View source for Uranium hexafluoride

{{Chembox
|Watchedfields = changed
|verifiedrevid = 476998888
|Name = Uranium hexafluoride
|ImageFile = Uranium hexafluoride dimensions.svg
|ImageSize = 144px
|ImageClass = skin-invert
|ImageFile1 = Uranium hexafluoride crystals sealed in an ampoule.jpg
|ImageClass1 = bg-transparent
|ImageSize1 = 150px
|ImageFile2 = 
|ImageClass2 = bg-transparent
|ImageSize2 = 150px
|IUPACName = Uranium hexafluoride<br/>Uranium(VI) fluoride
|Section1={{Chembox Identifiers
|Abbreviations = hex
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|ChemSpiderID = 22966
|PubChem = 24560
|InChI = 1/6FH.U/h6*1H;/q;;;;;;+6/p-6/rF6U/c1-7(2,3,4,5)6
|InChIKey = SANRKQGLYCLAFE-IIYYNVFAAT
|ChEBI_Ref = {{ebicite|correct|EBI}}
|ChEBI = 30235
|EINECS = 232-028-6
|Gmelin = 2923
|SMILES = F[U](F)(F)(F)(F)F
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|StdInChI = 1S/6FH.U/h6*1H;/q;;;;;;+6/p-6
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
|StdInChIKey = SANRKQGLYCLAFE-UHFFFAOYSA-H
|CASNo = 7783-81-5
|CASNo_Ref = {{cascite|correct|CAS}}
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = N06GJ1D94J
|RTECS = YR4720000
|UNNumber = 2978 (<1% {{chem2|^{235}U}})<br/>2977 (>1% {{chem2|^{235}U}})}}
|Section2={{Chembox Properties
|Formula = {{chem2|UF6}}
|MolarMass = {{nowrap|352.02 g/mol}}
|Appearance = Colorless solid
|Density = 5.09{{nbsp}}g/cm<sup>3</sup>, solid
|Solubility = Hydrolyzes
|SolubleOther = {{ubl|Soluble in [[chloroform]], [[carbon tetrachloride|{{chem2|CCl4}}]], liquid [[chlorine]], and [[bromine]]|Dissolves in [[nitrobenzene]];
151{{nbsp}}kPa<ref>{{cite web |url=http://www.ibilabs.com/Uranium%20Hexafluoride.htm |title=Uranium Hexafluoride |access-date=2013-08-08 |url-status=dead |archive-url=https://web.archive.org/web/20130916221434/http://ibilabs.com/Uranium%20Hexafluoride.htm |archive-date=2013-09-16}}</ref>}}
|BoilingPtC = 56.5
|BoilingPt_notes = (sublimes, at atmospheric pressure)}}
|Section3={{Chembox Structure
|CrystalStruct = [[Orthorhombic]], [[Pearson symbol|oP28]]
|SpaceGroup = Pnma, No. 62
|Coordination = Octahedral (''O<sub>h</sub>'')
|Dipole = 0}}
|Section4={{Chembox Thermochemistry
|DeltaHf = {{ubl
|Solid, −2197.7±1.8{{nbsp}}[[Joule|kJ]]·mol<sup>−1</sup><ref name="Enthalpie">{{cite journal |last1=Johnson |first1=Gerald K. |year=1979 |title=The Enthalpy of Formation of Uranium Hexafluoride |journal=[[The Journal of Chemical Thermodynamics]] |volume=11 |issue=5 |pages=483–490 |doi=10.1016/0021-9614(79)90126-5}}</ref>
|Gaseous, −2148.1±1.8{{nbsp}}kJ·mol<sup>−1</sup><ref name="Enthalpie"/>
}}
|Entropy = {{ubl
|Solid, 227.8±1.3{{nbsp}}J·K<sup>−1</sup>·mol<sup>−1</sup><ref name="Enthalpie"/>
|Gaseous, 377.8±1.3{{nbsp}}J·K<sup>−1</sup>·mol<sup>−1</sup><ref name="Enthalpie"/>
}}}}
|Section7={{Chembox Hazards
|MainHazards = Toxic, corrosive, radioactive<ref>Uranium(VI) fluoride</ref>
|ExternalSDS = [https://www.inchem.org/documents/icsc/icsc/eics1250.htm ICSC 1250]
|GHSPictograms = {{GHS skull and crossbones}}{{GHS health hazard}}{{GHS environment}}
|GHSSignalWord = Danger
|HPhrases = {{H-phrases|300|330|373|411}}
|PPhrases = {{P-phrases|}}
|NFPA-H = 4
|NFPA-F = 0
|NFPA-R = 2
|NFPA-S = W+OX
|FlashPt = Non-flammable
}}
|Section9={{Chembox Related
|OtherAnions = [[Uranium hexachloride]]
|OtherCations = {{ubl|[[Neptunium hexafluoride]]|[[Plutonium hexafluoride]]|[[Americium hexafluoride]]|[[Curium hexafluoride]]|[[Tungsten hexafluoride]]}}
|OtherFunction = {{ubl|[[Uranium(III) fluoride]]|[[Uranium(IV) fluoride]]|[[Uranium(V) fluoride]]}}
|OtherFunction_label = uranium fluorides
}}
}}

