Chicago Pile-1 was the experimental nuclear reactor that achieved a controlled self-sustaining fission chain reaction on 2 December 1942, in a structure beneath the University of Chicago's Stagg Field stands. The project was led by Enrico Fermi.
Uranium supplied fissile material, graphite moderated neutrons, and neutron-absorbing control rods adjusted the chain reaction. Gradually changing the absorber configuration let the team test approach to criticality. The scientific milestone was controlled neutron multiplication sufficient to sustain the reaction; the pile was an experimental assembly, not a power-producing station.[ref]
https://www.ne.anl.gov/About/early-days/. Argonne institutional account of Chicago Pile-1 and criticality date
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Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor, designed and built as part of the Manhattan Project during World War II. It was located under the stands of Stagg Field at the University of Chicago and became operational on December 2, 1942. CP-1 was constructed by a team of scientists and engineers, including notable figures such as Enrico Fermi, who led the project.
- youtu.be/mnScq24BEmc?t=114 the main cost for the reactor was the graphite. Presumably they already had the uranium in hand?. Edit, no, it is because it was a specialized graphite: Video 2. "German graphite from The Genius Behind the Bomb (1992)", i.e. nuclear graphite.