On this date in 1942, Chicago Pile-1 (CP-1) became the first nuclear reactor to achieve a self-sustaining nuclear chain reaction. Built under the supervision of Enrico Fermi as part of the Manhattan Project, CP-1 was a literal pile, with layers of graphite blocks being alternated with mixed layers of graphite and uranium metal and uranium oxide (see first image). Also included were control rods of cadmium sheets. The resulting structure was 6.1 m tall with an oval cross section. The reaction was triggered as the team slowly removed the control rods and ran critical for 4.5 minutes at about 0.5 W. (Image credit: US Department of Energy).
