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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
Melting point	64.8 °C (338.0 K)
Boiling point	56.5 °C (329.7 K) (sublimes)
Triple point	64°C (337 K), 150 kPa
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 offormation Δf''H''°solid	-2317 kJ/mol
Standard molar entropy ''S''°solid	228 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)

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

Contents

Application in the nuclear fuel cycle

Storage in gas cylinders

Chemistry

Other uranium fluorides

See also

External links

Notes

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

External links

★ National Pollutant Inventory - Fluoride and compounds fact sheet

Notes

1. http://www.usec.com/v2001_02/HTML/Aboutusec_enrichment.asp Uranium Enrichment and the Gaseous Diffusion Process
2. http://web.ead.anl.gov/uranium/faq/storage/faq16.cfm
3. [http://web.ead.anl.gov/uranium/documents/index.cfm
4. http://www.ieer.org/sdafiles/vol_5/5-2/deararj.html
5. http://web.ead.anl.gov/uranium/faq/health/faq30.cfm
6. Large & Associates, ''Uranium Hexafluoride (UF₆) Tailings, Characteristics,Transport and Storage at the Siberian Chemical Combine (Sibkhimkombinat) Tomsk'', November 2005
7. http://web.ead.anl.gov/uranium/faq/storage/faq22.cfm
8. http://web.ead.anl.gov/uranium/faq/mgmt/faq27.cfm
9. Neutron powder structural studies of UF₆, MoF₆ and WF₆ at 77 K, J.H. Levy, J.C Taylor and A.B Waugh, , , Journal of Fluorine Chemistry, 1983
10. The oxidising and fluoride ion acceptor properties of uranium hexafluoride in acetonitrile, Berry JA, Poole RT, Prescott A, Sharp DWA, Winfield JM, , , J. Chem. Soc. Dalton Trans., 1976 x
11. From Molecules to Frameworks: Variable Dimensionality in the UO₂(CH₃COO)₂·2H₂O/HF(aq)/Piperazine System. Syntheses, Structures, and Characterization of Zero-Dimensional (C₄N₂H₁₂)UO₂F₄·3H₂O, One-Dimensional (C₄N₂H₁₂)₂U₂F₁₂·H2O, Two-Dimensional (C₄N₂H₁₂)₂(U₂O₄F₅)₄·11H₂O, and Three-Dimensional (C₄N₂H₁₂)U₂O₄F₆, Walker SM, Halasyamani PS, Allen S, O'Hare D, , , J. Am. Chem. Soc., 1999 x
12. Crystallographic parameters in α-UF₅ and U₂F₉ by multiphase refinement of high-resolution neutron powder data, Howard CJ, Taylor JC, Waugh AB, , , Journal of Solid State Chemistry, 1982 x
13. , Laveissiere J, , , Bulletin de la Societe Francaise de Mineralogie et de Cristallographie, 1967
14. Structure of fluorides .17. NEUTRON-DIFFRACTION STUDY OF ALPHA-URANIUM OXIDE TETRAFLUORIDE, Levy JH, Taylor JC, Wilson PW, , , Journal of Inorganic and Nuclear Chemistry, 1977

References

★ Structure of fluorides. Part XII. Single-crystal neutron diffraction study of uranium hexafluoride at 293 K, Levy JH, , , J. Chem. Soc. Dalton Trans., 1976 x (xstal structure)

★ Synthetic methods and reactions. 46. Oxidation of organic compounds with uranium hexafluoride in haloalkane solutions, Olah GH, Welch J, , , J. Am. Chem. Soc., 1978 x (selective oxidant of CFCs)