Reduction Of UO2 Research Articles

A Study of Graphite as Anode in the Electro-Deoxidation of Solid UO2 in LiCl-Li2O Melt

Electrochemical behavior of graphite in LiCl and LiCl melts containing up to 1 wt% Li2O was studied by using cyclic voltammetry and electrolysis, in the context of its probable use as anode in the electrochemical reduction of solid UO2 in the molten salt at 650°C. The lithium deposition and chlorine evolution potentials of LiCl melt were determined by CV of tantalum (cathodic polarization) and graphite (anodic polarization) working electrodes as −2.19 V and +1.21 V vs Ni/NiO reference electrode respectively. In LiCl-Li2O melts, carbon dioxide was formed on the graphite WE from > +0.75 V onwards and its current and potential range of formation increased with increase in the Li2O content of the melt. Cathodic polarization of a UO2 disc WE in LiCl-1 wt% Li2O melt showed that the reduction of UO2 to U occurs at −2.18 V and the lithium metal deposition on it at −2.36 V. Electro-reduction experiments carried out with a dense UO2 pellet cathode and graphite anode in both LiCl and LiCl-Li2O melts proved that the surface of the pellet was reduced to U metal at ≥ −2.2 V. The reduction became increasingly difficult with increase in the Li2O content of the melt.

Alternate Anodes for the Electrolytic Reduction of UO2

The electrolytic reduction process of UO2 employs a platinum anode and a stainless steel cathode in molten LiCl-LiO2 maintained at 973 K (700 °C). The degradation of platinum under the severely oxidizing conditions encountered during the process is an issue of concern. In this study, Inconel 600 and 718, stainless steel alloy 316, tungsten, nickel, molybdenum, and titanium, were investigated though electrochemical polarization techniques, electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy to serve as potential anode materials. Of the various materials investigated, only tungsten exhibited sufficient stability at the required potential in the molten electrolyte. Tungsten anodes were further studied in molten LiCl-LiO2 electrolyte containing 2, 4, and 6 wt pct of Li2O. In LiCl-2 wt pct Li2O tungsten was found to be sufficiently stable to both oxidation and microstructural changes and the stability is attributed to the formation of a lithium-intercalated tungsten oxide surface film. Increase in the concentration of Li2O was found to lead to accelerated corrosion of the anode, in conjunction with the formation of a peroxotungstate oxide film.

Use of a single fuel containment material during pyroprocessing tests

The use of a single stainless steel (STS) wire mesh basket as the fuel containment material for a series of pyroprocessing steps has been studied. The use of a single basket minimizes fuel loss and was enabled by transporting and using the basket containing the fuel from one test to the next without unloading it. The series of tests consisted of electrolytic reduction, LiCl distillation, electrorefining, LiCl–KCl distillation, and finally a second electrolytic reduction and a subsequent LiCl distillation step. While the electrolytic reduction of UO2 was conducted in a LiCl–Li2O molten salt electrolyte at 650°C using the STS wire mesh basket as the cathode, the electrorefining was carried out in a LiCl–KCl–UCl3 molten salt electrolyte at 500°C, using the STS wire mesh basket as the anode. During the salt (LiCl and LiCl–KCl) distillation processes, the product of electrolytic reduction/electrorefining in the basket, which included metallic U and residual salts, was distilled at 850°C under vacuum. The electrolytic reduction, electrorefining, and salt distillation processes were successfully demonstrated with the use of a single STS wire mesh basket through the entire cycle. However, an unstable intermetallic U–Fe layer was observed between the reduction product and the STS basket, when a cross section of the basket was studied after the salt distillation steps. The influence of the U–Fe layer on the electrolysis steps needs to be studied further in order to understand and quantify the lifetime of a single STS wire mesh basket during pyroprocessing.

Electrochemical behavior of liquid Sb anode system for electrolytic reduction of UO2

A liquid Sb anode was investigated in this study to evaluate its ability to promote the electrolytic reduction of UO2. The liquid Sb anode consumed O2− ions to convert UO2 into metallic U during electrolytic reduction in an LiCl–Li2O electrolyte at 650 °C. However, simultaneous electroplating of Sb at the cathode prohibited the complete conversion, leaving residual UO2.

Inhibition of U(VI) reduction by synthetic and natural pyrite.

