Uranium hexafluoride

Uranium hexafluoride, 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.

Uranium hexafluoride
Names
IUPAC names
Uranium hexafluoride
Uranium(VI) fluoride
Identifiers
3D model (JSmol)
Abbreviations hex
ChEBI
ChemSpider
ECHA InfoCard 100.029.116
EC Number
  • 232-028-6
2923
RTECS number
  • YR4720000
UNII
UN number 2978 (<1% 235U)
2977 (>1% 235U)
  • InChI=1S/6FH.U/h6*1H;/q;;;;;;+6/p-6 checkY
    Key: SANRKQGLYCLAFE-UHFFFAOYSA-H checkY
  • InChI=1/6FH.U/h6*1H;/q;;;;;;+6/p-6/rF6U/c1-7(2,3,4,5)6
    Key: SANRKQGLYCLAFE-IIYYNVFAAT
  • F[U](F)(F)(F)(F)F
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
Occupational safety and health (OHS/OSH):
Main hazards
 Toxic, corrosive, radioactive[3]
GHS labelling:
GHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard
Danger
H300, H330, H373, H411
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
4
0
2
W
OX
Flash point Non-flammable
Safety data sheet (SDS) ICSC 1250
Related compounds
Other anions
 Uranium hexachloride
Other cations
Related uranium fluorides
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)
Infobox references

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.

The Honeywell Uranium Hexafluoride Processing Facility uses a different process.

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

U + 2 ClF3 → UF6 + Cl2

Properties

Physical properties

At atmospheric pressure, UF6 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), 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
Phase diagram of UF6

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 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 (DUF6 or D-UF6) as a waste product. The long-term storage of D-UF6 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 D-UF6 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 $15 million to $450 million.[20]

  • Ruptured 14-ton UF6 shipping cylinder. 1 fatality, dozens injured. ~29500 lbs of material released. Sequoyah Fuels Corporation 1986.
    Ruptured 14-ton UF6 shipping cylinder. 1 fatality, dozens injured. ~29500 lbs of material released. Sequoyah Fuels Corporation 1986.
  • DUF6 storage yard from afar
    DUF6 storage yard from afar
  • DUF6 cylinders: painted (left) and corroded (right)
    DUF6 cylinders: painted (left) and corroded (right)

References
  1. "Uranium Hexafluoride". Archived from the original on 2013-09-16. Retrieved 2013-08-08.
  2. 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. Bibcode:1948JChPh..16..429B. doi:10.1063/1.1746914.
  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. Bibcode:1968JChPh..48.4001K. doi:10.1063/1.1669727.
  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". web.evs.anl.gov. Retrieved 2022-08-18.
  13. "Uranium Enrichment and the Gaseous Diffusion Process". USEC Inc. Archived from the original on 2007-10-19. Retrieved 2007-09-24.
  14. "Unit converter molar mass calculator". TranslatorsCafé. Mississauga, Ontario, Canada: ANVICA Software Development. 1 February 2021.
  15. "How much depleted uranium hexafluoride is stored in the United States?". Depleted UF6 FAQs. Argonne National Laboratory.
  16. "Depleted UF6 Management Program Documents". Archived from the original on 2008-02-16. Retrieved 2006-05-17.
  17. "What is DUF6? Is it dangerous and what should we do with it?". Institute for Energy and Environmental Research. 2007-09-24.
  18. "Have there been accidents involving uranium hexafluoride?". Depleted UF6 FAQs. Argonne National Laboratory. Archived from the original on 2017-06-09.
  19. "What is going to happen to the uranium hexafluoride stored in the United States?". Depleted UF6 FAQs. Argonne National Laboratory.
  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.

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