Chicago Pile-1 — The first nuclear reactor was erected in 1942 in the West Stands section of Stagg Field at the University of Chicago. On December 2, 1942 a group of scientists achieved the first self-sustaining chain reaction and thereby initiated the controlled release of nuclear energy. The reactor consisted of uranium and uranium oxide lumps spaced in a cubic lattice embedded in graphite. In 1943 it was dismantled and reassembled at the Palos Park unit of the Argonne National Laboratory.
Photo Credit: By Melvin A. Miller of the Argonne National Laboratory - Public Domain, https://commons.wikimedia.org/w/index.php?curid=8147703