For more information, see The Science Behind the Atom Bomb. The chain reaction starts when neutrons strike the heavy uranium or plutonium nucleus which splits releasing a tremendous amount of energy along with two or more neutrons which, in turn split more nuclei, and so on. This is the minimum amount of fissionable material needed to start a chain reaction. An explosive nuclear chain reaction occurs when a sufficient quantity of nuclear fuel, such as uranium or plutonium, is brought together to form a critical mass.
