Amorphous UO3 Research Articles

Ultrasonically assisted conversion of uranium trioxide into uranium(vi) intrinsic colloids.

Under oxidizing conditions, the corrosion of spent nuclear fuel may lead to the leaching of radionuclides including soluble uranyl-based species. The speciation of the generated chemical forms is complex and the related potential formation of colloidal species appears surprisingly poorly reported in the literature. Their formation could however contribute significantly to the mobility of radionuclides in the environment. A better knowledge in the speciation and reactivity of these species appears particularly relevant. This study describes the preparation and characterization of intrinsic uranium(vi) colloids from amorphous and crystalline UO3 in pure water assisted by 20 kHz ultrasound. In the presence of carbon monoxide preventing the sonochemical formation of hydrogen peroxide, ultrasonic treatment boosts the conversion of UO3 powder into (meta-)schoepite precipitates and yields very stable and notably concentrated uranium(vi) nanoparticles in the liquid phase. Using HR-TEM, SAXS and XAS techniques, we confirmed that the colloidal suspension is composed of quasi-spherical nanoparticles measuring ca. 3.8 ± 0.3 nm and exhibiting a schoepite-like crystallographic structure. The proposed method demonstrates the possible formation of environmentally relevant U(vi) colloidal nanoparticles appearing particularly interesting for the preparation of reference systems in the absence of added ions and capping agents.

Oxidation as an Early Stage in the Multistep Thermal Decomposition of Uranium(IV) Oxalate into U3O8.

The thermal decomposition of actinide oxalates is greatly dependent on the oxidation state of the cation, the gas involved, and the physical characteristics of the precursor. In the actinides series, uranium(IV) oxalate U(C2O4)2·6H2O can be viewed as a peculiar case, as its sensibility toward oxidation leads to a specific series of reactions when heating under an oxygen atmosphere. In order to clarify the disagreements existing in the literature, particularly concerning potential carbonate intermediates and the possible transitory existence of UO3, we show here an extended characterization of the different intermediates through a combination of X-ray diffraction, vibrational spectroscopies and X-ray absorption near-edge spectroscopy. In this frame, uranium oxidation was found to occur at low temperature (200 °C) concomitantly to the onset of oxalate groups decomposition, leading to an amorphous oxo-oxalato compound. Pursuing the thermal conversion up to 350 °C led to complete oxidation of U(IV) into U(VI), then to the formation of amorphous UO3 still bearing adsorbed carbonates. The first pure oxide formed during the thermal conversion was further identified to substoichiometric UO3-δ after heating at 550 °C. Finally, U3O8 was obtained as the final stable phase after heating above 660 °C. The mechanism of thermal conversion of uranium(IV) oxalate into oxide under oxygen is then driven by a complex interplay between redox reactions and decomposition of the organic fractions. Such chemical reactions were also found to significantly modify the morphology of the powder through high-temperature environmental scanning electron microscopy observations: decomposition led to a 20% reduction in the size of the aggregates, while uranium oxidation clearly promoted growth within the agglomerates.

The conversion of ammonium uranate prepared via sol-gel synthesis into uranium oxides

A combination of simultaneous thermal analysis, evolved gas analysis and non-ambient XRD techniques was used to characterise and investigate the conversion reactions of ammonium uranates into uranium oxides. Two solid phases of the ternary system NH3−UO3−H2O were synthesised under specified conditions. Microspheres prepared by the sol-gel method via internal gelation were identified as 3UO3⋅2NH3⋅4H2O, whereas the product of a typical ammonium diuranate precipitation reaction was associated to the composition 3UO3⋅NH3⋅5H2O. The thermal decomposition profile of both compounds in air feature distinct reaction steps towards the conversion to U3O8, owing to the successive release of water and ammonia molecules. Both compounds are converted into α-U3O8 above 550 °C, but the crystallographic transition occurs differently. In compound 3UO3⋅NH3⋅5H2O (ADU) the transformation occurs via the crystalline β-UO3 phase, whereas in compound 3UO3⋅2NH3⋅4H2O (microspheres) an amorphous UO3 intermediate was observed. The new insights obtained on these uranate systems improve the information base for designing and synthesising minor actinide-containing target materials in future applications.

