Uranium gas centrifuge enrichment units. This stage in the production of fuel for nuclear power plants takes place at a gas centrifuge enrichment plant (GCEP). In its natural state, uranium contains three isotopes. Of these, U-235 is used for fission but only accounts for 0.7 percent of mined uranium. During the process of enrichment, the mined uranium is converted into uranium hexafluoride gas. This is spun in a centrifuge and the molecules containing heavier U-238 move outwards, with the gas richer in U-235 passed to further stages of purification before conversion into usable fuel. Photographed in January 1984, at the GCEP in Piketon, Ohio, USA.