B Reactor is the world's first production-scale nuclear reactor, built in 1943-1944, as part of the Manhattan Project, the United States' wartime program to build a nuclear weapon. It produced plutonium used in the world's first nuclear detonation, the Trinity test, at Alamogordo, New Mexico, on July 16, 1945, and in the bomb that destroyed Nagasaki, Japan, on August 9, 1945. B Reactor served as the design model for early nuclear facilities constructed during the Cold War.

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Information courtesy of the National Park Service.