Uranium hexafluoride
374388
224845112
2008-07-10T17:54:02Z
Nergaal
4761569
/* Chemistry */
{{Chembox new
| Name = Uranium hexafluoride
| ImageFile = Uranium-hexafluoride-2D.png
| ImageSize = 125px
| IUPACName = Uranium hexafluoride<br />Uranium(VI) fluoride
| Section1 = {{Chembox Identifiers
| CASNo = 7783-81-5
| Density = 5.09 g/cm<sup>3</sup>, solid
| Solubility = Decomposes
| MeltingPt = 64.8 °C (338.0 K)
| BoilingPt = 56.5 °C (329.7 K) (sublimes)
| Formula = UF<sub>6</sub>
| MolarMass = 352.02 g/mol
| Appearance = colorless solid
}}
| Section3 = {{Chembox Structure
| MolShape = [[Octahedral molecular geometry|Octahedral]]
| CrystalStruct = [[Hexagonal (crystal system)|Hexagonal]] [[close-packing|close packed]] (HCP)
| Dipole = zero
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = -2317 kJ/mol
| Entropy = 228 J.K<sup>−1</sup>.mol<sup>−1</sup>
}}
| Section7 = {{Chembox Hazards
| EUClass =
}}
| Section8 = {{Chembox Related
| OtherAnions = [[Uranium(VI) chloride]]
| OtherCations = [[Thorium(IV) fluoride]]<br />[[Protactinium(V) fluoride]]<br />[[Neptunium(VI) fluoride]]<br />[[Plutonium(VI) fluoride]]
| OtherCpds = [[Uranium trifluoride]]<br />[[Uranium tetrafluoride]]<br />[[Uranium pentafluoride]]
}}
}}
'''Uranium hexafluoride''' (UF<sub>6</sub>), referred to as "hex" in the nuclear industry, is a compound used in the [[uranium]] [[Isotope separation#Centrifugal Force|enrichment]] process that produces fuel for [[nuclear reactor]]s and [[nuclear weapon]]s. 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 AlF<sub>3</sub> that resists further reaction.
Milled uranium ore — U<sub>3</sub>O<sub>8</sub>, or "[[yellowcake]]" — is dissolved in [[nitric acid]], yielding a solution of [[uranyl nitrate]] UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>. Pure uranyl nitrate is obtained by [[solvent extraction]], then treated with [[ammonia]] to produce [[ammonium diuranate]] ("ADU", (NH<sub>4</sub>)<sub>2</sub>U<sub>2</sub>O<sub>7</sub>). Reduction with [[hydrogen]] gives UO<sub>2</sub>, which is converted with [[hydrofluoric acid]] (HF) to [[uranium tetrafluoride]], UF<sub>4</sub>. Oxidation with [[fluorine]] finally yields UF<sub>6</sub>.
[[Image:Uranium hexafluoride crystals sealed in an ampoule.jpg|thumb|left|UF<sub>6</sub> in a glass [[ampoule]].]]
==Application in the nuclear fuel cycle==
[[Image:Uranium hexafluoride phase diagram.gif|thumb|left|250px|[[Phase diagram]] of UF<sub>6</sub>.]]
<!-- please explain why its triple point makes it useful in these methods. -->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 [[isotopologue]]s of UF<sub>6</sub> differ in their molecular weight based solely on the uranium [[isotope]] present.<ref>{{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}}</ref>
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==
[[Image:DUF6 storage yard far.jpg]]
About 95% of the [[depleted uranium]] produced to date is stored as uranium hexafluoride, DUF<sub>6</sub>, 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 UF<sub>6</sub>. In the U.S. alone, 560,000 tonnes of depleted UF<sub>6</sub> 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.<ref>{{cite web | work = Depleted UF<sub>6</sub> 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]]}}</ref><ref>[http://web.ead.anl.gov/uranium/documents/index.cfm Documents<!-- Bot generated title -->]</ref> The long-term storage of DUF<sub>6</sub> presents environmental, health, and safety risks because of its chemical instability. When UF<sub>6</sub> is exposed to moist air, it reacts with the water in the air to produce UO<sub>2</sub>F<sub>2</sub> ([[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.<ref>{{cite web | publisher = Institute for Energy and Environmental Research | date = December 1997 | title = What is DUF<sub>6</sub>? 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}}</ref>
[[Image:DUF6 cylinder leak.gif|255px|right]]
There have been several accidents involving uranium hexafluoride in the United States.<ref>{{cite web | work = Depleted UF<sub>6</sub> FAQs | title = Have there been accidents involving uranium hexafluoride? | url = http://web.ead.anl.gov/uranium/faq/health/faq30.cfm | publisher = [[Argonne National Laboratory]]}}</ref><ref>{{cite web | title = Uranium Hexafluoride (UF<sub>6</sub>) 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}}</ref> The U.S. government has been converting DUF<sub>6</sub> to solid uranium oxides for disposal.<ref>{{cite web | work = Depleted UF<sub>6</sub> 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]]}}</ref> Such disposal of the entire DUF<sub>6</sub> inventory could cost anywhere from [[United States dollar|$]]15 million to $450 million.<ref>{{cite web | work = Depleted UF<sub>6</sub> 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]]}}</ref>
==Chemistry==
The solid state structure was reported by J.H. Levy, J.C Taylor and A.B Waugh.<ref>{{cite journal| author= J.H. Levy, J.C Taylor and A.B Waugh | title=Neutron powder structural studies of UF<sub>6</sub>, MoF<sub>6</sub> and WF<sub>6</sub> at 77 K| journal= Journal of Fluorine Chemistry| year= 1983| pages=29–36| volume=23|doi=10.1016/S0022-1139(00)81276-2 }}</ref> In this paper [[neutron diffraction]] was used to determine the structures of UF<sub>6</sub>, MoF<sub>6</sub> and WF<sub>6</sub> at 77K.
