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.
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.
U3O8 can be oxidized at 500 °C with oxygen. Note that above 750 °C even in 5 atm O2 UO3 decomposes into U3O8. 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. 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.
Uranium trioxide is shipped between processing facilities in the form of a gel, most often from mines to conversion plants. When used for conversion, all uranium oxides are often called reprocessed uranium (RepU).
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 uranium dioxide, uranium trioxide, and coffinite. In pure water, schoepite (UO2)8O2(OH)12·12(H2O) is formed in the first week and then after four months studtite (UO2)O2·4(H2O) was produced. This alteration of uranium oxide also leads to the formation of metastudtite, a more stable uranyl peroxide, often found in the surface of spent nuclear fuel exposed to water. Reports on the corrosion of uranium metal have been published by the Royal Society.
Health and safety hazards
Like all hexavalent uranium compounds, UO3 is hazardous by inhalation, ingestion, and through skin contact. It is a poisonous, slightly 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. However, once ingested, uranium is mainly toxic for the kidneys and may severely affect their function.
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 to 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.
Hydrates
Several hydrates of uranium trioxide are known, e.g., UO3·6H2O, which are commonly known as "uranic acid" in older literature due to their similarity in formula to various metal oxyacids, although they are not in fact particularly acidic.
Bond valence parameters
It is possible by bond valence calculations to estimate how great a contribution a given oxygen atom is making to the assumed valence of uranium. 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 RO 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.
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.
2 U3O8(s) + O2(g) 6 UO3(g)