The Manhattan Project
U.S. and British researchers were investigating the feasibility of nuclear weapons as early as 1939. Practical development began in earnest in 1942 when these efforts were transferred to the authority of the U.S. Army and became the Manhattan Project. The weapons-development portion of this project was located at the Los Alamos Laboratory in northern New Mexico, though much other development and production work was carried out at the Clinton Engineer Works near Oak Ridge, Tennessee (the separation of uranium-235); the Hanford Engineer Works near Hanford, Washington (the production and separation of plutonium-239); in and near Chicago, Illinois (at the University of Chicago and the Argonne National Laboratories); and at the University of California, Berkeley.
These research, development, and production efforts focused both on the development of the necessary fissile materials to power the nuclear chain reactions in the atomic bombs and on the design, testing, and manufacture of the bombs themselves.
From January 1944 until July 1945, large-scale production plants were set in operation, and the fissile material thus produced was then used to determine the features of the weapons. Multi-pronged research was undertaken to pursue several possibilities for bomb design. Early decisions about weapon design had been based on minute quantities of uranium-235 and plutonium that had been created in pilot plants and in physics-laboratory cyclotrons. From these experimental results, it was thought that the creation of a bomb was as simple as forming a critical mass of fissile material.
The production of both uranium-235 and plutonium-239 were enormous undertakings given the technology of the 1940s and accounted for 80% of the total costs of the project. Theoretically, enriching uranium was feasible through pre-existing techniques in physics (e.g., modifying particle accelerator technology), though it proved difficult to scale to industrial levels and was extremely costly.
Plutonium, by contrast, could theoretically be produced most easily in nuclear reactors, but the technology and science involved was wholly new. The first experimental nuclear reactor had been developed and constructed by Enrico Fermi and his team of co-workers by the end of 1942 at the University of Chicago (Chicago Pile-1), which proved that there were no obvious physical limitations to producing a slow-neutron nuclear chain reaction. Work began on constructing large plutonium-breeding reactors at Hanford, Washington, in October 1943. The first reactor-bred plutonium was produced in the B-Reactor, the first full-scale plutonium-production reactor in the world. The first large batch of plutonium was refined at Hanford in the "221-T plant", using the bismuth phosphate process, from December 26, 1944, to February 2, 1945. This was delivered to Oppenheimer's team at the Los Alamos laboratory on February 5, 1945. In the meantime, the X-10 Graphite Reactor, a scaled-down version of the Hanford reactors, was built in Oak Ridge, Tennessee, and went into operation in November 1943.
Plutonium is a synthetic element not found in nature in appreciable quantities. It also has relatively complicated physics, chemistry, and metallurgy compared to most other elements. The only prior plutonium isolated for the project had been produced in cyclotrons in very minute amounts. In April 1944, Emilio Segrè received the first sample of reactor-bred plutonium from the X-10 reactor and discovered that it was not as pure as cyclotron-produced plutonium by a significant degree. Specifically, the longer the plutonium remained irradiated inside the reactor — which is necessary for high yields of the metal — the greater its content of the isotope plutonium-240. Pu-240 undergoes spontaneous fission at an appreciable rate, and that releases excess neutrons. These extra neutrons implied a high probability that a gun-type bomb with plutonium would detonate too early, before a critical mass was formed, scattering the plutonium and producing a small "fizzle" of a nuclear explosion many times smaller than a full explosion. The practical result was that a simple gun-type atomic bomb (the proposed Thin Man) would not work as had been hoped.
The impossibility of solving this problem of a gun-type bomb with plutonium was decided upon in a meeting in Los Alamos on June 17, 1944. This forced a search for a different, more practical design for a plutonium-fueled bomb, and an implosion-type atomic-bomb design (i.e., the Fat Man design) was selected as the most practical one at that time. This required a great deal of research work and experimentation in engineering and hydrodynamics before a practical design could be worked out.
In an implosion bomb, a small spherical core of plutonium would be surrounded by high explosives that burned with different speeds. By alternating the faster and slower burning explosives in a carefully calculated spherical configuration, they would produce a compressive wave upon their simultaneous detonation. This "lensing" effect focused the explosive force inward with enough force to compress the plutonium core to several times its original density. This would rapidly reduce the necessary size of the critical mass of the material, making it supercritical. It would also activate a small neutron source at the center of the core, which would assure that the chain reaction began in earnest.
The advantage of the implosion method was that it was far more efficient in use of material — only 6.2 kg of plutonium would be needed for a full explosion, compared to the 64 kg of enriched uranium used in the "Little Boy" weapon. The engineering difficulties, though, were daunting. Though explosive lenses had been pursued during the war, the art was still very new, and the tolerances required in terms of timing and symmetry were unprecedented. Should the timing or symmetry be off, the bomb would not detonate fully, and instead just disperse the plutonium into the surrounding area. The entire Los Alamos laboratory was reorganized in 1944 to focus on designing a workable implosion bomb.
Scientists were confident that an implosion device would work, but these new design difficulties were great. It was decided that a full-sized test would be required before any military use, even though it would sacrifice one of a very small number of bombs. In early 1944, plans for a July 1945 test were finalized. The uranium weapon would not be tested; its success could be more or less guaranteed by measurements ahead of time.
The actual Fat Man atomic bomb, which used the same design as in the Trinity test, was exploded over Nagasaki on August 9, 1945, after the Trinity test had proven its operability in an atomic explosion.
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