<P> In the United States , where Fermi and Szilárd had both emigrated , this led to the creation of the first man - made reactor , known as Chicago Pile - 1 , which achieved criticality on December 2 , 1942 . This work became part of the Manhattan Project , a massive secret US government military project to make enriched uranium by building large reactors to breed plutonium for use in the first nuclear weapons . The US tested atom bombs and eventually these weapons were used to attack the cities of Hiroshima and Nagasaki . </P>
when was the first nuclear power plant opened
December 2 , 1942