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| Names | |
|---|---|
| IUPAC names Uranium dioxide Uranium(IV) oxide | |
| Other names Urania Uranous oxide | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.014.273 |
| EC Number |
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| RTECS number |
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| UNII | |
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| Properties | |
| UO2 | |
| Molar mass | 270.03 g/mol |
| Appearance | black powder |
| Density | 10.97 g/cm3 |
| Melting point | 2,865 °C (5,189 °F; 3,138 K) |
| insoluble | |
| Structure | |
| Fluorite (cubic),cF12 | |
| Fm3m, No. 225 | |
a = 547.1 pm[1] | |
| Tetrahedral (O2−); cubic (UIV) | |
| Thermochemistry | |
Std molar entropy(S⦵298) | 78 J·mol−1·K−1[2] |
Std enthalpy of formation(ΔfH⦵298) | −1084 kJ·mol−1[2] |
| Hazards | |
| GHS labelling: | |
   | |
| Danger | |
| H300,H330,H373,H410 | |
| P260,P264,P270,P271,P273,P284,P301+P310,P304+P340,P310,P314,P320,P321,P330,P391,P403+P233,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Flash point | N/A |
| Safety data sheet (SDS) | ICSC 1251 |
| Related compounds | |
Otheranions | Uranium(IV) sulfide Uranium(IV) selenide |
Othercations | Protactinium(IV) oxide Neptunium(IV) oxide |
| Triuranium octoxide Uranium trioxide | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Uranium dioxide oruranium(IV) oxide (UO2), also known asurania oruranous oxide, is anoxide ofuranium, and is a black,radioactive,crystalline powder that naturally occurs in the mineraluraninite. It is used innuclear fuel rods innuclear reactors. A mixture of uranium andplutonium dioxides is used asMOX fuel. It has been used as an orange, yellow, green, and black color inceramic glazes andglass.
Uranium dioxide is produced byreducinguranium trioxide withhydrogen. This reaction often createstriuranium octoxide as an intermediate.[3][4][5]
This reaction plays an important part in the creation ofnuclear fuel throughnuclear reprocessing anduranium enrichment.[5]
The solid isisostructural with (has the same structure as)fluorite (calcium fluoride), where each U is surrounded by eight O nearest neighbors in a cubic arrangement. In addition, the dioxides ofcerium,thorium, and thetransuranic elements fromneptunium throughcalifornium have the same structures.[6] No other elemental dioxides have the fluorite structure. Upon melting, the measured average U-O coordination reduces from 8 in the crystalline solid (UO8 cubes), down to 6.7±0.5 (at 3270 K) in the melt.[7] Models consistent with these measurements show the melt to consist mainly of UO6 and UO7 polyhedral units, where roughly2⁄3 of the connections between polyhedra are corner sharing and1⁄3 are edge sharing.[7]
Uranium dioxide isoxidized in contact withoxygen to formtriuranium octoxide:[8]
Theelectrochemistry of uranium dioxide has been investigated in detail as thegalvanic corrosion of uranium dioxide controls the rate at which usednuclear fuel dissolves.[clarification needed] Seespent nuclear fuel for further details.Water increases the oxidation rate ofplutonium and uranium metals.[9]
Uranium dioxide reacts withcarbon at high temperatures, forminguranium carbide andcarbon monoxide.[10]
This process must be done under aninert gas as uranium carbide is easily oxidized back intouranium oxide.
UO2 is used mainly asnuclear fuel, specifically as UO2 or as a mixture of UO2 and PuO2 (plutonium dioxide) called a mixed oxide (MOX fuel), in the form offuel rods innuclear reactors.[11]
Thethermal conductivity of uranium dioxide is very low when compared with elementaluranium,uranium nitride,uranium carbide andzircaloy cladding material as well as most uranium-based alloys.[12][13][14] This low thermal conductivity can result in localised overheating in the centres of fuel pellets.[15]
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.[citation needed]
Uranium oxide (urania) was used to color glass and ceramics prior to World War II, and until the applications of radioactivity were discovered this was its main use. In 1958 the military in both the US and Europe allowed its commercial use again as depleted uranium, and its use began again on a more limited scale. Urania-based ceramic glazes are dark green or black when fired in a reduction or when UO2 is used; more commonly it is used in oxidation to produce bright yellow, orange and red glazes.[16] Orange-coloredFiestaware is a well-known example of a product with a urania-colored glaze.[17]Uranium glass is pale green to yellow and often has strong fluorescent properties.[18] Urania has also been used in formulations ofenamel andporcelain.[19] It is possible to determine with aGeiger counter if a glaze or glass produced before 1958 contains urania.