Energetics of hydration on uranium oxide and peroxide surfaces

Abstract

Thermodynamic studies of studtite thermal decomposition pathways via amorphous intermediates UO3, U2O7, and UO4

The thermal decomposition of studtite (UO2)O2(H2O)2·2H2O results in a series of intermediate X-ray amorphous materials with general composition UO3+x (x = 0, 0.5, 1). As an extension of a structural study on U2O7, this work provides detailed calorimetric data on these amorphous oxygen-rich materials since their energetics and thermal stability are unknown. These were characterized in situ by thermogravimetry, and mass spectrometry. Ex situ X-ray diffraction and infrared spectroscopy characterized their chemical bonding and local structures. This detailed characterization formed the basis for obtaining formation enthalpies by high temperature oxide melt solution calorimetry. The thermodynamic data demonstrate the metastability of the amorphous UO3+x materials, and explain their irreversible and spontaneous reactions to generate oxygen and form metaschoepite. Thus, formation of studtite in the nuclear fuel cycle, followed by heat treatment, can produce metastable amorphous UO3+x materials that pose the risk of significant O2 gas. Quantitative knowledge of the energy landscape of amorphous UO3+x was provided for stability analysis and assessment of conditions for decomposition.

Low-Temperature Oxidation of Fine UO2 Powders: A Process of Nanosized Domain Development.

The nanostructure and phase evolution in low-temperature oxidized (40-250 °C), fine UO2 powders (<200 nm) have been investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The extent of oxidation was also measured via in situ thermogravimetric analysis. The oxidation of fine powders was found to proceed differently as compared to oxidation of coarse-grained UO2. No discrete surface oxide layer was observed and no U3O8 was formed, despite the high degree of oxidation (up to O/U = 2.45). Instead, nanosized (5-15 nm) amorphous nuclei (interpreted as amorphous UO3), unmodulated and modulated U4O9, and a continuous range of U3O7-z phases with varying tetragonal distortion (c/a > 1) were observed. Oxidation involves formation of higher uranium oxides in nanodomains near the grain surface which, initially, have a disordered defect structure ("disordered U4O9"). As oxidation progresses, domain growth increases and the long-period modulated structure of U4O9 develops ("ordered U4O9"). A similar mechanism is understood to happen also in U3O7-z.

Energetics of metastudtite and implications for nuclear waste alteration.

Metastudtite, (UO2)O2(H2O)2, is one of two known natural peroxide minerals, but little is established about its thermodynamic stability. In this work, its standard enthalpy of formation, -1,779.6 ± 1.9 kJ/mol, was obtained by high temperature oxide melt drop solution calorimetry. Decomposition of synthetic metastudtite was characterized by thermogravimetry and differential scanning calorimetry (DSC) with ex situ X-ray diffraction analysis. Four decomposition steps were observed in oxygen atmosphere: water loss around 220 °C associated with an endothermic heat effect accompanied by amorphization; another water loss from 400 °C to 530 °C; oxygen loss from amorphous UO3 to crystallize orthorhombic α-UO2.9; and reduction to crystalline U3O8. This detailed characterization allowed calculation of formation enthalpy from heat effects on decomposition measured by DSC and by transposed temperature drop calorimetry, and both these values agree with that from drop solution calorimetry. The data explain the irreversible transformation from studtite to metastudtite, the conditions under which metastudtite may form, and its significant role in the oxidation, corrosion, and dissolution of nuclear fuel in contact with water.

Investigation of ammonium diuranate calcination with high-temperature X-ray diffraction

The thermal decomposition of ammonium diuranate (ADU) in air is investigated using in situ high-temperature X-ray diffraction (HT-XRD), thermogravimetry and differential thermal analysis. Data have been collected in the temperature range from 30 to 1000 °C, allowing the observation of phase transformation and the assessment of the energy changes involved in the calcination of ADU. The starting material 2UO3·NH3·3H2O undergoes a process involving several endothermic and exothermic reactions. In situ HT-XRD shows that amorphous UO3 is obtained after achieving complete dehydration at 300 °C, and denitration at about 450 °C. After cooling from heat treatment at 600 °C, a crystalline UO3 phase appears, as displayed by ex situ XRD. The self-reduction of UO3 into orthorhombic U3O8 takes place at about 600 °C, but a long heat treatment or higher temperature is required to stabilise the structure of U3O8 at room temperature. U3O8 remains stable in air up to 850 °C. Above this temperature, oxygen losses lead to the formation of U3O8−x, as demonstrated by subtle changes in the diffraction pattern and by a mass loss recorded by TGA.

Thermal and X-ray diffraction analysis studies during the decomposition of ammonium uranyl nitrate.

