Uranium hexafluoride: Difference between revisions

Uranium fluoride
Names
	IUPAC names Uranium hexafluoride; Uranium(VI) fluoride
Identifiers
CAS Number	7783-81-5 ;
3D model (JSmol)	Interactive image;
ChemSpider	22966;
ECHA InfoCard	100.029.116
PubChem CID	24560;
RTECS number	YR4720000;
UN number	2978 (<1% 235U); 2977 (>1% 235U)
CompTox Dashboard (EPA)	DTXSID4074566 ;
	InChI InChI=1/6FH.U/h6*1H;/q;;;;;;+6/p-6/rF6U/c1-7(2,3,4,5)6 Key: SANRKQGLYCLAFE-IIYYNVFAAT;
	SMILES F[U](F)(F)(F)(F)F;
Properties
Chemical formula	UF6
Molar mass	352.02 g/mol
Appearance	colorless solid
Density	5.09 g/cm3, solid
Melting point	64.05 °C (triple point)
Boiling point	56.5 °C (sublimes)
Solubility in water	reacts
Solubility	soluble in chloroform, CCl4, liquid chlorine and bromine ; dissolves in nitrobenzene
Structure
Crystal structure	Orthorhombic, oP28
Space group	Pnma, No. 62
Coordination geometry	octahedral (Oh)
Dipole moment	0
Thermochemistry
Std molar; entropy (S⦵298)	– solid: −430,4 ± 1,5 J·K−1·mol−1 – gaseous: −280,4 ± 1,5 J·K−1·mol−1
Std enthalpy of; formation (ΔfH⦵298)	– solid: −(2197,7 ± 1,8) kJ·mol−1 – gaseous: −(2148,1 ± 1,8) kJ·mol−1
Hazards
Flash point	Non-flammable
Related compounds
Other anions	Uranium hexachloride
Other cations	Neptunium hexafluoride; Plutonium hexafluoride
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

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Revision as of 15:22, 9 November 2010

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.

Preparation

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 yields UF₆.

Properties

Physical Properties

At room pressure, it sublimes at 56.5 °C.^[2] The triple point is at 64.05 °C and 1.5 bar.^[3]

The solid state structure was determined by neutron diffraction at 77 K an 293 K.^[4]^[5]

This is a simple mononuclear molecule
The crystal structure of uranium hexafluoride

Chemical Properties

It has been shown that uranium hexafluoride is an oxidant^[6] 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 copper(II) heptafluorouranate(VI), F₇Cu₃U.^[7]

Polymeric uranium(VI) fluorides containing organic cations have been isolated and characterised by X-ray diffraction.^[8]

Application in the nuclear fuel cycle

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 64.05 °C (147 °F, 337 K) and slightly higher than normal atmospheric pressure. 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.^[9]

All the other uranium fluorides are involatile solids which are coordination polymers.

Gaseous diffusion requires about 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 (fluoride volatility) 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, 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.^[10]^[11] 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.^[12]

There have been several accidents involving uranium hexafluoride in the United States.^[13]^[14] The U.S. government has been converting DUF₆ to solid uranium oxides for disposal.^[15] Such disposal of the entire DUF₆ inventory could cost anywhere from $15 million to $450 million.^[16]

References

^ ^a ^b ^c ^d Gerald K. Johnson: "The Enthalpy of Formation of Uranium Hexafluoride", The Journal of Chemical Thermodynamics, 1979, 11 (5), p. 483–490; doi:10.1016/0021-9614(79)90126-5.
^ http://nuclearweaponarchive.org/Library/Glossary
^ Uranium Hexafluoride: Source: Appendix A of the PEIS (DOE/EIS-0269): Physical Properties
^ J. H. Levy, John C. Taylor, Paul W. Wilson (1976). "Structure of Fluorides. Part XII. Single-Crystal Neutron Diffraction Study of Uranium Hexafluoride at 293 K". J. Chem. Soc. Dalton Trans.: 219–224. doi:10.1039/DT9760000219.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ J. H. Levy, J. C. Taylor and A. B. Waugh (1983). "Neutron Powder Structural Studies of UF₆, MoF₆ and WF₆ at 77 K". Journal of Fluorine Chemistry. 23: 29–36. doi:10.1016/S0022-1139(00)81276-2.
^ G. H. Olah, J. Welch (1978). "Synthetic methods and reactions. 46. Oxidation of organic compounds with uranium hexafluoride in haloalkane solutions". J. Am. Chem. Soc. 100 (17): 5396–5402. doi:10.1021/ja00485a024.
^ J. A. Berry, R. T. Poole, A. Prescott, D. W. A. Sharp, J. M. Winfield (1976). "The oxidising and fluoride ion acceptor properties of uranium hexafluoride in acetonitrile". J. Chem. Soc. Dalton Trans.: 272–274. doi:10.1039/DT9760000272.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ S. M. Walker, P. S. Halasyamani, S. Allen, D. O'Hare (1999). "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". J. Am. Chem. Soc. 121 (45): 10513–10521. doi:10.1021/ja992145f.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ "Uranium Enrichment and the Gaseous Diffusion Process". USEC Inc. Retrieved 2007-09-24.
^ "How much depleted uranium hexafluoride is stored in the United States?". Depleted UF₆ FAQs. Argonne National Laboratory.
^ Documents
^ "What is DUF₆? Is it dangerous and what should we do with it?". Institute for Energy and Environmental Research. 2007-09-24.
^ "Have there been accidents involving uranium hexafluoride?". Depleted UF₆ FAQs. Argonne National Laboratory.
^ "Uranium Hexafluoride (UF₆) Tailings: Characteristics, Transport and Storage at the Siberian Chemical Combine (Sibkhimkombinat) Tomsk" (briefing note). Large and Associates. 5 November 2005. Retrieved 2007-09-24.
^ "What is going to happen to the uranium hexafluoride stored in the United States?". Depleted UF₆ FAQs. Argonne National Laboratory.
^ "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?". Depleted UF₆ FAQs. Argonne National Laboratory.

@@ Line 84: / Line 84: @@
 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, 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:DUF6PH38.jpg|255px|thumb|right|UF<sub>6</sub>-cylinder]]
+[[Image:DUF6PH38.jpg|255px|thumb|left|UF<sub>6</sub>-cylinder]]
 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:UF6_cylinder_rupture_bw.png|255px|thumb|Ruptured 14-ton UF<sub>6</sub> shipping cylinder. 1 fatality, dozens injured. ~29500 lbs of material released. 1986]]
+[[Image:UF6_cylinder_rupture_bw.png|255px|left|thumb|Ruptured 14-ton UF<sub>6</sub> shipping cylinder. 1 fatality, dozens injured. ~29500 lbs of material released. 1986]]
 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>
+{{-}}
 == References ==
 {{reflist}}