Cascade converters at a nuclear fuel enrichment plant in Kentucky, USA. Naturally-occuring uranium contains only 0.7% of the fissionable U-235 isotope, the remainder being the heavier U-238. The metal is reacted with fluorine to form uranium hexafluoride gas. This is then passed through the series of cascade converters, where the heavier U-238 hexafluoride molecules pass through diffusion filters and are removed. This process eventually produces a concentration of about 3% U-235 hexafluoride. This final product is then reconverted back to uranium metal for use in nuclear reactor fuel rods.