Uranium hexafluoride

Chembox new
Name = Uranium hexafluoride
ImageFile = Uranium-hexafluoride-2D.png ImageSize = 125px
IUPACName = Uranium hexafluoride
Uranium(VI) fluoride
Section1 = Chembox Identifiers
CASNo = 7783-81-5
Density = 5.09 g/cm³, solid
Solubility = Decomposes
MeltingPt = 64.8 °C (338.0 K)
BoilingPt = 56.5 °C (329.7 K) (sublimes)
Formula = UF₆
MolarMass = 352.02 g/mol
Appearance = colorless solid
Section3 = Chembox Structure
MolShape = Octahedral
CrystalStruct = Hexagonal close packed (HCP)
Dipole = zero
Section4 = Chembox Thermochemistry
DeltaHf = -2317 kJ/mol
Entropy = 228 J.K⁻¹.mol⁻¹
Section7 = Chembox Hazards
EUClass =
Section8 = Chembox Related
OtherAnions = Uranium(VI) chloride
OtherCations = Thorium(IV) fluoride
Protactinium(V) fluoride
Neptunium(VI) fluoride
Plutonium(VI) fluoride
OtherCpds = Uranium trifluoride
Uranium tetrafluoride
Uranium pentafluoride

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 aluminium, 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.

Application in the nuclear fuel cycle

[
Phase diagram of UF₆.] UF₆ 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. [cite web | title = Uranium Enrichment and the Gaseous Diffusion Process | publisher = USEC Inc | url = http://www.usec.com/v2001_02/HTML/Aboutusec_enrichment.asp | accessdate = 2007-09-24]

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.

torage in gas cylinders

About 95% of the depleted uranium produced to date is stored as uranium hexafluoride, DUF₆, 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. [cite web | work = Depleted UF₆ FAQs | title = How much depleted uranium hexafluoride is stored in the United States? | url = http://web.ead.anl.gov/uranium/faq/health/faq16.cfm | publisher = Argonne National Laboratory] [ [http://web.ead.anl.gov/uranium/documents/index.cfm Documents ] ] 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. [cite web | publisher = Institute for Energy and Environmental Research | date = December 1997 | title = What is DUF₆? Is it dangerous and what should we do with it? | url = http://www.ieer.org/sdafiles/vol_5/5-2/deararj.html | date = 2007-09-24]

There have been several accidents involving uranium hexafluoride in the United States. [cite web | work = Depleted UF₆ FAQs | title = Have there been accidents involving uranium hexafluoride? | url = http://web.ead.anl.gov/uranium/faq/health/faq30.cfm | publisher = Argonne National Laboratory] [cite web | title = Uranium Hexafluoride (UF₆) Tailings: Characteristics, Transport and Storage at the Siberian Chemical Combine (Sibkhimkombinat) Tomsk | format = briefing note | publisher = Large and Associates | date = 5 November 2005 | url = http://www.largeassociates.com/R3139-a1%20frontispiece.pdf | accessdate = 2007-09-24] The U.S. government has been converting DUF₆ to solid uranium oxides for disposal. [cite web | work = Depleted UF₆ FAQs | title = What is going to happen to the uranium hexafluoride stored in the United States? | url = http://web.ead.anl.gov/uranium/faq/health/faq22.cfm | publisher = Argonne National Laboratory] Such disposal of the entire DUF₆ inventory could cost anywhere from $15 million to $450 million. [cite web | work = Depleted UF₆ FAQs | title = Are there any currently-operating disposal facilities that can accept all of the depleted uranium oxide that would be generated from conversion of DOE's depleted UF6 inventory?| url = http://web.ead.anl.gov/uranium/faq/health/faq27.cfm | publisher = Argonne National Laboratory]

Chemistry

The solid state structure was reported by J.H. Levy, J.C Taylor and A.B Waugh. [cite journal| author= J.H. Levy, J.C Taylor and A.B Waugh | title=Neutron powder structural studies of UF₆, MoF₆ and WF₆ at 77 K| journal= Journal of Fluorine Chemistry| year= 1983| pages=29–36| volume=23|doi=10.1016/S0022-1139(00)81276-2 ] 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. [cite journal| author= Berry JA, Poole RT, Prescott A, Sharp DWA, Winfield JM| title= The oxidising and fluoride ion acceptor properties of uranium hexafluoride in acetonitrile| journal= J. Chem. Soc. Dalton Trans.| year= 1976| pages=272|doi=10.1039/DT9760000272 x]

Polymeric uranium(VI) fluorides containing organic cations have been isolated and characterised by X-ray diffraction. [cite journal| author= Walker SM, Halasyamani PS, Allen S, O'Hare D| title= 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| journal=J. Am. Chem. Soc.| year= 1999| pages= 10513| volume= 121|doi=10.1021/ja992145f x]

At room pressure, it sublimes at 56.5 C. [http://nuclearweaponarchive.org/Library/Glossary] The triple point is at 64 oC. [ [http://web.ead.anl.gov/uranium/guide/ucompound/propertiesu/hexafluoride.cfm Uranium Hexafluoride: Source: Appendix A of the PEIS (DOE/EIS-0269): Physical Properties ] ]

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. [cite journal| author= Howard CJ, Taylor JC, Waugh AB | title= Crystallographic parameters in α-UF₅ and U₂F₉ by multiphase refinement of high-resolution neutron powder data| journal=Journal of Solid State Chemistry | year=1982| pages=396–398| volume=45|doi=10.1016/0022-4596(82)90185-2 x]

The trifluoride of uranium was characterised by J. Laveissiere. [cite journal| author= Laveissiere J| title=| journal= Bulletin de la Societe Francaise de Mineralogie et de Cristallographie | year= 1967| pages= 304–307| volume=90] The structure of UOF₄ was reported by J.H. Levy, J.C. Taylor, and P.W. Wilson. [cite journal| author= Levy JH, Taylor JC, Wilson PW | title= Structure of fluorides .17. NEUTRON-DIFFRACTION STUDY OF ALPHA-URANIUM OXIDE TETRAFLUORIDE| journal=Journal of Inorganic and Nuclear Chemistry| year=1977| pages=1989–1991| volume=39| doi= 10.1016/0022-1902(77)80531-9]

ee also

*Depleted uranium
*Uranium

References

Further reading

*cite journal| author= Levy JH | title= Structure of fluorides. Part XII. Single-crystal neutron diffraction study of uranium hexafluoride at 293 K| journal= J. Chem. Soc. Dalton Trans.| year= 1976| pages= 219| doi=10.1039/DT9760000219x (xstal structure)
*cite journal| author= Olah GH, Welch J | title= Synthetic methods and reactions. 46. Oxidation of organic compounds with uranium hexafluoride in haloalkane solutions| journal=J. Am. Chem. Soc. | year= 1978| pages= 5396| volume= 100|doi=10.1021/ja00485a024 x (selective oxidant of CFCs)