American scientists, many of them refugees from fascist regimes in Europe, took steps in 1939 to organize a project to exploit the newly recognized fission process for military purposes. In the summer of 1939, Albert Einstein was persuaded by his fellow scientists to use his influence and present the military potential of an uncontrolled fission chain reaction to President Franklin D. Roosevelt. In February 1940, $6,000 was made available to start research. After the U.S. entry into the World war II, the War Department was given joint responsibility for the project. In June 1942 the Corps of Engineers’ Manhattan District was initially assigned management of the construction work (because much of the early research had been performed at Columbia University, in Manhattan). “Manhattan Project” became the code name for research work that would extend across the country.
Only method available for the production of the fissionable material plutonium-239, was developed at the metallurgical laboratory of the University of Chicago. In December 1942, Enrico Fermi, the Italian-born Nobel Prize-winning physicist, finally succeeded in producing and controlling a fission chain reaction in this reactor pile at Chicago. Upon succesful completion of the experiment, a coded message was transmitted to President Roosevelt: “The Italian navigator has landed in the new world.”
The large-scale production reactors were built on an isolated tract on the Columbia River north of Pasco, Washington—the Hanford Engineer Works, for the quantity production of plutonium-239. By the summer of 1945, amounts of plutonium-239 sufficient to produce a nuclear explosion had become available from the Hanford Works, and weapon development and design were sufficiently far advanced so that an actual field test of a nuclear explosive could be scheduled. By this time the original $6,000 authorized for the Manhattan Project had grown to $2 billion. The first atomic bomb was exploded at 5:30 am on July 16, 1945, at a site on the Alamogordo air base 120 miles (193 km) south of Albuquerque, New Mexico, followed by Hiroshima and Nagasaki explosions in the next month.
Reference:
http://www.britannica.com/EBchecked/topic/362098/Manhattan-Project
http://www.history.com/this-day-in-history/fermi-produces-the-first-nuclear-chain-reaction
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