Defect chemistry of uranium oxides

Uranium oxides are known as nonstoichiometric compounds whose composition changes according to external conditions such as temperature and oxygen partial pressure. The change of composition caused by the formation of defect structure results in a change of their properties. In this paper, the compositional changes of UO₂ and doped UO₂ [(U, M)O₂; M=La, Ti, Pu, Th, Nb, Cr, etc.] and also those of other uranium oxides (U₄O₉, U₃O₈) are shown against oxygen partial pressure. From the results of doped UO₂, it is concluded that the valence control rule holds to a first approximation. The defect structures are estimated both from log x vs. log Po₂ (x: deviation from the stoichiometric composition and Po₂: oxygen partial pressure) and log vs. log Po₂ (: electrical conductivity) relations. The defect structures of UO₂ and doped UO₂ are derived based on the Willis model for UO_2+x. The detect structure of U₄O₉ phase is similar to that of UO_2+x, but the defect structures of U₃O₈ phase are complicated due to the existence of many higher-order phase transitions. The thermodynamic data such as the partial molar enthalpy and entropy and the heat capacity are important to characterize the defect structure. The high temperature heat capacities of UO₂ doped with Gd show pronounced increases at high temperatures the onset temperature decreases as the dopant content increases. The increase of heat capacity is interpreted to be due to the formation of lattice defects. The heat capacity measurements on U₄O₉ and U₃O₈ clucidate the presence of the phase transition. The mechanisms of these phase transitions are discussed.     

Determination of Oxygen to Uranium Ratio in Uranium Dioxide Nuclear Fuel by Differential Pulse Polarography

A differential pulse polarographic method for determination of oxygen to uranium ratio in uranium oxides is fully described. An accuracy ?2.62% to +4.35% was achieved by calibrating the method against standard U₃O₃, replicate test runs with UO₂, gave a precision of ±4.59% or better. The method is now successfully used in routine analysis for UO² fuel.     

Comparative extraction efficiencies of tri-<Emphasis Type="Italic">n</Emphasis>-butyl phosphate and <Emphasis Type="Italic">N,N</Emphasis>-dihexyloctanamide for uranium recovery using supercritical CO<Subscript>2</Subscript>

Extraction of uranium from tissue paper, synthetic soil, and from its oxides (UO₂, UO₃ and U₃O₈) was carried out using supercritical carbon dioxide modified with methanol solutions of extractants such as tri-n-butyl phosphate (TBP) or N,N-dihexyl octanamide (DHOA). The effects of temperature, pressure, extractant/nitric acid (nitrate) concentration, and of hydrogen peroxide on uranium extraction were investigated. The dissolution and extraction of uranium in supercritical CO₂ modified with TBP, from oxide samples followed the order: UO₃ ≫ UO₂ > U₃O₈. Addition of hydrogen peroxide in the modifier solution enhanced the dissolution/extraction of uranium in dynamic mode. DHOA appeared better than TBP for recovery of uranium from different oxide samples. Similar enhancement in uranium extraction was observed in static mode experiments in the presence of hydrogen peroxide. Uranium estimation in the extracted fraction was carried out by spectrophotometry employing 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) as the chromophore.     

Thermal behaviour of co-precipitated mixtures of chromium(III) and uranium(VI)

In the CrUO system, besides the phases reported earlier, a triuranate CrU₃O_10-x (x ～ 0.3) could be identified. It is unstable above 70o°C and decomposes to a mixture of CrUO₄ and U₃O₈. Under reducing atmospheres up to 1600°C, the uranium—chromium—oxygen system gives a mixture of Cr₂O₃ and UO₂. No new phase could be identified. The compound CrUO₄ is unstable under reducing conditions and decomposes to a mixture of Cr₂O₃ and UO₂.     

Methodology for uranium compounds characterization applied to biomedical monitoring

Chronic exposure to uranium compounds led to the development of a methodology in order to characterize those compounds. This methodology, based on the recommendation of the I.C.R.P,¹ involves three main steps: the measurement of the uranium concentration and the particle size distribution at workstations; the characterization of the industrial compound, i.e. its physico-chemical properties; and the study of in-vitro solubility using chemical and cellular tests. Different methods for uranium analysis are presented. Results and comments on UF₄, UO₃, U₃O₈, UO₂ and U+UO₂ are given.     

