Discuss the two major isotopes of Uranium, 235 and 238. Define that the fissionable one is 238, which is found in uranium ore, like UF6 at a % of 0.7. For reactor use, 3% is required, and for weapon use 90%. Discuss methods for uranium enrichment, separation of the two isotopes. Discuss and use diagrams to indicate that they can only be separated physically, by gas diffusion and/or by centrifugation. Each method can take years to separate the isotopes, with centrifugation being the newer and quicker method. Discuss critical mass needed to produce a chain reaction. Use diagrams and images to show how the critical masses can be produced by colliding subcritical masses together, as in a bomb.