Portsmouth Gaseous Diffusion Plant, aerial photograph. This site, in Ohio, USA, was used to enrich nuclear fuel from 1954 to 2001. Naturally-occurring uranium is reacted with fluorine to form uranium hexafluoride gas. This is then passed through the series of cascade converters, where the heavier uranium isotope is removed, increasing the concentration of the U-235 used in nuclear weapons and power stations. This view shows the large process buildings and other facilities. Image published in 2015.