Most commercial uranium enrichment processes (gaseous diffusion and the gas centrifuge method) require the uranium to be in a gaseous form, therefore the uranium oxide concentrate must be first converted to uranium hexafluoride, which is a gas at relatively low temperatures. At atmospheric pressure, uranium hexafluoride sublimes at 56.5 °C. At this stage of the cycle the uranium hexafluoride conversion product still has the natural isotopic mix, i.e. it contains only 0.71% of fissile isotope 235U. In the enrichment process gaseous uranium hexafluoride is separated into two streams, one being enriched to the required level and known as low-enriched uranium; the other stream is progressively depleted in uranium-235 and is called ‘tails’. For lower temperatures, uranium hexafluoride forms solid grey crystals at standard temperature and pressure, is highly toxic, reacts with water, and is corrosive to most metals. For transportation, it will be put in a special container and then it will be carried to the enrichment plant.
Uranium enrichment produces large quantities of depleted uranium hexafluoride, or DUF6, as a waste product. The long-term storage of DUF6 presents environmental, health, and safety risks because of its chemical instability.