Nuclear fission. Numerical simulation of the splitting of atomic nuclei, dense regions made up of neutrons and protons found at the centre of an atom. The sphere shown is made up of 36, 000 radioactive nuclei (blue). Some of these nuclei are undergoing spontaneous fission, a type of radioactive decay, and are emitting neutrons (pink). These neutrons hit surrounding nuclei, inducing fission and leading to the release of more neutrons (red). The neutrons go on to hit more nuclei in a chain reaction. The rate of this chain reaction is plotted on the graph at bottom left.