'''Uranium hexafluoride''', sometimes called '''hex''', is the [[inorganic compound]] with the formula {{chem2|UF6|auto=1}}. Uranium hexafluoride is a volatile, white solid that is used in [[uranium enrichment|enriching uranium]] for [[nuclear reactor]]s and [[nuclear weapon]]s.<ref name=Ull />

==Preparation==
[[Uranium dioxide]] is converted with [[hydrofluoric acid]] (HF) to [[uranium tetrafluoride]]:<ref name=Ull>{{cite book |doi=10.1002/14356007.a27_281.pub2 |chapter=Uranium, Uranium Alloys, and Uranium Compounds |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2007 |last1=Peehs |first1=Martin |last2=Walter |first2=Thomas |last3=Walter |first3=Sabine |last4=Zemek |first4=Martin |isbn=978-3-527-30385-4 }}</ref>
:{{chem2|UO2  +  4 HF  ->  UF4  + 2 H2O}}
The resulting {{chem2|UF4}} is subsequently oxidized with [[fluorine]] to give the hexafluoride:
:{{chem2|UF4  +  F2  ->  UF6}}
In samples contaminated with [[uranium trioxide]], [[uranyl fluoride]],  an [[oxyfluoride]] compound is produced in the HF step:
:{{chem2|UO3  +  2 HF  ->  UO2F2  +  H2O}}
which can be fluorinated to produce the same product, uranium hexafluoride.
:{{chem2|UO2F2  + 2 F2  ->  UF6  +  O2}}
The fluorination steps in both reactions above are highly [[exothermic]].

==Properties==

=== Physical properties ===
At [[atmospheric pressure]], {{chem2|UF6}} [[sublimation (phase transition)|sublimes]] at 56.5&nbsp;°C.<ref>{{cite journal |last1=Brickwedde |first1=Ferdinand G. |last2=Hoge |first2=Harold J. |last3=Scott |first3=Russell B. |year=1948 |title=The Low Temperature Heat Capacities, Enthalpies, and Entropies of UF<sub>4</sub> and UF<sub>6</sub> |journal=[[J. Chem. Phys.]] |volume=16 |issue=5 |pages=429–436 |doi=10.1063/1.1746914 |bibcode=1948JChPh..16..429B|doi-access=free}}</ref>

The solid-state structure was determined by [[neutron diffraction]] at 77 K and 293 K.<ref>{{cite journal |author1=J. H. Levy |author2=John C. Taylor |author3=Paul W. Wilson |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=219–224 |doi=10.1039/DT9760000219 |issue=3}}</ref><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><ref name="UF6_1973">J. C. Taylor, P. W. Wilson, J. W. Kelly: „The structures of fluorides. I. Deviations from ideal symmetry in the structure of crystalline UF<sub>6</sub>: a neutron diffraction analysis", ''[[Acta Crystallogr.]]'', '''1973''', ''B29'', p.&nbsp;7–12; {{doi|10.1107/S0567740873001895}}.</ref>

