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|---|---|
| Systematic name | Uranium trioxide |
| Other names | Uranium(VI) oxide |
| Molecular formula | UO3 |
| Molar mass | 286.03 g/mol |
| CAS number | |
| Density | 5.5-8.7 g/cm |
| Solubility (water) | Insoluble |
| Solubility (dog lung fluid) | < 5 days half time |
| Melting point | ca. 500 °C decomp.(s) |
| Disclaimer and references | |
Uranium trioxide (UO3), also called uranyl oxide and uranic oxide, is an oxide of uranium. Uranyl oxide is a toxic, teratogenic, and radioactive solid, obtained by heating uranyl nitrate to 400 °C, and a gas produced when uranium burns in air, condensing at standard temperature and pressure.
In one of its common solid phase forms, uranyl oxide is a yellow-orange powder.
Cameco Corporation, which operates at the world's largest uranium refinery at Blind River, Ontario, produces high-purity uranium trioxide.
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.
Chemistry
Solid UO3 may be obtained by heating uranyl nitrate (UO2(NO3)2·6H2O) at 400 °C. This conversion is used in the reprocessing of nuclear fuel, which begins with the dissolution of the fuel rods in HNO3.
Uranium trioxide is also an intermediary compound in the conversion of sodium diuranate yellowcake (Na2U2O7·6H2O) to uranium hexafluoride and is shipped between processing facilities in the form of a UO3 gel. In the jargon of the uranium refining industry, the chemical solution containing the concentrated uranium trioxide is called "OK liquor". Upon heating, this material liberates ammonia, giving UO3.
Chemical and structural properties
The only well characterized binary trioxide of any actinide is UO3, of which several polymorphs are known. At 800-900 °C, UO3 releases some O2 to give green-colored U3O8. Heating at 700 °C under H2 gives dark brown uranium dioxide (UO2), which is used in MOX nuclear fuel rods.
Combustion product of uranium burning in air
Gaseous monomeric UO3 is produced by combustion of uranium metal in air from 2200-2800 Kelvin (Ackermann et al. 1960; Mouradian et al. 1963.) The production of UO3 gas vapor is "not infrequently ignored" (Gmelin vol. U-C1, p. 98). UO3(g) molecules condense.
According to Wilson (1961), Ackermann et al. (1960) show that U3O8 crystals result from the two step process:
- 1/3 U3O8(s) + 1/6 O2(g) → UO3(g) at T1
- UO3(g) → 1/3 U3O8 + 1/6 O2(g) at T2
- where T2 < T1.
Uranium trioxide gas molecules are a known intermediate in the chemical transition reactions forming U3O8 crystals from combustion products including uranium oxides.
Individual UO3 gas vapor molecules will not decompose below the burning temperature of uranium in air, because uranium monoxide requires additional energy to form, as does the release of O2 by a single UO3 molecule. (Hoekstra and Siegel 1958; Wanner and Forest (2004) p. 98.)
Uranium oxide 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, 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.
Related anions and cations
Uranium oxide is amphoteric and reacts as acid and as a base, depending on the conditions.
As an acid:
- UO3 + H2O → UO4 + H
Dissolving uranium oxide in a strong base like sodium hydroxide forms the doubly negatively charged uranate anion (UO4). Uranates tend to agglomerate, forming diuranate, U2O7
As a base:
- UO3 + H2O → UO2 + 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)2 • 6H2O 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.
See also
References
- Peer-reviewed
- Wilson, W.B. (1961) "High-Pressure High-Temperature Investigation of the Uranium-Oxygen System," Journal Of Inorganic and Nuclear Chemistry, 19, 212-222.
- R.J. Ackermann, et al., "Free Energies of Formation of Gaseous Uranium, Molybdenum, and Tungsten Trioxides," Journal of Physical Chemistry, vol. 64 (1960) pp. 350-355
- Mouradian and Baker (1963) "Burning Temperatures of Uranium and Zirconium in Air," Nuclear Science and Engineering, 15, 388-394
- Morrow et al. (1972) "Inhalation studies of uranium trioxide" Health Physics 23, 273-280
- Stuart et al. (1979) "Solubility and Hemolytic Activity of Uranium Trioxide." Environmental Research 18, 385-396
- Nakajima, K.; Arai, Y. (2001) "Mass-spectrometric investigation of UO{sub 3}(g)", Journal of Nuclear Materials, 294, 250-255.
- Green, D.W. (1980) "Relationship between spectroscopic data and thermodynamic functions; application to uranium, plutonium, and thorium oxide vapor species," Journal of Nuclear Materials, 88, 51-63.
- Ackermann, R.J. and A.T. Chang (1973) "Thermodynamic Characterization of U3O8-z Phase," Journal Of Chemical Thermodynamics, 5, 873-890.
- Chapman, A.T. and Meadows, R.E. (1964) "Volatility of UO2+/-x and Phase Relations in the System Uranium Oxygen," Journal of the American Ceramic Society, 47, 614-621.
- Drowart, J., A. Pattoret, and S. Smoes (1964) "Heat of sublimation of uranium and consistency of thermodynamic data for uranium compounds," Journal of Nuclear Materials, 12, 319-322.
- Roberts, L.E.J. and A.J. Walter (1961) "Equilibrium Pressures and Phase Relations in the Uranium Oxide System," Journal of Inorganic and Nuclear Chemistry, 22, 213-229.
- Hoekstra, H.R. and Siegel, S. (1970) "Uranium-Oxygen System at high pressure," Journal of Inorganic and Nuclear Chemistry, 32, 3237-3248.
- United Nations invitation-only
- H. R. Hoekstra and S. Siegel (1958) "Recent Developments in the Chemistry of the Uranium-Oxygen System," in the Proceedings of the Second International Conference on Peaceful Uses of Atomic Energy, (Geneva: UN) 7, 394-400.
- Other
- H. Wanner and I. Forest, eds. (2004) Chemical Termodynamics of Uranium (Paris: OECD and French Nuclear Energy Agency)
- Gmelin Handbook of Inorganic Chemistry, 8th ed., English translation, vol. U-C1 (1977), page 98
- «Gmelin Handbuch der anorganischen Chemiek» 8th ed., vol. U-C2, pp. 118-120
- Gilchrist, R.L., J.A. Glissmyer, and J. Mishima (1979) "Characterization of Airborne Uranium from Test Firings of XM774 Ammunition," Technical report no. PNL-2944, Richland, WA: Battelle Pacific Northwest Laboratory, November 1979.
- URANIUM and CERAMICS by Edouard Bastarache
- Uranium and Insoluble Compounds from the Occupational Safety and Health Administration; note that UO3 is moderately soluble (Morrow, 1972.)