Fission device

From a nontechnical standpoint, nuclear fission is the mechanism that causes the intense energy release of a fission weapon. In this context, the nucleus of a radioactive element, such as plutonium, is struck by a subatomic particle, a neutron. When the unstable nucleus captures the neutron, it splits into two new nuclei, releases energy, and emits two new neutrons.

If the fission were only of one nucleus, the energy release would be infinitesimal. When the system is constructed such that the emitted neutrons hit other nuclei and cause additional fissions, the process of a chain reaction exists. In a nuclear power reactor, the rate of the chain reaction is carefully controlled.