Uranium hexafluoride

Uranium hexafluoride, sometimes called hex, is the inorganic compound with the formula Template:Chem2. Uranium hexafluoride is a volatile, white solid that is used in enriching uranium for nuclear reactors and nuclear weapons.<ref name=Ull />

Preparation

Uranium dioxide is converted with hydrofluoric acid (HF) to uranium tetrafluoride:<ref name=Ull>Template:Cite book</ref>

Template:Chem2

The resulting Template:Chem2 is subsequently oxidized with fluorine to give the hexafluoride:

Template:Chem2

In samples contaminated with uranium trioxide, uranyl fluoride, an oxyfluoride compound is produced in the HF step:

Template:Chem2

which can be fluorinated to produce the same product, uranium hexafluoride.

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The fluorination steps in both reactions above are highly exothermic.

Properties

Physical properties

At atmospheric pressure, Template:Chem2 sublimes at 56.5 °C.<ref>Template:Cite journal</ref>

The solid-state structure was determined by neutron diffraction at 77 K and 293 K.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="UF6_1973">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; {{#invoke:CS1 identifiers|main|_template=doi}}.</ref>

File:Uranium-hexafluoride-unit-cell-3D-balls.png

Packing of UF₆ in its unit cell.<ref name="UF6_1968">Template:Cite journal</ref>

Chemical properties

UF₆ reacts with water, releasing hydrofluoric acid. The compound reacts with aluminium, forming a surface layer of [[aluminium fluoride|Template:Chem2]] that resists any further reaction from the compound.

Uranium hexafluoride is a mild oxidant.<ref>Template:Cite journal</ref> It is a Lewis acid as evidenced by its binding to form heptafluorouranate(VI), Template:Chem2.<ref>Template:Cite journal</ref>

Polymeric uranium(VI) fluorides containing organic cations have been isolated and characterized by X-ray diffraction.<ref>Template:Cite journal</ref>

Application in the fuel cycle

File:Uranium hexafluoride phase diagram.gif

Phase diagram of Template:Chem2

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 Template:Chem2, which is 64 °C (147 °F; 337 K) and 152 kPa (22 psi; 1.5 atm),<ref>Template:Cite web</ref> 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 Template:Chem2 differ in their molecular weight based solely on the uranium isotope present.<ref>Template:Cite web</ref> 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 Template:Chem2 isotopologue of Template:Chem2 ("hex") to "U mass" is 0.676.<ref>Template:Cite web</ref>

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 (DTemplate:Chem2 or D-Template:Chem2) as a waste product. The long-term storage of D-Template:Chem2 presents environmental, health, and safety risks because of its chemical instability. When Template:Chem2 is exposed to moist air, it reacts with the water in the air to produce Template:Chem2 (uranyl fluoride) and HF (hydrogen fluoride) both of which are highly corrosive and toxic. In 2005, about 686,000 tonnes of D-Template:Chem2 was housed in 57,122 storage cylinders located near Portsmouth, Ohio; Oak Ridge, Tennessee; and Paducah, Kentucky.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> Storage cylinders must be regularly inspected for signs of corrosion and leaks. The estimated lifetime of the steel cylinders is measured in decades.<ref>Template:Cite web</ref>

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 Template:Chem2 escaped.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref> The US government has been converting DTemplate:Chem2 to solid uranium oxides for disposal.<ref>Template:Cite web</ref> Such disposal of the entire DTemplate:Chem2 stockpile could cost anywhere from $15 million to $450 million.<ref>Template:Cite web</ref> {{#invoke:Gallery|gallery}}

See also

• Enriched uranium

Further reading

• Template:Cite book
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• Template:Cite report

• Template:Cite book
• Template:Cite patent
• Template:Cite patent

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]