Compositions of the condensed medium and gas phase forming upon heating of uranium oxides (UO2, UO3, U3O8, and U4O9) in argon and oxygen atmosphere: Computer experiment

Thermodynamic modeling methods have been used for calculating the compositions of the condensed medium and gas phase forming upon heating of the oxides UO₂, UO₃, U₃O₈, and U₄O₉ at constant pressure (p = 0.1 MPa) in the temperature range 300–2000 K in an inert (Ar) or oxidative (O₂) atmosphere. The stability of uranium oxides and the state of aggregation of the condensed medium have been studied as a function of temperature. Original Russian Text ? G.K. Moiseev, A.B. Shubin, T.V. Kulikova, A.L. Ivanovskii, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 6, pp. 985–989.     

Single step conversion of UO2 and U3U8 into uranyl sulphate solution

A direct and simple method for the conversion of UO₂ and U₃O₈ powder into uranyl sulphate solution is described, eliminating many tedious chemical steps. UO₂ and U₃O₈ are not soluble in concentrated or dilute sulphuric acid, as uranium in lower oxidation state does not react with sulphuric acid. However, nitric acid oxidizes uranium from lower valency to higher valency state, i.e., tetravalent to the hexavalent uranyl ion in solution. Sufficient amount of sulphuric acid present in the reaction mixture makes it possible for uranyl ions, formed by oxidation of nitric acid, to react with sulphuric acid forming uranyl sulphate.     

Specific absorption parameters for uranium octoxide and dioxide applicable to the ICRP Publication 66 respiratory tract model

Literature data from in vivo chest measurements and urinary excretion rates of individuals exposed to U₃O₈ and UO₂ were used to compare the results predicted by different models with empirical observations in humans. As a result the use of the respiratory tract model proposed in ICRP Publication 66 with its default absorption parameters underestimates urinary excretion of inhaled U₃O₈ and UO₂. The new respiratory tract model also overpredicts the Fecal/Urine activity ratio, independently of the systemic model. For U₃O₈ and UO₂ the choice of systemic model has very little influence on the predicted urinary excretion of inhaled compounds. On the other way, the choice of the respiratory tract model does influence the predicted urinary excretion significantly. In this work specific absorption parameters for U₃O₈ and UO₂ were derived to be used in the respiratory tract model proposed in ICRP Publication 66. The predicted biokinetics of these compounds were compared with those derived for Type M and Type S compounds of uranium.     

Magnetic susceptibilities of UO2ThO2ZrO2 solid solutions

Magnetic susceptibilities were measured from 2.2 K to room temperature for solid solutions of UO₂ThO₂ZrO₂ of which the lattice parameters are the same as that of UO₂, i.e., Th_0.7yZr_0.3yU_1−yO₂ solid solutions. The Néel temperature decreases linearly with decreasing uranium concentration, the critical concentration being 69 mole% UO₂. The Néel temperatures of the present solid solutions are nearly in the middle of UO₂ThO₂ solid solutions and UO₂ZrO₂ solid solutions, which indicates that the magnetic dilution effect of ZrO₂ is larger than that of ThO₂. The effective magnetic moment decreases with decreasing uranium concentration, which is due to a decrease in the magnetic interactions with adjacent uranium ions, not due to a change of the strength of crystalline field. The Weiss constant decreases almost linearly with decreasing uranium concentration.     

Template‐Free Synthesis and Mechanistic Study of Porous Three‐Dimensional Hierarchical Uranium‐Containing and Uranium Oxide Microspheres

A novel type of uranium‐containing microspheres with an urchin‐like hierarchical nano/microstructure has been successfully synthesized by a facile template‐free hydrothermal method with uranyl nitrate hexahydrate, urea, and glycerol as the uranium source, precipitating agent, and shape‐controlling agent, respectively. The as‐synthesized microspheres were usually a few micrometers in size and porous inside, and their shells were composed of nanoscale rod‐shaped crystals. The growth mechanism of the hydrothermal reaction was studied, revealing that temperature, ratios of reactants, solution pH, and reaction time were all critical for the growth. The mechanism study also revealed that an intermediate compound of 3 UO₃?NH₃?5 H₂O was first formed and then gradually converted into the final hydrothermal product. These uranium‐containing microspheres were excellent precursors to synthesize porous uranium oxide microspheres. With a suitable calcination temperature, very uniform microspheres of uranium oxides (UO_2+x, U₃O₈, and UO₃) were successfully synthesized.     

