Nuclear fission. Numerical simulation of the decay of a sphere of 5000 radioactive uranium nuclei (blue) in a process called nuclear fission. This process is the splitting of the nucleus of an atom. The nucleus of an atom contains neutrons and protons. Spontaneous decay of the heavy and unstable uranium nuclei causes the release of neutrons (pink). These neutrons hit surrounding nuclei, inducing fission and leading to the release of more neutrons (red). This leads to a nuclear chain reaction, shown in the graph at lower left. The induced neutron generation rate jumps to a maximum. As the number of uranium nuclei left decreases, the decay rate drops over time. Nuclear fission is used in power stations and atomic weapons. For a series of animations with differing number of uranium atoms, see K003/7258 to K003/7260.