Uranium hexafluoride
Uranium hexafluoride (), colloquially known as "hex" in the nuclear industry, is the inorganic compound with the formula UF. Hex is a volatile white solid. It reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with aluminium, forming a thin surface layer of AlF that resists any further reaction from the compound. UF is used in the process of enriching uranium, which produces fuel for nuclear reactors and nuclear weapons. Preparation. Milled uranium ore—UO or "yellowcake"—is dissolved in nitric acid, yielding a solution of uranyl nitrate UO(NO). Pure uranyl nitrate is obtained by solvent extraction, then  treated with ammonia to produce ammonium diuranate ("ADU", (NH)UO). Reduction with hydrogen gives UO, which is converted with hydrofluoric acid (HF) to uranium tetrafluoride, UF. Oxidation with fluorine yields UF. During nuclear reprocessing, uranium is reacted with chlorine trifluoride to give UF: Properties. Physical properties. At atmospheric pressure, it sublimes at 56.5 °C. 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 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), Cu(UF). Polymeric uranium(VI) fluorides containing organic cations have been isolated and characterised by X-ray diffraction. Application in the nuclear fuel cycle. UF is used in both of the main uranium enrichment methods—gaseous diffusion and the gas centrifuge method—because its triple point is at temperature 64.05 °C (147 °F, 337 K) and only slightly higher than normal atmospheric pressure. Fluorine has only a single naturally occurring stable isotope, so isotopologues of UF differ in their molecular weight based solely on the uranium isotope present. All the  other uranium fluorides are nonvolatile solids that are coordination polymers. The conversion factor for UF ("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. 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, or DUF, as a waste product. The long-term storage of DUF presents environmental, health, and safety risks because of its chemical instability. When UF is exposed to moist air, it  reacts with the water in the air to produce UOF (uranyl fluoride) and HF (hydrogen fluoride) both of which are highly corrosive and toxic. In 2005, 686,500 tonnes of DUF was housed in 57,122 storage cylinders located near Portsmouth, Ohio; Oak Ridge, Tennessee; 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 in 1986. The U.S. government has been  converting DUF to solid uranium oxides for disposal. Such disposal of the entire DUF inventory could cost anywhere from $15 million to $450 million.