Study On Electrolytic Extraction And Separation Of La,Pr And U On Al Cathode In LiCl-KCl Molten Salt

Nuclear energy,as a kind of green and clean power,is regarded as the promising candidate for the future energy supply in virtue of the huge energy output,excellent economic benefit,and no greenhouse gas generation.At the same time,reprocessing of spent nuclear fuel in an economic and safe manner is also one of the challenges facing the sustainable development of nuclear energy today.Based on the high solubility and the excellent radiation resistance,molten salt-based pyrochemical separation processes is the most promising reprocessing technology.The content of lanthanides（Ln）is high in fission products,and separating them from actinides（An）can greatly improve the utilization of spent nuclear fuel.Due to the similar properties of lanthanides and actinides,the separation of Ln/An is efficiently difficult.Therefore,this dissertation concentrates on the research of electrochemical behavior of two lanthanides（La,Pr）and U on Al electrode in Li Cl-KCl molten salt to provide basic experimental data for separation and extraction of Ln/An.The main research works of this dissertation are as follows:The electro-reduction process of La（III）and Pr（III）were studied on W,solid Al,and liquid Al electrodes in Li Cl-KCl molten salt by different electrochemical methods,all of which were rapid transfer three-electron processes.La（III）and Pr（III）were reduced to the corresponding metal on the W electrode,and under-potential deposition occurred on both the solid and liquid Al electrodes to generate Al-Ln（La,Pr）alloys.The relationship between the apparent electrode potential and temperature of the La（III）/La（0）and Pr（III）/Pr（0）systems on the W electrode was established by the open circuit chronopotentiometry and the Nernst equation.The relationship between temperature and activity coefficient of La and Pr on the liquid Al electrode was obtained via coulometric titration and electromotive force method.Different electrochemical methods were used to investigate the electrochemical reduction behaviors of La（III）,Pr（III）and U（IV）on the solid Al electrode in Li Cl-KCl molten salt.The Al-La,Al-Pr and Al-U alloys were prepared by potentiostatic electrolysis.The co-reduction behaviors of La（III）,Pr（III）,U（IV）and Al（III）were investigated in Li Cl-KCl-Al Cl₃ molten salt by using La₂O₃,Pr₆O₁₁,U₃O₈ as ion sources,respectively.The thermodynamic parameters of Al-La,Al-Pr and Al-U intermetallic compounds were calculated according to the electromotive force method.La,Pr and U in La₂O₃,Pr₆O₁₁,U₃O₈ were directly extracted by potentiostatic electrolysis,and the electrolysis products of Al-La,Al-Pr and Al-U alloys were characterized and analyzed by XRD and SEM-EDS.The extraction efficiency was calculated based on the results of ICP analysis:η_La=96.10%,η_Pr=97.78%,η_U=92.03%,respectively.The electrochemical behaviors of Li Cl-KCl-Al Cl₃-UCl₄-La Cl₃ and Li Cl-KCl-Al Cl₃-UCl₄-Pr Cl₃ multi-component molten salt system were explored on the solid Al electrode via cyclic voltammetry and square wave voltammetry.U was extracted and separated by potentiostatic electrolysis.According to the ICP results,the separation factor of U/La and U/Pr was calculated as:SF_U/La=537.81,SF_U/Pr=376.12.The results show that Al electrode is a potential electrode material for molten salt-based electrolytic extraction and Ln/An separation.