Workers load uranium slugs into the X-10 Graphite Reactor's concrete face. Built as part of the Manhattan Project, X-10 was the first-ever production reactor, and acted as a proof of concept for the reactors that would produce materials for the first nuclear bombs. The concrete face is 44 by 44 feet (13.4 by 13.4 metres). Part of the Oak Ridge National Laboratory in Oak Ridge, Tennessee, since 1966 it has been a designated National Historic Landmark.Photograph: Ed Westcott / US Army