Uranium hexafluoride
Chemical compound
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Through Exothermic process, Atmospheric pressure and Sublimation
Preparation
Uranium dioxide is converted with hydrofluoric acid (HF) to uranium tetrafluoride:
- UO2 + 4 HF → UF4 + 2 H2O
The resulting UF4 is subsequently oxidized with fluorine to give the hexafluoride:
- UF4 + F2 → UF6
In samples contaminated with uranium trioxide, uranyl fluoride, an oxyfluoride compound is produced in the HF step:
- UO3 + 2 HF → UO2F2 + H2O
which can be fluorinated to produce the same product, uranium hexafluoride.
- UO2F2 + 2 F2 → UF6 + O2
The fluorination steps in both reactions above are highly exothermic.
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Properties
Physical properties
At atmospheric pressure, UF6 sublimes at 56.5 °C.
The solid-state structure was determined by neutron diffraction at 77 K and 293 K.
Chemical properties
UF6 reacts with water, releasing hydrofluoric acid. The compound reacts with aluminium, forming a surface layer of AlF3 that resists any further reaction from the compound.
Uranium hexafluoride is a mild oxidant. It is a Lewis acid as evidenced by its binding to form heptafluorouranate(VI), [UF7]−.
Polymeric 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 methods, namely gaseous diffusion and the gas centrifuge process. Since the triple point of UF6, which is 64 °C (147 °F; 337 K) and 152 kPa (22 psi; 1.5 atm), 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. 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.
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, about 686,000 tonnes of D-UF6 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.
Accidents and disposal
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 lb (13,400 kg) of gaseous UF6 escaped. The US 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.