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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 chemis ...
with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive
hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepres ...
. The compound reacts mildly with
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
, 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 reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...
s and
nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bomb ...
s.


Preparation

Milled uranium ore—U3O8 or "
yellowcake Yellowcake (also called urania) is a type of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. It is a step in the processing of uranium after it has been mined but before f ...
"—is dissolved in
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
, yielding a solution of
uranyl nitrate Uranyl nitrate is a water-soluble yellow uranium salt with the formula . The hexa-, tri-, and dihydrates are known. The compound is mainly of interest because it is an intermediate in the preparation of nuclear fuels. Uranyl nitrate can be prepa ...
UO2(NO3)2. Pure uranyl nitrate is obtained by solvent extraction, then treated with
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
gives UO2, which is converted with
hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepres ...
(HF) to
uranium tetrafluoride Uranium tetrafluoride is the inorganic compound with the formula UF4. It is a green solid with an insignificant vapor pressure and low solubility in water. Uranium in its tetravalent ( uranous) state is important in various technological process ...
, 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 reactiv ...
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, th ...
, uranium is reacted with
chlorine trifluoride Chlorine trifluoride is an interhalogen compound with the formula ClF3. This colorless, poisonous, corrosive, and extremely reactive gas condenses to a pale-greenish yellow liquid, the form in which it is most often sold (pressurized at room temp ...
to give UF6: :U + 2 ClF3 → UF6 + Cl2


Properties


Physical properties

At
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
, it sublimes at 56.5 °C. The solid state structure was determined by
neutron diffraction Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of thermal or cold neutrons to ob ...
at 77 K and 293 K. Image:Uranium-hexafluoride-unit-cell-3D-balls.png,
Ball-and-stick model In chemistry, the ball-and-stick model is a molecular model of a chemical substance which displays both the three-dimensional position of the atoms and the bonds between them. The atoms are typically represented by spheres, connected by rods ...
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.'', 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 An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
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 ty ...
; for instance, the reaction of copper(II) fluoride 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.
Polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
ic 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 A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centrifugal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radiu ...
method. Since the
triple point In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the ...
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 In thermodynamics, work is one of the principal processes by which a thermodynamic system can interact with its surroundings and exchange energy. An exchange of energy is facilitated by a mechanism through which the system can spontaneously exer ...
. Fluorine has only a single naturally occurring stable isotope, so
isotopologue In chemistry, isotopologues are molecules that differ only in their isotopic composition. They have the same chemical formula and bonding arrangement of atoms, but at least one atom has a different number of neutrons than the parent. An exampl ...
s of UF6 differ in their molecular weight based solely on the uranium
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
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 ...
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), which was developed in the
Czech Republic The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as Bohemia, it is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. The ...
. In this process,
spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...
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 Solvation (or dissolution) describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of t ...
-based methods or electrochemically. 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 ...
) 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 Portsmouth is a city in and the county seat of Scioto County, Ohio, United States. Located in southern Ohio south of Chillicothe, it lies on the north bank of the Ohio River, across from Kentucky, just east of the mouth of the Scioto River. ...
;
Oak Ridge, Tennessee Oak Ridge is a city in Anderson County, Tennessee, Anderson and Roane County, Tennessee, Roane counties in the East Tennessee, eastern part of the U.S. state of Tennessee, about west of downtown Knoxville, Tennessee, Knoxville. Oak Ridge's popu ...
; and
Paducah, Kentucky Paducah ( ) is a home rule-class city in and the county seat of McCracken County, Kentucky. The largest city in the Jackson Purchase region, it is located at the confluence of the Tennessee and the Ohio rivers, halfway between St. Louis, Miss ...
. 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 for disposal. Such disposal of the entire DUF6 stockpile could cost anywhere from $15 million to $450 million.


References


Further reading

* '' Gmelins Handbuch der anorganischen Chemie'', System Nr. 55, Uran, Teil A, p. 121–123. * ''Gmelins Handbuch der anorganischen Chemie'', System Nr. 55, Uran, Teil C 8, p. 71–163. * R. DeWitt: ''Uranium hexafluoride: A survey of the physico-chemical properties'', Technical report, GAT-280; Goodyear Atomic Corp., Portsmouth, Ohio; 12. August 1960; . * Ingmar Grenthe, Janusz Drożdżynński, Takeo Fujino, Edgar C. Buck,
Thomas E. Albrecht-Schmitt Thomas may refer to: People * List of people with given name Thomas * Thomas (name) * Thomas (surname) * Saint Thomas (disambiguation) * Thomas Aquinas (1225–1274) Italian Dominican friar, philosopher, and Doctor of the Church * Thomas the Ap ...
, Stephen F. Wolf
Uranium
, in: Lester R. Morss, Norman M. Edelstein, Jean Fuger (Hrsg.): ''The Chemistry of the Actinide and Transactinide Elements'', Springer, Dordrecht 2006; , p. 253–698; (p. 530–531, 557–564). * US-Patent 2535572

26. December 1950. * US-Patent 5723837

3. March 1998.


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