Uranium hexafluoride
Encyclopedia
Uranium hexafluoride referred to as "hex" in the nuclear industry, is a compound used in the uranium
enrichment process that produces fuel for nuclear reactor
s and nuclear weapon
s. It forms solid grey crystals at standard temperature and pressure (STP), is highly toxic, reacts violently with water and is corrosive to most metals. It reacts mildly with aluminium
, forming a thin surface layer of AlF3 that resists further reaction.
"—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.
is at 64.05 °C and 1.5 bar.
The solid state structure was determined by neutron diffraction
at 77 K and 293 K.
, for instance the reaction of copper(II) fluoride
with uranium hexafluoride in acetonitrile
is reported to form copper(II) heptafluorouranate(VI), Cu(UF7)2.
Polymer
ic uranium(VI) fluorides containing organic cations have been isolated and characterised by X-ray diffraction.
and the gas centrifuge
method, because it has a triple point at 64.05 °C (147 °F, 337 K) and slightly higher than normal atmospheric pressure. Fluorine has only a single stable naturally occurring isotope, so isotopologue
s of UF6 differ in their molecular weight based solely on the uranium isotope
present.
All the other uranium fluorides are involatile solids which are coordination polymers
.
Gaseous diffusion requires about 60 times as much energy as the gas centrifuge process; even so, this is just 4% of the energy that can be produced by the resulting enriched uranium
.
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 is then distilled to separate the different classes of material.
produced to date is stored as uranium hexafluoride, DUF6, in steel cylinders in open air yards close to enrichment plants. Each cylinder contains up to 12.7 tonnes (or 14 US tons) of solid UF6. In the U.S. alone, 560,000 tonnes of depleted UF6 had accumulated by 1993. In 2005, 686,500 tonnes in 57,122 storage cylinders were located near Portsmouth, Ohio
, Oak Ridge, Tennessee
, and Paducah, Kentucky
. 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 soluble and toxic. 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 United States, 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.
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
enrichment process that produces fuel for nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s and nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
s. It forms solid grey crystals at standard temperature and pressure (STP), is highly toxic, reacts violently with water and is corrosive to most metals. It reacts mildly with aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, forming a thin surface layer of AlF3 that resists further reaction.
Preparation
Milled uranium ore—U3O8 or "yellowcakeYellowcake
Yellowcake is a kind of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores...
"—is dissolved in nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
, yielding a solution of uranyl nitrate
Uranyl nitrate
Uranyl nitrate is a water soluble yellow uranium salt. The yellow-green crystals of uranium nitrate hexahydrate are triboluminescent.Uranyl nitrate can be prepared by reaction of uranium salts with nitric acid...
UO2(NO3)2. Pure uranyl nitrate is obtained by solvent extraction, then treated with ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
to produce ammonium diuranate
Ammonium diuranate
Ammonium diuranate or ' , is one of the intermediate chemical forms of uranium produced during yellowcake production. The name 'yellowcake' originally given to this bright yellow substance, now applies to mixtures of uranium oxides which are actually hardly ever yellow...
("ADU", (NH4)2U2O7). Reduction with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gives UO2, which is converted with hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
(HF) to uranium tetrafluoride
Uranium tetrafluoride
Uranium tetrafluoride is a green crystalline solid compound of uranium with an insignificant vapor pressure and very slight solubility in water. Uranium in its tetravalent state is very important in different technological processes...
, UF4. Oxidation with fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
yields UF6.
Physical Properties
At room pressure, it sublimes at 56.5 °C. The triple pointTriple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...
is at 64.05 °C and 1.5 bar.
The solid state structure was determined by neutron diffraction
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 obtain a diffraction pattern that provides information of...
at 77 K and 293 K.
Chemical properties
It has been shown that uranium hexafluoride is an oxidant and a Lewis acid which is able to bind to fluorideFluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
, for instance the reaction of copper(II) fluoride
Copper(II) fluoride
Copper fluoride is an inorganic compound with the chemical formula CuF2. It is a white or green, crystalline, hygroscopic solid. It has a rutile-type crystal structure similar to other fluorides of chemical formulae MF2.-Uses:...
with uranium hexafluoride in acetonitrile
Acetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...
is reported to form copper(II) heptafluorouranate(VI), Cu(UF7)2.
Polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
ic uranium(VI) 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 diffusionGaseous diffusion
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride through semi-permeable membranes. This produces a slight separation between the molecules containing uranium-235 and uranium-238 . By use of a large cascade of many stages, high separations...
and the gas centrifuge
Gas centrifuge
A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centripetal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container.A prominent use of gas...
method, because it has a triple point at 64.05 °C (147 °F, 337 K) and slightly higher than normal atmospheric pressure. Fluorine has only a single stable naturally occurring isotope, so isotopologue
Isotopologue
Isotopologues are molecules that differ only in their isotopic composition. Simply, the isotopologue of a chemical species has at least one atom with a different number of neutrons than the parent....
s of UF6 differ in their molecular weight based solely on the uranium isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
present.
All the other uranium fluorides are involatile solids which are coordination polymers
Coordination polymers
A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands, extending in an array. It can also be described as a polymer whose repeat units are coordination complexes...
.
Gaseous diffusion requires about 60 times as much energy as the gas centrifuge process; even so, this is just 4% of the energy that can be produced by the resulting enriched uranium
Enriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
.
In addition to its use in enrichment
Enrichment
Enrichment may mean:*Education. *The process of adding nutrients to cereals or grain: see food fortification.*The process of adding sugar to grape must during winemaking in order to achieve a higher alcohol content of the wine, more commonly referred to as chaptalization.*Behavioral...
, uranium hexafluoride has been used in an advanced reprocessing method (fluoride volatility
Fluoride volatility
Fluoride volatility is jargon that describes the volatility of fluorides, which is relevant to the separation of radionuclides. Hexafluorides and pentafluorides have much lower boiling points than the lower-valence fluorides. Most difluorides and trifluorides have high boiling points, while most...
) which was developed in the Czech Republic
Czech Republic
The Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
. In this process, used oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
is treated with fluorine gas to form a mixture of fluorides. This is then distilled to separate the different classes of material.
Storage in gas cylinders
About 95% of the depleted uraniumDepleted uranium
Depleted uranium is uranium with a lower content of the fissile isotope U-235 than natural uranium . Uses of DU take advantage of its very high density of 19.1 g/cm3...
produced to date is stored as uranium hexafluoride, DUF6, in steel cylinders in open air yards close to enrichment plants. Each cylinder contains up to 12.7 tonnes (or 14 US tons) of solid UF6. In the U.S. alone, 560,000 tonnes of depleted UF6 had accumulated by 1993. In 2005, 686,500 tonnes in 57,122 storage cylinders were located near Portsmouth, Ohio
Portsmouth, Ohio
Portsmouth is a city in the U.S. state of Ohio and the county seat of Scioto County. The municipality is located on the northern banks of the Ohio River and east of the Scioto River in Southern Ohio. The population was 20,226 at the 2010 census.-Foundation:...
, Oak Ridge, Tennessee
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 Knoxville. Oak Ridge's population was 27,387 at the 2000 census...
, and Paducah, Kentucky
Paducah, Kentucky
Paducah is the largest city in Kentucky's Jackson Purchase Region and the county seat of McCracken County, Kentucky, United States. It is located at the confluence of the Tennessee River and the Ohio River, halfway between the metropolitan areas of St. Louis, Missouri, to the west and Nashville,...
. 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
Uranyl fluoride
Uranyl fluoride , a compound of uranium, is an intermediate in the conversion of uranium hexafluoride UF6 to an uranium oxide or metal form and is a direct product of the reaction of UF6 with moisture in the air. It is very soluble in water. Uranyl fluoride also is hygroscopic and changes in color...
) and HF (hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
) both of which are highly soluble and toxic. 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 United States, including a cylinder-filling accident and material release at the Sequoyah Fuels Corporation
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 :...
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 $
United States dollar
The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....
15 million to $450 million.
Further reading
- Uranium Hexafluoride in www.webelements.com
- US-Patent 2535572: Preparation of UF6; 26. December 1950.
- US-Patent 5723837: Uranium Hexafluoride Purification; 3. March 1998.
- Simon Cotton (Uppingham School, Rutland, UK): Uranium Hexafluoride.
- Import of Western depleted uranium hexafluoride (uranium tails) to Russia
- Uranium Hexafluoride (UF6) – Physical and chemical properties of UF6, and its use in uranium processing – Uranium Hexafluoride and Its Properties