What Keeps Nuclear Weapons from Proliferating The hardest step in making a nuclear bomb

Bill explains that the hardest step is making the proper type of uranium. Weapons and power plants require uranium that contains a greater amount of the isotope uranium-235 than found in natural uranium, which is mostly uranium-238. He outlines the key difficulty in separating the two isotope: They have nearly identical properties. He explains the two key methods for separation: Gas diffusion and centrifuges.