A nuclear chain reaction occurs when one nuclear reaction causes one or more subsequent nuclear reactions, leading to a self-propagating series or "positive feedback loop" of these reactions. In a fission chain reaction, a free neutron interacts with the nucleus of an atom and causes that nucleus to split apart into two smaller nuclei, releasing additional neutrons. These neutrons can be absorbed by other fissionable nuclei, releasing still more neutrons, and the process repeats. The chain reaction is self-sustaining when the number of neutrons released in a given time equals or exceeds the number of neutrons lost by absorption in non-fissionable material or by escape from the system.

The specific nuclear reaction that causes the chain reaction may be the fission of heavy isotopes, such as uranium-235. A nuclear chain reaction releases several million times more energy per reaction than any chemical reaction. The energy released from each fission includes kinetic energy of fission products, gamma rays, kinetic energy of the neutrons, and energy from fission products.

Chain reactions can naturally give rise to reaction rates that grow (or shrink) exponentially, whereas a nuclear power reactor needs to be able to hold the reaction rate reasonably constant. To maintain a constant reaction rate, control rods made of materials such as boron or cadmium are inserted into the reactor core to absorb excess neutrons and slow down the chain reaction.