Mechanochemical effects in U3O8

The effect of the mechanical treatment of U₃O₈ in a planetary ball mill in air or as a suspension in benzene solution of thyolilthreefluoroacetone (TTA) on the nature of the oxide and on the leaching of U and ²³⁴Th into diluted aqueous solutions of HCl, Na₂EDTA and NaCl has been studied. Transformation of U₃O₈ to UO₂, is much stronger expressed when the mechanoactivation is performed in air is established. The leaching behavior of U and Th depends significantly on the activation mode and on the leaching reagent nature. The role of mechanochemically enhanced UO₂-ThO₂ solid solution formation for the observed effects is discussed.     

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.     

Oxidative leaching of uranium from SIMFUEL using Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>–H<Subscript>2</Subscript>O<Subscript>2</Subscript> solution

A feasibility and basic study to find a possibility to develop such a process for recovering U alone from spent fuel by using the methods of an oxidative leaching and a precipitation of U in high alkaline carbonate media was newly suggested with the characteristics of a highly enhanced proliferation-resistance and more environmental friendliness. This study has focused on the examination of an oxidative leaching of uranium from SIMFUEL powders contained 16 elements (U, Ce, Gd, La, Nd, Pr, Sm, Eu, Y, Mo, Pd, Ru, Zr, Ba, Sr, and Te) using a Na₂CO₃ solution with hydrogen peroxide. U₃O₈ was dissolved more rapidly than UO₂ in a carbonate solution. However, in the presence of H₂O₂, we can find out that the leaching rates of the reduced SIMFUEL powder are faster than the oxidized SIMFUEL powder. In carbonate solutions with hydrogen peroxide, uranium oxides were dissolved in the form of uranyl peroxo-carbonato complexes. UO₂(O₂) _x (CO₃) _y ^2−2x−2y , where x/y has 1/2, 2/1.     

A device for uranium series leaching from glass fiber in HEPA filter

For the disposal of a high efficiency particulate air (HEPA) glass filter into the environment, the glass fiber should be leached to lower its radioactive concentration to the clearance level. To derive an optimum method for the removal of uranium series from a HEPA glass fiber, five methods were applied in this study. That is, chemical leaching by a 4.0?M HNO₃?C0.1?M Ce(IV) solution, chemical leaching by a 5 wt% NaOH solution, chemical leaching by a 0.5?M H₂O₂?C1.0?M Na₂CO₃ solution, chemical consecutive chemical leaching by a 4.0?M HNO₃ solution, and repeated chemical leaching by a 4.0?M HNO₃ solution were used to remove the uranium series. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after leaching for 5?h by the 4.0?M HNO₃?C0.1?M Ce(IV) solution were 2.1, 0.3, 1.1, and 1.2?Bq/g. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after leaching for 36?h by 4.0?M HNO₃?C0.1?M Ce(IV) solution were 76.9, 3.4, 63.7, and 71.9?Bq/g. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after leaching for 8?h by a 0.5?M H₂O₂?C1.0?M Na₂CO₃ solution were 8.9, 0.0, 1.91, and 6.4?Bq/g. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after consecutive leaching for 8?h by the 4.0?M HNO₃ solution were 2.08, 0.12, 1.55, and 2.0?Bq/g. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after three repetitions of leaching for 3?h by the 4.0?M HNO₃ solution were 0.02, 0.02, 0.29, and 0.26?Bq/g. Meanwhile, the removal efficiencies of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th from the waste solution after its precipitation?Cfiltration treatment with NaOH and alum for reuse of the 4.0?M HNO₃ waste solution were 100, 100, 93.3, and 100%.     

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.     

Dissolution of uranium oxides under alkaline oxidizing conditions

Bench scale experiments were conducted to determine the dissolution characteristics of UO₂, U₃O₈, and UO₃ in aqueous peroxide-containing carbonate solutions. The experimental parameters investigated included carbonate countercation (NH₄ ⁺, Na⁺, K⁺, and Rb⁺) and H₂O₂ concentration. The carbonate countercation had a dramatic influence on the dissolution behavior of UO₂ in 1 M carbonate solutions containing 0.1 M H₂O₂, with the most rapid dissolution occurring in (NH₄)₂CO₃ solution. The initial dissolution rate (y) of UO₂ in 1 M (NH₄)₂CO₃ increased linearly with peroxide concentration (x) ranging from 0.05 to 2 M according to: y = 2.41x + 1.14. The trend in initial dissolution rates for the three U oxides under study was UO₃ ≫ U₃O₈ > UO₂.     

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.     

