Electrochemical and spectroscopic properties of technetium in fused alkali metal chlorides

The behavior of metallic technetium in chloride melts was studied for the first time. The experiments were conducted in NaCl-2CsCl eutectic melt at 550 °C using high temperature electronic absorption spectroscopy, cyclic voltammetry and chronopotentiometry. The anodic dissolution of Tc results in the formation of a mixture of Tc(IV) and a lower oxidation state species. Cyclic voltammograms of technetium-containing melts show two cathodic and two associated anodic peaks. One pair is around -0.35 V vs. Ag/AgCl reference electrode and is associated with the deposition of Tc metal. ©The Electrochemical Society.Physical and Analytical Electrochemistry;Electrodeposition;Energy Technolog

Tungsten chemistry in alkali chloride melts

Speciation of tungsten in alkali chloride melts (based on NaCl-2CsCl, NaCl-KCl and 3LiCl-2KCl mixtures) was studied between 550 and 750æC using electronic absorption spectroscopy. Only W(IV) and W(V) chloro and oxychloro species can be stabilized under the conditions studied. Tungsten(IV) chloride ions are very sensitive to oxide/hydroxide impurities present in the melt. Anodic dissolution of W metal at anodic current densities in the range of 0.005 - 0.1 A/cm2 produces only [WCl6]2- ions that can be electrochemically (on a glassy carbon anode) oxidized to [WCl 6]-. Small amounts of oxide ions present in the melt result in a gradual conversion of W(IV) chloro species into W(IV) oxychloro species. In the presence of O2 in the atmosphere [WCl 6]2- is oxidized into the tungsten(V) species [WOCl 5]2-. Dissolution of tungsten hexachloride, WCl 6, in an NaCl-2CsCl melt initially yields [WCl6] 2- (due to disproportionation). Reaction of metallic tungsten with Pd(II)-containing melts results in the formation of [WCl6] 2- species. The main spectroscopic parameters of [WCl 6]2- and [WCl6]- complex ions were calculated. © 2007 Verlag der Zeitschrift für Naturforschung

The effect of fission product elements on the behavior of uranyl species in alkali chloride melts: A contribution towards reprocessing spent oxide fuels

The reactions of uranyl(VI) containing chloride melts with molybdenum, niobium, zirconium and palladium were studied using high temperature electronic absorption spectroscopy. Depending on the nature of the added element uranium is reduced to uranyl(V) and uranium(IV) chloro-species and UO2. Palladium, niobium and zirconium can all be removed from a uranyl(VI)-containing melt using molybdenum metal and the melt can then be purified from Mo(III) ions by bubbling Cl2 gas. Such approach can be employed for removal a number of fission product elements from molten chloride baths during reprocessing spent oxide fuels. ©The Electrochemical Society.Physical and Analytical Electrochemistry;Electrodeposition;Energy Technolog

On the formation of uranium(V) species in alkali chloride melts

Uranyl(V) species are normally unstable in solutions but are here shown to be stable in high-temperature chloride melts. Reactions leading to the formation of UO2Cl4 3- ions were studied, including thermal decomposition and chemical reduction of uranyl(VI) chlorospecies in various alkali chloride melts (LiCl, 3LiCl-2KCl, NaCl-KCl, and NaCl-2CsCl) at 550-850 °C. Decomposition of UO2Cl4 2- species under reduced pressure, with inert gas bubbling through the melt or using zirconium getter in the atmosphere results in the formation of UO 2Cl4 3- and UO2. Elemental tellurium, palladium, silver, molybdenum, niobium, zirconium, and hydrogen, as well as niobium and zirconium ions were tested as the reducing agents. The outcome of the reaction depends on the reductant used and its electrochemical properties: uranyl(VI) species can be reduced to uranyl(V) and uranium(IV) ions, and to uranium dioxide. © 2010 IUPAC

Speciation of rhenium in chloride melts: Spectroscopic and electrochemical study

Speciation of rhenium in high-temperature alkali chloride-based melts was studied using electronic absorption and IR spectroscopy of molten salts and diffuse reflectance spectroscopy of quenched melts. Rhenium was added to the melts by anodic dissolution of the metal (at anodic current densities of 0.005-0.05 A/cm2), by reacting Re and ReO2 with Cl 2 and HCl, and by dissolving K2[ReCl6]. The melts included 3LiCl-2KCl and NaCl-2CsCl eutectics, an NaCl-KCl equimolar mixture, and pure NaCl, KCl and CsCl between 450 and 850 °C. Rhenium was present in the melts as Re(IV) hexachloro-ions, [ReCl6]2-; no evidence of species containing rhenium in oxidation states below four was obtained. The kinetics of [ReCl6]2- disproportionation in molten alkali chlorides were investigated, and the IR spectra of [ReO 4]- ions in molten CsCl-CsI and CsI were measured for the first time. © 2008 Verlag der Zeitschrift für Naturforschung, Tübingen

