Uranium dioxide

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Uranium dioxide
Names
IUPAC names
Uranium dioxide
Uranium(IV) oxide
Other names
Urania
Uranous oxide
Identifiers
3D model (
JSmol
)
ChemSpider
ECHA InfoCard
 100.014.273 Edit this at Wikidata
EC Number
  • 215-700-3
RTECS number
  • YR4705000
UNII
  • InChI=1S/2O.U
    Key: FCTBKIHDJGHPPO-UHFFFAOYSA-N
  • O=[U]=O
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
78 J·mol−1·K−1[2]
Std enthalpy of
formation
(ΔfH⦵298)
 −1084 kJ·mol−1[2]
Hazards
GHS labelling:
GHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard
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)
NFPA 704 four-colored diamondHealth 4: Very short exposure could cause death or major residual injury. E.g. VX gasFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazard OX: Oxidizer. E.g. potassium perchlorate
4
0
0
OX
Flash point N/A
Safety data sheet (SDS) ICSC 1251
Related compounds
Other anions
Uranium(IV) selenide
Other cations
 Protactinium(IV) oxide
Neptunium(IV) oxide
Related uranium oxides
 Triuranium octoxide
Uranium trioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Uranium dioxide or uranium(IV) oxide (UO2), also known as urania or uranous oxide, is an

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.

Production

Uranium dioxide is produced by reducing uranium trioxide with hydrogen.

UO3 + H2 → UO2 + H2O at 700 °C (973 K)

This reaction plays an important part in the creation of

uranium enrichment
.

Chemistry

Structure

The solid is isostructural 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 of cerium, thorium, and the transuranic elements from neptunium through californium have the same structures.[3] 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.[4] Models consistent with these measurements show the melt to consist mainly of UO6 and UO7 polyhedral units, where roughly 2⁄3 of the connections between polyhedra are corner sharing and 1⁄3 are edge sharing.[4]

  • Uranium dioxide
    Uranium dioxide
  • Sintered uranium dioxide pellet
    Sintered uranium dioxide pellet

Oxidation

Uranium dioxide is

triuranium octaoxide
.

3 UO2 + O2 → U3O8 at 700 °C (970 K)

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.[5][6]

Carbonization

Uranium dioxide is carbonized in contact with carbon, forming uranium carbide and carbon monoxide.

.

This process must be done under an inert gas as uranium carbide is easily oxidized back into uranium oxide.

Uses

Nuclear fuel

UO2 is used mainly as

fuel rods in nuclear reactors
.

The

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.[citation needed
]

The thermal conductivity of zirconium metal and uranium dioxide as a function of temperature
  • Uranium oxide fuel pellet
    Uranium oxide fuel pellet
  • Starting material containers for uranium dioxide fuel pellet production at a plant in Russia
    Starting material containers for uranium dioxide fuel pellet production at a plant in Russia

Color for glass ceramic glaze

Geiger counter (kit without housing) audibly reacting to an orange Fiestaware shard.

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.

Fiestaware is a well-known example of a product with a urania-colored glaze. Uranium glass is pale green to yellow and often has strong fluorescent properties. Urania has also been used in formulations of enamel and porcelain. It is possible to determine with a Geiger counter
if a glaze or glass produced before 1958 contains urania.

Other uses

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.[citation needed]

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.[citation needed
]

Depleted uranium dioxide can be also used as a

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.[8]

The use of uranium dioxide as a material for

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.[citation needed
]

Uranium dioxide displays strong

antiferromagnetic state, observed at cryogenic temperatures below 30 kelvins. 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.[9] 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.[10]

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 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.[11]

The

dielectric constant of uranium dioxide is about 22, which is almost twice as high as of silicon (11.2) and GaAs (14.1). 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

Peltier elements
.

The

VLSI chips. 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.[citation needed
]

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[citation needed].

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

p-n-p transistor of UO2 were successfully manufactured in a laboratory.[12]

Toxicity

Uranium dioxide is known to be absorbed by phagocytosis in the lungs.[13]

See also

References

  1. S2CID 97183844
    .
  2. ^
    ISBN 978-0-618-94690-7
    .
  3. S2CID 118365366
    .
  4. ^
    S2CID 206561628
    .
  5. doi:10.2172/756904. Retrieved 2009-06-06. {{cite journal}}: Cite journal requires |journal= (help
    )
  6. doi:10.1016/S0925-8388(00)01222-6
    .
  7. ^ Örtel, Stefan. Uran in der Keramik. Geschichte - Technik - Hersteller.
  8. S2CID 4299921
    .
  9. PMID 28740123
    .
  10. S2CID 231812027
    .
  11. PMID 21668262
    .
  12. doi:10.1016/j.vacuum.2008.04.005
    .
  13. ISBN 0-8493-7302-6[page needed
    ]

Further reading

External links
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