Uranium hexafluoride

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Uranium hexafluoride
Image:Uranium hexafluoride crystals sealed in an ampoule.jpg
General
Systematic name	Uranium hexafluoride Uranium(VI) fluoride
Molecular formula	UF6
Molar mass	352.02 g/mol
Appearance	colorless solid
CAS number	[7783-81-5]
Properties
Density and phase	5.09 g/cm3, solid
Solubility in water	Decomposes
Melting point	64.8 °C (338.0 K)
Boiling point	56.5 °C (329.7 K) (sublimes)
Vapor pressure	16.7 kPa at 25°C
Structure
Molecular shape	Octahedral
Coordination geometry	Pseudo-octahedral
Crystal structure	Hexagonal close packed (HCP)
Dipole moment	zero
Thermodynamic data
Standard enthalpy of formation ΔfH°solid	? kJ/mol
Standard molar entropy S°solid	? J.K−1.mol−1
Hazards
RADIOACTIVE
EU classification	not listed
NFPA 704
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Uranium(VI) chloride
Other cations	Thorium(IV) fluoride Protactinium(V) fluoride Neptunium(VI) fluoride Plutonium(VI) fluoride
Related compounds	Uranium trifluoride Uranium tetrafluoride Uranium pentafluoride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Uranium hexafluoride, or UF₆, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure (STP), is highly toxic, reacts violently with water and is corrosive to most metals. It reacts mildly with aluminum, forming a thin surface layer of AlF₃ that resists further reaction.

Milled uranium ore — U₃O₈, or "yellowcake" — is dissolved in nitric acid, yielding a solution of uranyl nitrate UO₂(NO₃)₂. Pure uranyl nitrate is obtained by solvent extraction, then treated with ammonia to produce ammonium diuranate (ADU). Reduction with hydrogen gives UO₂, which is converted with hydrofluoric acid (HF) to UF₄. Oxidation with fluorine finally yields UF₆.

Contents 1 Application in the nuclear fuel cycle 2 Chemistry 3 Other uranium fluorides 4 See also 5 External links 6 References

Application in the nuclear fuel cycle

It is used in both of the main uranium enrichment methods, gaseous diffusion and the gas centrifuge method, because it has a triple point at 64 °C (147 °F, 337 K) and slightly higher than normal atmospheric pressure. Additionally, fluorine has only a single stable naturally occurring isotope, so isotopomeres of UF₆ differ in their molecular weight based solely on the uranium isotope present.

It is important to note that all the other uranium fluorides are involatile solids which are coordination polymers.

Gaseous diffusion requires ca. 60 times as much energy as the gas centrifuge process; even so, this is just 4% of the energy that can be produced by the resulting enriched uranium. Image:Uranium hexafluoride phase diagram.gif

In addition to its use in enrichment, uranium hexafluoride has been used in an advanced reprocessing method which was developed in the Czech Republic. In this process used oxide nuclear fuel is treated with fluorine gas to form a mixture of fluorides. This is then distilled to separate the different classes of metals.

Chemistry

The solid state strucutre was reported by J.H. Levy, J.C Taylor and A.B Waugh.Template:Ref In this paper neutron diffraction was used to determine the structures of UF_{6, MoF6 and WF6 at 77K.}

Image:UF6solid.jpg

It has been shown that uranium hexafluoride is an oxidant and a lewis acid which is able to bind to fluoride, for instance the reaction of copper fluoride with uranium hexafluoride in acetonitrile is reported to form Cu[UF_{7]2.5MeCN.Template:Ref}

It is interesting to note that polymeric uranium(VI) fluorides containing organic cations have been isolated and characterised by X-ray diffraction.Template:Ref

Other uranium fluorides

The pentafluoride of uranium (UF_{5) and diuranium nonofluoride (U2F9) has been characterised by C.J. Howard, J.C Taylor and A.B. Waugh.Template:Ref}

Image:UF5solid.jpg

Image:U2F9solid.jpg

Image:UF4solid.jpg

The trifluoride of uranium was characterised by J. Laveissiere.Template:Ref

Image:UF3solid.jpg

The structure of UOF_{4 was reported by J.H. Levy, J.C. Taylor, and P.W. Wilson.Template:Ref}

See also

• Depleted uranium
• Uranium

External links

• National Pollutant Inventory - Fluoride and compounds fact sheet

References

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• Template:Cite journalDOI (xstal structure)
• Template:Cite journal DOI (selective oxidant of CFCs)de:Uranhexafluorid

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