Discuss the need to separate the U-235 from the U-238 in a sample of uranium. The 235 is fissionable using most neutrons, while the 238 requires very high speed neutrons. Only 0.7% of a sample of uranium is the isotope 235, which must be separated. The 2 methods of separation are gaseous diffusion and centrifuging. The term diffusion had to be explained, and then the gaseous diffusion method discussed in detail. The centrifuge method was also briefly discussed, as to how it works and differs from the gaseous diffusion method.