A
nuclear chain reaction occurs when on average more than one
neutron from a
nuclear fission reaction causes another fission reaction. The rate of reactions will accelerate exponentially if left unmoderated. An uncontrolled chain reaction within a sufficiently large amount of fission fuel (critical mass) can lead to an explosive energy release and is the concept behind
nuclear weapons. The chain reaction could also be adequately controlled and used as an energy source (
nuclear reactor).
See also nuclear physics, Enrico Fermi, Manhattan Project