Uranium trioxide

Uranium trioxide (UO₃), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO₃, is a yellow-orange powder.

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Production and use

There are three methods to generate uranium trioxide. As noted below, two are used industrially in the reprocessing of nuclear fuel and uranium enrichment.

1. U₃O₈ can be oxidized at 500°C with oxygen.^[1] Note that above 750°C even in 5 atm O₂ UO₃ decomposes into U₃O₈.^[2]
2. Uranyl nitrate, (UO₂(NO₃)₂·6H₂O) can be heated to yield UO₃. This occurs during the reprocessing of nuclear fuel. Fuel rods are dissolved in HNO₃ to separate uranyl nitrate from plutonium and the fission products (the PUREX method). The pure uranyl nitrate is converted to solid UO₃ by heating at 400 °C. After reduction with hydrogen (with other inert gas present) to uranium dioxide, the uranium can be used in new MOX fuel rods.
3. Ammonium diuranate or sodium diuranate (Na₂U₂O₇·6H₂O) may be decomposed. Sodium diuranate, also known as yellowcake, is converted to uranium trioxide in the enrichment of uranium. Uranium dioxide and uranium tetrafluoride are intermediates in the process which ends in uranium hexafluoride.^[3]

Uranium trioxide is shipped between processing facilities in the form of a gel.

Cameco Corporation, which operates at the world's largest uranium refinery at Blind River, Ontario, produces high-purity uranium trioxide.

It has been reported that the corrosion of uranium in a silica rich aqueous solution forms both uranium dioxide and uranium trioxide.^[4] In pure water, schoepite {(UO₂)₈O₂(OH)₁₂.12(H₂O)} is formed^[5] in the first week and then after four months studtite {(UO₂)O₂·4(H₂O)} was formed. Reports on the corrosion of uranium metal have been published by the Royal Society.^[6][7]

Health and safety hazards

Like all hexavalent uranium compounds, UO₃ is hazardous by inhalation, ingestion, and through skin contact. It is a poisonous, radioactive substance, which may cause shortness of breath, coughing, acute arterial lesions, and changes in the chromosomes of white blood cells and gonads leading to congenital malformations if inhaled.^[8][9]

Structure

Solid state structure

The only well characterized binary trioxide of any actinide is UO₃, of which several polymorphs are known. Solid UO₃ loses O₂ on heating to give green-colored U₃O₈: reports of the decomposition temperature in air vary from 200–650 °C. Heating at 700 °C under H₂ gives dark brown uranium dioxide (UO₂), which is used in MOX nuclear fuel rods.

Alpha

Beta

Gamma

Delta

High pressure form

There is a high-pressure solid form with U₂O₂ and U₃O₃ rings in it.^{[15] [16]}

Hydrates

Several hydrates of uranium trioxide are known, e.g., UO₃•6H₂O.^[2]

Bond valence parameters

It is possible by bond valence calculations^[17] to estimate how great a contribution a given oxygen atom is making to the assumed valence of uranium. Zachariasen, J. Less Common Met., 1978, 62, 1-7. Lists the parameters to allow such calculations to be done for many of the actinides. Bond valence calculations use parameters which are estimated after examining a large number of crystal structures of uranium oxides (and related uranium compounds), note that the oxidation states which this method provides are only a guide which assists in the understanding of a crystal structure.

The formula to use is

The sum of the s values is equal to the oxidation state of the metal centre.

For uranium binding to oxygen the constants R_O and B are tabulated in the table below. For each oxidation state use the parameters from the table shown below.

It is possible to do these calculations on paper or software. A program which does it can be obtained free of charge.^[18][19]

Molecular forms

While uranium trioxide is encountered as a polymeric solid under ambient conditions, some work has been done on the molecular form in the gas phase, in matrix isolations studies, and computationally.

Gas phase

At elevated temperatures gaseous UO₃ is in equilibrium with solid U₃O₈ and molecular oxygen.

