Uranium hexafluoride


Uranium hexafluoride, 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.

Preparation

Milled uranium ore—U3O8 or "yellowcake"—is dissolved in nitric acid, yielding a solution of uranyl nitrate UO22. Pure uranyl nitrate is obtained by solvent extraction, then treated with ammonia to produce ammonium diuranate. Reduction with hydrogen gives UO2, which is converted with hydrofluoric acid to uranium tetrafluoride, UF4. Oxidation with fluorine yields UF6.
During nuclear reprocessing, uranium is reacted with chlorine trifluoride to give UF6:

Properties

Physical properties

At atmospheric pressure, it sublimes at 56.5 °C.
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The solid state structure was determined by neutron diffraction at 77 K and 293 K.

Chemical properties

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 fluoride with uranium hexafluoride in acetonitrile is reported to form copper heptafluorouranate, Cu2.
Polymeric uranium fluorides containing organic cations have been isolated and characterised by X-ray diffraction.

Application in the nuclear fuel cycle

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 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, 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.
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 and HF 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.