Study On Extraction Of Fission Fragment Gadolinium,Neodymium And Samarium On Sn/Pb Electrodes By Molten Salt Electrolysis

Nuclear fuel dry reprocessing is one of the most important reprocessing candidate technologies in advanced nuclear fuel cycle strategy.In order to avoid the adverse impact of rare earth elements（neutron poisons）in fission products in the chain reaction and the transmutation efficiency in reprocessing,these elements must be separated and extracted.It is difficult to separate them by conventional chemical methods because the physical and chemical properties of rare earth elements and actinides are similar.While the active cathode shows excellent performance in the process of electrochemical extraction and separation of lanthanum-actinides from high temperature molten salt,therefore,the fission elements Sm,Gd and Nd were selected to investigate electrochemical behavior,kinetics and thermodynamic.Then the Rare earth elements are efficiently extracted by molten salt electrolysis on the active cathode Pb and Sn electrodes.In this paper,electrochemical methods such as cyclic voltammetry,square wave voltammetry and open circuit chronopotentiometry were used to study the electroreduction process of Sm,Gd and Nd in high temperature molten salts,and electrochemical parameters were calculated.The main contents are as follows:（1）The electrochemical properties of Pb Cl₂,Sn Cl₂ and Gd Cl₃ on tungsten electrodes in molten salts were studied.The experiments showed that the reduction of Pb（II）and Sn（II）ions in molten salts is a one-step,two-electron transfer reversible process controlled by diffusion.While the reduction of Gd（III）on the tungsten electrode is a one-step three-electron transfer,reversible process controlled by diffusion.The diffusion coefficient,gibbs free energy of formation,activation energy of diffusion,enthalpy of formation and entropy of formation at different temperatures were provided,respectively.The electrochemical properties of Gd on Pb and Sn cathodes were then studied.The thermodynamic parameters such as the formation Gibbs free energy,formation enthalpy and formation entropy of alloys Pb₃Gd and Sn₃Gd were determined according to OCP curves at different temperature.According to the electrochemical research results,the rare earth Gd was extracted by potentiostatic/galvanostatic electrolysis on the Pb and Sn electrodes.The concentration of rare earth elements in the molten salt before and after extraction was analyzed by ICP-AES,and the extraction rates of Gd were 99.02%and 99.21%,respectively.（2）The electrochemical properties of Nd Cl₃ on tungsten electrode,active cathodes Pb and Sn were studied respectively.The electrochemical test results showed that the reduction process of Nd（III）on tungsten electrodes is a one-step three-electron transfer and a subject to diffusion-controlled reversible process.Kinetic and thermodynamic data such as diffusion coefficient,diffusion activation energy,gibbs free energy of formation,enthalpy of formation and entropy of formation were determined by cyclic voltammetry and open circuit chronopotentiometry at different temperatures.At the same time,the thermodynamic parameters such as Gibbs free energy of formation,enthalpy of formation and formation entropy of alloys Pb₃Nd and Sn₃Nd were also calculated respectively.According to the electrochemical research results,Nd was extracted by potentiostatic/galvanostatic electrolysis on Pb and Sn electrodes,and the concentration of Nd in melts before and after extraction was analyzed by ICP-AES.The extraction rates of Nd on Pb and Sn electrodes were were 99.03%and 98.70%.（3）The electrochemical properties of variable valence element Sm on tungsten electrode and active cathode Pb and Sn were researched,respectively.The open circuit chronopotentiometry（OCP）curves in the system show that five potential platforms of intermetallic compounds Pb₃Sm,Pb₂Sm,Pb₁₀Sm₁₁,Pb₄Sm₅ and Pb₃Sm₅ were detected.And four potential platforms were appeared in OCP curve of Sn-Sm,corresponding to four different forms of intermetallic compounds.According to the potential of the alloy,the potentiostatic electrolysis at-2.0 V was selected for extraction.Pb-Sm alloy and Sn-Sm alloy were formed in the electrolysis products.The extraction rate of Sm on the Pb and Sn film electrodes after electrolysis for 25 h was calculated to be 98.50%and 97.30%,respectively.It shows that Pb and Sn electrodes are also a good choice for the extraction of variable valence rare earth Sm.