Reductive precipitation is an effective method of attenuating the mobility of uranium (U) in subsurface environments. The reduction of U(VI) by synthetic and naturally occurring pyrite was investigated at pH 3.0-9.5. In contrast to thermodynamic calculations that were used to predict UO2(s) precipitation, a mixed U(IV) and U(VI) product (e.g., U3O8/U4O9/U3O7) was only observed at pH 6.21-8.63 and 4.52-4.83 for synthetic and natural pyrite, respectively. Under acidic conditions, the reduction of UO2(2+) by surface-associated Fe(2+) may not be favored because the mineral surface is nearly neutral or not negative enough. At high pH, the sorption of negatively charged U(VI) species is not favored on the negatively charged mineral surface. Thus, the redox reaction is not favored. Trace elements generally contained within the natural pyrite structure can affect the reactivity of pyrite and lead to a different result between the natural and synthetic pyrite. Because UO2(s) is extremely redox-sensitive toward U(VI), the observed UO2+x(s) phase reduction product indicates a surface reaction that is largely controlled by reaction kinetics and pyrite surface chemistry. These factors may explain why most laboratory experiments have observed incomplete U(VI) reduction on Fe(II)-bearing minerals.

Propagation of U(V)-reduction in the presence of U(IV) aggregate in a weakly acidic solution

Reduction processes of UO22+ in weakly acidic solutions were investigated based on electrochemical and spectrophotometric measurements. A reversible one-electron reduction of UO22+ to UO2+ and a further irreversible reduction of UO2+ were observed voltammetrically at a gold microdisk electrode in weakly acidic perchlorate solutions of pH from 2.0 to 3.5. Aggregates of U(IV) were formed as a deposit on the electrode and a colloid in the bulk solution, when the controlled-potential bulk electrolysis was carried out at a gold gauze electrode, even though the potential applied was that available for the first one-electron reduction wave of UO22+ observed at a gold microdisk electrode. It was elucidated that the aggregate was produced by the combination of the one-electron reduction to UO2+ and the disproportionation of UO2+ producing U(IV) and UO22+. The aggregate enhanced the rate of the disproportionation of UO2+, and hence the reduction current of UO22+ increased abruptly when a definite amount of aggregate was formed on the electrode, in the solution, or both. The aggregate of a two-electron reduction product of UO22+ was determined to be poorly crystallized UO2 of fluorite-type structure, and the characteristics of the aggregate were different from the crystalline UO2

Experimental and Theoretical Approaches to Redox Innocence of Ligands in Uranyl Complexes: What Is Formal Oxidation State of Uranium in Reductant of Uranyl(VI)?

Redox behavior of [UO2(gha)DMSO](-)/UO2(gha)DMSO couple (gha = glyoxal bis(2-hydroxanil)ate, DMSO = dimethyl sulfoxide) in DMSO solution was investigated by cyclic voltammetry and UV-vis-NIR spectroelectrochemical technique, as well as density functional theory (DFT) calculations. [UO2(gha)DMSO](-) was found to be formed via one-electron reduction of UO2(gha)DMSO without any successive reactions. The observed absorption spectrum of [UO2(gha)DMSO](-), however, has clearly different characteristics from those of uranyl(V) complexes reported so far. Detailed analysis of molecular orbitals and spin density of the redox couple showed that the gha(2-) ligand in UO2(gha)DMSO is reduced to gha(•3-) to give [UO2(gha)DMSO](-) and the formal oxidation state of U remains unchanged from +6. In contrast, the additional DFT calculations confirmed that the redox reaction certainly occurs at the U center in other uranyl(V/VI) redox couples we found previously. The noninnocence of the Schiff base ligand in the [UO2(gha)DMSO](-)/UO2(gha)DMSO redox couple is due to the lower energy level of LUMO in this ligand relative to those of U 5f orbitals. This is the first example of the noninnocent ligand system in the coordination chemistry of uranyl(VI).

A Novel Technique for Estimation of Metallic Uranium Using Proton Exchange Membrane Based Hydrogen Sensor

Direct electrochemical reduction of UO2 to U metal is being attempted in molten salts. In this context, a new technique for the estimation of uranium metal has been developed. In this technique, the concentration of H2 released during the reaction of a metal with HBr is measured online as a function of time using an in-house developed proton exchange membrane hydrogen sensor. The method was developed by reacting different known masses of Zn with HBr and measuring the quantity of liberated hydrogen. Subsequently the methodology was employed for the estimation of U. From the calibration plots of number of moles of hydrogen released versus number of moles of Zn or U taken, an unknown mass of Zn or U was estimated. Uranium metal was estimated by the method with accuracy comparable with the results from inductively coupled plasma optical emission spectroscopic analysis. The technique was found to be simple, accurate and hands free for the estimation of uranium metal. The paper discusses the details of the sensor assembly, calibration, testing, methodology development and results of estimation of zinc and uranium metals using the sensor.

