Fissionable uranium 235 only exists in about 0.7% of a uranium sample. The U-235 is needed for nuclear reactors and nuclear weapons. The U-235 is extracted from a mixture of uranium by a process of uranium enrichment. This process involves a physical separation of U-235 from a gaseous uranium compound by gaseous diffusion. The process of gaseous diffusion was explained, along with separation by centrifugation.