Like all radioactive elements, Uranium comes in several isotopes. The most common isotope is Uranium 238, accounting for 99.284%. The isotope which allows for the chain reaction necessary to make an atom bomb is Uranium 235, accounting for 0.711%. In order to make an atom bomb, it is necessary to increase the proportion of Uranium 235 to more than 85%. This is called 'highly enriched uranium'. Uranium enriched to a lower level is used in nuclear power stations.
Enrichment is carried out by reacting Uranium with Fluorine, producing a gas. Molecules of different masses can be separated using either diffusion or centrifugal processes.