Electrochemical reduction of UO2 in LiCl–Li2O molten salt using porous and nonporous anode shrouds

Electrochemical reductions of uranium oxide in a molten LiCl–Li2O electrolyte were carried out using porous and nonporous anode shrouds. The study focused on the effect of the type of anode shroud on the current density by running experiments with six anode shrouds. Dense ceramics, MgO, and MgO (3wt%) stabilized ZrO2 (ZrO2–MgO) were used as nonporous shrouds. STS 20, 100, and 300 meshes and ZrO2–MgO coated STS 40 mesh were used as porous shrouds. The current densities (0.34–0.40Acm−2) of the electrolysis runs using the nonporous anode shrouds were much lower than those (0.76–0.79Acm−2) of the runs using the porous shrouds. The ZrO2–MgO shroud (600–700MPa at 25°C) showed better bending strength than that of MgO (170MPa at 25°C). The high current densities achieved in the electrolysis runs using the porous anode shrouds were attributed to the transport of O2− ions through the pores in meshes of the shroud wall. ZrO2–MgO coating on STS mesh was chemically unstable in a molten LiCl–Li2O electrolyte containing Li metal. The electrochemical reduction runs using STS 20, 100, and 300 meshes showed similar current densities in spite of their different opening sizes. The STS mesh shrouds which were immersed in a LiCl–Li2O electrolyte were stable without any damage or corrosion.

Mixed-Valence Uranium(V,VI) and Uranyl Oxyhydroxides Synthesized under High-Temperature, High-Pressure Hydrothermal Conditions: Na5[U5O16(OH)2] and Na5[U5O17(OH)

An unusaul mixed-valence uranium(V,VI) oxyhydroxide, Na5[U5O16(OH)2], was synthesized via reduction of UO2(2+) with zinc under hydrothermal conditions at 570 °C and 150 MPa. Its structure consists of extended sheets of edge-sharing U(VI) pentagonal bipyramids and U(V) square bipyramids. The overall anion topology is the same as that of the uranyl mineral sayrite, but the squares are populated by U(V)(OH)2(3+) ions. The valence state of uranium was confirmed by X-ray photoelectron spectroscopy. A hydrothermal reaction without zinc under the same reaction conditions yielded a uranyl oxyhydroxide, Na5[U5O17(OH)], with a new sheet structure. The sheet anion topology, which is closely related to that of the mixed-valence compound, contains chains of edge-sharing pentagons as well as dimers of corner-sharing squares.

Formation of (Cr, Al)UO4 from doped UO2 and its influence on partition of soluble fission products

CrUO4 and (Cr,Al)UO4 have been fabricated by a sol–gel method, studied using diffraction techniques and modelled using empirical pair potentials. Cr2O3 was predicted to preferentially form CrUO4 over entering solution into hyper-stoichiometric UO2+x by atomic scale simulation. Further, it was predicted that the formation of CrUO4 can proceed by removing excess oxygen from the UO2 lattice. Attempts to synthesise AlUO4 failed, instead forming U3O8 and Al2O3. X-ray diffraction confirmed the structure of CrUO4 and identifies the existence of a (Cr,Al)UO4 phase for the first time (with a maximum Al to Cr mole ratio of 1:3). Simulation was subsequently used to predict the partition energies for the removal of fission products or fuel additives from hyper-stoichiometric UO2+x and their incorporation into the secondary phase. The partition energies are consistent only with smaller cations (e.g. Zr4+, Mo4+ and Fe3+) residing in CrUO4, while all divalent cations are predicted to remain in UO2+x. Additions of Al had little effect on partition behaviour. The reduction of UO2+x due to the formation of CrUO4 has important implications for the solution limits of other fission products as many species are less soluble in UO2 than UO2+x.

Mechanism of Direct Electrochemical Reduction of Solid UO2 to Uranium Metal in CaCl2-48mol% NaCl Melt

Electrochemical reduction of high dense UO2 pellets (95% TD) was carried out in molten CaCl2-48mol% NaCl at 923 K by constant voltage and constant current modes of electrolysis using graphite as the anode with an aim to check the feasibility of reduction and to determine the reduction mechanism. Electro-reduction experiments were carried out at 3.3 V for different durations of time (6, 12, 19, 25, 35 and 44 h) in order to obtain partially reduced samples. The UO2 pellets electrolysed for 35 h and above were fully reduced whereas those samples electrolysed for lower durations of time showed only surface reduction. Low current galvanostatic electrolysis of UO2 pellet showed gradual polarization of the cathode to higher cathodic values with time within a band of +0.250 V and 0.000 V vs (Na-Ca)/(Na+,Ca2+). No ternary intermediate compounds of the type Ca-U-O or Na-U-O or sub-oxides were observed in the partially reduced samples. Unlike in pure CaCl2 melt at 1173 K, the consumption of graphite anode and the carbon contamination of the melt were found to be minimum in the present study. Based on the experimental data, a probable mechanism of electrochemical reduction of UO2 to U in molten CaCl2-48mol% NaCl has been proposed.

