Uranium hexafluoride (), (sometimes called "hex") is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

with the formula UF₆. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive

hydrofluoric acid Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...

. 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 Ammonium diuranate or (ADU) ((NH4)2U2O7), is one of the intermediate chemical forms of uranium produced during yellowcake production. The name "yellowcake" originally given to this bright yellow salt, now applies to mixtures of uranium oxides which ...

("ADU", (NH₄)₂U₂O₇). Reduction with hydrogen gives UO₂, which is converted with

(HF) to uranium tetrafluoride, UF₄. Oxidation with

fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...

yields UF₆. During

nuclear reprocessing Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the ...

, uranium is reacted with chlorine trifluoride to give UF₆: :U + 2 ClF₃ → UF₆ + Cl₂

Properties

Physical properties

At atmospheric pressure, it

sublimes Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point i ...

at 56.5 °C. The solid state structure was determined by neutron diffraction at 77 K and 293 K. Image:Uranium-hexafluoride-unit-cell-3D-balls.png, Ball-and-stick model of the unit cell of uranium hexafluorideJ. 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. ''Acta Crystallographica'' is a series of peer-reviewed scientific journals, with articles centred on crystallography, published by the International Union of Crystallography (IUCr). Originally established in 1948 as a single journal called ''Acta ...

'', 1973, ''B29'', p. 7–12; . Image:Uranium hexafluoride dimensions.svg, Bond lengths and angles of gaseous uranium hexafluoride

Chemical properties

It has been shown that uranium hexafluoride is an oxidant and a

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

that is able to bind to

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts typ ...

; for instance, the reaction of

copper(II) fluoride Copper(II) fluoride is an inorganic compound with the chemical formula CuF2. The anhydrous form is a white, ionic, crystalline, hygroscopic solid with a distorted rutile-type crystal structure, similar to other fluorides of chemical formulae MF2 ...

with uranium hexafluoride in

acetonitrile Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not clas ...

is reported to form copper(II) heptafluorouranate(VI), Cu(UF₇)₂. Polymeric uranium(VI) fluorides containing organic cations have been isolated and characterized by X-ray diffraction.

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 Enrichment may refer to: * Behavioral enrichment, the practice of providing animals under managed care with stimuli such as natural and artificial objects * Data enrichment, appending or enhancing data with relevant context from other sources, se ...

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)— 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. This difference is the basis for the physical separation of isotopes in enrichment. All the other uranium fluorides are nonvolatile solids that are

coordination polymer A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, o ...

s. The conversion factor for the ²³⁸U isotopologue of UF₆ ("hex") to "U mass" is .676. 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 Fluoride volatility is the tendency of highly fluorinated molecules to vaporize at comparatively low temperatures. Heptafluorides, hexafluorides and pentafluorides have much lower boiling points than the lower- valence fluorides. Most difluor ...

), 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 product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...

s 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 Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

. Uranium enrichment produces large quantities of

depleted uranium hexafluoride Depleted uranium hexafluoride (DUHF; also referred to as depleted uranium tails, depleted uranium tailings or DUF6) is a byproduct of the processing of uranium hexafluoride into enriched uranium. It is one of the chemical forms of depleted uranium ...

, 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 Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

) 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 Oak Ridge is a city in Anderson and Roane counties in the eastern part of the U.S. state of Tennessee, about west of downtown Knoxville. Oak Ridge's population was 31,402 at the 2020 census. It is part of the Knoxville Metropolitan Area. Oak ...

; 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 Sequoyah Fuels Corporation owned and operated a uranium processing plant near Gore, Oklahoma. The company was created in 1983 as a subsidiary of Kerr-McGee. In 1988 it was sold to General Atomics. Uranium processing plant The plant is located ...

in 1986. The U.S. government has been converting DUF₆ to solid

uranium oxides Uranium oxide is an oxide of the element uranium. The metal uranium forms several oxides: * Uranium dioxide or uranium(IV) oxide (UO2, the mineral uraninite or pitchblende) * Diuranium pentoxide or uranium(V) oxide (U2O5) * Uranium trioxide or u ...

for disposal. Such disposal of the entire DUF₆ stockpile could cost anywhere from $15 million to $450 million.

References

External links

* Simon Cotton (Uppingham School, Rutland, UK)

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
ead link 30 June 2017
Uranium Hexafluoride in www.webelements.com
{{fluorine compounds Uranium(VI) compounds Hexafluorides Actinide halides Octahedral compounds Nuclear materials