Uranium hexafluoride (UF₆)

Uranium hexafluoride (UF₆) is the chemical form of uranium used in the enrichment stage of the nuclear fuel cycle. It is produced by converting yellowcake (U₃O₈) into a compound of uranium and fluorine. UF₆ is used because it becomes a gas at a low temperature, and enrichment requires uranium in gaseous form.

Properties of uranium hexafluoride

At room temperature, UF₆ is a white crystalline solid. At atmospheric pressure it sublimes, turning directly from solid to gas, at about 56.5°C. This makes it easy to move between solid form for storage and gas form for enrichment. Fluorine has only one naturally occurring isotope, so differences in the mass of UF₆ molecules come only from the uranium isotope, which is what allows centrifuges to separate uranium-235 from uranium-238.

UF₆ reacts with moisture in the air to form hydrogen fluoride, a corrosive and toxic gas, and uranyl fluoride. Its chemical hazard is greater than its radiological hazard, so it is handled under strict controls.

How UF₆ is stored and shipped

UF₆ is stored and transported as a solid in sealed steel cylinders built to international standards. Natural UF₆ from conversion facilities typically ships in large 48-inch cylinders, and enriched UF₆ ships in smaller 30-inch cylinders inside protective overpacks.

Where UF₆ sits in the fuel cycle

Converting U₃O₈ into UF₆ is carried out at a small number of conversion facilities worldwide. The UF₆ is then enriched, and the enriched UF₆ is converted again into uranium dioxide (UO₂) powder and fabricated into fuel pellets. UF₆ sits between the traded commodity form of uranium, yellowcake, and finished reactor fuel. It is not the form in which uranium is bought and sold on the spot market. See the nuclear fuel cycle.

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