Electrochemical reduction of UO2 to U in a LiCl–KCl-Li2O molten salt

An electrochemical reduction of UO₂ to U in a LiCl–KCl-Li₂O molten salt has been investigated in this study. A diagram showing equilibrium potentials (relative to Cl₂/Cl^?) plotted versus the negative logarithms of oxide-ion activity (pO^2?) was constructed. The crushed UO₂ pellets in the cathode basket of an electrolytic reducer were successfully reduced to U. The reduction of UO₂ is proved to proceed mainly through chemical reaction with in situ generated Li and K at the cathode. The control of cathode potential is essential to prevent the deposition and subsequent vaporization of K metal at the cathode for the applications of a LiCl–KCl-Li₂O molten salt as an electrolyte for the metal production from its oxide sources.     

Morphology of lithium droplets electrolytically deposited in LiCl–KCl–Li2O melt

We report the growth of electrochemically deposited liquid lithium droplet in LiCl–KCl–Li₂O melt at 673–723 K. To understand the transient behavior of liquid lithium in the electrolyte, the interface between the electrodeposited molten metal phase and the molten salt system was observed in situ using a high-speed digital microscope. We found that the droplets on the electrode are slightly flattened, when the colloidal Li content decreases due to an increasing Li₂O content. This mechanism indicates that the heterogeneous distribution of the colloid Li may be due to the local Li solubility in the electrolyte.     

Distillation of LiCl from the LiCl–Li2O molten salt of the electrolytic reduction process

Electrolytic reduction of the uranium oxide in LiCl–Li₂O molten salt for the treatment of spent nuclear fuel requires the separation of the residual salt from the reduced metal product, which contains about 20 wt% salt. In order to separate the residual salt and reuse it in the electrolytic reduction, a vacuum distillation process was developed. Lab-scale distillation equipment was designed and installed in an argon atmosphere glove box. The equipment consisted of an evaporator in which the reduced metal product was contained and exposed to a high temperature and reduced pressure; a receiver; and a vertically oriented condenser that operated at a temperature below the melting point of lithium chloride. We performed experiments with LiCl–Li₂O salt to evaluate the evaporation rate of LiCl salt and varied the operating temperature to discern its effect on the behavior of salt evaporation. Complete removal of the LiCl salt from the evaporator was accomplished by reducing the internal pressure to <100 mTorr and heating to 900 °C. We achieved evaporation efficiency as high as 100 %.     

Electrochemical separation of uranium and cerium in molten LiCl–KCl

The electrochemical separation of uranium from cerium in LiCl–KCl eutectic and the electrochemical behavior of Ce(III) were studied. According to the cyclic voltammogram of Ce(III) and the former result of U(III), electrodeposition potential was determined at ?1.65 V (vs Ag/AgCl). The uranium metal was successfully deposited and separated from cerium. The morphology of deposit and cross section of electrode were investigated by SEM, firstly uranium deposit alloys with stainless steel and forms a thin transition layer, and secondly the uranium metal layer grows from the transition layer. The separation factors of uranium/cerium on different recovery ratios were determined through a series of steps. It was found that the content of cerium in the deposit and separation factors declined with increasing the initial concentration of U³⁺ in molten salts; the separation factors remained stable at around 20 in different uranium recovery ratios.     

Underpotential deposition of Li in a molten LiCl–Li2O electrolyte for the electrochemical reduction of U from uranium oxides

Reactive metal oxides are conventionally reduced to metal by metallothermic reduction. This paper presents on the efficient reduction method based on the electrochemical reaction in a molten LiCl–Li₂O electrolyte at 650 °C. An underpotential deposition of Li on uranium oxides was observed that enabled the mass electrochemical reduction of U₃O₈ to U. An advantage of using in-situ generated Li as a reductant is that a high-speed electrochemical reduction could be achieved with a wider operating voltage window when compared to a direct electrochemical reduction.     

Electrochemical properties and extraction of Dy on liquid Sn electrode in LiCl–KCl molten salt

Journal of Solid State Electrochemistry - To extract dysprosium (Dy) from LiCl–KCl molten salt, the electrochemical properties of Dy on liquid Sn electrode were explored by various...     

Electrochemical reactivity of Li–Mn–O and Li–Fe–Mn–O spinels

Electrochemical reductive dissolution of Li–Mn–O and Li–Fe–Mn–O spinels and Li⁺ extraction/insertion in these oxides were performed using voltammetry of microparticles. Both electrochemical reactions are sensitive to the Fe/(Fe+Mn) ratio, specific surface area, Li content in tetrahedral positions, and Mn valence, and can be used for electrochemical analysis of the homogeneity of the elemental and phase composition of synthetic samples. The peak potential (E _P) of the reductive dissolution of the Li–Mn–O spinel is directly proportional to the logarithm of the specific surface area. E _P of Li–Fe–Mn–O spinels is mainly controlled by the Fe/(Fe+Mn) ratio. Li⁺ insertion/extraction can be performed with Mn-rich Li–Fe–Mn–O spinels in aqueous solution under an ambient atmosphere and it is sensitive to the regularity of the spinel structure, in particularly to the amount of Li in tetrahedral positions and the Mn valence. Electronic Publication     

Electrochemical extraction of ytterbium from LiCl–KCl-YbCl3-ZnCl2 melt by forming Zn–Yb alloys

Journal of Solid State Electrochemistry - The electrochemical behavior of Yb(III) was studied at a W electrode in LiCl–KCl–ZnCl2 molten salt system. Cyclic voltammetry, square wave...     

