Nuclear fuel production. Gas diffusion machinery and workers in a factory used to enrich uranium for use in nuclear reactors. Uranium consists of two natural isotopes: mostly U-238 and less than 1 percent U-235. The U-235 needs to be increased to around 3-5 percent of the overall mixture to make a controlled nuclear fission reaction feasible. Here, the enrichment is carried out by gas diffusion by heating solid uranium hexafluoride (UF6). The UF6 molecules containing U-235 are lighter and diffuse faster through the pipelines and filters, producing a UF6 gas that is enriched with U-235. After cooling and solidifying, the enriched UF6 is used to make fuel for nuclear power stations. This is Eurodif's George Besse factory at the Tricastin site in southern France. Photographed in 2004.