Uranium hexafluoride ( UF6 ) , colloquially known in the nuclear industry as ” hex “, is a compound used in the process of enriching uranium, which is the fuel for nuclear reactors and nuclear weapons. produces.
Hex forms solid gray crystals at standard temperature and pressure, is highly toxic, reacts with water, and is corrosive to most metals. The compound reacts mildly with aluminum, forming a thin surface layer of AlF that resists any further reaction from the compound.
preparation
Milled uranium ore – U 3 O 8 or “yellowcake” – dissolves in nitric acid, leaving a solution of uranyl nitrate U 2 (NO 3 ) 2 . Pure uranyl nitrate is obtained by solvent extraction, then treated with ammonia to produce ammonium duranate (” AdU “, ( NH4 ) 2U2O7 ) . Reduction with hydrogen gives UO2 , which with hydrofluoric acid (HF) is converted to uranium tetrafluoride, UF4 . Oxidation with fluorine yields UF 6 .
During nuclear reprocessing, uranium reacts with chlorine trifluoride to give UF 6 :
U + 2 Clf 3 → Uf 6 + Cl 2
Property
physical properties
At atmospheric pressure, it sublimes at 56.5 °C. [3]UF
6 in a glass ampoule.
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 [8] and a Lewis acid capable of binding fluoride; For example, the reaction of copper (II) fluoride with uranium hexafluoride in acetonitrile is reported to form copper (II) heptafluorouranate (VI), Cu (Uf 7 ) 2 . [9]
Polymeric uranium (VI) fluoride containing organic cations has been isolated and characterized by X-ray diffraction. [10]
applications in the nuclear fuel cycle
UF 6 is used in both main uranium enrichment methods – gaseous diffusion and gas centrifuge method – because its triple point is at 64.05 °C (147 °F, 337 K) and only slightly above normal atmospheric pressure. . Therefore fluorine has only one naturally occurring stable isotope, the different isotopes present in the isotopologs Uf of 6 based on their molecular weight uranium. [11 1]
All other uranium fluorides are non-volatile solids which are coordination polymers.
Gaseous diffusion requires approximately 60 times more energy than a gas centrifuge process: gaseous diffusion-produced nuclear fuel produces 25 times more energy than that used in the diffusion process, while centrifuge-produced The fuel generates 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 instability), which was developed in the Czech Republic. In this process, the used oxide nuclear fuel is treated with fluorine gas to form a mixture of fluoride. This mixture is then distilled to separate the different classes of material.
Uranium enrichment produces large amounts of reduced uranium hexafluoride, or DUF 6 , as a waste product . Long-term storage of DUF 6 presents environmental, health and safety risks due to its chemical instability. When UF6 is exposed to moist air, it reacts with 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 tons of DUF 6 were placed in 57,122 storage cylinders located near Portsmouth, Ohio; Oak Ridge, Tennessee ; and Paduka, Kentucky. [12] [13]Storage cylinders should be inspected regularly for signs of corrosion and leakage. The estimated lifetime of steel cylinders is measured in decades. [14]
There have been several accidents involving uranium hexafluoride in the US, including a cylinder filling accident and material release at Sequoyah Fuels Corporation in 1986. [15] The US government is converting DUF 6 to solid uranium oxide for disposal. [16] The cost of such disposal of the entire DUF 6 inventory can be anywhere from $15 million to $450 million.
- Broken 14-ton UF 6 shipping cylinder. 1 death, dozens injured. ~29500 lbs of material released. Sequoyah Fuel Corporation 1986.
- DUF 6 Storage Yard Remotely
- DUF 6 Cylinder: Pictured (left) and sculpted (right)
Observation
Enriched uranium is a type of uranium in which the percentage composition of uranium-235 is increased through a process of isotope separation . Natural uranium is 99.284% of the U isotope, with U only constituting about 0.711% of its mass . U is the only nuclide present in nature (in any appreciable amount) that is fissionable with thermal neutrons . Enriched uranium is an important component for both nuclear power production and military nuclear weapons. Ensuring the safety of nuclear power generation and the proliferation of nuclear weapons
The International Atomic Energy Agency attempts to monitor and control enriched uranium supplies and processes in efforts to prevent
Uranium was enriched with uranium, a shortened version of the Oak Ridge alloy codenamed Orloy , after the plant was replaced by uranium during the Manhattan Project . The term Orloy is sometimes used to refer to enriched uranium. There are approximately 2,000 tons (T, Mg) of highly enriched uranium in the world, mostly produced for nuclear power, nuclear weapons, naval propulsion, and small quantities for research reactors.
The remaining U after enrichment is known as uranium (DU), and is even significantly less radioactive than natural uranium, although there is still a very dense and extremely dangerous form of uranium – such granule shear. The natural by-products of the verb are what makes it useful for armor-penetrating weapons and radiation protection. Currently, 95 percent of the world’s depleted uranium reserves remain in safe storage.
The fission of the three isotopes 2 3 5 U ratio (0.72%) to naturally occurring uranium (mass number 234,235,238) leads to artificially elevated uranium. The percentage of 2 3 5 U, called enrichment, ranges from a few percentages of less enriched uranium to HEU 90% or even less. Gaseous uranium hexafluoride UF 6 diffuses through the perforated walls, using the difference in diffusion rate arising from the mass difference of uranium to a concentration of 2 3 5 U, this process is called gas diffusion .In addition to the enriched uranium is repeated thousands of times method, a cylinder in pouring uranium hexafluoride gas, the cylinder is rotated at high speed, pushing the heavy 2 3 8 U to the cylindrical wall , collecting 2 3 5 U in the central part. It has been developed for gas centrifugation. In this method, if the circumferential speed of the cylinder is operated approximately 27 times at 400 meters per second, approximately 5% enriched uranium is obtained. Electric power can be reduced to 1/10th of that of the diffusion method, but the amount of gas sent to the cylinder is small, hence the need for multiple centrifugal separators. Production cost is more expensive but 2 3 As nuclear fuel for 8U small reactors are small enough to absorb neutrons, have the advantage of ease of design, are widely used in power reactors or more. Since concentrated uranium is in a gaseous state with UF6 , it is converted back to uranium dioxide UO2 ( powder form) and made into nuclear reactor fuel. → Related commodities nuclear fuel | breeding reactor | Conversion Furnace | depleted uranium
