The results obtained by different methods are fairly close to each other.The electrochemical behaviors of LiCl-KCl-CeCl3 molten salts were investigated on the W electrodes in LiCl-KCl molten salts in the temperature range of 748～823 K and the Ce(Ⅲ)concentration(CCe(Ⅲ))range of 7.00×10-5～2.13×10-4 mol·cm-3.The values of j0 and α of Ce(Ⅲ)/Ce reaction were calculated by Tafel method and LP method under different temperatures and different concentrations of Ce(Ⅲ).However,the power function relationship between j0 and Cce(Ⅲ)calculated by these two methods cannot obtain a reasonable value of a.This may be due to the fact that the influence of mass transfer effect is not considered in both the Tafel method and the LP method,resulting in inaccurate calculation results.The Butler-Volmer equation was re-derived after considering the mass transfer effect,and a non-simplified kinetic equation was obtained.By fitting the nonlinear least squares algorithm to the equation,the values of j0 and α that is most consistent with the experimental data were obtained.The values of j0 and CCe(Ⅲ)show reasonable power function relations at different temperatures:j0=104.49CCe(Ⅲ)0.87(748K),j0=133.31CCe(Ⅲ)0.86(773 K)j0=162.12CCe(Ⅲ)0.86(798 K),j0=176.06CCe(Ⅲ)0.85(823 K).The relationship between the rate constants of Ce(Ⅲ)/Ce reactions and temperatures was calculated by the power function relations between j0 and Cce(Ⅲ)at different temperatures:lnk0=-4359.5/T-2.07.The deposition potentials of fourteen lanthanides except promethium on liquid Zn electrodes were obtained through the CV curves at 773 K.The deposition potential gradually becomes more positive with the increase of the atomic number,but extremely negative values appear at Eu and Yb,showing a "bimodal effect" as a whole.Combining the Hume-Rothery rules and the relationship between alloy formation energy and electronegativity and atomic size,a mathematical equation was established based on the result of multiple linear regression analysis,which indicates that the deposition potentials of lanthanides on liquid Zn electrodes are correlated with the the atomic radius difference and electronegativity difference between Zn and lanthanides.The deposition potentials of 5 lanthanides on the Al electrodes and the Cu electrodeds were obtained by CV and SWV at 773 K.The relationship equations between the deposition potentials and the atomic radius difference and electronegativity difference between Al/Cu and lanthanides were obtained through multiple linear regression analyses.The values of the deposition potentials of remaining lanthanides were calculated by the equations,and the errors are small compared with the experimental values.