:[[Image:Uranium-hexafluoride-unit-cell-3D-balls.png|thumb|left|Packing of UF<sub>6</sub> in its unit cell.<ref name="UF6_1968">{{cite journal |last1=Kimura |first1=Masao |last2=Schomaker |first2=Werner |last3=Smith |first3=Darwin W. |last4=Bernard |year=1968 |title=Electron-Diffraction Investigation of the Hexafluorides of Tungsten, Osmium, Iridium, Uranium, Neptunium, and Plutonium |url=https://authors.library.caltech.edu/71173/ |journal=[[J. Chem. Phys.]] |volume=48 |issue=8 |pages=4001–4012 |doi=10.1063/1.1669727 |bibcode=1968JChPh..48.4001K |access-date=2020-10-10 |archive-date=2023-01-11 |archive-url=https://web.archive.org/web/20230111083844/https://authors.library.caltech.edu/71173/ |url-status=dead }}</ref>]]

===Chemical properties===
UF<sub>6</sub> [[hydrolysis|reacts with water]], releasing [[hydrofluoric acid]]. The compound reacts with [[aluminium]], forming a surface layer of [[aluminium fluoride|{{chem2|AlF3}}]] that resists any further reaction from the compound. 

Uranium hexafluoride is a mild [[oxidant]].<ref>{{cite journal |author1=G. H. Olah |author2=J. Welch |title=Synthetic methods and reactions. 46. Oxidation of organic compounds with uranium hexafluoride in haloalkane solutions |journal=[[J. Am. Chem. Soc.]] |year=1978 |pages=5396–5402 |volume=100 |issue=17 |doi=10.1021/ja00485a024}}</ref> It is a [[Lewis acids and bases|Lewis acid]] as evidenced by its binding to form heptafluorouranate(VI), {{chem2|[UF7]-}}.<ref>{{cite journal |author1=J. A. Berry |author2=R. T. Poole |author3=A. Prescott |author4=D. W. A. Sharp |author5=J. M. Winfield |title=The oxidising and fluoride ion acceptor properties of uranium hexafluoride in acetonitrile |journal=J. Chem. Soc., Dalton Trans. |year=1976 |pages=272–274 |doi=10.1039/DT9760000272 |issue=3}}</ref>

[[Polymer]]ic uranium(VI) fluorides containing organic cations have been isolated and characterized by X-ray diffraction.<ref>{{cite journal |author1=S. M. Walker |author2=P. S. Halasyamani |author3=S. Allen |author4=D. O'Hare |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>·H<sub>2</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=10513–10521 |volume=121 |issue=45 |doi=10.1021/ja992145f}}</ref>

==Application in the fuel cycle==
[[File:Uranium hexafluoride phase diagram.gif|thumb|left|250px|[[Phase diagram]] of {{chem2|UF6}}]]
As one of the most [[volatility (chemistry)|volatile]] compounds of uranium, uranium hexafluoride is relatively convenient to process and is used in both of the main [[uranium enrichment]] methods, namely [[gaseous diffusion]] and the [[gas centrifuge]] method. Since the [[triple point]] of {{chem2|UF6}}, which is 64 °C (147 °F; 337 K) and 152 kPa (22 psi; 1.5 atm),<ref>{{cite web |title=Uranium Hexafluoride: Source: Appendix A of the PEIS (DOE/EIS-0269): Physical Properties |url=https://web.evs.anl.gov/uranium/guide/ucompound/propertiesu/hexafluoride.cfm |access-date=2022-08-18 |website=web.evs.anl.gov}}</ref> is close to ambient conditions, phase transitions can be achieved with little [[work (thermodynamics)|thermodynamic work]].

Fluorine has only a single naturally occurring stable isotope, so [[isotopologue]]s of {{chem2|UF6}} 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 |access-date=2007-09-24 |url-status=dead |archive-url=https://web.archive.org/web/20071019081831/http://www.usec.com/v2001_02/HTML/Aboutusec_enrichment.asp |archive-date=2007-10-19}}</ref> This difference is the basis for the physical separation of isotopes in enrichment.

All the other uranium fluorides are nonvolatile solids that are [[coordination polymer]]s.

The conversion factor for the {{chem2|^{238}U}} isotopologue of {{chem2|UF6}} ("hex") to "U mass" is 0.676.<ref>{{cite web
|website=TranslatorsCafé
|date=1 February 2021
|title=Unit converter molar mass calculator
|url=https://www.translatorscafe.com/unit-converter/en-US/molar-mass/?q=UF6
|publisher=ANVICA Software Development
|location=Mississauga, Ontario, Canada
}}</ref>

Gaseous diffusion requires about 60 times as much energy as the gas centrifuge process: gaseous diffusion-produced nuclear fuel produces 25 times more energy than is used in the diffusion process, while centrifuge-produced fuel produces 1,500 times more energy than is used in the centrifuge process.

