Graphite, uranium, and careful control produced the first self-sustaining nuclear chain reaction.
Year
1942
People
Enrico Fermi, Leona Woods, Arthur Compton, George Weil
Ideas at stake
chain reaction, nuclear reactor, criticality, control rod
A pile built for a number
On 2 December 1942, beneath the stands of the University of Chicago’s unused football field, a team assembled layers of graphite and uranium. They were not trying to make an explosion. They wanted the neutron multiplication factor k to reach exactly one.
Below one, the chain reaction fades. Above one, it grows. At one, each generation replaces itself.
Graphite slowed neutrons so uranium could absorb them more effectively. Cadmium control rods captured neutrons and adjusted the reaction. As George Weil withdrew the final rod, Enrico Fermi watched the instruments and calculations.
Chicago Pile-1 became the first human-made self-sustaining nuclear chain reaction. It operated at very low power and was shut down after less than half an hour.
The dramatic phrase is “first reactor.” The deeper accomplishment was feedback: measure the multiplication, move the absorber, and keep a potentially growing process at a chosen state. Nuclear engineering began not merely by releasing energy, but by learning to govern its rate.
