Uranium hexafluoride

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Uranium hexafluoride
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
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]] Î"fH°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 s	Uranium(VI) chloride
Other s	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 â€" U₃O₈, or "yellowcake" â€" 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.

Application in the nuclear fuel cycle

Phase diagram of UF6.

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.

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.

Storage in gas cylinders

About 95% of the depleted uranium produced to date is stored as uranium hexafluoride, (D)UF₆, in steel gas cylinders in open air yards close to enrichment plants. Each cylinder contains up to 12.7 tonnes (or 14 US tons) of UF₆. In the U.S. alone, 560,000 tonnes of depleted UF₆ had accumulated by 1993. In 2005, 686,500 tonnes in 57,122 storage cylinders were located near Portsmouth, Ohio, Oak Ridge, Tennessee, and Paducah, Kentucky. http://web.ead.anl.gov/uranium/faq/storage/faq16.cfm [http://web.ead.anl.gov/uranium/documents/index.cfm The long-term storage of DUF₆ presents environmental, health, and safety risks because of its chemical instability. When UF₆ is exposed to moist air, it reacts with the water in the air to produce UO₂F₂ (uranyl fluoride) and HF (hydrogen fluoride) both of which are highly soluble and toxic. Storage cylinders must be regularly inspected for signs of corrosion and leaks. The estimated life time of the steel cylinders is measured in decades. http://www.ieer.org/sdafiles/vol_5/5-2/deararj.html

There have been several accidents involving uranium hexafluoride in the United States. http://web.ead.anl.gov/uranium/faq/health/faq30.cfm The U.S. government has been converting DUF₆ to solid uranium oxides for disposal. http://web.ead.anl.gov/uranium/faq/storage/faq22.cfm Such disposal of the entire DUF₆ inventory could cost anywhere from 15 million to 450 million US dollars. http://web.ead.anl.gov/uranium/faq/mgmt/faq27.cfm

Chemistry

The solid state structure was reported by J.H. Levy, J.C Taylor and A.B Waugh. In this paper neutron diffraction was used to determine the structures of UF₆, MoF₆ and WF₆ at 77K.Image:UF6solid.jpg|This is a simple mononuclear moleculeImage:Uranium-hexafluoride-crystal-3D-vdW.png|The crystal structure of uranium hexafluoride

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[UF₇]₂.5MeCN. x

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

Other uranium fluorides

The pentafluoride of uranium (UF₅) and diuranium nonofluoride (U₂F₉) has been characterised by C.J. Howard, J.C Taylor and A.B. Waugh. x

It is clear that the solid is a 1D coordination polymer

This is U2F9 which is a coordination polymer

This is UF4 which is a coordination polymer

The trifluoride of uranium was characterised by J. Laveissiere.

This is UF3 which is a coordination polymer

The structure of UOF₄ was reported by J.H. Levy, J.C. Taylor, and P.W. Wilson.

See also

*Depleted uranium
*Uranium

External links

*National Pollutant Inventory - Fluoride and compounds fact sheet

Notes

References

*x (xstal structure)
* x (selective oxidant of CFCs)