In the United States , where Fermi and Szilárd had both emigrated , the discovery of the nuclear chain reaction led to the creation of the first man-made reactor , the research reactor known as Chicago Pile-1 , which achieved self-sustaining power/criticality on December 2 , 1942 . The reactor 's development was part of the Manhattan Project , the Allied effort to create atomic bombs during World War II . It led to the building of larger single-purpose production reactors , such as the X-10 Pile , for the production of weapons-grade plutonium for use in the first nuclear weapons . The United States tested the first nuclear weapon in July 1945 , the Trinity test , with the atomic bombings of Hiroshima and Nagasaki taking place one month later
Trinity test
Trinity (nuclear test)
Nuclear power