Uranium dioxide

Uranium dioxide (UO₂), an oxide of uranium, also known as urania or uranic oxide is a black, radioactive, crystalline powder. It occurs naturally in the mineral uraninite. It has a melting point of 2800°C and, if produced from enriched uranium it is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used as MOX fuel. Prior to 1960 it was used as yellow and black color in ceramic glazes and glass.

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Production

Uranium dioxide is produced by reducing uranium trioxide with hydrogen.

UO₃ + H₂ → UO₂ + H₂O at 700°C (970K)

This reaction takes part in the reprocessing of nuclear fuel and enrichment of uranium for nuclear fuel.

Chemistry

Structure

The solid is isostructural with (has the same structure as) fluorite (calcium fluoride). In addition, the dioxides of plutonium and neptunium have the same structures

Oxidation with oxygen

Uranium dioxide is oxidized in contact with oxygen to the triuranium octaoxide.

3UO₂ + O₂ → U₃O₈ at 700°C (970K)

Aqueous electrochemistry

The electrochemistry of uranium dioxide has been investigated in detail as the galvanic corrosion of uranium dioxide controls the rate at which used nuclear fuel dissolves. See the spent nuclear fuel page for further details.

Oxidation of uranium metal

It has been reported that water causes the rate of the oxidation of both plutonium and uranium metal to increase when compared with the situation which exists when water is absent.[1]

Uses

Nuclear Fuel

Enriched UO₂ is used mainly as nuclear fuel, specifically as UO₂ or as a mixture of UO₂ and PuO₂ (plutonium dioxide) called a mixed oxide (MOX fuel) for fuel rods in nuclear reactors.

Note that the thermal conductivity of uranium dioxide is very low when compared with uranium, uranium nitride, uranium carbide and zirconium cladding material. This low thermal conductivity can result in localised overheating in the centres of fuel pellets. The graph below shows the different temperature gradients in different fuel compounds. For these fuels the thermal power density is the same and the diameter of all the pellets are the same.

Colour for ceramics glaze

All uranium oxides were used to colour glass and ceramics. Uranium oxide-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. Uranium oxide has also been used in formulations of enamel, uranium glass, and porcelain.

Prior to 1960, uranium oxides were used as coloured glazes. It is possible to determine with a Geiger counter if a glaze or glass contains uranium oxides.

Other use

Depleted UO₂ (DUO₂) can be used as a material for radiation shielding. For example, DUCRETE is a "heavy concrete" material where gravel is replaced with uranium dioxide aggregate; this material is investigated for use for casks for radioactive waste. Casks can be also made of DUO₂-steel cermet, a composite material made of an aggregate of uranium dioxide serving as radiation shielding, graphite and/or silicon carbide serving as neutron radiation absorber and moderator, and steel as the matrix, whose high thermal conductivity allows easy removal of decay heat.

Depleted uranium dioxide can be also used as a catalyst, eg. for degradation of volatile organic compounds in gaseous phase, oxidation of methane to methanol, and removal of sulfur from petroleum. It has high efficiency and long-term stability when used to destroy VOCs when compared with some of the commercial catalysts, such as precious metals, TiO₂, and Co₃O₄ catalysts. Much research is being done in this area, DU being favoured for the uranium component due to its low radioactivity. ^[2] Use of uranium dioxide as a material for rechargeable batteries is investigated. The batteries could have high power density and potential of 4.7V per cell.

Another investigated application is in photoelectrochemical cells, for solar-assisted hydrogen production. UO₂ is used as a photoanode.

Semiconductor properties

Uranium dioxide is a semiconductor material. Its band gap is about 1.3 eV, which lies between the band gap for silicon and gallium arsenide, near the optimum for efficiency vs band gap curve for absorption of solar radiation, suggesting its possible use for very efficient solar cells based on Schottky diode structure; it also absorbs at five different wavelengths, including infrared, further enhancing its efficiency. Its intrinsic conductivity at room temperature is about the same as of single crystal silicon.

Its dielectric constant is about 22, which is almost twice as high as of silicon (11.2) and GaAs (14.1), which poses an advantage over Si and GaAs for construction of integrated circuits, as it may allow higher density integration with higher breakdown voltages and with lower susceptibility to the CMOS tunneling breakdown.

The Seebeck coefficient of uranium dioxide at room temperature is about 750 µV/K, a value significantly higher than the 270 µV/K of thallium tin telluride (Tl₂SnTe₅) and thallium germanium telluride (Tl₂GeTe₅) and of bismuth-tellurium alloys, other materials promising for thermopower applications and Peltier elements.

The radioactive decay impact of the ²³⁵U and ²³⁸U on its semiconducting properties was not measured as of 2005. Due to the slow decay rate of these isotopes, it should not meaningfully influence the properties of uranium dioxide solar cells and thermoelectric devices, but it may become an important factor for VLSI chips. Use of depleted uranium oxide is necessary for this reason. The accumulation of decay products, eg. helium, in the crystal lattice may also cause gradual long-term changes in its properties.

The stoichiometry of the material dramatically influences its electrical properties. For example, the electrical conductivity of UO_1.994 is orders of magnitude lower at higher temperatures than the conductivity of UO_2.001.

Uranium dioxide, like U₃O₈, is a ceramic material capable of withstanding high temperatures (about 2300 °C, in comparison with at most 200 °C for silicon or GaAs), making it suitable for high-temperature applications like thermophotovoltaic devices.

Uranium dioxide is also resistant to radiation damage, making it useful for rad-hard devices for special military and aerospace applications.

A Schottky diode of U₃O₈ and a p-n-p transistor of UO₂ were successfully manufactured in a laboratory.

References

1. ^ Barrett SA, Jacobson AJ, Tofield BC, Fender BEF (1982). "The preparation and structure of barium uranium oxide BaUO_3+x". Acta Crystallographica B 38: 2775-2781. DOI
2. ^ Hutchings GJ (1996). "A Uranium-Oxide-Based Catalysts for the Destruction of Volatile Chloro-Organic compounds". Nature 384: 341-343.