Reduction Of UO2 Research Articles

Chemical reactivity of uranium trioxide polymorphs in reduction by hydrogen

The α, β, γ and amorphous polymorphs of uranium trioxide have been prepared and their reactivity towards reduction to UO 2 and further conversion to UF 4 studied. The UO 3 polymorphs are characterized by X-ray, porosity measurements and density determinations. Reduction of UO 3 to UO 2 was carried out by differential thermal analysis and the results of the DTA experiments were analysed in terms of the reactivity of the UO 3 sample. The reactivity of the product UO 2 towards hydrofluorination was measured by its oxygen content upon reoxidation after two weeks of air exposure. The ratio O:U depends upon the UO 3 polymorph concerned and the temperature of reduction to UO 2.

Oxygen mass transport effects during hydrogen reduction of UO2 + x

Reduction of UO 2 + x was studied in a molecular beam system, in which a collisionless flux of atomic hydrogen strikes a portion of the surface of the solid and the reaction product water is detected by an in-situ mass spectrometer. The experiment revealed kinetic limitations due to both surface chemical reaction and to transport processes, the latter including surface diffusion of oxygen as well as bulk diffusion. A quantitative model of the reduction process showed that the surface reaction rate was linear in the H atom beam intensity and in the excess O/U ratio above a minimum value for detectable reaction. The bulk diffusion coefficient of oxygen determined from the data at 585°C agreed well with the measurement of Lay. The surface diffusion coefficient at the same temperature was found to be ~1 cm 2 /s, suggesting ideal two-dimensional-gas behavior of oxygen on the urania surface.

Kinetics on carbothermic reduction of UO2+C powders and compacts to UC2.

The rate for the carbothermic synthesis of UC2 from UO2+C powders and compacts was determined from the amount of CO gas released into flowing He. The reduction rate was found to be greatly influenced by the characteristics of oxide powders. For the UO2 powder having a small particle size, the reaction at the UO2-UC2 interface in every particle may control the reduction. For the powder of a large particle size, about 100;μm, the reduction was found to be controlled by the diffusion of oxygen through the carbide layer to the surface of the particle. The shape of compacted samples also influences the reduction behavior of UO2+C mixtures to UC2. The reduction was controlled by the reaction at the interface between the produced UC2+C layer and the unreacted core in the sample compacted using the fine UO2 powder at 80–100 MPa. In the case of the sample compacted at 470 MPa, the reduction was controlled by the diffusion of CO gas from the interface of the compact through the UC2+C layer. The activation energy wa...

Thermoanalytical study on the oxidation of uranium dioxides derived from uranyl nitrate, uranyl acetate and ammonium diuranate

The oxidation of UO 2 was investigated by TG, DSC and X-ray diffraction . UO 2 samples were prepared by the reduction of UO 3 at P H 2 + P N 2 = 100 + 50 mm Hg and 5°C min −1 up to 800°C. In order to obtain six UO 2 samples with different preparative histories, UNH, UAH and ADU were used as starting materials and their thermal decomposition was carried out at 450–625°C for 0–9 h at an air flow rate of 100 ml min −1. α-UO 3, γ-UO 3, UO 3 - 2 H 2O, and their mixtures were obtained. The reduction of UO 3 gave β-UO 2+ x with different x values from 0.030 to 0.055. The oxidation carried out at P O 2 = 150 mm Hg was found to consist of oxygen uptake at room temperature. UO 2 - U 3O 7 (Step I) and U 3O 7 → U 3O 8 (Step II). TG and DSC curves of the oxidation showed two plateaus and two exothermic peaks corresponding to Steps I and II. In the case of two of the samples, the DSC peak of Step II split into two substeps, which were assumed to be due to the different reactivities of U 3O- formed from α-CO 3 and that from other types of UO 3. The increase in O/U ratio due to the oxygen uptake at room temperature changed from 0.010 to 0.042 except for a sample prepared from ADU which showed an extraordinarily large value of 0.445. TG curves showed an increase in O/U from room temperature to near 250°C for Step I and the plateau at 250–350°C where O/U was about 2.42, and showed a sharp increase in O/U above 350°C for Step II and the plateau above 100°C where O/U was 2.72–2.75. It is thought that the prepared UO 2 had a defective structure with a large interstitial volume to accommodate the excess oxygen.

