Chemistry:Uranium hexafluoride
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| Names | |
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| IUPAC names
Uranium hexafluoride
Uranium(VI) fluoride | |
| Identifiers | |
3D model (JSmol)
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| Abbreviations | hex |
| ChEBI | |
| ChemSpider | |
| EC Number |
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| 2923 | |
PubChem CID
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| RTECS number |
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| UNII | |
| UN number | 2978 (<1% 235 U) 2977 (>1% 235 U) |
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| Properties | |
| UF 6 | |
| Molar mass | 352.02 g/mol |
| Appearance | Colorless solid |
| Density | 5.09 g/cm3, solid |
| Boiling point | 56.5 °C (133.7 °F; 329.6 K) (sublimes, at atmospheric pressure) |
| Hydrolyzes | |
| Solubility |
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| Structure | |
| Orthorhombic, oP28 | |
| Pnma, No. 62 | |
| Octahedral (Oh) | |
| 0 | |
| Thermochemistry | |
Std molar
entropy (S |
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Std enthalpy of
formation (ΔfH⦵298) |
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| Hazards | |
| Main hazards | Toxic, corrosive, radioactive[3] |
| Safety data sheet | ICSC 1250 |
| GHS pictograms |   
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| GHS Signal word | Danger |
| H300, H330, H373, H411 | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Related compounds | |
Other anions
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Uranium hexachloride |
Other cations
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Related uranium fluorides
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
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| Infobox references | |
Uranium hexafluoride, sometimes called hex, is an inorganic compound with the formula UF
6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with aluminium, forming a thin surface layer of AlF
3 that resists any further reaction from the compound. UF
6 is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons.
Preparation
Milled uranium ore—U
3O
8 or "yellowcake"—is dissolved in nitric acid, yielding a solution of uranyl nitrate UO
2(NO
3)
2. Pure uranyl nitrate is obtained by solvent extraction, then treated with ammonia to produce ammonium diuranate ("ADU", [NH
4]
2U
2O
7). Reduction with hydrogen gives UO
2, which is converted with hydrofluoric acid (HF) to uranium tetrafluoride, UF
4. Oxidation with fluorine yields UF
6.
The Honeywell Uranium Hexafluoride Processing Facility uses a different process.
During nuclear reprocessing, uranium is reacted with chlorine trifluoride to give UF
6:
- U + 2 ClF
3 → UF
6 + Cl
2
Properties
Physical properties
At atmospheric pressure, UF
6 sublimes at 56.5 °C.[4]
The solid-state structure was determined by neutron diffraction at 77 K and 293 K.[5][6]
Ball-and-stick model of the unit cell of uranium hexafluoride[7]
Bond lengths and angles of gaseous uranium hexafluoride[8]
Chemical properties
It has been shown that uranium hexafluoride is an oxidant[9] and a Lewis acid that is able to bind to fluoride; for instance, the reaction of copper(II) fluoride with uranium hexafluoride in acetonitrile is reported to form copper(II) heptafluorouranate(VI), Cu2+[UF−
7]
2.[10]
Polymeric uranium(VI) fluorides containing organic cations have been isolated and characterized by X-ray diffraction.[11]
Application in the fuel cycle
As one of the most 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 UF
6—64 °C(147 °F; 337 K) and 152 kPa (22 psi; 1.5 atm)[12]— is close to ambient conditions, phase transitions can be achieved in a processing facility with little thermodynamic work.
Fluorine has only a single naturally occurring stable isotope, so isotopologues of UF
6 differ in their molecular weight based solely on the uranium isotope present.[13] This difference is the basis for the physical separation of isotopes in enrichment.
All the other uranium fluorides are nonvolatile solids that are coordination polymers.
The conversion factor for the 238
U isotopologue of UF
6 ("hex") to "U mass" is 0.676.[14]
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 products 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 electrochemically.
Uranium enrichment produces large quantities of depleted uranium hexafluoride (DUF
6 or D-UF
6) as a waste product. The long-term storage of D-UF
6 presents environmental, health, and safety risks because of its chemical instability. When UF
6 is exposed to moist air, it reacts with the water in the air to produce UO
2F
2 (uranyl fluoride) and HF (hydrogen fluoride) both of which are highly corrosive and toxic. In 2005, 686,500 tonnes of D-UF
6 was housed in 57,122 storage cylinders located near Portsmouth, Ohio; Oak Ridge, Tennessee; and Paducah, Kentucky.[15][16] Storage cylinders must be regularly inspected for signs of corrosion and leaks. The estimated lifetime of the steel cylinders is measured in decades.[17]
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 UF6 escaped.[18] [19] The U.S. government has been converting DUF
6 to solid uranium oxides for disposal.[20] Such disposal of the entire DUF
6 stockpile could cost anywhere from $15 million to $450 million.[21]
Ruptured 14-ton UF
6 shipping cylinder. 1 fatality, dozens injured. ~29500 lbs of material released. Sequoyah Fuels Corporation 1986.
DUF
6 storage yard from afar
DUF
6 cylinders: painted (left) and corroded (right)
References
- ↑ "Uranium Hexafluoride". http://www.ibilabs.com/Uranium%20Hexafluoride.htm.
