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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]

A nuclear reactor sustains and controls a nuclear chain reaction to produce heat or a useful neutron field. Nuclear fission supplies fresh neutrons; absorption and neutron leakage remove them. A steady neutron population requires a balance between production and losses, the condition of nuclear criticality.

The functions of its components are different. Fuel supplies fissioning atomic nuclei; control rods change neutron absorption; coolant transports heat. In a thermal reactor, a neutron moderator reduces neutron energies before subsequent reactions. Slowing neutrons and cooling the fuel are separate physical functions even when the same material performs both.

Chicago Pile-1 demonstrated a controlled self-sustaining chain reaction in 1942 using a uranium–graphite assembly and neutron-absorbing controls. The decisive observation was the neutron population's response to changing absorption, rather than generation of useful electrical power.^[ref] A power reactor adds heat-transfer and conversion equipment; decay heat persists after the chain reaction is stopped.