Thermal Reduction Of CeO₂ And UO₃ And Anodic Dissolution Of Reduction Product Al-Ce（U） Alloy

Dry reprocessing technology for spent fuel is the most promising one to realize advanced fuel cycle,recycle uranium and plutonium from spent fuel,and reduce the hazard of spent fuel.Among them,molten salt electrolysis is the core of dry reprocessing technology,which is considered to be the most reliable means to treat MOX spent fuel in fast reactors.In this work,Ce was a substitute for Pu to study the reprocessing process of MOX spent fuel（UO₂-Pu O₂）,including the thermal reduction of the oxide（CeO₂ and UO₃）obtained after the head treatment and the anodic dissolution process of the reduction products of Al-An（U/P）alloys.The main works are as follows:A thermodynamic feasibility of Li and Ca metals used as reducing agents was confirmed.The alloys of Al/Zn/Bi-Li and Zn-Ca were prepared by molten salt electrolysis,and the reduction of CeO2 was performed using Al/Zn/Bi-Li and Zn-Ca as reductant.The reduction products were characterized by XRD and SEM-EDS,which demonstrated that CeO₂ could be reduced to metallic Ce,and the obtained Al/Zn/Bi-Ce alloys contained Al₁₁Ce₃,Ce Zn₁₁ and Bi Ce intermetallic compounds,respectively.The reduction efficiency of the Al-Li alloy is higher than those of Zn/Bi-Li alloys,while the reduction efficiency of Zn-Ca alloy was lower due to the generation of the intermediate product Ca_0.1Ce_0.9O_1.9.Meanwhile,UO₃ was reduced by Al-Li alloy.XRD and TEM-EDS results indicated the reduction product Al-U alloy contained Al₄U intermetallic compound.Furthermore,the reduction mechanism of UO₃reduced by Al-Li alloy was speculated.The anodic dissolution of Al-Ce alloy was investigated by cyclic voltammetry and anodic dissolution voltammetry,the dissolution potential and order of alloy components were obtained.These results showed that the dissolution order of Al-Ce alloy was confirmed to be Ce→Al₁₁Ce₃intermetallic compound→Al.The exchange current density of Al-Ce alloy was calculated by Tafel method under different temperature.It was found that the exchange current density increased with the increasing temperature.The diffusion coefficient of Ce atoms in Al-Ce alloy was calculated by anodic chronopotentiometry.Open-circuit chronopotentiometry was performed to calculate the activity of Ce in Al-Ce alloys and the standard molar Gibbs free energy of Al-Ce intermetallic compound.The anodic passivation of Al-Ce alloy was investigated,and it was found that the passivation film composition was Al₂O₃ by analyzing the surface morphologies of Al-Ce alloy before and after the dissolution.The morphology and structure of Al-Ce alloy could change with the change of current density.The effect of F^-introduced into the melt on passivation was investigated,and it was found that the addition of F^-could inhibit passivation.The change of ion concentration in the melt was monitored by square wave voltammetry during the anodic dissolution.No obvious reduction peak related to Al（III）/Al was detected,indicating that the anodic dissolution of Al-Ce alloy could be carried out selectively.The dissolution of Al-U alloy was investigated by various electrochemical methods,and the dissolution order of Al-U alloy was obtained U→Al₄U intermetallic compound→Al.U was dissolved into U（III）,and then oxidized to U（IV）.The dissolution potential and passivation region of the alloy were studied using anodic polarization curves.Anodic passivation of Al-U alloy was found during its dissolution.The exchange current densities of Al-U alloy were calculated by the linear polarization method under different temperatures,and it was found that the exchange current density increased with the increasing temperature.The diffusion coefficient of U atoms in the Al-U alloy was also calculated by the anodic chronopotentiometry.Moreover,open-circuit chronopotentiometry was performed to calculate the activities of U in Al-U alloys and the standard molar generation Gibbs free energies of Al-U intermetallic compounds.