The Manhattan Project was the code name for the U.S. effort during World War II to produce the atomic bomb. It was named for the Manhattan Engineer District of the U.S. Army Corps of Engineers, because much of the early research was done in New York City. Sparked by refugee physicists in the United States, the program was slowly organized after nuclear fission was discovered by German scientists in 1938, and many U.S. scientists expressed the fear that Hitler would attempt to build a fission bomb.

The program was first under the leadership of Vannevar Bush, head of the National Defense Research Committee and the Office of Scientific Research and Development, and then under Gen. Leslie Groves of the Army Corps of Engineers. Groves immediately purchased a site at Oak Ridge, Tenn., for facilities to separate the necessary uranium-235 from the much more common uranium-238, and he consolidated the research done in many East Coast universities under the direction of Arthur Compton at the University of Chicago. He also appointed theoretical physicist J. Robert Oppenheimer as director of the weapons laboratory, built on an isolated mesa at Los Alamos, N.Mex. After much difficulty a porous barrier suitable for separating isotopes of uranium was developed and installed in the Oak Ridge gaseous diffusion plant. Finally, in 1945, uranium-235 of bomb purity was shipped to Los Alamos, where it was fashioned into a gun-type weapon. In a barrel one piece of uranium was fired at another, together forming a supercritical, explosive mass.

Another type of atomic bomb was also constructed using the synthetic element plutonium. Enrico Fermi built a reactor at Chicago in late 1942, the prototype of five production reactors erected at Hanford, Wash. These reactors manufactured plutonium by bombarding uranium-238 with neutrons. At Los Alamos the plutonium was surrounded with high explosives to compress it into a superdense, supercritical mass far faster than could be done in a gun barrel. The result was tested at Alamogordo, N.Mex., on July 16, 1945 — the first explosion of an atomic bomb.

Lawrence Badash

Bibliography: Groves, Leslie R., Now It Can Be Told (1962; repr. 1983); Hacker, Barton C., The Dragon's Tail: Radiation Safety in the Manhattan Project, 1942 –1946 (1987); Hales, Peter Bacon, Atomic Spaces: Living on the Manhattan Project (1997); Howes, Ruth H., and Herzenberg, Caroline L., Their Day in the Sun: Women of the Manhattan Project (1999); Stoff, Michael, et al., eds., The Manhattan Project: A Documentary Introduction to the Atomic Age (1991); Szasz, Ferenc Morton, The Day the Sun Rose Twice (1984; repr. 1995).