Review the formula for uranium fission and chain reactions. Present that the U-235 isotope does not undergo fission, while the U-238 isotope does. Discuss the separation by physical means only, not chemically. Present that uranium ore contains only 0.7% of U-235, the fissionable isotope. Use diagrams and sketches to elicit that the isotopes are separated by gas diffusion through a porous membrane. In order for a sample of uranium to act in a reactor, it must be at least 3%, and act as a bomb, 90%. Separation of the isotopes can take years of diffusion just to obtain a small amount. Discuss the newer method of separation using centrifuges.