Fission chain reaction. (Source: © Peter Hermes Furian / stock.adobe.com)

A fission chain reaction, which is not interfered with externally. This usually refers to the course of a fission reaction in an atomic bomb, where the concentration of fissile material is high enough and a so-called supercritical state is reached — more neutrons come out of each generation of fission than went into it, and the amount of energy released by fission increases rapidly.

In a nuclear power plant, the fission reaction is controlled using a neutron absorber and a moderator.