Scientists in Germany discovered nuclear fission in late 1938. Refugee scientists Leo Szilard, Edward Teller and Eugene Wigner believed that the energy released in nuclear fission might be used in bombs by the Germans.
They persuaded Albert Einstein, America's most famous physicist, to warn President Franklin Roosevelt of this danger in an August 2, 1939, letter. In response to the warning, Roosevelt ordered increased research in nuclear physics.
Under the auspices of National Bureau of Standards chief Lyman Briggs, small research programs had begun in 1939 at the Naval Research Laboratory in Washington, where physicist Philip Abelson explored uranium isotope separation. At Columbia University Italian nuclear physicist Enrico Fermi built prototype nuclear reactors using various configurations of graphite and uranium.
Vannevar Bush, director of the Carnegie Institution of Washington, organized the National Defense Research Committee in 1940 to mobilize the United States' scientific resources in support of the war effort.
New laboratories were created, including the Radiation Laboratory at the Massachusetts Institute of Technology, which aided the development of radar, and the Underwater Sound Laboratory at San Diego, which developed sonar.
The National Defense Research Council (NDRC) also took over the uranium project, as Briggs' program in nuclear physics was called. In 1940, Bush and Roosevelt created the Office of Scientific Research and Development to expand these efforts.
The uranium project had not made much progress by the summer of 1941, when word came from Britain of calculations by Otto Frisch and Fritz Peierls that showed that a very small amount of the fissionable isotope of uranium, U-235 - could produce an explosion equivalent to that of several thousand tons of TNT.
The National Academy of Sciences proposed an all-out effort to build nuclear weapons. Bush created a special committee, the S-1 Committee, to guide the effort. No sooner was this decision made than the Japanese bombed Pearl Harbor on December 7th, 1941. The war had begun for the United States.
At the University of Chicago Metallurgical Laboratory, the University of California Radiation Laboratory and Columbia University's physics department, efforts to prepare the nuclear materials for a weapon were accelerated.
Uranium 235 had to be separated from uranium ore and plutonium made by neutron bombardment of natural uranium. Beginning in 1942, huge plants were built at Oak Ridge (Site X) in Tennessee and Hanford (Site W) outside of Richland, Washington, to produce these materials.
When the United States entered World War II in December 1941, several projects were under way to investigate the separation of fissionable uranium 235 from uranium 238, the manufacture of plutonium, and the feasibility of nuclear piles and explosions.
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