December 2, 1942

Chicago Pile-1

Chicago Pile-1 achieves the first controlled, self-sustaining nuclear chain reaction

The first controlled nuclear reactor ran at roughly half the power of a small light bulb — and was shut down, deliberately, after 28 minutes.

2 December 1942: criticality in a squash court

On 2 December 1942, in the squash-rackets court beneath the west stands of the University of Chicago's Stagg Field, Enrico Fermi's team progressively withdrew Chicago Pile-1's neutron-absorbing control rods. On Fermi's instruction, George Weil pulled the final cadmium-plated rod, a few inches at a time — and the neutron count showed the pile had gone critical and self-sustaining.

Forty-nine people were present. The pile itself was enormous for its whisper of output: almost 400 tons of graphite, 6 tons of uranium metal and 50 tons of uranium oxide in 57 completed layers, operating at approximately half a watt.

28 minutes, then the rod goes back in

The team let the controlled reaction run for 28 minutes before reinserting the rod and shutting it down. Reaching criticality did not produce an explosion: Fermi kept the power extremely low, demonstrating that a natural-uranium, graphite-moderated chain reaction could be created and controlled.

Sources give slightly different clock times for the moment of criticality — about 3:20, 3:22 or 3:25 p.m. — but the date, sequence and achievement are firmly documented.

'The Italian navigator has just landed'

Arthur Compton, director of the Metallurgical Laboratory, telephoned James B. Conant at Harvard and reported success in code: 'The Italian navigator has just landed in the New World.' The line went to a university president — to Conant at Harvard, not to President Roosevelt.

The witnesses celebrated with a bottle of Chianti and signed its woven basket, which survives in the National Archives. CP-1 validated the reactor principle that the X-10 reactor at Oak Ridge and the plutonium-production reactors at Hanford would later scale up; CP-1 was dismantled in early 1943, and its material was used in the shielded CP-2 reactor at Site A in the Argonne Forest Preserve.

Sources

Researched 23 Aug 2026 8 sources 3 audit passes

How this was checked

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What the sources leave uncertain

Contemporary and commemorative sources give slightly different clock times for criticality, including approximately 3:20, 3:22, and 3:25 p.m. The date, sequence, and achievement itself are firmly documented.
The wording “first human-made” distinguishes CP-1 from naturally occurring prehistoric fission reactors discovered much later at Oklo.