Polarographic Behaviour and Determination of Stability Constants of Uranyl‐Ascorbate Complex

AbstractThe complex species of UO2(HA)(H2A)+ and UO2(HA)2 were identified in the ascorbic acid solution of uranyl ion at pH<2.1 and pH>2.1, respectively. Polarographic wave was proved to be the simultaneous reduction of UO2+2 and UO2(HA)(H2A)+ at pH <2.1. However, at pH>2.1, the wave is due to the reduction of U02(HA)2 The stability constants of the two complex species were found to be 5.1×10+ and 1.0×105, respectively. The hydrolysis constant of uranyl ion in the solution of ascorbic acid was determined.

Reaction of thorium and ThCl 4 with UO 2 and (Th, U)O 2 in fused chloride salts

An experimental and theoretical investigation of the reactions of thorium and ThCl 4 with UO 2 and (Th,U)O 2 in fused KCl-LiCl and NaCl-MgCl 2 eutectic salts has been carried out. The equilibrium ThCl 4 + ThO 2 ⇄ 2ThOCl 2 has been considered in some detail and the maximum solubility of ThOCl 2 as a function of the ThCl 4 concentration in KCl-LiCl calculated. The activity coefficient of ThOCl 2 was estimated from experimental data. Similar calculations were made for the solubility of UOCl 2. The reaction of ThCl 4 with UO 2 involves three independent reactions. The equilibrium concentration of components in KCl-LiCl salt for the overall reaction Th + UO 2 → ThO 2 + U can be described in terms of two independent reactions and the experimental data are consistent with available thermodynamic data. The reduction of UO 2 and (Th,U)O 2 to produce uranium and ThO 2 has been demonstrated. Methods for separating the uranium from the ThO 2 precipitate are considered briefly.

ChemInform Abstract: KINETIC MEASUREMENTS FOR THE CARBOTHERMIC SYNTHESIS OF UN, PUN, AND (U,PU)N

AbstractDie Kinetik der Bildung von UN, PuN und (U,Pu)Nbei der carbotherrnischen Reduktion von U02 und PuOz in strömendem N2 wird untersucht.

Kinetic Measurements for the Carbothermic Synthesis of UN, PuN, and (U,Pu)N

A kinetic study of the synthesis of nitrides using carbothermic reduction of UO2 and PuO2 in flowing N2 was conducted. Reactio n rate constants as functions of the reaction temperature and N2 flow rate were determined. The measured rate constants varied with temperature according to the Arrhenius equation at constant N2 flow rates. In logarithmic form, the measured rate constants also varied linearly with N2 flow rate at constant temperature, up to a flow rate of 120 cm3/min-g of reactant. The rate constant for a particular reaction temperature and N2 flow rate can be calculated from the logarithmic form of the Arrhenius equation by adding a flow- rate term, F. The expression ln k = (-86,000/ RT) + 18.1 + 0.021F fits the kinetic data from 1430 to 1625 deg C. Close agreement was obtained between calculated and experimenta rate constants determined for 50-g batch production runs. The reaction rates measured are considered rapid enough for an economical nitride process.

Crystallographic shear and ordered reduction of UO2

Abstract Electron microscopy was used to study the surfaces (with replicas) and the near surface regions (with transmission) of UO2 annealed under reducing conditions. Parallel striations were observed with both techniques following isothermal or pulse annealing to temperatures between 1480° and 1950°c, and simultaneously a splitting of the spots in the diffraction patterns occurred. These observations are consistent with the formation of planar faults in a crystallographic shear structure of partly reduced UO2. A reduction to UO1·98 is indicated for regions near the surface on annealing UO2 to 1500°c in H2. The surface structure is probably due to additional etching to minimize the misfit. Implications of the ordered reduction on physical measurements sensitive to surface conditions (especially of uranium self-diffusion), and on the understanding of the defect structure of UO2, are discussed.

