| The separation and extraction of lanthanides or actinides by liquid cathode is one of the most effective methods for spent fuel reprocessing.The neutron absorption cross-section of the lanthanides in the spent fuel is relatively large,which prevents transmutation elements from capturing neutrons,thus reducing transmutation efficiency.In addition,lanthanides can form low melting point alloys with metal protective layer of nuclear fuel,which affects the sealing of fuel core.Therefore,the lanthanides must be separated in the reprocessing of spent fuel.One of the important goals of spent fuel reprocessing is to recover and reuse actinides in the advanced nuclear fuel cycle system,which can reduce the amount of spent fuel and radioactive toxicity.Herein,the electrochemical behaviors and separation coefficients of lanthanides（La,Pr）and U on liquid Ga electrodes in LiCl-KCl molten salts were investigated.The thermodynamics,kinetics and diffusion coefficients of lanthanides（La,Pr）and U on inert W electrodes were studied by electrochemical methods.The results indicated that the reduction process of La（Ⅲ）and Pr（Ⅲ）is one-step-three-electron transfer process by controlled diffusion.The reduction process of U（IV）→U（0）is divided into two steps.The first step is U（IV）→U（Ⅲ）,which is reversible,and U（Ⅲ）reduced to U（0）is irreversible.The apparent electrode potentials,the Gibbs free energies and activity coefficients of lanthanides（La,Pr）and U on inert W electrode at different temperatures were calculated by open circuit chronopotentiometry tests.The interaction between La3+and O2-was studied by potentiometric titration.The potentiometric titration curve showed that the oxide precipitate was La OCl.The apparent electrode potentials and activity coefficients were obtained from the experimental results,E-p O2–diagram of La in LiCl-KCl molten salt at 723 K was established.The electrode process of La（Ⅲ）,Pr（Ⅲ）and U（IV）on liquid Ga electrode were studied by cyclic voltammetry and square wave voltammetry.The results indicated that the reduction of La（Ⅲ）,Pr（Ⅲ）,U（IV）are both one-step processes on liquid Ga electrode.Meanwhile,the reduction of La（Ⅲ）and U（IV）are quasi-reversible,the reduction of Pr（Ⅲ）is irreversible on the liquid Ga electrode.The diffusion coefficient of La（Ⅲ）in LiCl-KCl molten salt is 7.65×10-5 cm2·s-1,the diffusion coefficient of Pr（Ⅲ）in LiCl-KCl molten salt is 1.2×10-5 cm2·s-1.The apparent electrode potentials,the Gibbs free energies and activity coefficients of lanthanides（La,Pr）and U on liquid Ga electrode at different temperatures（723-813 K）were calculated by open circuit chronopotentiometry tests.In the meantime,the separation factor of La/U and Pr/U were calculated The results indicated that uranium is mainly concentrated in the metal phase,La and Pr elements are mainly concentrated in the molten salt. |
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