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 (pO2−) 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.

Application of a boron doped diamond (BDD) electrode as an anode for the electrolytic reduction of UO2 in Li2O–LiCl–KCl molten salt

A boron doped diamond thin film electrode was employed as an inert anode to replace a platinum electrode in a conventional electrolytic reduction process for UO2 reduction in Li2O–LiCl molten salt at 650°C. The molten salt was changed into Li2O–LiCl–KCl to decrease the operation temperature to 550°C at which the boron doped diamond was chemically stable. The potential for oxygen evolution on the boron doped diamond electrode was determined to be approximately 2.2V vs. a Li–Pb reference electrode whereas that for Li deposition was around −0.58V. The density of the anodic current was low compared to that of the cathodic current. Thus the potential of the cathode might not reach the potential for Li deposition if the surface area of the cathode is too wide compared to that of the anode. Therefore, the ratio of the surface areas of the cathode and anode should be precisely controlled. Because the reduction of UO2 is dependent on the reaction with Li, the deposition of Li is a prerequisite in the reduction process. In a consecutive reduction run, it was proved that the boron doped diamond could be employed as an inert anode.

Determination of the extent of reduction of dense UO2 cathodes from direct electrochemical reduction studies in molten chloride medium

Electro-reduction of solid UO2 to U has been studied with molten CaCl2 or LiCl as the electrolyte medium. Electro-reduction of thick (>3mm), powder compacted and sintered pellets of UO2 showed incomplete reduction resulting in a mixture of uranium metal and UO2. The extent of reduction of UO2 to U was determined by employing a novel method called ‘metal estimation by hydrogen sensor (MEHS)’, in which the hydrogen evolved during the reaction of U metal in the reduced product with con. HBr was measured using an in-house developed polymer electrolyte based amperometric hydrogen sensor. The results of our investigations on incompletely reduced UO2 pellets in both CaCl2 and LiCl melts showed that the extent of reduction of different regions of the oxide pellet was different. It varied from 88.3% on the surface of the pellet as against 3.7% towards the centre bulk during electro-reduction in CaCl2 (at 1173K). The metallisation was found restricted to the surface of the pellets reduced in LiCl melt (at 923K). Electro-reduction of small chunks of UO2 pellet in CaCl2 melt resulted in products with lower extent of reduction. Based on the measurements, a probable mechanism on the propagation of reduction through the solid UO2 matrix during the electrochemical reduction process has been proposed.

Electrochemical deposition of uranium oxide in highly concentrated calcium chloride

The coordination circumstances and redox reactions of UO2 2+ in the aqueous solution concentrated by calcium chloride, such as CaCl2·6H2O (6.9 M CaCl2), were studied by Raman spectroscopy and electrochemical methods. The frequency of the O=U=O symmetrical stretching vibration suggested that the complex formation of UO2 2+ with Cl− leads to the weakening of U=O bond. In the electrochemical measurements, two-step cathodic currents were observed at −0.090 and −0.4 V (vs. Ag|AgCl) corresponding to the reduction of UO2 2+ to UO2 + and that of UO2 + to UO2, respectively. It was found that UO2 + formed at first cathodic current was disproportionated to form UO2 2+ and UO2. The UO2 was identified by X-ray diffraction analysis. Electrolytic deposition of UO2 was observed in 6.9–4.7 M CaCl2 and in 14 M LiCl. When small amount of proton, i.e., 0.005 M was coexisted in 6.9 M CaCl2, UO2 2+ was reduced to form U4+ instead of UO2.

