What was the first nuclear reactor to produce a self-sustaining chain reaction?

Chicago Pile-1 was the first nuclear reactor to produce a self-sustaining chain reaction.

Physicists led by Enrico Fermi achieved the milestone on 2 December 1942 beneath the stands of Stagg Field at the University of Chicago. The reactor used graphite blocks to moderate neutrons and uranium as fuel. Control rods absorbed neutrons and allowed the team to regulate the reaction.

The achievement demonstrated that a controlled nuclear chain reaction was possible. It became a major step in the Manhattan Project and in the later development of nuclear reactors for research, electricity generation, and isotope production.

Chicago Pile-1 was not a commercial power station and did not generate electricity for a public grid. It also differed from later reactors in size, shielding, cooling, and engineering. The name “pile” referred to its stacked construction of graphite and uranium, not to an electrical battery.