Two types of ammonium uranyl nitrate (NH4)2UO2(NO3)4·2H2O and NH4UO2(NO3)3, were thermally decomposed and reduced in a TG-DTA unit in nitrogen, air, and hydrogen atmospheres. Various intermediate phases produced by the thermal decomposition and reduction process were investigated by an X-ray diffraction analysis and a TG/DTA analysis. Both (NH4)2UO2(NO3)4·2H2O and NH4UO2(NO3)3 decomposed to amorphous UO3 regardless of the atmosphere used. The amorphous UO3 from (NH4)2UO2(NO3)4·2H2O was crystallized to γ-UO3 regardless of the atmosphere used without a change in weight. The amorphous UO3 obtained from decomposition of NH4UO2(NO3)3 was crystallized to α-UO3 under a nitrogen and air atmosphere, and to β-UO3 under a hydrogen atmosphere without a change in weight. Under each atmosphere, the reaction paths of (NH4)2UO2(NO3)4·2H2O and NH4UO2(NO3)3 were as follows: under a nitrogen atmosphere: (NH4)2UO2(NO3)4·2H2O → (NH4)2UO2(NO3)4·H2O → (NH4)2UO2(NO3)4 → NH4UO2(NO3)3 → A-UO3 → γ-UO3 → U3O8, NH4UO2(NO3)3 → A-UO3 → α-UO3 → U3O8; under an air atmosphere: (NH4)2UO2(NO3)4·2H2O → (NH4)2UO2(NO3)4·H2O → (NH4)2UO2(NO3)4 → NH4UO2(NO3)3 → A-UO3 → γ-UO3 → U3O8, NH4UO2(NO3)3 → A-UO3 → α-UO3 → U3O8; and under a hydrogen atmosphere: (NH4)2UO2(NO3)4·2H2O → (NH4)2UO2(NO3)4·H2O → (NH4)2UO2(NO3)4 → NH4UO2(NO3)3 → A-UO3 → γ-UO3 → α-U3O8 → UO2, NH4 UO2(NO3)3 → A-UO3 → β-UO3 → α-U3O8 → UO2.

External Gelation 방법을 이용한 구형 UO3Gel 입자 제조

HTGR (High Temperature Gas-cooled Reactor) is spotlighted to next generation nuclear power plant for producing the clean hydrogen gas and the electricity. In this study, the spherical UO₃ gel particles were prepared by the external gelation process, and the characteristics of these particles were analyzed the particle shape, composition of precipitate, and thermal decomposition characteristics with the Streoscope, FT-IR, and X-ray diffractometer. Raw material of the ADUN (Acid Deficient Uranyl Nitrate) solution, which has [NO₃]/[U] mole ratio = 1.75, was obtained from dissolution of the U₃O 8 powder with concentrated HNO₃, and its concentration is 3.5 M-U/l. The broth solution is prepared with the ADUN, urea, PVA, and THFA solution. The droplets of the broth solution was made through a nozzle system. From this study, we obtained the following results; 1) an externel chemical gelation process is a suitable method in the spherical UO₃ particle production, 2) the particle shape are changed by an urea mixing time, THFA volume, and the viscosity of the broth solution, 3) the amorphous UO₃ particles obtained from these experiments was converted to U₃O 8 and then UO₂ by heat treatment in hydrogen atmosphere at 600℃.

Sol-Gel법을 이용한 구형 UO2Kernel 제조에 관한 기초연구

HTGR(High Temperature Gas-Cooled Reactor) is highlighted to next generation power plant for producing the clean hydrogen gas. In this study, the spherical UO₂ kernel via UO₃ gel particles was prepared by the sol-gel process. Raw material of slightly Acid Deficient Uranyl Nitrate (ADUN) solution, which has pH = 1.10 and [NO₃]/[U] mole ratio = 1.93, was obtained from dissolution of U₃O 8 powder with conc.-HNO₃. The surface of these spherical UO₃ gel particles, which was prepared from the broth solution, consisted of 1 M-uranium, 1 M-HMTA, and urea, were covered with the fine crystallite aggregates, and these particles were so hard that crushed well. But the other UO₃ gel particles prepared with the broth solution, consisted of 2 M-uranium, 2 M-HMTA, and urea, have soft surface characteristics and an amorphous phase. This type of UO₃ gel particles is some chance of doing possibility of high density from the compaction. The amorphous UO₃ gel particles was converted to U₃O 8 and then UO₂ by calcination at 600℃ in 4%-H₂+N₂ atmosphere.

