Sketch of the worlds first reactor Due to wartime secrecy there are no photographs of the completed reactor. The reactor itself was composed of layers of graphite interspersed with Uranium. Criticality, that is, self sustained chain reaction, was achieved on December 2nd 1942, when the 57th layer was added. This gave rise to the early name for reactor ' piles ' because of piling one layer on another. Achieving control of the release of the power of the atom was a major breakthrough in what was then a race to make an atomic weapon. Word of the success was telephoned immediately to Washington in this cryptic language ' The Italian Navigator has landed in the New World and found the natives very friendly', The historic event took place in an improvised laboratory in the racquets court under the West Stands of the University of Chicago's Stagg Field