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Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactivity, powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used as MOX fuel. It has been used as an orange, yellow, green, and black color in uranium tile and uranium glass.
This reaction plays an important part in the creation of nuclear fuel through nuclear reprocessing and uranium enrichment.
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 spent nuclear fuel for further details. Water increases the oxidation rate of plutonium and uranium metals.
This process must be done under an inert gas as uranium carbide is easily oxidized back into uranium oxide.
The thermal conductivity of uranium dioxide is very low when compared with elemental uranium, uranium nitride, uranium carbide and zircaloy cladding material as well as most uranium-based alloys. 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.
Depleted uranium 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 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, e.g. 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 , such as , titanium dioxide, and cobalt oxide catalysts. Much research is being done in this area, DU being favoured for the uranium component due to its low radioactivity.
The use of uranium dioxide as a material for rechargeable batteries is being investigated. The batteries could have a high power density and a reduction potential of -4.7 V per cell. Another investigated application is in photoelectrochemical cells for solar-assisted hydrogen production where UO2 is used as a photoanode. In earlier times, uranium dioxide was also used as heat conductor for current limitation (URDOX-resistor), which was the first use of its semiconductor properties.
Uranium dioxide displays strong piezomagnetism in the antiferromagnetic state, observed at cryogenic temperatures below 30 . Accordingly, the linear magnetostriction found in UO2 changes sign with the applied magnetic field and exhibits magnetoelastic memory switching phenomena at record high switch-fields of 180,000 Oe. The microscopic origin of the material magnetic properties lays in the face-centered-cubic crystal lattice symmetry of uranium atoms, and its response to applied magnetic fields.
The dielectric constant of uranium dioxide is about 21.5, which is almost twice as high as of silicon (11.7) and GaAs (12.4). This is an advantage over Si and GaAs in the construction of integrated circuits, as it may allow higher density integration with higher breakdown voltages and with lower susceptibility to the CMOS tunnelling 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 the −170 μV/K (n-type) / 160 μV/K (p-type) of bismuth telluride, other materials promising for thermoelectric power generation applications and .
The radioactive decay impact of the 235U and 238U on its semiconducting properties was not measured . 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 high-performance integrated circuits. Use of depleted uranium oxide is necessary for this reason. The capture of alpha particles emitted during radioactive decay as helium atoms 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 UO1.994 is orders of magnitude lower at higher temperatures than the conductivity of UO2.001.
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 yellowcake and a p-n-p transistor of UO2 were successfully manufactured in a laboratory.
If inhaled, short term effects include irreversible kidney damage or acute necrotic arterial lesions. Inhalation of large particles of uranium materials or chronic exposure to uranium powders may result in radiation damage to internal tissues, especially the lungs and bones. Long term, in addition to effects from short term exposure, damage may include pulmonary fibrosis and malignant pulmonary neoplasia, anemia and blood disorders, liver damage, bone effects, sterility, and cancers. Skin contact with uranium powders may result in dermatitis. If ingested, it may cause kidney damage or acute necrotic arterial lesions. Ingestion may also affect the liver, and cause radiation damage to internal tissues.
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Semiconductor properties
The band gap of uranium dioxide is comparable to those of 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 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.
Health dangers
Uranium dioxide is dangerous in two ways: heavy metal toxicity, and radiation. Uranium dioxide decays primarily by emission of alpha particles and gamma radiation, which is cumulatively dangerous to biologic organisms including animals and humans. It can be severely toxic or even fatal if swallowed, inhaled, absorbed through the skin and eyes.
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