Recognizing that a project to build atomic bombs would require immense industrial resources, the Americans took the lead. They organized the manhattan Engineer District of the Army Corps of Engineers in 1942. Vannevar Bush, head of the Office of Scientific Research and Development (OSRD), appointed Brigadier General Leslie Richard Groves, who had overseen construction of the Pentagon, to direct the manhattan Project.
Sustaining and controlling a nuclear chain reaction was the first crucial technical step. Fermi's team accomplished this at the Metallurgical Laboratory at the University of Chicago on 2 December 1942 (the actual nuclear pile occupied a squash court at Stagg Field). Fermi thus proved that a larger reactor could produce enough of a highly fissionable isotope of plutonium (239Pu) to make a bomb. Work began on a reactor complex at Hanford, Washington, to produce the required plutonium.
The uranium 235 (235U) isotope mentioned in the Frisch-Peierls memorandum also held considerable promise as bomb material and was pursued simultaneously. Separation of 235U from 238U was accomplished at Oak Ridge, Tennessee, by gaseous diffusion and electromagnetic separation in an immense plant that covered 43 acres and employed 31,000 people. As work progressed on 239Pu and 235U production, Groves recognized that a central laboratory was needed to design, develop, and assemble the bombs. He chose Julius Robert Oppenheimer to direct what became known as Los Alamos Laboratory. Work began there in the spring of 1943.
Intense secrecy and compartmentalization characterized the manhattan Project, but at Los Alamos, Oppenheimer fostered a spirit of collaboration, camaraderie, and open communication. Design and assembly of the 235U bomb was straightforward in that a guntype method could be used to initiate the explosion. The time-consuming process of separating 235U was the chief difficulty, but Oak Ridge eventually succeeded in isolating enough 235U for Oppenheimer's team to assemble "Little Boy," the bomb used against Hiroshima on 6 August 1945.
By mid-1945, Hanford had produced enough 239Pu for three bombs, but they required a complex implosion device with multiple detonators firing simultaneously to create compression waves that would initiate a core explosion. In 1944, to tackle the implosion design challenge, Oppenheimer called on George Kistiakowsky to head the effort to produce the necessary shaped charges. His design was successfully tested at Alamogordo, New Mexico, on 16 July 1945 and used in "Fat Man," the plutonium bomb that devastated Nagasaki on 9 August 1945. At a cost of $2 billion, manhattan Project scientists and engineers had achieved the seemingly impossible—producing three atomic devices by August 1945 that fundamentally changed the nature of warfare, vastly enlarging humanity's capacity for destruction.
Debates about whether the atomic bomb attacks were needed to end the war continue to rage. Certainly, even after it became apparent by 1944 that Germany had abandoned its effort to produce atomic bombs, nearly all members of the manhattan Project team continued to press ahead. The ultimate decision to use the bombs rested with President Harry S Truman, who never doubted that they were a major factor in Japan's decision to surrender, thereby saving tens of thousands of Allied lives.
The project's technical success strengthened an emerging military-industrial complex in the United States and led to the formation of the Atomic Energy Commission (AEC) in 1946. Further research into nuclear weapons production led to the successful test of a hydrogen bomb in 1952, ushering in a new and frightening thermonuclear age.
William J. Astore
Groves, Leslie R. Now It Can Be Told: The Story of the Manhattan Project. New York: Harper and Brothers, 1962.; Hawkins, David, Edith C. Truslow, and Ralph Carlisle Smith. Project Y: The Los Alamos Story—Part I: Toward Trinity, Part II: Beyond Trinity. Los Angeles, CA: Tomash Publishers, 1983.; Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon and Schuster, 1986.; Stoff, Michael B., Jonathan F. Fanton, and R. Hal Williams. The Manhattan Project: A Documentary Introduction to the Atomic Age. New York: McGraw-Hill, 1991.