The nature of the chemical bond in UO2

The nature of the chemical bond in UO₂ was analyzed taking into account the X-ray photoelectron spectroscopy (XPS) structure parameters of the valence and core electrons, as well as the relativistic discrete variation electronic structure calculation results for this oxide. The ionic/covalent nature of the chemical bond was determined for the UO₈ (D_4h) cluster, reflecting uranium's close environment in UO₂, and the U₁₃O₅₆ and U₆₃O₂₁₆ clusters, reflecting the bulk of solid uranium dioxide. The bar graph of the theoretical valence band (from 0 to ~35 eV) of XPS spectrum was built such that it was in satisfactory agreement with the experimental spectrum of a UO₂ single crystalline thin film. It was shown that unlike the crystal field theory results, the covalence effects in UO₂ are significant due to the strong overlap of the U 6p and U 5f atomic orbitals with the ligand orbitals, in addition to the U 6d atomic orbital (AO). A quantitative molecular orbital (MO) scheme for UO₂ was built. The contribution of the MO electrons to the chemical bond covalence component was evaluated on the basis of the bond population values. It was found that the electrons of inner valence molecular orbitals (IVMO) weaken the chemical bond formed by the electrons of outer valence molecular orbitals (OVMO) by 32% in UO₈ and by 25% in U₆₃O₂₁₆.     

UO2, NpO2 and PuO2 preparation in aqueous nitrate solutions in the presence of hydrazine hydrate

It was established that heating to 90 °C of nitrate solutions of U, Np and Pu in the presence of hydrazine hydrate results in the formation of hydrated dioxides of these elements. On ignition under inert or reducing conditions in the temperature range of 280–800 °C hydrated uranium dioxide transmogrify into crystalline UO₂. On ignition in air atmosphere UO₂·nH₂O turns into UO₃ at 440 °C and into U₃O₈ at 570–800 °C. It was shown that thermolysis of the solution containing a mixture of uranium, neptunium and plutonium nitrates at 90 °C in the presence of hydrazine hydrate allows one to prepare hydrated dioxides (U, Np, Pu)O₂·nH₂O which on heating to ~300 °C transmogrify into crystalline product of UO₂, NpO₂ and PuO₂ solid solution. The technique of preparation of solid solutions of U and Pu dioxides is very promising as simple and effective method of production of MOX-fuel for.     

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.     

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.     

Implication of volume changes in uranium oxides: A density functional study

In severe nuclear accident scenarios (in air environments and high temperatures) UO₂ fuel pellets oxidise to produce uranium oxides with higher oxygen content, e.g., U₄O₉ or U₃O₈. As a first step in investigating the microstructural changes following UO₂ oxidation to hexagonal high temperature phase of U₃O₈, density functional quantum mechanical calculations of the structure, elastic properties and electronic structure of U₃O₈ have been performed. The calculated properties of hexagonal phase of U₃O₈ are compared to those of the orthorhombic pseudo-hexagonal phase which is stable at room temperature. The total energy technique based on the local density approximation plus Hubbard U as implemented in the CASTEP code is used to investigate changes in the lattice constants. The first-principles calculations predict a 35–42% increase in volume per uranium atom as a result of the transformation from UO₂ to U₃O₈, in agreement with experimental data. The implications of this prediction on the linear expansion and fragmentation of fuel are discussed.     

Thermal and kinetic analysis of uranium salts

Thermal decomposition of U(C₂O₄)₂·6H₂O was studied using TG method in nitrogen, air, and oxygen atmospheres. The decomposition proceeded in five stages. The first three stages were dehydration reactions and corresponded to removal of four, one, and one mole water, respectively. Anhydrous salt decomposed to oxide products in two stages. The decomposition products in nitrogen atmosphere were different from those in air and oxygen atmospheres. In nitrogen atmosphere UO_1.5(CO₃)_0.5 was the first product and U₂O₅ was the second product, while these in air and oxygen atmospheres were UO(CO₃) and UO₃, respectively. The second decomposition products were not stable and converted to stable oxides (nitrogen: UO₂, air–oxygen: U₃O₈). The kinetics of each reaction was investigated with using Kissinger–Akahira–Sunose and Flynn–Wall–Ozawa methods. These methods were combined with modeling equations for thermodynamic functions, the effective models were investigated and thermodynamic values were calculated.     

Uranyl-catalyzed chemiluminescent reaction of U4+ oxidation by dioxygen in aqueous HClO4 solution

Chemiluminescence (CL) accompanying the reaction of U⁴⁺ with O₂ in 0.0004–0.1M HClO₄ was studied. It was found that the electron-excited uranyl ion (UO₂ ²⁺)^* is the CL emitter. The fact that the reaction rate and the CL yield increase as the solution acidity decreases was explained by different reactivities of the U _aq ⁴⁺ aquation and the products of its stepwise hydrolysis, UOH³⁺ and U(OH)₂ ²⁺, toward O₂. Based on the results of analysis of the chain-radical mechanism of the reaction between U⁴⁺ and O₂, it was concluded that transfer of an electron from the UO₂ ⁺ ion to the oxidizing agent (a ·OH radical) is the most plausible elementary step of the reaction of (UO₂ ²⁺)^* formation. It was found that the reaction rate, as well as the CL yield, increase substantially in the presence of uranyl ion. Catalytic action of UO₂ ²⁺ was explained by the formation of a UO₂ ²⁺·UO₂ ⁺ complex, which reduces the rate of the UO₂ ⁺ disproportionation reaction (UO₂ ⁺ is an intermediate of the reaction and is involved in chain propagation), and by regeneration of the active center, UO₂ ⁺, in the reaction of UO₂ ²⁺ with U⁴⁺. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1522–1528, September, 2000.     

