Electrochemical Behavior Of Uranium And Typical Fission Elements In Chloride Melting Salt System

Nuclear energy is one of the efficient approaches for solving the energy demand problem that human has to be faced. Nuclear fuel cycle is the core issue for the nuclear energy research. The spent fuel post-process technology is the most important method to increase coefficient of utilization for nuclear fuel and decrease the radioactivity for nulcear waste. The post-process technology must to be developed for the nuclear energy in high-speed and health way. The pyrometallurgical process is the important research direction for spent nuclear fuel of new type nuclear reactors. The cole issue for pyrometallurgical process research is electrochemistry of molten salt. This paper fouces on the electrochemical behavoirs study of uranium and typical nuclear fission elements in molten chloride salts. The main results are listed below:In this paper, actinide (U) and lanthanide (Ce) informations on the structural and transport characteristics in the molten salt mixtures were calculated by the molecular dynamics simulation. The electrochemical behaviors of lanthanide (Lu, La) and fission element (Zr) in the molten salt mixtures were studied by the electrochemical methods.In the molten LiCl-KCl, LiCl-KCl-UCl3 and LiCl-KCl-CeCl3 mixtures, the relationship formula of temperature and density, composition and density were obtained. The first peak for g U-Cl(r) was located at 0.266nm and the corresponding first coordination number of U3+was～7.1. The first peak for g Ce-C1(r) was located at 0.259 nm and the corresponding first coordination number of U3+ was-6.9. These inconsistencies between MD data and experimental data could be attributed to that our values are obtained in the molten LiCl-KCl-UCl3 (CeCl3) mixtures, and interaction between U3+(Ce3+) and Cl- was more powerful than that in pure molten UC13 (CeCl3). For self-diffusion coefficients, the activation energy of U3+ was 25.8 kJ-mol-1, and the activation energy of Ce3+ was 22.8 kJ-mol"1. Furthermore, the preexponential factors for U3+decreased from 46.2E-5 to 32.2E-5 cm2·s-1 as the molar fraction of U3+ increased from 0.005 to 0.05, and the preexponential factors for Ce3+decreased from 31.9E-5 to 21.8E-5 cm2·s-1 as the molar fraction of Ce3+increased from 0.005 to 0.05.The electrochemical behavior of Lu (III) ions on a molybdenum electrode in LiCl-KCl-LuCl3 and LiCl-KCl-MgCl2-LuCl3melts at 873 K was explored by various electrochemical techniques. The number of exchanged electrons in this electrochemical process was 2.92 (close to 3). The reduction of Lu (Ⅲ) to Lu (0) was quasi-reversible by a one-step mechanism. The results of XRD and SEM-EDS analysis demostrated that the Mg-Lu alloy samples were prepared by gavanostastic electrolysis on a Mo electrode, and an intermetallic compound of MgLu can be formed. Moreover, the Mg-Lu alloy layers were prepared by potentiostatic electrolysis on an Mg electrode, whereas two kinds of intermetallic compounds (Mg24Lu5 and MgLu) and Lu metal could be formed. Due to the relationship of electrolysis time and deposited thickness, the rate of deposition was slowed down with the increase of electrolysis time, because the reduction of Lu (III) ions on a Lu surface required a potential more negative than that on a Mg surface. Formation of Mg-Lu alloy layer could be controlled by applied potential and time. The mechanism of formation of Mg-Lu alloy layer was confirmed. Potentiostatic electrolysis was an effective method for electrochemical extraction of Lu.To investigate the electrochemical co-reduction mechanism of Mg-Zr alloys, the electrochemical behaviors of Zr (Ⅳ) in the KCl-MgCl2-K2ZrF6 and KCl-MgCl2-K2ZrF6-KF-ZrO2 melts were studied on a molybdenum electrode at 1023 K, respectively. Cyclic voltammograms (CVs) and square-wave voltammograms (SWVs) showed that Zr (IV) was reduced to Zr metal by a two-step mechanism corresponding to the Zr (IV)/Zr (II) and Zr (Ⅱ)/Zr (0) in the KCl-MgCl2-K2ZrF6 melts. The dissolution of ZrO2 in the KCl-MgCl2-K2ZrF6-KF melts could be observed from CVs and the concentrations of Zr (IV) were measured by inductively coupled plasma atomic emission spectrometer (ICP-AES). The Mg-Zr alloys were obtained by galvanostatic electrolysis in KCl-MgCl2-K2ZrF6-KF-ZrO2 melts and characterized by XRD and SEM-EDS. The content of zirconium in sample analyzed by ICP-AES could reach about 7.16 wt.%. Formation of Mg-Zr alloys could be controlled by applied time and composition. Direction electrolysis of ZrO2 to obtained Mg-Zr alloy in KCl-MgCl2-K2ZrF6-KF-ZrO2 melts was confirmed to be feasibility.Electrochemical and thermodynamic studies on the formation of La-Ni intermetallic compounds in molten LiCl-KCl-LaCl3 (3.5 wt.%) melts at 773 K were preformed by electrochemical techniques. The electrochemical reduction of La (Ⅲ) ions was investigated on inert W and reactive Ni electrodes by cyclic voltammetry. The reduction potential of La (III)/La on a Ni electrode was observed at more positive potential values than those on W electrode, due to the formation of La-Ni intermetallic compounds when La ions reacted with the Ni substrate. And square wave voltammetry, chronopotentiometry and open circuit chronopotentiometry put into evidence the formation of La-Ni intermetallic compounds. Potentiostatic electrolysis on a nickel electrode led to the formation of three La-Ni intermetallic compounds, LaNis, La7Ni16 and La2Ni3, characterized by XRD and SEM-EDS. The standard free energies of formation for the intermetallic compounds were calculated from open-circuit chronopotentiometric measurements. And using the Gibbs-Helmholtz equation and Hess law, the Gibbs free energy for the La-Ni intermetallic compounds was estimated. Formation of La-Ni alloy layer could be controlled by applied potential and time.The mechanism of formation of La-Ni alloy was confirmed. Potentiostatic electrolysis was an effective method for electrochemical extraction of La.The structure informations and thermodynamics properties of actinide (uranium, U) and lanthanide (cerium, Ce) and the electrochemical behaviors of lanthanide (Lu, La) and fission element (Zr) in the molten salt mixtures obtained in this paper, are very important to recognize molten salt and direct the study of post-precessing for spent nuclear fuel more deeply, which present many important data for the designment of post-processing flowsheet for spent nuclear fuel.