Chemistry,crystal structure,and structural evolution versus temperature of the uranyl diselenite UO2Se2O5: Structural relationships with (UO2)Se2O5·2H2O,UO2SeO3, and β-U3O8

The chemistry and structural chemistry versus temperature in the system UO₃SeO₂H₂O were determined. A proposal has been presented for the structural transformations of various selenites, i.e., UO₂Se₂O₅·2H₂O, UO₂Se₂O₅, UO₂SeO₃, and the U₃O₈ uranium oxide final product in its β form. The unit-cell of the hydrated uranyl diselenite has been determined from an indexed powder pattern: it crystallizes in the triclinic system with a = 9.40(4)Å, b = 11.85(5)Å, c = 6.69(5)Å, α = 94.3(3)°, β = 90.3(3)°, and γ = 114.5(3)°, V = 676ų. On this basis, a structure derived from that of UO₂Se₂O₅ is proposed, corresponding to a reasonable packing of oxygen, water molecules, and lone pairs.     

Die Reduktion von Ammoniumpolyuranat zu Urandioxid

Zusammenfassung Es werden die Resultate des isothermen Zerfalles und der Reduktion vonstandardisiertem Ammoniumpolyuranat im Bereich von 285 bis 463° C (in Wasserstoff) wiedergegeben. Der Zerfall zu UO₃ wurde schon bei einer Temp. unter 290° C festgestellt, diese Phase blieb jedoch darauf stabil bis 320° C. Zwischen 320° C und 380° C verläuft die Reduktion zu U₃O₈, über 380° C aber zu UO₂. Die Aktivierungsenergien bei der Reduktion von UO₃ zu U₃O₈ und von U₃O₈ zu UO₂ wurden berechnet, und zwar 32,2 kcal/g-mol und 41,7 kcal/g-mol. Die Ergebnisse können mit den Literaturangaben für die Reduktion der einzelnen Phasen UO₃ und U₃O₈ verglichen werden. Die beobachteten Unterschiede weisen auf den Einfluß der Aktivität der Präparate hin.

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The isothermal decomposition and reduction of ammonium polyuranate (ADU) was investigated in the temperature interval 285–463° C in hydrogen. The formation of UO₃ was noticed below 290° C and this product was stable up to 320° C. U₃O₈ was stable from this temperature on up to 380° C, where the reduction to UO₂ was observed. The activation energies 32,2 Kcal/mole and 41.7 Kcal/mole were calculated for the reduction of UO₃ to U₃O₈ and for the reduction of U₃O₈ to UO₂, respectively. The results are comparable with the published data on reduction of separate phases UO₃ and U₃O₈. Some differences noticed show the influence of the activities of the products.

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Mit 4 Abbildungen     

Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U38

The ex‐situ qualitative study of the kinetic formation of the poly‐oxo cluster U₃₈, has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X‐ray absorption near edge structure (XANES), extended X‐ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO₂ from t=3 h, after the formation of unknown solid phases at an early stage. The crystallization of U₃₈ occurred from t=4 h at the expense of UO₂, and is completed after t=8 h. Starting from pure uranium(IV) species in solution (t=0–1 h), oxidation reactions are observed with a U^IV/U^VI ratio of 70:30 for t=1–3 h. Then, the ratio is inversed with a U^IV/U^VI ratio of 25/75, when the precipitation of UO₂ occurs. Thorough SEM observations of the U₃₈ crystallites showed that the UO₂ aggregates are embedded within. This may indicate that UO₂ acts as reservoir of uranium(IV), for the formation of U₃₈, stabilized by benzoate and THF ligands. During the early stages of the U₃₈ crystallization, a transient crystallized phase appeared at t=4 h. Its crystal structure revealed a new dodecanuclear moiety (U₁₂), based on the inner hexanuclear core of {U₆O₈} type, decorated by three additional pairs of dinuclear U₂ units. The U₁₂ motif is stabilized by benzoate, oxalates, and glycolate ligands.     

Extraction of pertechnetates from HNO<Subscript>3</Subscript> solutions into ionic liquids

The effect of the oxidation temperature of sintered UO₂ pellets on the powder properties of U₃O₈ was studied in the temperature range 250–900 °C in air. The U₃O₈ was obtained at 450 °C after 180 min and its particle size and surface area are respectively, 35 µm and 0.7 m²/g. The reduction of the U₃O₈ powder resulted in UO₂ after 30 min with a surface area of 0.8 m²/g. This value was improved more than 3.5 times by applying five alternating oxidation–reduction cycles.     