The Key Role of U28 in the Aqueous Self‐Assembly of Uranyl Peroxide Nanocages

For 11 years now, the structural diversity and aesthetic beauty of uranyl–peroxide capsules have fascinated researchers from the diverse fields of mineralogy, polyoxometalate chemistry, and nuclear fuel technologies. There is still much to be learned about the mechanisms of the self‐assembly process, and the role of solution parameters including pH, alkali template, temperature, time, and others. Here we have exploited the high solubility of the UO₂²⁺/H₂O₂/LiOH aqueous system to address the effect of the hydroxide concentration. Important techniques of this study are single‐crystal X‐ray diffraction, small‐angle X‐ray scattering, and Raman spectroscopy. Three key phases dominate the solution speciation as a function of time and the LiOH/UO₂²⁺ ratio: the uranyl–triperoxide monomer [UO₂(O₂)₃]^4?and the two capsules [(UO₂)(O₂)(OH)]₂₄^24?(U₂₄) and [(UO₂)(O₂)_1.5]₂₈^28?(U₂₈). When the LiOH/U ratio is around three, U₂₈ forms rapidly and this cluster can be isolated in high yield and purity. This result was most surprising and challenges the hypothesis that alkali templating is the most important determinant in the cluster geometry. Moreover, analogous experiments with KOH, NH₄OH, and TEAOH (TEA=tetraethylammonium) also rapidly yield U₂₈, which suggests that U₂₈ is the kinetically favored species. Complete mapping of the pH–time phase space reveals only a narrow window of the U₂₈ dominance, which is why it was previously overlooked as an important kinetic species in this chemical system, as well as others with different counterions.     

Determination of oxygen in ternary uranium oxides by a gravimetric alkaline earth addition method

The applicability of a gravimetric method based on alkaline earth metal addition for the determination of oxygen in ternary uranium oxides of the type M—U—O (M=La, Ce and Th) is described. The oxide sample is mixed with MgO or Ba_2.8UO_5.8 and heated in air under suitable conditions. Because uranium is completely oxidized to the hexavalent state during the reaction, oxygen can be determined from the weight change. Oxygen in La_yU_1-y O_2+x is determined up to y = 0.8 with a standard deviation for x of ±0.006 with MgO. For Th_yU_1-y O_2+x, the value of x is determined with Ba_2.8UO_5.8 with a standard deviation of ±0.01 at y = 0.8. For Ce_yU_1-y O_2+x, the method can be applied only for low cerium concentrations where y = 0–0.2; the value for x with Ba_2.8UO_5.8 at y = 0.2 showed a standard deviation of ±0.002.     

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.     

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     

Stabilization of uranium(IV) in heteropoly anions*

Summary The heteropoly anions [U^IVMo₁₂O₄₂]^8–(UMo₁₂), [U^IVW₁₀O₃₆]^8– (UW₁₀), [U^IV(PW₁₁O₃₉)₂]¹⁰ [U(PW₁₁)₂] and [U^IV(SiW₁₁O₃₉)₂]¹² [U(SiW₁₁)₂] were examined by cyclic voltammetry on a wax-impregnated carbon electrode. Reversible one-electron oxidations were observed for UMo₁₂ (E = +0.91 V vs see at pH = ca. 0), U(PW₁₁ )2 (E = +0.60 V at pH 4.4) and U(SiW₁₁)₂ (E = +0.19 V at pH 4.4). No oxidation of UW₁₀ was detected at potentials prior to oxygen discharge (ca. +0.9 V at pH 7). Controlled potential oxidation of aqueous solutions of UMo₁₂ gave unstable solutions of [U^VMo₁₂O₄₂]^7–. Oxidation of U(PW₁₁)2 was achieved in aqueous and nonaqueous (acetonitrile, propylene carbonate) solution. The electronic spectra of U^VMo₁₂ and U^V(PW₁₁)₂ are reported and are discussed in terms of UO₁₂ (/y) and UO₈ (D4d) chromophores respectively. Possibilities for geometrical and optical isomers of U(XW 11)2 anions are considered. Solutions of brucinium salts of U(PWI I)2 and UW₁₀ in dimethyl formamide show induced Cotton effects at wavelengths corresponding to the f-f transitions of UIV. The voltammograms of UMo₁₂, ThMo₁₂ and CeMo₁₂ show an irreversible twelve-electron reduction at -0.25 V. The pale brown reduced solutions cannot be reoxidized to the original heteropoly anions.Taken from the Ph. D. Thesis of S.C.T., Georgetown University, 1977. Presented in part at the 17th International Conference on Coordination Chemistry, Hamburg, September 1976.     

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.