Prior to the realisation of the harmfulness of radiation, uranium was included in false teeth and dentures, as its slight fluorescence made the dentures appear more like real teeth in a variety of lighting conditions.[20]
Depleted UO2 (DUO2) can be used as a material forradiation shielding. For example,DUCRETE is a "heavyconcrete" material wheregravel is replaced with uranium dioxide aggregate; this material is investigated for use forcasks forradioactive waste.[21] Casks can be also made of DUO2-steelcermet, acomposite material made of anaggregate of uranium dioxide serving as radiation shielding,graphite and/orsilicon carbide serving asneutron radiation absorber and moderator, and steel as the matrix, whose high thermal conductivity allows easy removal of decay heat.[22]
Depleted uranium dioxide can be also used as acatalyst, e.g. for degradation ofvolatile organic compounds in gaseous phase,oxidation ofmethane tomethanol, and removal ofsulfur frompetroleum. It has high efficiency and long-term stability when used to destroy VOCs when compared with some of the commercialcatalysts, such asprecious metals,TiO2, andCo3O4 catalysts. Much research is being done in this area, DU being favoured for the uranium component due to its low radioactivity.[23]
The use of uranium dioxide as a material forrechargeable batteries is being investigated.[24] The batteries could have a highpower density and areduction potential of -4.7 V per cell.[25] Another investigated application is inphotoelectrochemical cells for solar-assisted hydrogen production where UO2 is used as aphotoanode. 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.[citation needed]
Uranium dioxide displays strongpiezomagnetism in theantiferromagnetic state, observed at cryogenic temperatures below 30kelvins. Accordingly, the linearmagnetostriction 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.[26] 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.[27]
Theband gap of uranium dioxide is comparable to those ofsilicon andgallium arsenide, near the optimum for efficiency vs band gap curve for absorption of solar radiation, suggesting its possible use for very efficientsolar cells based onSchottky 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 ofsingle crystal silicon.[28]
Thedielectric constant of uranium dioxide is about 21.5,[29] which is almost twice as high as of silicon (11.7)[30] and GaAs (12.4).[31] This is an advantage over Si and GaAs in the construction ofintegrated circuits, as it may allow higher density integration with higherbreakdown voltages and with lower susceptibility to theCMOStunnelling breakdown.[32]
TheSeebeck coefficient of uranium dioxide at room temperature is about -750 μV/K, a value significantly higher than the -270 μV/K ofthallium tin telluride (Tl2SnTe5) andthallium germanium telluride (Tl2GeTe5)[32] and the −170 μV/K (n-type) / 160 μV/K (p-type) ofbismuth telluride,[33] other materials promising forthermoelectric power generation applications[32] andPeltier elements.[citation needed]
Theradioactive decay impact of the235U and238U on its semiconducting properties was not measured as of 2005[update]. 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-performanceintegrated circuits. Use ofdepleted 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.[32]
Thestoichiometry 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.[32]
Uranium dioxide, like U3O8, is aceramic material capable of withstanding high temperatures (about 2300 °C, in comparison with at most 200 °C for silicon or GaAs),[32] making it suitable for high-temperature applications like thermophotovoltaic devices.[citation needed]
Uranium dioxide is also resistant toradiation damage,[32] making it useful forrad-hard devices[citation needed] for special military andaerospace applications.[32]
ASchottky diode ofU3O8 and ap-n-p transistor of UO2 were successfully manufactured in a laboratory.[34]
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.[35]
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.[35]
