Natural uranium hexafluoride (UF6) arrives at the enrichment plants in 48Y steel cylinders which also serve as operational cylinders. The most useful part of the uranium isotope for use in a nuclear reactor is called U‐235. In natural uranium the U‐235 isotope is only about 0,711% by mass and as such natural uranium cannot be used as a fuel for nuclear reactors. Before use in a nuclear reactor the UF6 has to be enriched by either gaseous diffusion or gas centrifuge. This process separates the uranium isotopes by mass. The natural UF6 is therefore enriched from 0.711% to ca. 5%. Once enriched, 30B cylinders (30 inches in diameter) are filled with the product. These cylinders are placed into overpacks (protective shipping packages) for transportation to fuel fabrication plants.