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).
The first self-sustaining nuclear chain reaction was initiated a team led by Enrico Fermi below the bleachers of Stagg Field at the University of Chicago on December 2, 1942 during the Manhattan Project.
See also nuclear physics