Effects Of Ce³⁺,Gd³⁺ And Sm³⁺on Electrodeposition Of Uo₂ In LiCl-KCl Molten Salt

In recent years,dry reprocessing technology has been widely studied in spent fuel of fast reactor.Molten salt electrolysis is considered to be one of the most effective methods for dry reprocessing of spent fuel.In spent fuel,the existence of rare earth elements seriously hinders the recovery of actinides.In order to realize the recovery and reuse of uranium resources,it is necessary to investigate the electrochemical properties of actinides and rare earth elements at high temperature molten salt.In this work,U₃O₈ and rare earth oxides（La₂O₃,Sm₂O₃,Gd₂O₃and CeO₂）as raw material,the fluorination reactions,the electrochemical behavior of UO₂²⁺and RE³⁺in LiCl-KCl molten salt,and the effect of RE³⁺on the electrodeposition of UO₂ were studied.The contents of this paper are as follows:（1）The reaction feasibility of U₃O₈ and rare earth oxides（La₂O₃,Sm₂O₃,Gd₂O₃ and CeO₂）with NH₄HF₂ was studied based on thermodynamic data.The fluorination reaction mechanism of U₃O₈ was investigated.Under different fluorination conditions,the reactions of U₃O₈ and rare earth oxides were explored,the products were UO₂F₂ and REF₃（LaF₃,SmF₃,GdF₃ and CeF₃）.Comparison of the fluorination effects,the optimum fluorination conditions were obtained.The fluorination rate of U₃O₈ was estimated to be 96.2%,and the fluorination rate of rare earth oxide was more than 96.0%.（2）In LiCl-KCl molten salt,the electrochemical behaviors of UO₂²⁺and Ce³⁺on W electrode were studied by cyclic voltammetry,square wave voltammetry and reverse chronopotentiometry.It was found that UO₂²⁺was reduced to UO₂ by two steps,namely:UO₂²⁺+e^-=UO₂⁺,UO₂⁺+e^-=UO₂,and the reduction of Ce³⁺was one-step 3-electron process with diffusion controlled.The diffusion coefficient of Ce³⁺in LiCl-KCl molten salt was calculated to be 5.38×10^-6 cm² s^-1 in the presence of F^-.The effect of Ce³⁺on electrodeposition of UO₂ in CeF₃-UO₂F₂-LiCl-KCl molten salt system was studied.It is found that in the UO₂F₂-CeF₃-LiCl-KCl system,when the concentration of Ce³⁺was high,Ce³⁺could react to form CeO⁺at the molten salt interface,while the potential difference between the reduction potentials of CeO⁺/CeO（-0.28 V）and UO₂⁺/UO₂（-0.2 V）was small,it was hard to separate U and Ce.Moreover,the CeO deposited on the electrode was found to be very unstable in air and easily oxidized to form CeO₂.Meanwhile,the composition of uranium and rare earth in the real spent fuel was simulated.The electrochemical formation of UO₂ was carried out in CeF₃-UO₂F₂-LiCl-KCl molten salt,at low concentration of Ce³⁺,the product was only UO₂.（3）The electrode reaction process of Gd³⁺and Sm³⁺in LiCl-KCl molten salt was studied by cyclic voltammetry,square wave voltammetry and reverse chronopotentiometry,respectively.Gd³⁺was reduced to metal Gd through one-step three-electron process,while Sm³⁺could only reduce to Sm²⁺by one electron transfer.In the presence of F^-,the diffusion coefficients of Gd³⁺and Sm³⁺in LiCl-KCl molten salt were evaluated to be 7.21×10^-6 cm² s^-1and 4.29×10^-6 cm² s^-1,respectively.Compared with LiCl-KCl molten salt,the diffusion coefficients of Gd³⁺and Sm³⁺were an order of magnitude smaller in the presence of F^-,which showed that there is a strong complexation between F^-and rare earth ion.The effects of Gd³⁺and Sm³⁺on the electrodeposition of UO₂ were studied.When the concentration of Gd³⁺was higher,Gd³⁺was converted to GdO⁺.The reduction potential difference of GdO⁺/GdO（-0.28V vs Ag/AgCl）and UO₂²⁺/UO₂ was small,it was hard to separate U and Gd.Gd O deposited on the electrode was easily oxidized to Gd₂O₃ in air.The product was comprised of UO₂ and Gd₂O₃.When the concentration of Gd³⁺and Sm³⁺in molten salt was low,it was difficult that Gd³⁺and Sm³⁺converted into GdO⁺and SmO⁺,avoiding the electrochemical formation of GdO and SmO.The electrolytic product only consisted of UO₂.