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 first be converted to uranium hexafluoride, a gas at relatively low temperatures. At atmospheric pressure, uranium hexafluoride sublimes at 56.5 °C. At this cycle stage, 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 carried to the enrichment plant.
As a waste product, uranium enrichment produces large quantities of depleted uranium hexafluoride or DUF6. The long-term storage of DUF6 presents environmental, health, and safety risks because of its chemical instability.