Who achieved the first controlled nuclear chain reaction in 1942?

Enrico Fermi achieved the first controlled, self-sustaining nuclear chain reaction in 1942.

The experiment took place on December 2, 1942, beneath the stands of Stagg Field at the University of Chicago. Fermi led a team that built Chicago Pile-1, the world's first artificial nuclear reactor. The pile used layers of natural uranium and ultra-pure graphite, which slowed neutrons so they were more likely to trigger further uranium fissions.

When the reactor became critical, the chain reaction sustained itself for about 4.5 minutes at roughly half a watt. Control rods absorbed neutrons and allowed the scientists to regulate or stop the reaction. The experiment demonstrated that nuclear energy could be controlled, a crucial step toward nuclear reactors and the Manhattan Project's later weapons work.

A common mix-up is naming J. Robert Oppenheimer, who directed the Manhattan Project's weapons laboratory, or Leo Szilard, who helped develop the chain-reaction concept. Fermi, however, led the successful Chicago Pile-1 demonstration.