| The prospect of nuclear energy development is becoming more and more promising.However,the development of nuclear energy is faced with the shortage of resources and the disposal problem of nuclear waste.Compared with uranium extraction by wet process such as in-situ leaching method,molten salt electrolysis method plays an important role in the preparation of uranium metal and its alloys.The method can also solve the problems in the extraction of uranium resources.It has obvious advantages,for example,the uranium metal product is much more uniform;the phase composition of the alloys can be controlled,and molten salt is reusable and friendly to environment.These advantages accelerate development of the electrolysis of uranides in recent years.At present,what we need to do is to realize the optimization of various technological parameters in the electrolysis process,to ensure that the reactions in electrodes and electrolytes are carried out in the reduction process,and to conduct in-depth research on the kinetic and thermodynamic theories of electrochemistry,so as to produce a large number of uranium metal.Based on these requirements,the thesis adopt such electrochemical methods as cyclic voltammetry,square wave voltammetry and chronopotentiometry to study the reduction process of uranium compounds in Li Cl-KCl molten salt system at 773K.And potentiometric titration method is adopted to investigate the oxidation process of uranium ions in Li Cl-KCl molten salt at high temperature.The thermodynamic and kinetic parameters of the reactions are calculated.The reaction mechanism was analyzed and the electrolytic parameters were continuously optimized to prepare uranium,U-Fe and U-Ga alloys.The specific work content is as follows:Cyclic voltammetry,square wave voltammetry and chronopotentiometry were used at773K in Li Cl-KCl-U3O8 molten salt system to verify the electrochemical reduction behaviors and reduction process of U3O8.It was found that U3O8 are reduced to U by two steps.In the first step,uranium trioxide is reduced to uranium dioxide,and the reduction potential is-1.70V.In the second step,uranium dioxide is reduced to uranium metal with the reduction potential of-2.45 V.Uranium metal was prepared by potentiostatic method on molybdenum electrode by-2.60 V and-3.2 V.The XRD test results proved the feasibility of accelerating the preparation of uranium metal under the mixed reduction process of‘Li Reduction Progress-Electrochemical Reduction'.Because U3O8 dissolves incompletely in Li Cl-KCl molten sat at 773 K,‘Chemical and Electrochemical'indirect reduction method was used in this part to make product UCl4completely dissolved in Li Cl-KCl molten salt.Firstly,U3O8 powder reacts with CCl4 gas and transformed to UCl4.Then in Li Cl-KCl-UCl4 at 773 K,the electrochemical behaviors and reduction process of U(IV)ions on the metal molybdenum are studied by the use of the cyclic vo1tammtery,chronopotentiometry,and open circuit potential method.On the soluble-soluble system,the Nernst equation,Randles-Shevchik equation,Berzins Delahay equation and based on Yang-Hudson for the extrapolation of infinite dilute solution method are all used in the study of thermodynamics and kinetics of related reduction research,such as diffusion coefficient D,transferred electron number n,transfer factor?and thermodynamic properties of?G,etc.,and used to study sedimentary process of the U(IV)ions on the inert metal interface.Based on the calculations,the electrolytic current was set to 100 m A,and the block metal was observed to be produced in the Li Cl-KCl-UCl4 molten salt system.Due to the different forms of U-O and U-Cl in the high-temperature molten salt environment,it has a great influence on the electrochemical tests and the preparation of metallic uranium.The transformation of U-O compounds and U-Cl compounds in Li Cl-KCl-UCl4 molten salt at 773 K was investigated by electrochemical potentiometric titration of U(IV)ions.When O2-ions reacts with U(IV)ions,the potential gradually increases.When the concentration of oxygen ions reaches the equivalent point?=2.391,the potential rapidly approaches 0 V,resulting in the formation of UO2 with poor conductivity.According to Faraday's law,the diffusion equation and schrodinger equation,Gran function,etc.,to calculate the corresponding Nernst slope s of YSZME electrode,electrode constant k,standard formal potential E*,electromotive force value emf and oxygen ion activityaO2-in blank component reactions in order to calculate the p O2-formula of UO2 to U which is E=-3.678+0.0383×p O2-.And according to the calculated UO2 stability constant of 1.653,the p O2-formula of UO2/U(IV)is E=-2.107+0.154×p O2-.Based on theoretical calculation and experimental data,the Littlewood diagram of U-O compounds in Li Cl-KCl-UCl4 molten salt was drawn,and the stable U-O and uranium ions phase regions under unit activity in Li Cl-KCl molten salt system were proposed.The recovery of uranium and other elements needs different electrodes in the reprocessing.Electrochemical methods such as cyclic voltammetry,square wave voltammetry and chronopotentiometry were used to study the electrochemical reduction and deposition conditions of prepared uranium alloys at 773K in Li Cl-KCl molten salt.The reduction potentials of Fe2U and Fe U6 were determined to be-0.90 V and-1.25 V respectively under the present experimental conditions using inert molybdenum metal and solid STS stainless steel as cathode,and the Fe2U and Fe U6 alloys were successfully prepared by setting the Potentiostatic potential-2.60 V.Chronopotentiometry was used as the cathode to conduct 10m A,40 m A and 100 m A electrolysis in a short time at the active liquid metal Ga electrode,and Ga3U,Ga2U and Ga3U2 intermetallic compounds were formed.UGa3 and metal U elements were prepared by galvanostatic method of 100 m A.At the same time,U-Ni and U-Co alloys were prepared by active cathode alloying method,and the relationship equation between equilibrium potential and atomic percentage of uranium,radius active metal were confirmed:C=e(2.329-E)/0.975 and E=-39.389/r+29.503. |
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