Electrochemical studies of U(VI)/U(V) in saturated Na2CO3solution at gold nanoparticles embedded CTA-modified electrode

AbstractGold nanoparticles (AuNPs) were prepared in the matrix of cellulose triacetate (CTA) membrane containing a liquid anion exchanger trioctylmethylammonium chloride (Aliquat-336). The AuCl4−ions were transferred in the membrane matrix by anion-exchange process, and then subsequently reduced with NaBH4to form AuNPs in the membrane. The AuNPs-embedded CTA (AuNPs-CTA) membrane was characterized by UV-visible spectroscopy, XRD and AFM. The electrochemical properties of AuNPs-CTA modified electrode were evaluated by studying redox behavior of UO22+/UO2+couple in saturated Na2CO3solution, using voltammetric techniques. In carbonate solution, the predominant species of UO22+is [UO2(CO3)3]4−, and a stable UO2(CO3)35−complex is formed by one-electron reduction of UO2(CO3)34−. Previous reports show that the UO22+/UO2+couple, which is electrochemically reversible in less complexing media, becomes electrochemically irreversible in aqueous CO32−solution. In this study, an electrocatalytic reduction of UO22+to UO2+in saturated Na2CO3solution was observed at AuNPs-CTA-modified electrode with higher current density and faster heterogeneous electron-transfer kinetics than that at bare Au electrode. The standard heterogeneous rate constant,kº, for the reduction process at AuNPs-CTA modified electrode was about 25 times higher than that of bare Au electrode. Therefore, it is concluded that AuNPs-CTA membrane has improved the interfacial electron-transfer properties of electrode, resulting in a better electrochemical response than bare Au electrode.

Electrochemical Corrosion Behavior of Refractory Metals in LiCl-Li2O Molten Salt

In a typical electrochemical cell, platinum wire is used as an anode for electrolytic reduction of UO2 spent LWR fuel in a molten salt system consisting of LiCl + 2-8 wt% Li2O. During the electrolysis, the metallic lithium migrates to the anode and attacks the platinum wire. Stability of the anode material is critical for sustained operation of the electrolytic UO2 reduction cell for reprocessing spent nuclear oxide fuels. This investigation aims at developing dimensionally stable anode materials for sustained operation under aggressive conditions. The investigated materials are: tungsten, molybdenum, hafnium, and tantalum, and stainless steels type 304 and 316. Corrosion properties of the candidate materials are compared with that of platinum using electrochemical polarization and impedance spectroscopy results. In order to simulate the lithium attack on the anode material, lithium was cathodically deposited and equilibrated for about one hour before starting the anodic polarization test in LiCl + 2 wt% Li2O molten salt at 650°C. Platinum samples showed an extensive surface cracking due to penetration of lithium. Type 316 stainless steel samples showed higher impedance and smaller passive current density than that of platinum at lower anodic potentials. Tungsten samples showed higher electrochemical impedance modulus and better passivity than any other materials tested in this study.

Application of Electrochemical Reduction to Produce Metal Fuel Material from Actinide Oxides

The electrochemical reduction process has been recently developed for converting oxide nuclear fuels to metals. In order to characterize the reduction mechanism and to investigate appropriate conditions for improving the reduction rate, several reduction tests were conducted in a LiCl-Li2O electrolyte at 650°C using various types of cathode baskets containing 10 to 100 g of UO2. The reduction progressed from the outside to the center of the cathode basket, and the reduction rate might be determined by the transportation of oxygen ion to the bulk salt. It was verified that feeding in small UO2 particles and reducing the thickness of the UO2 layer in the cathode basket improved the reduction rate. The completion of UO2 reduction was indicated by the open circuit potential of the cathode basket exhibiting lithium deposition potential for a long time. A salt distillation test was conducted using the reduction product comprising a mixture of porous uranium metal particles and the electrolyte. The reduction product loaded in an yttria crucible was heated to 1400°C in an argon stream. The residue in the crucible consisted of a uranium metal ingot and a small amount of dross. The adhering LiCl seemed to be completely removed. Consequently, it was demonstrated in the electrochemical reduction followed by the salt distillation that a uranium metal ingot could be produced from the UO2 feed with a high degree of efficiency.

Phase separation of metal-added corium and its effect on a steam explosion

To simulate a relocation of molten core material and its interaction phenomenon with water during a severe accident in a nuclear reactor, a typical corium of UO 2/ZrO 2/Zr/Stainless steel mixed at a 62 wt%, 15 wt%, 12 wt% and 11 wt%, respectively, was melted and then cooled down to become a solidified ingot. It was shown that the molten corium was separated into two layers, of which the upper layer was oxide mixtures and the lower layer was metal alloys. The upper layer was UO 2 and ZrO 2 and the lower layer mostly consisted of metal mixtures such as uranium, zirconium and stainless steel. Iron content varied with the positions and about a half of it existed as an alloy such as Fe 2U. Uranium metal was produced by reduction of UO 2 by zirconium metal. The average densities of the upper oxide layer and the lower metal layer were 8.802 and 9.411 g/cm 3, respectively. In another test, metal-added molten corium was poured into water and it showed that a steam explosion could occur by applying an external trigger.