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Names | |
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IUPAC names Uranium hexafluoride Uranium(VI) fluoride | |
Identifiers | |
3D model (JSmol) | |
Abbreviations | hex |
ChEBI | |
ChemSpider | |
ECHA InfoCard | 100.029.116 |
EC Number |
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2923 | |
PubChem CID | |
RTECS number |
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UNII | |
UN number | 2978 (<1% 235U) 2977 (>1% 235U) |
CompTox Dashboard (EPA) | |
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Properties | |
UF6 | |
Molar mass | 352.02 g/mol |
Appearance | Colorless solid |
Density | 5.09 g/cm3, solid |
Boiling point | 56.5 °C (133.7 °F; 329.6 K) (sublimes, at atmospheric pressure) |
Hydrolyzes | |
Solubility |
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Structure | |
Orthorhombic, oP28 | |
Pnma, No. 62 | |
Octahedral (Oh) | |
0 | |
Thermochemistry | |
Std molar entropy (S⦵298) | |
Std enthalpy of formation (ΔfH⦵298) | |
Hazards | |
Occupational safety and health (OHS/OSH): | |
Main hazards | Toxic, corrosive, radioactive [3] |
GHS labelling: | |
Danger | |
H300, H330, H373, H411 | |
NFPA 704 (fire diamond) | |
Flash point | Non-flammable |
Safety data sheet (SDS) | ICSC 1250 |
Related compounds | |
Other anions | Uranium hexachloride |
Other cations | |
Related uranium fluorides | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Uranium hexafluoride, sometimes called hex, is an inorganic compound with the formula U F 6. Uranium hexafluoride is a volatile and toxic white solid that reacts with water, releasing corrosive hydrofluoric acid. 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.
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 ("ADU", [NH4]2U2O7). Reduction with hydrogen gives UO2, which is converted with hydrofluoric acid (HF) to uranium tetrafluoride, UF4. Oxidation with fluorine yields UF6.
The Honeywell Uranium Hexafluoride Processing Facility uses a different process.
During nuclear reprocessing, uranium is reacted with chlorine trifluoride to give UF6:
At atmospheric pressure, UF6 sublimes at 56.5 °C. [4]
The solid-state structure was determined by neutron diffraction at 77 K and 293 K. [5] [6]
It has been shown that uranium hexafluoride is an oxidant [9] and a Lewis acid that is able to bind to fluoride; for instance, the reaction of copper(II) fluoride with uranium hexafluoride in acetonitrile is reported to form copper(II) heptafluorouranate(VI), Cu2+[UF−7]2. [10]
Polymeric uranium(VI) fluorides containing organic cations have been isolated and characterized by X-ray diffraction. [11]
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 UF6; 64 °C(147 °F; 337 K) and 152 kPa (22 psi; 1.5 atm) [12] ; 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 UF6 differ in their molecular weight based solely on the uranium isotope present. [13] 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 238U isotopologue of UF6 ("hex") to "U mass" is 0.676. [14]
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 (DUF6 or D-UF6) as a waste product. The long-term storage of D-UF6 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 corrosive and toxic. In 2005, 686,500 tonnes of D-UF6 was housed in 57,122 storage cylinders located near Portsmouth, Ohio; Oak Ridge, Tennessee; and Paducah, Kentucky. [15] [16] Storage cylinders must be regularly inspected for signs of corrosion and leaks. The estimated lifetime of the steel cylinders is measured in decades. [17]
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 UF6 escaped. [18] [19] The U.S. government has been converting DUF6 to solid uranium oxides for disposal. [20] Such disposal of the entire DUF6 stockpile could cost anywhere from $15 million to $450 million. [21]
Uranium is a chemical element; it has symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium radioactively decays by emitting an alpha particle. The half-life of this decay varies between 159,200 and 4.5 billion years for different isotopes, making them useful for dating the age of the Earth. The most common isotopes in natural uranium are uranium-238 and uranium-235. Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite.
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research. By tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. In the following text, mainly uranium enrichment is considered. This process is crucial in the manufacture of uranium fuel for nuclear power plants, and is also required for the creation of uranium-based nuclear weapons. Plutonium-based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or grade.
Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Naturally-occurring uranium is composed of three major isotopes: uranium-238, uranium-235, and uranium-234. 235U is the only nuclide existing in nature that is fissile with thermal neutrons.
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.
