Uranium hexafluoride

Uranium hexafluoride (UF₆), referred to as "hex" in the nuclear industry, 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", (NH₄)₂U₂O₇). Reduction with hydrogen gives UO₂, which is converted with hydrofluoric acid (HF) to uranium tetrafluoride, UF₄. Oxidation with fluorine finally yields UF₆.

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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 147 °F (64 °C, 337 K) and slightly higher than normal atmospheric pressure. Additionally, fluorine has only a single stable naturally occurring isotope, so isotopologues of UF₆ differ in their molecular weight based solely on the uranium isotope present.^[1]

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 material.

Storage in gas cylinders

About 95% of the depleted uranium produced to date is stored as uranium hexafluoride, (D)UF₆, in steel cylinders in open air yards close to enrichment plants. Each cylinder contains up to 12.7 tonnes (or 14 US tons) of solid 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.^[2][3] 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.^[4]

There have been several accidents involving uranium hexafluoride in the United States.^[5][6] The U.S. government has been converting DUF₆ to solid uranium oxides for disposal.^[7] Such disposal of the entire DUF₆ inventory could cost anywhere from 15 million to 450 million US dollars.^[8]

Chemistry

The solid state structure was reported by J.H. Levy, J.C Taylor and A.B Waugh.^[9] 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[UF₇]₂.5MeCN.^[10]

Polymeric uranium(VI) fluorides containing organic cations have been isolated and characterised by X-ray diffraction.^[11]

Other uranium fluorides

The pentafluoride of uranium (UF₅) and diuranium nonafluoride (U₂F₉) has been characterised by C.J. Howard, J.C Taylor and A.B. Waugh.^[12]

The trifluoride of uranium was characterised by J. Laveissiere.^[13]

The structure of UOF₄ was reported by J.H. Levy, J.C. Taylor, and P.W. Wilson.^[14]

See also

• Depleted uranium
• Uranium

References

Further reading

• Levy JH (1976). "Structure of fluorides. Part XII. Single-crystal neutron diffraction study of uranium hexafluoride at 293 K". J. Chem. Soc. Dalton Trans.: 219. doi:10.1039/DT9760000219.x (xstal structure)
• Olah GH, Welch J (1978). "Synthetic methods and reactions. 46. Oxidation of organic compounds with uranium hexafluoride in haloalkane solutions". J. Am. Chem. Soc. 100: 5396. doi:10.1021/ja00485a024. x (selective oxidant of CFCs)