Study of the reduction of UO2 by magnesium or calcium dissolved in molten chlorides

The reduction of solid UO2 to uranium by magnesium or calcium dissolved in their molten chlorides has been studied. The rate of reduction per unit area of UO2 surface, at constant temperature and concentration of reductant in the molten chloride, was found to increase with time to a constant value. The rate of reduction per unit area was observed to be proportional to the concentration of reductant in the molten salt. The small increase observed in the reaction rate over the temperature range 750° to 850°C, suggests that the reduction is controlled by transport of the reductant to the reaction site. Solidified salt, containing UO2 pellets which had been partially reduced, was sectioned, polished, and examined microscopically. The products of the reduction reaction form concentric layers around the UO2 pellets. Layers of metallic uranium and oxide containing small amounts of dispersed salt alternated with layers of salt containing small amounts of metallic uranium and oxide. The layers ruptured, presumably because the volume of the products, uranium and oxide, is greater than the volume of the UO2. Therefore, an impervious layer did not form on the oxide surface to inhibit the reduction reaction.

Oxygen in Uranium Carbide during Carbothermic Reduction Processes

It has been considered so far that oxygen retained in UC is intrinsic to the carbothermic reduction of UO2, so that some have regarded this reaction as inherently unsuitable for the preparation of pure UC powder, unless the release of CO gas can be kept within prescribed limits during irradiation. Nevertheless, quantitative considerations on the attainable lower limits of oxygen content in the UC have so far been scarcely reported. The author has estimated the partial pressure of CO gas in the carbothermic reduction and the solubility of oxygen in UC from thermodynamic considerations. It has been established as a result that the reaction has a large temperature dependency, and that the reduction is easily promoted under conditions of high temperature and low pressure, only 0.00456 ppm of oxygen being soluble in UC at 1,550°C under 1×10−5 mmHg.

Cerium-Stabilized UO_2^+2:Nd^+3 Laser Glass

The ground-state absorption at 1.06 μ in UO2+2:Nd+3 laser glass is directly related to the uranium content of the glass. Valence reduction of UO2+2 to U+5 (and possibly U+4) is responsible; however, the oxidized state can be stabilized with cerium. The percentage of the total uranium present as the UO2+2 species was increased from 63% to 91% in a particular soda-lime-silica base.

酸化ウランのカルシウム蒸気還元法によるウランの製錬に関する研究

酸化ウランのカルシウム蒸気還元法によるウランの製錬に関する研究

Oxidation of uranium dioxide by carbon dioxide in a reactor flux

Exposure of microcrystalline UO2 to CO2 in a reactor flux between 40 and 500°C leads to the formation of CO and UO2+x, where x does not exceed 0.25. The UO2+x, a disordered cubic phase which is thermodynamically unstable at low temperatures, is stable to annealing at temperatures below 650°C, but when heated to 750°C recrystallizes into the thermodynamically stable forms U4O9 and UO2. The initial rate of oxidation is proportional to surface area of the UO2 and independent of CO2 pressure, the geometry of the system and of inert gas diluents. UO2 is oxidized by products of radiolysis of chemisorbed CO2. Active species produced in the gas phase remote from the solid do not take part in the oxidation. A radiation-assisted reduction of UO2+x by CO also occurs, the rate being dependent on temperature. The steady-state composition of UO2+x at a given temperature is controlled by the relative rates of the oxidation by CO2 and the reduction by CO. High G values for γ-radiolysis of CO2 in this system arise from direct energy transfer to the gas from the solid surface.