Seventy one years ago — on December 2, 1942, at 3:25 pm — Enrico Fermi and his team achieved the first controlled, man-made, self sustaining chain reaction in a simple reactor. In recognition of that historical event, several of my nuclear colleagues refer to December 2 as “Critmass” (short for critical mass).
The first nuclear reactor — CP-1 (Critical Pile number 1) — was actually a carefully constructed pile consisting of graphite bricks, uranium oxide pseudospheres, and uranium metal pseudospheres. The pile was built on a 30 by 60 foot squash court located under the stands at Stagg Field, the former home of the University of Chicago’s short-lived, but successful, football program.
The construction period was remarkably brief; the stacking process started on November 16, 1942, slightly more than two weeks before the criticality experiment.
In 2012, the Argonne National Laboratory produced a short history video that included interviews of Harold Agnew and Warren Nyers, two of the atomic pioneers who were part of the team that built the pile and produced the world-changing demonstration. ANL also has a Flickr page of CP-1 related images worth perusing.