- ↑ 2.0 2.1 2.2 2.3 Johnson, Gerald K. (1979). "The Enthalpy of Formation of Uranium Hexafluoride". The Journal of Chemical Thermodynamics 11 (5): 483–490. doi:10.1016/0021-9614(79)90126-5.
- ↑ Uranium(VI) fluoride
- ↑ Brickwedde, Ferdinand G.; Hoge, Harold J.; Scott, Russell B. (1948). "The Low Temperature Heat Capacities, Enthalpies, and Entropies of UF4 and UF6". J. Chem. Phys. 16 (5): 429–436. doi:10.1063/1.1746914. Bibcode: 1948JChPh..16..429B.
- ↑ 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. (3): 219–224. doi:10.1039/DT9760000219.
- ↑ 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.
- ↑ J. C. Taylor, P. W. Wilson, J. W. Kelly: „The structures of fluorides. I. Deviations from ideal symmetry in the structure of crystalline UF6: a neutron diffraction analysis", Acta Crystallogr., 1973, B29, p. 7–12; doi:10.1107/S0567740873001895.
- ↑ Kimura, Masao; Schomaker, Werner; Smith, Darwin W.; Bernard (1968). "Electron-Diffraction Investigation of the Hexafluorides of Tungsten, Osmium, Iridium, Uranium, Neptunium, and Plutonium". J. Chem. Phys. 48 (8): 4001–4012. doi:10.1063/1.1669727. Bibcode: 1968JChPh..48.4001K. https://authors.library.caltech.edu/71173/. Retrieved 2020-10-10.
- ↑ 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. (3): 272–274. doi:10.1039/DT9760000272.
- ↑ 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.
- ↑ "Uranium Hexafluoride: Source: Appendix A of the PEIS (DOE/EIS-0269): Physical Properties". https://web.evs.anl.gov/uranium/guide/ucompound/propertiesu/hexafluoride.cfm.
- ↑ "Uranium Enrichment and the Gaseous Diffusion Process". USEC Inc. http://www.usec.com/v2001_02/HTML/Aboutusec_enrichment.asp.
- ↑ "Unit converter molar mass calculator". Mississauga, Ontario, Canada: ANVICA Software Development. 1 February 2021. https://www.translatorscafe.com/unit-converter/en-US/molar-mass/?q=UF6.
- ↑ "How much depleted uranium hexafluoride is stored in the United States?". Depleted UF6 FAQs. Argonne National Laboratory. https://web.evs.anl.gov/uranium/faq/mgmt/faq23.cfm.
- ↑ "Depleted UF6 Management Program Documents". https://web.evs.anl.gov/uranium/documents/.
- ↑ "What is DUF6? Is it dangerous and what should we do with it?". Institute for Energy and Environmental Research. 2007-09-24. http://www.ieer.org/sdafiles/vol_5/5-2/deararj.html.
- ↑ "The Sequoyah Corporation Fuels Release and the Church Rock Spill: Unpublicized Nuclear Releases in American Indian Communities" Authors Doug Brugge, Jamie L. deLemos, and Cat Bui. September 2007. PMCID:PMC1963288 PMID: 17666688
- ↑ "Have there been accidents involving uranium hexafluoride?". Depleted UF6 FAQs. Argonne National Laboratory. http://web.ead.anl.gov/uranium/faq/health/faq30.cfm.
- ↑ "What is going to happen to the uranium hexafluoride stored in the United States?". Depleted UF6 FAQs. Argonne National Laboratory. https://web.evs.anl.gov/uranium/faq/storage/faq22.cfm.
- ↑ "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. http://web.evs.anl.gov/uranium/faq/mgmt/faq27.cfm.
Further reading
- Gmelins Handbuch der anorganischen Chemie, System Nr. 55, Uran, Teil A, p. 121–123.
- Gmelins Handbuch der anorganischen Chemie, System Nr. 55, Uran, Teil C 8, p. 71–163.
- R. DeWitt: Uranium hexafluoride: A survey of the physico-chemical properties, Technical report, GAT-280; Goodyear Atomic Corp., Portsmouth, Ohio; 12. August 1960; doi:10.2172/4025868.
- Ingmar Grenthe, Janusz Drożdżynński, Takeo Fujino, Edgar C. Buck, Thomas E. Albrecht-Schmitt, Stephen F. Wolf: Uranium , in: Lester R. Morss, Norman M. Edelstein, Jean Fuger (Hrsg.): The Chemistry of the Actinide and Transactinide Elements, Springer, Dordrecht 2006; ISBN:1-4020-3555-1, p. 253–698; doi:10.1007/1-4020-3598-5_5 (p. 530–531, 557–564).
- US-Patent 2535572: Preparation of UF6; 26. December 1950.
- US-Patent 5723837: Uranium Hexafluoride Purification; 3. March 1998.
External links
- Simon Cotton (Uppingham School, Rutland, UK): Uranium Hexafluoride.
- Uranium Hexafluoride (UF6) – Physical and chemical properties of UF6, and its use in uranium processing – Uranium Hexafluoride and Its Properties
- Uranium Hexafluoride at WebElements
- Import of Western depleted uranium hexafluoride (uranium tails) to Russia [dead link 30 June 2017]
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