Principle of a chain reaction
Following the absorption of a neutron, a fissile nucleus of uranium 235 or plutonium 239 splits into two unequal parts and release several secondary neutrons. In the diagram above, two of these three 'secondary' neutrons lead to fission reactions which, in turn, release neutrons capable of causing tertiary fission reactions. When the proportion of fissile nuclei is very high this chain reaction can become explosive, a property used to full effect inside atomic bombs. In the core of a reactor containing less than 4% fissile nuclei, the fuel is designed in such a way that each fission produces exactly one ‘torch bearer” neutron, leading to a continuous but stable chain reaction.