¹/₃ U₃O₈(s) +¹/₆ O₂(g) UO₃(g)

With increasing temperature the equilibrium is shifted to the right. This system has been studied at temperatures between 900 °C and 2500 °C. The vapor pressure of monomeric UO₃ in equilibrium with air and solid U₃O₈ at ambient pressure, about 10⁻⁵ mbar (1 mPa) at 980 °C, rising to 0.1 mbar (10 Pa) at 1400 °C, 0.34 mbar (34 Pa) at 2100 °C, 1.9 mbar (193 Pa) at 2300 °C, and 8.1 mbar (809 Pa) at 2500 °C.^[20][21]

Matrix isolation

Infrared spectroscopy of molecular UO₃ isolated in an argon matrix indicates a T-shaped structure (point group C_2v) for the molecule. This is in contrast to the commonly encountered D_3h molecular symmetry exhibited by most trioxides. From the force constants the authors deduct the U-O bond lengths to be between 1.76 and 1.79 Å (176 to 179 pm).^[22]

Computational study

  Calculations predict that the point group of molecular UO₃ is C_2v, with an axial bond length of 1.75 Å, an equatorial bond length of 1.83 Å and an angle of 161 ° between the axial oxygens. The more symmetrical D_3h species is a saddle point, 49 kJ/mol above the C_2v minimum. The authors invoke a second-order Jahn-Teller effect as explanation.^[23]

Reactivity

Uranium trioxide reacts at 400 °C with freon-12 to form chlorine, phosgene, carbon dioxide and uranium(IV) fluoride. The freon-12 can be replaced with freon-11 which forms carbon tetrachloride instead of carbon dioxide. This is a case of a hard perhalogenated freon which is normally considered to be inert being converted chemically at a moderate temperature.^[24]

2 CF₂Cl₂ + UO₃ → UF₄ + CO₂ + COCl₂ + Cl₂

4 CF₂Cl₂ + UO₃ → UF₄ + 3COCl₂ + CCl₄ + Cl₂

Uranium trioxide can be dissolved in a mixture of tributyl phosphate and thenoyltrifluoroacetone in supercritical carbon dioxide, ultrasound was employed during the dissolution.^[25]

Electrochemical modification

The reversible insertion of magnesium cations into the lattice of uranium trioxide by cyclic voltammetry using a graphite electrode modified with microscopic particles of the uranium oxide has been investigated. This experiment has also been done for U₃O₈. This is an example of electrochemistry of a solid modified electrode, the experiment which used for uranium trioxide is related to a carbon paste electrode experiment. It is also possible to reduce uranium trioxide with sodium metal to form sodium uranium oxides.^[26]

It has been the case that it is possible to insert lithium^[27][28][29] into the uranium trioxide lattice by electrochemical means, this is similar to the way that some rechargeable lithium ion batteries work. In these rechargeable cells one of the electrodes is a metal oxide which contains a metal such as cobalt which can be reduced, to maintain the electroneutrality for each electron which is added to the electrode material a lithium ion enters the lattice of this oxide electrode.

Amphoterism/Reactivity to form related uranium(VI) anions and cations

Uranium oxide is amphoteric and reacts as acid and as a base, depending on the conditions.

As an acid

UO₃ + H₂O → UO₄²⁻ + H⁺

Dissolving uranium oxide in a strong base like sodium hydroxide forms the doubly negatively charged uranate anion (UO₄²⁻). Uranates tend to agglomerate, forming diuranate, U₂O₇²⁻, or other poly-uranates. Important diuranates include ammonium diuranate ((NH₄)₂U₂O₇), sodium diuranate (Na₂U₂O₇) and magnesium diuranate (MgU₂O₇), which forms part of some yellowcakes. It is worth noting that uranates of the form M₂UO₄ do not contain UO₄²⁻ ions, but rather flattened UO₆ octahedra, containing a uranyl group and bridging oxygens.^[30]

As a base

UO₃ + H₂O → UO₂²⁺ + OH⁻

Dissolving uranium oxide in a strong acid like sulfuric or nitric acid forms the double positive charged uranyl cation. The uranyl nitrate formed (UO₂(NO₃)₂ˑ6H₂O) is soluble in ethers, alcohols, ketones and esters; for example, tributylphosphate. This solubilty is used to separate uranium from other elements in nuclear reprocessing, which begins with the dissolution of nuclear fuel rods in nitric acid. The uranyl nitrate is then converted to uranium trioxide by heating.

From nitric acid one obtains uranyl nitrate, trans-UO₂(NO₃)₂·2H₂O, consisting of eight-coordinated uranium with two bidentate nitrato ligands and two water ligands as well as the familiar O=U=O core.

Uranium oxides in ceramics

UO₃-based ceramics become green or black when fired in a reducing atmosphere and yellow to orange when fired with oxygen. Orange-coloured Fiestaware is a well-known example of a product with a uranium-based glaze. UO₃-has also been used in formulations of enamel, uranium glass, and porcelain.

Prior to 1960, UO₃ was used as an agent of crystallization in crystalline coloured glazes. It is possible to determine with a Geiger counter if a glaze or glass was made from UO₃.

References