Chicago Pile-1 (CP-1) was the world's first man-made nuclear reactor. The construction of CP-1 was part of the Manhattan Project, and was carried out by the Metallurgical Laboratory at the University of Chicago.
CP-1 was built under the west stands of the original Stagg Field. The first self-sustaining nuclear chain reaction was initiated in CP-1 on 2 December 1942.
The reactor was a "pile" of uranium pellets and graphite blocks, assembled under the supervision of the renowned physicist Enrico Fermi, in collaboration with Leó Szilárd, discoverer of the chain reaction, and assisted by Martin Whittaker, Walter Zinn, and George Weil. It contained a critical mass of fissile material, together with control rods. The shape of the pile was intended to be roughly spherical, but as work proceeded Fermi calculated that critical mass could be achieved without finishing the entire pile as planned.
CP-1 was originally to be built in Red Gate Woods, a forest preserve outside the city, but a labor strike prevented this. So Fermi built the "pile" under the west stands of Stagg Field, the University's abandoned football stadium, in a space that had been used as a rackets court. In the pile, the neutron-producing uranium pellets were separated from one another by graphite blocks. Some of the free neutrons produced by the natural decay of uranium would be absorbed by other uranium atoms, causing nuclear fission of those atoms and the release of additional free neutrons. The graphite between the uranium pellets was a neutron moderator; it slowed the neutrons, increasing the chance they would absorbed. Fermi himself described the apparatus as "a crude pile of black bricks and wooden timbers."