The Oxidation Of U⁴⁺ And Electrochemical Behavior Of UO₂²⁺ In Fluoride Melts

Molten salt reactor（MSR）,a unique kind of reactor using liquid fuel in generation IV reactors,has been internationally recognized as one of most ideal reactor types for the implementation of thorium-uranium cycle.Due to the characteristics of thorium-uranium cycle,on-line or off-line fuel reprocessing should be employed for the effective utilization of nuclear fuel in MSR.As the most widely researched and successful pyroprocess technologies,molten salt electrochemical separation method is well-suited for processing for MSR spent nuclear fuel,and it has advantages as compactness,radiation resistance,criticality safety as well as waste minimization.Unfortunately,the direct electrochemical separation of U⁴⁺from MSR fuel has limitations due to the form of fuel as LiF-BeF₂-UF₄-ThF₄-FPF_x.In LiF-BeF₂（LiF:BeF₂=66:34 mol%,FLiBe）melt,the reduction of U⁴⁺exhibits two steps:U⁴⁺→U³⁺and U³⁺→U⁰,and the reduction potential of latter is closing to the electrochemical window potential of FLiBe melt(Be²⁺/Be⁰ couple and those of rare earths（REs）.A new idea about the employment of electrochemical method in the MSR fuel processing is proposed:first the U⁴⁺is converted into UO₂²⁺to make the reduction potential shift positively;then UO₂²⁺is electrochemically reduced to UO₂ to separate from the melt.Up to now,most of the researches concerning the molten salt electrochemistry of UO₂²⁺were focused on the chloride melts.Hardly any literature has reported the electrochemical properties of UO₂²⁺in the fluoride melts.In addition,it is also a key issue whether U⁴⁺can be converted into UO₂²⁺in the fluoride melt at high temperature.In this paper,LiF-NaF-KF（LiF:NaF:KF=46.5:11.5:42 mol%,FLiNaK）melt was used to simulate the FLiBe melt,the oxidation of UF₄ and the electrochemical behavior of UO₂²⁺as well as its electrolysis in FLiNaK melt were conducted.The results were listed as follow:1.UO₂F₂,the yellowish powder,was prepared by the reaction of UF₄ powder and oxygen at high temperature,and UO₂F₂-FLiNaK melt was prepared by dissolving this power in FLiNaK melt.The electrochemical behavior of UO₂²⁺in FLiNaK melt on Pt electrode was investigated at 550℃by cyclic voltammetry（CV）and square wave voltammetry（SWV）.The results showed that the reduction of UO₂²⁺in FLiNaK melt exhibited two steps both with one exchanged electron:UO₂²⁺+e^-→UO₂⁺and UO₂⁺+e^-→UO₂,with the reduction peak at about 2.02 V and 1.49 V（vs.K⁺/K）,respectively,which were more positive than those of rare earths（REs）and the electrochemical potential of FLiNaK melt.Both of them were reversible and diffusion-controlled.The UO₂F₂-FLiNaK melt was preliminarily electrolyzed by the potentiostatic method for 4h at 550℃,and separation ratio of uranium was 54.5%.X-ray diffraction（XRD）,energy dispersive spectrometer&scanning electron microscope（EDS-SEM）and energy dispersive spectrometer&transmission electron microscopy（EDS-TEM）showed that the obtained product was UO₂ in the form of single crystal,mainly with octahedral structure,which were in the range of several micrometers to several tens of micrometers.2.The UO₂F₂ powder was dissolved in FLiBe melt to prepare UO₂F₂-FLiBe melt.The electrochemical behavior of UO₂²⁺in FLiBe melt was investigated at 550℃.The results showed that UO₂²⁺was unstable during electrochemical measurements and the addition of KF could stabilize it.The electrochemical behavior of UO₂²⁺in KF-FLiBe melt was investigated by CV and SWV,revealing that the reduction of UO₂²⁺was a single step with two electrons:UO₂²⁺+2e^-→UO₂.The reduction peak in CV curve was at about 1.93 V(vs.Be²⁺/Be),which was far away from those of rare earths and the electrochemical window of melt.By potentiostatic electrolysis of UO₂F₂-KF-FLiBe melt for 4h at 550℃,XRD,SEM-EDS and EDS-TEM manifested that the product on cathode was octahedral UO₂ single crystals with micro size.3.The oxidation of UF₄ in LiF-NaF-KF melt was carried out at 550℃by inducing oxygen.To monitor the reaction process,the off-gas from the outlet of the furnace was detected by in-situ infrared spectrometer（IR）.The melts before and after oxidation were analyzed by XRD,UV-vis absorption spectrometer of molten salt（UAMS）,and the inductively coupled plasma atomic emission spectrometer（ICP-AES）.The results showed that the outlet gas of the reactor only contains HF;the concentrations of uranium in FLiNaK melt before and after reaction were almost unchanged;K₃UO₂F₅（3KF·UO₂F₂）was formed in molten salt phase;UAMS analysis suggested the presentation of UO₂²⁺.Combined with thermodynamic calculations,the reaction mechanism of UF₄ and O₂ in FLiNaK melt was estimated to be:2UF₄（l）+O₂（g）+2H₂O（g）=2UO₂F₂（l）+4HF（g）.4.The electrochemical behavior of UF₄-FLiNaK melt after oxidation was investigated by CV at 550℃.The electrochemical signals in this system were consistent with those in UO₂F₂-FLiNaK melt,which is a two-step reaction with one exchanged electron:UO₂²⁺+e^-→UO₂⁺and UO₂⁺+e^-→UO₂,indicating that the generation of UO₂²⁺after in-situ oxidation.Potentiostatic electrolysis was employed in the UF₄-FLiNaK melt after in-situ oxidation at 550℃.XRD and SEM-EDS manifested that the product on cathode was UO₂ nanostructure crystal with triangular surface,which are the triangular growth of UO₂ crystal on（111）face.In conclusion,U⁴⁺in FLiNaK melt can be converted into UO₂²⁺by chemical reaction and the reduction potential of UO₂²⁺is far away from the electrochemical window and those of rare earths,and UO₂ can be obtained by electrolysis with a relatively positive potential.The preliminary results indicated that the new idea proposed by this paper has certain feasibility.It was found that UO₂²⁺was unstable in FLiBe melt during electrochemical measurements,indicating that it is easy to be separated from FLiBe melt;on the other hand,it is possibly difficult to convert U⁴⁺to UO₂²⁺in FLiBe melt compared to that in FLiNaK melt.