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.

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.

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 till now 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. ^*

There have been several accidents involving uranium hexafluoride in the United States. The U.S. government has been converting DUF₆ to solid uranium oxides for disposal. US dollars. [http://web.ead.anl.gov/uranium/faq/mgmt/faq27.cfm" target="_blank" >^*

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.

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^*₂.5MeCN.

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

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.

The trifluoride of uranium was characterised by J. Laveissiere.

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

References

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

Uranium compounds | Nuclear materials | Fluorides | Metal halides

Uranhexafluorid | Uraniumhexafluoride | 六フッ化ウラン | Фторид урана(VI)