Über Perowskitphasen in den Systemen BaO–SE2O3–UO2,x mit SE = Seltene Erden,La und Y. II. Verbindungen der Zusammensetzung Ba2(SE0,67U0,33)UO6,17

In the BaO–SE₂O₃–UO_2.x system the formation of the compounds Ba₂(SE_0,67U_0.33)UO_6.17 is observed. They crystallize in a pseudocubic ordered perovskite lattice, and contain tetravalent and pentavalent uranium in the ratio 1:3. Their magnetic and spectroscopic properties are reported.     

Thermogravimetric study of uranium phosphates

The thermal decomposition of (UO₂)₃(PO₄)₂ and U(HPO₄)₂ ·xH₂O in the temperature range 25–1600?, was investigated. (UO₂)₃(PO₄)₂ decomposed first to 1/3[U₃O₈ + 3U₂O₃P₂O₇] and then to U₃O₅P₂O₇ before a loss of phosphorus was observed above 1350?. Decomposition in air and in inert atmospheres was nearly identical. Reduction with H₂ or with carbon black in argon gave U₃O₅P₂O₇ and [UO₂ + + (UO)₂P₂O₇] before pure UO₂ was formed. U(HPO₄)₂ ·xH₂O decomposed to UP₂O₇ in argon. It oxidized partly in air before the same product was obtained. The high temperature stability of UP₂O₇ and U₃(PO₄)₄ was also investigated.     

The Local Uranium Environment in Cesium Uranates: A Combined XPS, XAS, XRD, and Neutron Diffraction Analysis

The cesium uranates Cs₂UO₄, Cs₂U₂O₇, Cs₄U₅O₁₇ and Cs₂U₄O₁₂ were studied using X-ray Diffraction (XRD), neutron diffraction, X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) in an attempt to couple the crystallographic structure to the uranium valence state using the local uranium environment. The diffraction spectra were used for Rietveld refinement to determine the atomic positions and interatomic distances. These distances were subsequently used in Bond Valence Sum (BVS) calculations to determine the uranium valences. The XAS spectra give direct information on the local uranium environment regarding the U-O distances and the arrangement of the oxygen atoms around the central uranium. The difference between the monovalent uranates and the multivalent Cs₂U₄O₁₂ is clearly established in all spectra, as well as in the crystal structures. The different valences present can be assigned to individual uranium lattice sites, but some amount of disorder is required to balance the charges.     

Magnetic studies on ScyU1−yO2+x solid solutions

Magnetic susceptibilities of Sc_yU_1−yO_2+x solid solutions have been measured from 2.7 K to room temperature. The magnetic moment and Weiss constant have been determined in the temperature range in which the Curie-Weiss law holds. For the solid solutions showing antiferromagnetic transition, the Néel temperature has also been determined. The substitution of Sc³⁺ for U⁴⁺ was found to effect not only magnetic dilution of UO₂, but also oxidation of U⁴⁺ to U⁵⁺. Excess oxygen ions which entered the interstitial sites, weakened the antiferromagnetic interaction between uranium ions and oxidized U⁴⁺ to U⁵⁺. The effect of oxygen vacancies on the antiferromagnetic interaction was small in the concentration range of this experiment (0.8 a/o).     

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

The oxidation of UO₂ was investigated by TG, DSC and X-ray diffraction . UO₂ samples were prepared by the reduction of UO₃ at P_H2 + P_N2 = 100 + 50 mm Hg and 5°C min^?1 up to 800°C. In order to obtain six UO₂ 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-UO₃, γ-UO₃, UO₃ - 2 H₂O, and their mixtures were obtained. The reduction of UO₃ gave β-UO_2+x with different x values from 0.030 to 0.055. The oxidation carried out at P_O2 = 150 mm Hg was found to consist of oxygen uptake at room temperature. UO₂ - U₃O₇ (Step I) and U₃O₇ → U₃O₈ (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 U₃O- formed from α-CO₃ and that from other types of UO₃. 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 UO₂ had a defective structure with a large interstitial volume to accommodate the excess oxygen.     

Mechanochemistry of the 5f-element compounds

The effect of the mechanoactivation on UO₃ and U₃O₈ in agate or stainless steel vessels in air or in toluene is studied. UO₂(OH)₂ is the main product of UO₃·H₂O activation in steel vessel in air. The presence of toluene leads to strong amorphization and dispersity increase and, probably, to the formation of U₂O₅. The activation of U₃O₈ leads to its reduction to U₃O₇ which relative content in the reaction mixture depends on the mechanoactivation conditions.