Which country built the world's first known nuclear reactor?

The United States built the world's first known nuclear reactor, Chicago Pile-1. It achieved the first human-made, self-sustaining nuclear chain reaction on 2 December 1942 beneath the stands of Stagg Field at the University of Chicago.

The reactor was designed and built under the direction of Italian physicist Enrico Fermi as part of the Manhattan Project. It used blocks of graphite to moderate neutrons and uranium as fuel. Chicago Pile-1 had no cooling system because it produced only a very small amount of power, approximately half a watt during its first controlled reaction.

The experiment demonstrated that a controlled chain reaction was physically possible and became a crucial milestone in nuclear science. It was later dismantled and rebuilt at a site outside Chicago. Chicago Pile-1 was an experimental reactor, not a commercial power station, and it did not generate electricity for public use.