Uranium hexafluoride

Uranium hexafluoride
Names
	IUPAC names Uranium hexafluoride; Uranium(VI) fluoride
Identifiers
CAS Number	7783-81-5 ;
3D model (JSmol)	Interactive image;
Abbreviations	hex
ChEBI	CHEBI:30235 ;
ChemSpider	22966 ;
ECHA InfoCard	100.029.116
EC Number	232-028-6;
Gmelin Reference	2923
PubChem CID	24560;
RTECS number	YR4720000;
UNII	N06GJ1D94J ;
UN number	2978 (<1% 235U); 2977 (>1% 235U)
CompTox Dashboard (EPA)	DTXSID4074566 ;
	InChI InChI=1S/6FH.U/h61H;/q;;;;;;+6/p-6 Key: SANRKQGLYCLAFE-UHFFFAOYSA-H ; InChI=1/6FH.U/h61H;/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
Boiling point	56.5 °C (133.7 °F; 329.6 K) (sublimes, at atmospheric pressure)
Solubility in water	Hydrolyzes
Solubility	Soluble in chloroform, CCl4, liquid chlorine, and bromine; Dissolves in nitrobenzene; 151 kPa;
Structure
Crystal structure	Orthorhombic, oP28
Space group	Pnma, No. 62
Coordination geometry	Octahedral (Oh)
Dipole moment	0
Thermochemistry
Std molar; entropy (S⦵298)	Solid, 227.8±1.3 J·K−1·mol−1; Gaseous, 377.8±1.3 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
	Occupational safety and health (OHS/OSH):
Main hazards	Toxic, corrosive, radioactive
	GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H300, H330, H373, H411
NFPA 704 (fire diamond)	4 0 2 W; OX
Flash point	Non-flammable
Safety data sheet (SDS)	ICSC 1250
Related compounds
Other anions	Uranium hexachloride
Other cations	Neptunium hexafluoride; Plutonium hexafluoride; Americium hexafluoride; Curium hexafluoride; Tungsten 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). verify (what is ?) Infobox references

Uranium hexafluoride, sometimes called hex, is an inorganic compound with the formula U F₆. Uranium hexafluoride is a volatile and toxic 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.

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]

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 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 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]

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 DUF₆ to solid uranium oxides for disposal.^[20] Such disposal of the entire DUF₆ stockpile could cost anywhere from $15 million to $450 million.^[21]

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 cylinders: painted (left) and corroded (right)

References

^ "Uranium Hexafluoride". Archived from the original on 2013-09-16. Retrieved 2013-08-08.
^ ^a ^b ^c ^d 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. Bibcode:1948JChPh..16..429B. doi:10.1063/1.1746914.
^ 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. Bibcode:1968JChPh..48.4001K. doi:10.1063/1.1669727. Archived from the original on 2023-01-11. 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". web.evs.anl.gov. Retrieved 2022-08-18.
^ "Uranium Enrichment and the Gaseous Diffusion Process". USEC Inc. Archived from the original on 2007-10-19. Retrieved 2007-09-24.
^ "Unit converter molar mass calculator". TranslatorsCafé. Mississauga, Ontario, Canada: ANVICA Software Development. 1 February 2021.
^ "How much depleted uranium hexafluoride is stored in the United States?". Depleted UF₆ FAQs. Argonne National Laboratory.
^ "Depleted UF₆ Management Program Documents". Archived from the original on 2008-02-16. Retrieved 2006-05-17.
^ "What is DUF₆? Is it dangerous and what should we do with it?". Institute for Energy and Environmental Research. 2007-09-24.
^ Brugge, D.; Delemos, J. L.; Bui, C. (2007). "The Sequoyah Corporation Fuels Release and the Church Rock Spill: Unpublicized Nuclear Releases in American Indian Communities". American Journal of Public Health. 97 (9): 1595–1600. doi:10.2105/AJPH.2006.103044. PMC 1963288. PMID 17666688.
^ "Have there been accidents involving uranium hexafluoride?". Depleted UF₆ FAQs. Argonne National Laboratory. Archived from the original on 2017-06-09.
^ "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.