Study On The Electrochemical Behaviors Of Eu（Ⅲ）,Pr（Ⅲ） And UO₂²⁺ In Different Chloride Salts

Reprocessing of spent fuel is a key step to realize the closed cycle of nuclear fuel,and molten salt electrolysis is regarded as one of the most promising dry reprocessing technologies.In the process of molten salt electrolysis,the choice of molten salt electrolyte is very important.In order to achieve efficient separation of uranium and fission products,it is necessary to carry out electrochemical basic research of uranium and fission products in different molten salt systems.Therefore,in this paper,LiCl-NaCl,LiCl-KCl and LiCl-CsCl were used as molten salt systems to comparatively study the electrochemical behavior of UO22+ and fission products Eu(Ⅲ)and Pr(Ⅲ)in different in molten salt systems.The influences of molten salt system on the reaction mechanism,reaction reversibility,reduction peak potential and diffusion coefficient of UO22+,Eu(Ⅲ)and Pr(Ⅲ)were mainly investigated.Meanwhile,the size of the grain size and extraction rate of UO2 prepared by electrolysis in different molten salt systems were also explored.The main research work is as follows:The electrochemical behaviors of Eu(Ⅲ)in LiCl-NaCl,LiCl-KCl and LiCl-CsCl molten salt systems were studied on W electrode by electrochemical methods.The reduction peak potential and diffusion coefficient of Eu(Ⅲ)in three molten salt systems were compared,and the mean cationic radius in LiCl-NaCl,LiCl-KCl and LiCl-CsCl molten salt system were calculated to be 0.83 ?,1.02 ? and 1.36 ?.It was found that the reduction peak potential of Eu(Ⅲ)shifted negatively with the increase of the cationic radius of molten salt,EEu(Ⅲ)(LiClNaCl)>EEu(Ⅲ)(LiCl-KCl)>EEu(Ⅲ)(LiCl-CsCl).The diffusion coefficients of Eu(Ⅲ)in three molten salt systems decreased with the increase of cationic radius of molten salt,DEu(Ⅲ)(LiClNaCl)>DEu(Ⅲ)(LiCl-KCl)>DEu(Ⅲ)(LiCl-CsCl).In the temperature range from 773 K to 948 K,the diffusion coefficients of Eu(Ⅲ)in three molten salt systems increased with the increase of temperature.Moreover,the effect of molten salt system on the electrochemical behavior of Eu(Ⅲ)was theoretically analyzed.The electrochemical behaviors of Pr(Ⅲ)in the three molten salt systems were studied on W electrode by electrochemical methods.The reduction peak potential and diffusion coefficient of Pr(Ⅲ)in three molten salt systems were compared,found that EPr(Ⅲ)(LiCl-NaCl)>EPr(Ⅲ)(LiCl-KCl)>EPr(Ⅲ)(LiCl-CsCl),and DPr(Ⅲ)(LiCl-NaCl)>DPr(Ⅲ)(LiCl-KCl)>DPr(Ⅲ)(LiCl-CsCl).The equilibrium potentials of Pr(Ⅲ)/Pr in molten salts of LiCl-NaCl,LiClKCl and LiCl-CsCl at different temperatures were measured by open-circuit chronopotentiometry.The apparent Gibbs free energy of formation for PrCl3 was estimated,and according to the relationship of Gibbs energy and temperature,the apparent enthalpy of formation and entropy of formation for PrCl3 were obtained.The electrochemical reduction mechanisms of UO22+ in LiCl-NaCl,LiCl-KCl and LiClCsCl molten salts were studied on W electrode by electrochemical method.The diffusion coefficients of UO22+ in different molten salts were calculated at 873 K,and the diffusion coefficient of UO22+ decreased with the increase of cationic radius of molten salt.The products obtained by constant potential electrolysis in three kinds of molten salt systems were characterized by XRD,TEM and SEM,which showed that the composition was UO2 and preferential orientation along(Ⅲ)direction.The average grain size of UO2 obtained in LiClNaCl molten salt was the smallest and that in LiCl-CsCl molten salt was the largest.The extraction of UO2 from LiCl-NaCl,LiCl-KCl and LiCl-CsCl molten salts was monitored online by square-wave voltammetry and the extraction rate was calculated.The results showed that the extraction rate of UO2 decreased successively in LiCl-NaCl,LiCl-KCl and LiCl-CsCl molten salt.