Thermal decomposition of ammonium uranates

The thermal decomposition of ammonium uranates precipitated from uranyl nitrate solution on the addition of aqueous ammonium hydroxide and hexamine under various conditions has been studied by means of thermogravimetry, differential thermal analysis, infrared spectroscopy and X-ray diffraction. Although all precipitates show the composition corresponding to UO3 · NH3 · H2O, the precipitates with hexamine give X-ray diffraction patterns designed as types I and II, in which type I is similar to the precipitates with ammonia. As a result, it is concluded that ammonium uranates thermally decompose to amorphous UO3 at about 400°, and transform to U3O8 viaβ-UO3 and/orα-UO3, latter being formed in the case of type II only.

Investigation of hydrolysis products from uranium trioxide and uranyl salts by derivatography and infrared spectroscopy

It can be established from the data obtained by derivatography and infrared spectroscopy that the hydrolysis ofβ- and amorphous UO3 at room temperature, and that of uranyl salts in the presence of anion exchange resins, leads to compounds of UO3 · 2H2O composition with identical properties. This type of compound is of complex structure containing in addition to water molecules in two kinds of bonds, hydroxyl groups as well. The relative quantities of the components with different bond strengths show that the molecular structure can be described only by a trimeric formula, or by a formula corresponding to an integral multiple of a trimeric form. One kind of water has diffuse bond strength and is probably water between individual layers, while the other is a structural water of definite bond strength which may form various hydrogen bridge bonds.

Thermal decomposition of hydrated UO3

AbstractCalcination of ammonium polyuranate at temperatures between 350 and 600°, with a rate of temperature increase of 1°/min., yielded amorphous UO3 having surface area of about 30 m.2/g. More rapid increase of temperature gave mixtures of amorphous UO3+β‐UO3 with no reproducible surface area. To obtain pure amorphous UO3, the mixtures were hydrated in water or in steam and then dehydrated. During hydration, the surface area decreased and the decrease continued during subsequent calcination, to give a final surface area between 3 and 5 m.2/g.

Thermal decomposition of hydrated uranium peroxides

AbstractThe water vapour pressure of the equilibrium between the hydrates UO4.4H2O and UO4. 2H2O has been measured as a function of the temperature. The results can be expressed as: log pH2O(mm) = 11.020 − 3082/TThermal decomposition of these hydrates results in the formation of amorphous (A‐) UO3, α‐UO3 or the UO2.9‐phase. These transformations were studied by thermogravimetric and differential thermal analysis.At 520° recrystallization of these oxides takes place with formation of γ‐UO3.

The low temperature oxidation of UO2 and U4O9

The low temperature oxidation of UO2 to U3O7 is shown to be a diffusion-controlled diphasic reaction. Two tetragonal phases of U3O7 have been identified: α-U3O7, with a = 5·46 Å, c = 5·40 Å and β-U3O7 with a = 5·38 Å, c = 5·55 Å. Annealing experiments with β-U3O7 show the existence of a third tetragonal phase with a composition approximately UO2·3, which is stable to 500°C. Its parameters are a = 5·41 Å, c = 5·49 Å.The oxidation of U4O9 is also controlled by diffusion of oxygen through a second phase, UO2·3 which is in turn converted to β-U3O7.Evidence is given to show that the oxidized surface layer which forms on UO2 at 25°C is amorphous UO3.

八三酸化ウランの反応活性に関する研究, (I)

Uranouranic oxides prepared from various uranium compounds by various methods were physically and chemically examined.It is concluded that uranouranic oxide obtained from UPO is the most reactive, and that the difference of these reactivity does not depend upon impurities, but essentially upon some physical properties, such as the specific surface area and the crystal structure.Present paper concerns with the comparative study of the mechanism on the thermal decomposition of UNH, ADU and UPO by a semiautomatic recording thermal balance, a X-ray diffractometer and its high temperature apparatus. The results are as follows;(1) On the way of thermal decomposition of UNH, uranic oxide is formed in the temperature range of 320° to 570°C, and its compound may be of monoclinic.(2) On the thermal decomposition fo ADU, uranic oxide is formed in the range of 340° to 550°C, and its product may be of a new crystal system.(3) On the thermal decomposition of UPO, amorphous UO3 is formed in the range of 320° to 500°C.(4) The uranouranic oxides are always orthorhombic crystals.