<center><gallery>
Image:UF6solid.jpg|This is a simple mononuclear molecule
Image:Uranium-hexafluoride-crystal-3D-vdW.png|The crystal structure of uranium hexafluoride
</gallery></center>
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[UF<sub>7</sub>]<sub>2</sub>.5MeCN.<ref>{{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</ref>
[[Polymer]]ic uranium(VI) fluorides containing organic cations have been isolated and characterised by X-ray diffraction.<ref>{{cite journal| author= Walker SM, Halasyamani PS, Allen S, O'Hare D| title= From Molecules to Frameworks: Variable Dimensionality in the UO<sub>2</sub>(CH<sub>3</sub>COO)<sub>2</sub>·2H<sub>2</sub>O/HF(aq)/Piperazine System. Syntheses, Structures, and Characterization of Zero-Dimensional (C<sub>4</sub>N<sub>2</sub>H<sub>12</sub>)UO<sub>2</sub>F<sub>4</sub>·3H<sub>2</sub>O, One-Dimensional (C<sub>4</sub>N<sub>2</sub>H<sub>12</sub>)<sub>2</sub>U<sub>2</sub>F<sub>12</sub>·H2</sub>O, Two-Dimensional (C<sub>4</sub>N<sub>2</sub>H<sub>12</sub>)<sub>2</sub>(U<sub>2</sub>O<sub>4</sub>F<sub>5</sub>)<sub>4</sub>·11H<sub>2</sub>O, and Three-Dimensional (C<sub>4</sub>N<sub>2</sub>H<sub>12</sub>)U<sub>2</sub>O<sub>4</sub>F<sub>6</sub>| journal=[[J. Am. Chem. Soc.]]| year= 1999| pages= 10513| volume= 121|doi=10.1021/ja992145f}} x</ref>
At room pressure, it sublimes at 56.5 C.<ref>http://nuclearweaponarchive.org/Library/Glossary</ref> The triple point is at 64 oC.<ref>http://web.ead.anl.gov/uranium/guide/ucompound/propertiesu/hexafluoride.cfm</ref>
==Other uranium fluorides==
The pentafluoride of uranium (UF<sub>5</sub>) and diuranium nonafluoride (U<sub>2</sub>F<sub>9</sub>) has been characterised by C.J. Howard, J.C Taylor and A.B. Waugh.<ref>{{cite journal| author= Howard CJ, Taylor JC, Waugh AB | title= Crystallographic parameters in α-UF<sub>5</sub> and U<sub>2</sub>F<sub>9</sub> 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</ref>
[[Image:UF5solid.jpg|400px|It is clear that the solid is a 1D coordination polymer]][[Image:U2F9solid.jpg|350px|This is U2F9 which is a coordination polymer]][[Image:UF4solid.jpg|350px|This is UF4 which is a coordination polymer]][[Image:UF3solid.jpg|350px|This is UF3 which is a coordination polymer]]
The trifluoride of uranium was characterised by J. Laveissiere.<ref>{{cite journal| author= Laveissiere J| title=| journal= Bulletin de la Societe Francaise de Mineralogie et de Cristallographie | year= 1967| pages= 304–307| volume=90}}</ref> The structure of UOF<sub>4</sub> was reported by J.H. Levy, J.C. Taylor, and P.W. Wilson.<ref>{{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}}</ref>
==See also==
*[[Depleted uranium]]
*[[Uranium]]
==References==
{{reflist|2}}
==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/DT9760000219}}x (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)
[[Category:Uranium compounds]]
[[Category:Nuclear materials]]
[[Category:Fluorides]]
[[Category:Metal halides]]
[[de:Uranhexafluorid]]
[[es:Hexafluoruro de uranio]]
[[fa:هگزافلوراید اورانیوم]]
[[fr:Hexafluorure d'uranium]]
[[it:Esafluoruro di uranio]]
[[nl:Uraniumhexafluoride]]
[[ja:六フッ化ウラン]]
[[pl:Fluorek uranu(VI)]]
[[pt:Hexafluoreto de urânio]]
[[ru:Фторид урана(VI)]]
[[fi:Uraaniheksafluoridi]]
[[sv:Uranhexafluorid]]
[[zh:六氟化鈾]]