The distillation behavior of fluorides in LiCl–KCl molten salt

Journal of Radioanalytical and Nuclear Chemistry - The distillation behaviors of some fission product fluorides in LiCl–KCl system were investigated. For rare earth element, the corresponding...     

Electrochemical and spectroscopic investigations of Tb(III) in molten LiCl–KCl eutectic at high temperature

Electrochemical and spectroscopic properties of Tb(III) in molten LiCl–KCl eutectic at high temperature were investigated by cyclic voltammetry and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The diffusion coefficient of Tb(III) and the formal standard potential of Tb(III)/Tb⁰ were determined to be 2.06 ± 0.4 × 10^? 5 cm² s^? 1 and ? 2.83 ± 0.03 V vs. Cl₂/Cl^? at 887 K, respectively. Additionally, visible fluorescence of Tb(III) due to the electronic transitions from ⁵D₃ and ⁵D₄ to ⁷F_J was observed and measured by TRLFS for the first time. These results provide the first fluorescence spectroscopic evidence for a direct in situ quantification of Tb(III) in the high temperature molten salt system.     

Electrochemistry of acetylide anion and anodic formation of carbon films in a LiCl–KCl–CaCl2–CaC2 melt

The electrochemical deposition of carbon films on a nickel substrate was carried out through anodic oxidation of calcium acetylide dissolved in a LiCl–KCl–CaCl₂ melt at 823 K. Continuous and tenacious carbon films were prepared by a two-stage anodically potentiostatic deposition at a fast rate, and characterized by SEM, Raman spectroscopy, XRD and XPS. The results show the carbon films composed of micron-sized particles with graphitized and amorphous phases containing a mixture of sp³ and sp² carbon. The cyclic voltammetry behavior of acetylide anion on graphite and nickel electrodes indicated that C₂^2 − ions are oxidized more favorably on the nickel substrate due to the anodic depolarization from nickel carburization.     

Enhanced electrochemical reduction of rare earth oxides in simulated oxide fuel via co-reduction of NiO in Li2O–LiCl salt

Rare earth oxides in spent oxide fuel from nuclear plants have poor reducibility in the electrochemical reduction process due to their high oxygen affinity and thermodynamic stability. Here, we demonstrate that the extent of their reduction can be enhanced via co-reduction of NiO in a Li₂O–LiCl electrolyte for the electrochemical reduction of a simulated oxide fuel (simfuel). First, the electrochemical behaviors of Nd₂O₃, NiO, and Nd₂O₃–NiO were studied by cyclic voltammetry and voltage control electrolysis. Then, the electrochemical reduction of the simfuel containing UO₂ and rare earth oxides (Nd₂O₃, La₂O₃, and CeO₂) was conducted in molten LiCl salt with 1 wt.% Li₂O via the co-reduction of NiO. The extent of reduction of the rare earth oxides was found to be significantly improved.     

Investigation of the evaporation of rare earth chlorides in a LiCl–KCl molten salt

Uranium dendrites which were deposited at a solid cathode of an electrorefiner contained a certain amount of salts. These salts should be removed for the recovery of pure metal using a cathode processor. In the uranium deposits from the electrorefining process, there are actinide chlorides and rare earth chlorides in addition to uranium chloride in the LiCl–KCl eutectic salt. The evaporation behaviors of the actinides and rare earth chlorides in the salts should be investigated for the removal of salts in the deposits. Experiments on the salt evaporation of rare earth chlorides in a LiCl–KCl eutectic salt were carried out. Though the vapor pressures of the rare earth chlorides were lower than those of the LiCl and KCl, the rare earth chlorides were co-evaporized with the LiCl–KCl eutectic salt. The Hertz–Langmuir relation was applied for this evaporation, and also the evaporation rates of the salt were obtained. The co-evaporation of the rare earth chlorides and LiCl–KCl eutectic were also discussed.     

Electrochemical behavior of uranium(III) in NaCl–KCl molten salt

The electro-redox behavior of uranium(III) on Mo electrode in NaCl–KCl molten salt in the temperature range 973–1073 K has been investigated using cyclic voltammetry electrochemical method and so on, such research will help to understand uranium behavior in pyro-reprocessing. The results showed that UCl₃ could be reduced into uranium metal in a quasi-reversible one-step process exchanging three electrons. The diffusion coefficients of U(III) ions were determined and the activation energy for diffusion was found to be 55.794 kJ mol⁻¹. The apparent standard potentials of U(III)/U(0) at several temperatures were calculated. The thermodynamic properties of UCl₃ have also been investigated.

     

Reaction mechanisms of the oxygen reduction and evolution reactions in aprotic solvents for Li–O2 batteries

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in aprotic solvents are elementary reactions for the discharging and charging processes on the cathode of the lithium-oxygen batteries, respectively. Understanding the mechanisms of these reactions at a molecular level has now become a bottleneck that hinders the development of the battery. This short article briefly reviews recent progresses in the studies of the ORR/OER mechanism in aprotic solvents. Two reaction mechanisms, the electrochemical pathway and chemical (disproportionation) pathway, will be discussed with their contribution to the ORR process on the cathode surface. Furthermore, the origin of the OER overpotential will also be discussed. The solutions to reduce the OER overpotential are noted with development of redox mediators.