Fermi Achieves First Self-Sustaining Nuclear Chain Reaction

Context

The concept of a nuclear chain reaction originated with physicist Leo Szilard in 1933, but it gained urgency after the 1938 discovery of nuclear fission by Otto Hahn and Fritz Strassmann, with theoretical explanation by Lise Meitner and Otto Frisch. Early experiments at Columbia University by Fermi and Szilard demonstrated neutron multiplication in uranium, prompting work on a moderator to sustain the process. Fears that Nazi Germany might harness the technology for weapons drove the United States to organize the Manhattan Project, which consolidated efforts at the University of Chicago's Metallurgical Laboratory under physicist Arthur Compton.

What Happened

By late 1942 the Metallurgical Laboratory had assembled Chicago Pile-1, a roughly spherical stack of 45,000 graphite blocks interspersed with uranium metal and oxide, in a squash court under the west stands of Stagg Field. A team of about thirty scientists, including Fermi, Szilard, Leona Woods, Herbert Anderson, Walter Zinn, and Martin Whitaker, completed the structure in November. On December 2 the experiment began with all but one cadmium control rod inserted; physicist George Weil withdrew the final rod in measured increments while counters tracked neutron activity. As the reaction approached and then sustained criticality, Fermi monitored the rising neutron count and confirmed the chain reaction was self-sustaining at low power.

Aftermath

A coded message reporting success was sent to project leaders in Washington that afternoon. The pile operated briefly at roughly half a watt before being shut down; it was later dismantled, relocated to Site A near Chicago, and reconfigured as Chicago Pile-2 for continued low-power research. The demonstration immediately cleared the way for larger graphite-moderated production reactors at Hanford, Washington, which generated plutonium for the Nagasaki bomb.

Legacy

The Chicago Pile-1 experiment proved that a controlled nuclear chain reaction was possible, laying the technical foundation for both nuclear power reactors and the plutonium pathway in atomic weapons. It accelerated the Allied nuclear program while exposing the limitations of the German effort, which had underestimated graphite's potential as a moderator. The achievement is commemorated at the site by a Henry Moore sculpture and remains a landmark in the history of nuclear physics and twentieth-century technology.