Electrode and redox potentials of molybdenum and stability of molybdenum chloro-species in alkali chloride melts

Molybdenum electrode and Mo(III)/(IV) redox potentials were measured in fused alkali chlorides. Experiments were performed in individual salts (LiCl, NaCl, KCl, RbCl and CsCl) and in several binary and ternary eutectic or low-melting mixtures between 633 and 1173 K (depending on the melting point of the solvent salt). Formal standard electrode potentials E∗ Mo/Mo(III) and redox potentials E∗ Mo(III)/Mo(IV) in respect to Cl-/Cl2 couple and Gibbs free energy change of formation of molybdenum(III) chloride in alkali chloride melts were calculated. Electronic absorption spectra of Mo(III) ions were recorded, and spectroscopic parameters of MoCl6 3- complex ions determined. High temperature spectroscopy measurements were used to study the stability of Mo(III) chloro-species in fused chlorides and the reaction rates of Mo(III) ions disproportionation were also determined. Diffusion coefficients of molybdenum ions in LiCl-KCl-CsCl and NaCl-CsCl eutectic based melts were determined from the results of electrochemical measurements. © The Author(s) 2017

Spectroelectrochemical study of stainless steel corrosion in NaCl-KCl melt

High-temperature spectroelectrochemestry was applied to study corrosion of various types of stainless steel in molten salts. The electronic absorption spectra of products of anodic dissolution of stainless steel major components (iron, chromium, nickel, molybdenum, manganese, titanium) were measured in NaCl-KCl melt at 750 °C The effectiveness and limitations of applying spectroscopic method for studying alloys corrosion was demonstrated on example of anodic dissolution of AISI 316L, 316Ti and 321 austenitic steels. The major corrosion products of steels are iron, manganese and chromium species. Prolongation of anodic dissolution leads to increasing chromium-to-iron ratio in the melt. Titanium in steels forms very stable carbonitride species that aren't dissolved during anodic oxidation. © The Electrochemical Society.Physical and Analytical Electrochemistry;Electrodeposition;Energy Technolog

Effect of melt composition on the reaction of uranium dioxide with hydrogen chloride in molten alkali chlorides

The reaction of uranium dioxide with excess hydrogen chloride in alkali chloride melts (LiCl, 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl) has been studied between 450 and 750°C, and the reaction products were characterized by electronic absorption and X-ray absorption spectroscopy. Uranium(V), [UO 2Cl4]3-, and uranium(IV), [UCl 6]2-, species were formed. They depended upon the temperature and the radius of the alkali cations present. Uranium(V) ions predominated in melts with small cations (LiCl and 3LiCl-2KCl). © 2007 Verlag der Zeitschrift für Naturforschung

Study of causes of film formation on the electrolyte surface during niobium electrorefining

Formation of a conducting film on the surface of fused electrolyte during electrorefining of niobium in chloride melts was investigated. It was found that this film has metallic nature and is formed as a result of disproportionation of niobium(III) ions at a liquid-gas interface. Decreasing temperature gradient along the heated part of the electrolyser can be used to limit the influence of the disproportionation onto film formation. The metal film formation on solid ceramics can be used for creation of conducting layers on various oxide materials. copyright The Electrochemical Society.Electrochemical Society, Phys. Anal. Electrochem. Div.;Electrochemical Society, Electrodeposition Division;Electrochemical Society, High Temperature Materials Division;Electrochemical Society, Battery Division;Electrochemical Society, Energy Technology Divisio

Behaviour of rare earth elements in molten salts in relation to pyrochemical reprocessing of spent nuclear fuels

The kinetics of chlorination of lanthanide oxides (by Cl2 and HCl) and precipitation of lanthanide phosphates was studied by in situ electronic absorption spectroscopy in 3LiC1-2KC1 and NaCl-KCl melts at 400-750°C Products of the chlorination are the corresponding hexachlorospecies, LnCl63-, and the rate of chlorination increases with increasing temperature. Composition of the precipitated phosphates depends on the melt composition and 1.5-5 fold excess of phosphate, depending on the nature of lanthanide, is needed for the complete removal of the lanthanides from the melt. Over three hours are required for completing the reaction of phosphate precipitation. copyright The Electrochemical Society.Electrochemical Society, Phys. Anal. Electrochem. Div.;Electrochemical Society, Electrodeposition Division;Electrochemical Society, High Temperature Materials Division;Electrochemical Society, Battery Division;Electrochemical Society, Energy Technology Divisio