The concept of a nuclear chain reaction was hypothesized in 1933, shortly after Chadwick's discovery of the neutron. Only a few years later, in December 1938 nuclear fission was discovered by Otto Hahn and his assistant Fritz Strassmann. Hahn understood that a "burst" of the atomic nuclei had occurred. The first artificial self-sustaining nuclear chain reaction (Chicago Pile-1, or CP-1) took place in December 1942 under the leadership of Enrico Fermi.
