Uranium hexafluoride (UF6), colloquially known as "hex" in the nuclear industry, is a compound used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons. Hex forms solid grey crystals at standard temperature and pressure, is highly toxic, reacts with water, and is corrosive to most metals. The compound reacts mildly with aluminium, forming a thin surface layer of AlF3 that resists any further reaction from the compound.
3D model (JSmol)
|UN number||2978 (<1% 235U)|
2977 (>1% 235U)
|Molar mass||352.02 g/mol|
|Density||5.09 g/cm3, solid|
|Melting point||64.052 °C (147.294 °F; 337.202 K) (triple point at 151 kPa)|
|Boiling point||56.5 °C (133.7 °F; 329.6 K) (sublimes)|
|Solubility||soluble in chloroform, CCl4, liquid chlorine and bromine |
dissolves in nitrobenzene
|Pnma, No. 62|
|– solid: −430,4 ± 1,5 J·K−1·mol−1|
– gaseous: −280,4 ± 1,5 J·K−1·mol−1
Std enthalpy of
|– solid: −(2197,7 ± 1,8) kJ·mol−1|
– gaseous: −(2148,1 ± 1,8) kJ·mol−1
|Safety data sheet||ICSC 1250|
| T+ (T+)|
|R-phrases (outdated)||R26/28, R33, R51/53|
|S-phrases (outdated)||(S1/2), S20/21, S45, S61|
Related uranium fluorides
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
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.
- U + 2 ClF3 → UF6 + Cl2
Ball-and-stick model of the unit cell of uranium hexafluoride
Bond lengths and angles of gaseous uranium hexafluoride
It has been shown that uranium hexafluoride is an oxidant 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.
Application in the nuclear fuel cycleEdit
UF6 is used in both of the main uranium enrichment methods — gaseous diffusion and the gas centrifuge method — because its triple point is at temperature 64.05 °C (147 °F, 337 K) and only slightly higher than normal atmospheric pressure. 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.
All the other uranium fluorides are nonvolatile solids that are coordination polymers.
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, used oxide nuclear fuel is treated with fluorine gas to form a mixture of fluorides. This mixture is then distilled to separate the different classes of material.
Storage in cylindersEdit
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. Storage cylinders must be regularly inspected for signs of corrosion and leaks. The estimated lifetime of the steel cylinders is measured in decades.
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. The U.S. government has been converting DUF6 to solid uranium oxides for disposal. Such disposal of the entire DUF6 inventory could cost anywhere from $15 million to $450 million.
- "Archived copy". Archived from the original on 2013-09-16. Retrieved 2013-08-08.CS1 maint: Archived copy as title (link)
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- US-Patent 5723837: Uranium Hexafluoride Purification; 3. March 1998.
|Wikimedia Commons has media related to Uranium hexafluoride.|
- 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
- Import of Western depleted uranium hexafluoride (uranium tails) to Russia [dead link 30 June 2017]
- Uranium Hexafluoride in www.webelements.com