In addition to its use in enrichment, uranium hexafluoride has been used in an advanced reprocessing method ([[fluoride volatility]]), which was developed in the [[Czech Republic]]. In this process, [[spent nuclear fuel]] is treated with fluorine gas to transform the oxides or elemental metals into a mixture of fluorides. This mixture is then distilled to separate the different classes of material. Some [[fission product]]s form nonvolatile fluorides which remain as solids and can then either be prepared for storage as nuclear waste or further processed either by [[solvation]]-based methods or [[electrochemistry|electrochemically]].

Uranium enrichment produces large quantities of [[depleted uranium hexafluoride]] (D{{chem2|UF6}} or D-{{chem2|UF6}}) as a waste product. The long-term storage of D-{{chem2|UF6}} presents environmental, health, and safety risks because of its chemical instability. When {{chem2|UF6}} is exposed to moist air, it reacts with the water in the air to produce {{chem2|UO2F2}} ([[uranyl fluoride]]) and HF ([[hydrogen fluoride]]) both of which are highly corrosive and toxic. In 2005, about 686,000 tonnes of D-{{chem2|UF6}} was housed in 57,122 storage cylinders located near [[Portsmouth, Ohio|Portsmouth]], Ohio; [[Oak Ridge, Tennessee|Oak Ridge]], Tennessee; and [[Paducah, Kentucky|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=https://web.evs.anl.gov/uranium/faq/mgmt/faq23.cfm |publisher=[[Argonne National Laboratory]]}}</ref><ref>{{cite web |title=Depleted UF<sub>6</sub> Management Program Documents |url=https://web.evs.anl.gov/uranium/documents/ |access-date=2006-05-17 |url-status=live |archive-url=https://web.archive.org/web/20080216013118/http://web.ead.anl.gov/uranium/documents/index.cfm |archive-date=2008-02-16}}</ref> Storage cylinders must be regularly inspected for signs of corrosion and leaks. The estimated lifetime of the steel cylinders is measured in decades.<ref>{{cite web |publisher=Institute for Energy and Environmental Research |date=2007-09-24 |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| archive-url=https://web.archive.org/web/20071014065307/http://www.ieer.org/sdafiles/vol_5/5-2/deararj.html| archive-date = 14 October 2007}}</ref>

== Accidents and disposal ==

There have been several accidents involving uranium hexafluoride in the US, including a cylinder-filling accident and material release at the [[Sequoyah Fuels Corporation]] in 1986 where an estimated 29,500 pounds of gaseous {{chem2|UF6}} escaped.<ref>{{cite journal | pmc=1963288 | date=2007 | last1=Brugge | first1=D. | last2=Delemos | first2=J. L. | last3=Bui | first3=C. | title=The Sequoyah Corporation Fuels Release and the Church Rock Spill: Unpublicized Nuclear Releases in American Indian Communities | journal=American Journal of Public Health | volume=97 | issue=9 | pages=1595–1600 | doi=10.2105/AJPH.2006.103044 | pmid=17666688 }}</ref><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 |archive-url=https://web.archive.org/web/20170609171651/http://web.ead.anl.gov/uranium/faq/health/faq30.cfm |archive-date=2017-06-09}}</ref>  The US government has been converting D{{chem2|UF6}} 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=https://web.evs.anl.gov/uranium/faq/storage/faq22.cfm |publisher=Argonne National Laboratory}}</ref> Such disposal of the entire D{{chem2|UF6}} stockpile could cost anywhere from $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 UF<sub>6</sub> inventory? |url=http://web.evs.anl.gov/uranium/faq/mgmt/faq27.cfm |publisher=Argonne National Laboratory}}</ref>
{{Gallery
|width=250
|height=180
|File:UF6 cylinder rupture bw.png
|Ruptured 14-ton {{chem2|UF6}} shipping cylinder. 1 fatality, dozens injured. ~29500&nbsp;lb of material released. Sequoyah Fuels Corporation 1986.
|File:DUF6 storage yard far.jpg
|D{{chem2|UF6}} storage yard from afar
|File:Corroded DUF6 cylinder.jpg
|D{{chem2|UF6}} cylinders: painted (left) and corroded (right)<!-- (The left image is available on Commons; have to find the right image and then can make it look better as currently the images are not sized correctly.) -->
}}