Yellowcake 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 fuel fabrication or uranium enrichment. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores. Typically, yellowcakes are obtained through the milling and chemical processing of uranium ore, forming a coarse powder that has a pungent odor, is insoluble in water, and contains about 80% uranium oxide, which melts at approximately 2880 °C.
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 radius of a rotating container. A prominent use of gas centrifuges is for the separation of uranium-235 (235U) from uranium-238 (238U). The gas centrifuge was developed to replace the gaseous diffusion method of uranium-235 extraction. High degrees of separation of these isotopes relies on using many individual centrifuges arranged in series, that achieve successively higher concentrations. This process yields higher concentrations of uranium-235 while using significantly less energy compared to the gaseous diffusion process.
Gaseous diffusion is a technology that was used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) between the molecules containing uranium-235 (235U) and uranium-238 (238U). By use of a large cascade of many stages, high separations can be achieved. It was the first process to be developed that was capable of producing enriched uranium in industrially useful quantities, but is nowadays considered obsolete, having been superseded by the more-efficient gas centrifuge process (enrichment factor 1.05 to 1.2).
K-25 was the codename given by the Manhattan Project to the program to produce enriched uranium for atomic bombs using the gaseous diffusion method. Originally the codename for the product, over time it came to refer to the project, the production facility located at the Clinton Engineer Works in Oak Ridge, Tennessee, the main gaseous diffusion building, and ultimately the site. When it was built in 1944, the four-story K-25 gaseous diffusion plant was the world's largest building, comprising over 5,264,000 square feet (489,000 m2) of floor space and a volume of 97,500,000 cubic feet (2,760,000 m3).
The Honeywell Uranium Hexafluoride Processing Facility, a uranium conversion facility, is located 1.9 miles (3 km) northwest of Metropolis, Illinois, United States. The plant, Honeywell Specialty Chemicals in Metropolis, Illinois, has a nominal capacity of 15,000 tU as uranium hexafluoride per year. ConverDyn, a general partnership between affiliates of Honeywell and General Atomics, is the exclusive agent for conversion sales from the Honeywell Uranium Hexafluoride Processing Facility.
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
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 difluorides and trifluorides have high boiling points, while most tetrafluorides and monofluorides fall in between. The term "fluoride volatility" is jargon used particularly in the context of separation of radionuclides.
Uranyl nitrate is a water-soluble yellow uranium salt with the formula UO2(NO3)2 · n H2O. The hexa-, tri-, and dihydrates are known. The compound is mainly of interest because it is an intermediate in the preparation of nuclear fuels.
Molecular laser isotope separation (MLIS) is a method of isotope separation, where specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions of uranium hexafluoride molecules. It is similar to AVLIS. Its main advantage over AVLIS is low energy consumption and use of uranium hexafluoride instead of vaporized uranium.
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula UO2+
2. It has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands may be bound to the uranyl ion in an equatorial plane around the uranium atom. The uranyl ion forms many complexes, particularly with ligands that have oxygen donor atoms. Complexes of the uranyl ion are important in the extraction of uranium from its ores and in nuclear fuel reprocessing.
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 processes. In the uranium refining industry it is known as green salt.
Uranium compounds are compounds formed by the element uranium (U). Although uranium is a radioactive actinide, its compounds are well studied due to its long half-life and its applications. It usually forms in the +4 and +6 oxidation states, although it can also form in other oxidation states.
Plutonium hexafluoride is the highest fluoride of plutonium, and is of interest for laser enrichment of plutonium, in particular for the production of pure plutonium-239 from irradiated uranium. This isotope of plutonium is needed to avoid premature ignition of low-mass nuclear weapon designs by neutrons produced by spontaneous fission of plutonium-240.
Neptunium(VI) fluoride (NpF6) is the highest fluoride of neptunium, it is also one of seventeen known binary hexafluorides. It is an orange volatile crystalline solid. It is relatively hard to handle, being very corrosive, volatile and radioactive. Neptunium hexafluoride is stable in dry air but reacts vigorously with water.
ConverDyn is a general partnership between American multinational firms General Atomics and Honeywell that provides uranium hexafluoride (UF6) conversion and related services to utilities operating nuclear power plants in North America, Europe, and Asia. The company is the sole marketing agent of UF6 produced at the Honeywell Uranium Hexafluoride Processing Facility in Metropolis, Illinois.
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 (up to 73-75%), along with depleted triuranium octoxide (up to 25%) and depleted uranium metal (up to 2%). DUHF is 1.7 times less radioactive than uranium hexafluoride and natural uranium.