Chemistry:Uranium hexafluoride

From HandWiki
Uranium hexafluoride
Uranium-hexafluoride-2D-V2.svg
Uranium-hexafluoride-3D-vdW.png
Uranium-hexafluoride-crystal-3D-vdW.png
Names
IUPAC names
Uranium hexafluoride
Uranium(VI) fluoride
Identifiers
3D model (JSmol)
Abbreviations hex
ChEBI
ChemSpider
EC Number
  • 232-028-6
2923
RTECS number
  • YR4720000
UNII
UN number 2978 (<1% 235U)
2977 (>1% 235U)
Properties
UF6
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
Structure
Orthorhombic, oP28
Pnma, No. 62
Octahedral (Oh)
0
Thermochemistry
  • Solid, 227.8±1.3 J·K−1·mol−1[2]
  • Gaseous, 377.8±1.3 J·K−1·mol−1[2]
  • Solid, −2197.7±1.8 kJ·mol−1[2]
  • Gaseous, −2148.1±1.8 kJ·mol−1[2]
Hazards
Main hazards Toxic, corrosive, radioactive[3]
Safety data sheet ICSC 1250
GHS pictograms GHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard
GHS Signal word Danger
H300, H330, H373, H411
NFPA 704 (fire diamond)
Flash point Non-flammable
Related compounds
Other anions
Uranium hexachloride
Other cations
Neptunium hexafluoride
Plutonium hexafluoride
Related uranium fluorides
Uranium(III) fluoride
Uranium(IV) fluoride
Uranium(V) fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☑Y verify (what is ☑Y☒N ?)
Infobox references
Tracking categories (test):

Uranium hexafluoride (UF6), (sometimes called "hex") is an inorganic compound with the formula UF6. 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 AlF3 that resists any further reaction from the compound. UF6 is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons.

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.

During nuclear reprocessing, uranium is reacted with chlorine trifluoride to give UF6:

U + 2 ClF3 → UF6 + Cl2

Properties

Physical properties

At atmospheric pressure, it sublimes at 56.5 °C.[4]

UF6 in a glass ampoule.

The solid state structure was determined by neutron diffraction at 77 K and 293 K.[5][6]

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), Cu(UF7)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 UF6—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 UF6 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 238U isotopologue of UF6 ("hex") to "U mass" is .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, or DUF6, as a waste product. 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 corrosive and toxic. In 2005, 686,500 tonnes of DUF6 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]

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.[18] The U.S. government has been converting DUF6 to solid uranium oxides for disposal.[19] Such disposal of the entire DUF6 stockpile could cost anywhere from United States dollar 15 million to $450 million.[20]

References

  1. "Uranium Hexafluoride". http://www.ibilabs.com/Uranium%20Hexafluoride.htm. 
  2. 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. 
  3. Uranium(VI) fluoride
  4. 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. Bibcode1948JChPh..16..429B. 
  5. 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. 
  6. 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. 
  7. 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.
  8. 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. Bibcode1968JChPh..48.4001K. https://authors.library.caltech.edu/71173/. 
  9. 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. 
  10. 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. 
  11. 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. 
  12. "Uranium Hexafluoride: Source: Appendix A of the PEIS (DOE/EIS-0269): Physical Properties". https://web.evs.anl.gov/uranium/guide/ucompound/propertiesu/hexafluoride.cfm. 
  13. "Uranium Enrichment and the Gaseous Diffusion Process". USEC Inc. http://www.usec.com/v2001_02/HTML/Aboutusec_enrichment.asp. 
  14. TranslatorsCafé (1 February 2021). "Unit converter molar mass calculator". Mississauga, Ontario, Canada: ANVICA Software Development. https://www.translatorscafe.com/unit-converter/en-US/molar-mass/?q=UF6. 
  15. "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. 
  16. Documents
  17. "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. 
  18. "Have there been accidents involving uranium hexafluoride?". Depleted UF6 FAQs. Argonne National Laboratory. http://web.ead.anl.gov/uranium/faq/health/faq30.cfm. 
  19. "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. 
  20. "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