==See also==
* [[Enriched uranium]]
==References==
{{Reflist}}

==Further reading==
{{refbegin|30em}}
* {{cite book
 | title = [[Gmelins Handbuch der anorganischen Chemie]]
 | volume = System Nr. 55, Uran,  Teil A
 | pages = 121–123
 | publisher = Verlag Chemie, Gmelin-Institut für Anorganische
 | location = Weinheim
 | year = 1955
|orig-year=1817
 | language = de
 | editor-first= Leopold
|editor-last =Gmelin
}}
* {{cite book
 | title = [[Gmelins Handbuch der anorganischen Chemie]]
 | volume = System Nr. 55, Uran,  Teil C
 | pages = 71–163
 | publisher = Verlag Chemie, Gmelin-Institut für Anorganische
 | location = Weinheim
|orig-year=1817
 | language = de
 | editor-first= Leopold
|editor-last =Gmelin
}}
* {{cite report
 | last = DeWitt
 | first = R.
 | title = Uranium hexafluoride: A survey of the physico-chemical properties
 | publisher = Goodyear Atomic Corporation
 | location = Portsmouth, Ohio
 | date = 1960-08-12
 | type = Technical report
 | series = GAT-280
 | doi = 10.2172/4025868
 | url = https://doi.org/10.2172/4025868
}}

* {{cite book
 | last1 = Grenthe
 | first1 = Ingmar
 | last2 = Drożdżynński
 | first2 = Janusz
 | last3 = Fujino
 | first3 = Takeo
 | last4 = Buck
 | first4 = Edgar C.
 | last5 = Albrecht-Schmitt
 | first5 = Thomas E.
 | last6 = Wolf
 | first6 = Stephen F.
 | chapter = Uranium
 | chapter-url = http://radchem.nevada.edu/classes/rdch710/files/Uranium.pdf
 | archive-url = https://web.archive.org/web/20120118213655/http://radchem.nevada.edu/classes/rdch710/files/Uranium.pdf
 | archive-date = 2012-01-18
 | editor1 = Lester R. Morss
 | editor2 = Norman M. Edelstein
 | editor3 = Jean Fuger
 | title = The Chemistry of the Actinide and Transactinide Elements
 | publisher = Springer
 | location = Dordrecht
 | year = 2006
 | pages = 253–698
 | isbn = 1-4020-3555-1
 | doi = 10.1007/1-4020-3598-5_5
 | language = en
 
}}
* {{cite patent
 | country = US
 | number = 2535572
 | title = Preparation of UF<sub>6</sub>
 | gdate = 1950-12-26
 | url = https://www.freepatentsonline.com/2535572.html
 
}}
* {{cite patent
 | country = US
 | number = 5723837
 | title = Uranium Hexafluoride Purification
 | gdate = 1998-03-03
 | url = https://www.freepatentsonline.com/5723837.html
}}
{{refend}}

==External links==
{{subject bar|auto=y|d=y}}
* Simon Cotton (Uppingham School, Rutland, UK): [http://www.chm.bris.ac.uk/motm/uf6/uf6v.htm Uranium Hexafluoride].
* [https://web.evs.anl.gov/uranium/guide/uf6/ Uranium Hexafluoride (UF<sub>6</sub>) – Physical and chemical properties of UF<sub>6</sub>, and its use in uranium processing – Uranium Hexafluoride and Its Properties]
* [https://www.webelements.com/compounds/uranium/uranium_hexafluoride.html Uranium Hexafluoride] at WebElements
* [http://ecoperestroika.ru/english/details-import-of-uranium-tails-2013-and-our-protests/ Import of Western depleted uranium hexafluoride (uranium tails) to Russia] [dead link 30 June 2017]

{{Hexafluorides}}
{{Uranium compounds}}
{{Fluorides}}
{{Actinide halides}}

[[Category:Actinide halides]]
[[Category:Hexafluorides]]
[[Category:Nuclear materials]]
[[Category:Octahedral compounds]]
[[Category:Uranium(VI) compounds]]