The Manhattan Project was an epic, secret, wartime effort to design and build the world's first nuclear weapon. Commanding the efforts of the world's greatest physicists and mathematicians during World War II, the 20-billion-dollar project resulted in the production of the first uranium and plutonium bombs. The American quest for nuclear explosives was driven by the fear that Hitler's Germany would invent them first and thereby gain a decisive military advantage. The monumental project took less than four years, and encompassed construction of vast facilities in Oak Ridge, Tennessee, and Hanford, Washington, that were used for the purpose of obtaining sufficient quantities of the isotopes uranium-235 and plutonium-239, necessary to produce the fission chain reaction that released the bombs' destructive energy. After a successful test in Alamogordo, New Mexico, the United States exploded a nuclear bomb over the Japanese city of Hiroshima on August 6, 1945. Three days later another bomb dropped over the Japanese city of Nagasaki spurred the Japanese surrender that ended World War II.
In the 1930s and early 1940s, fundamental discoveries regarding the neutron and atomic physics allowed for the possibility of induced nuclear chain reactions. Danish physicist Neils Bohr' compound nucleus theory, for example, laid the foundation for the theoretical exploration of fission, the process whereby the central part of an atom, the nucleus, absorbs a neutron, then breaks into two equal fragments. In certain elements, such as plutonium-239, the fragments release other neutrons quickly break up more atoms, creating a chain reaction that releases large amounts of heat and radiation.
Hungarian physicist Leo Szilard conceived the idea of the nuclear chain reaction in 1933, and immediately became concerned that, if practical, nuclear energy could be used to make weapons of war. Szilard, who fled Nazi persecution first in his native Hungary, then again in Germany, conveyed his concerns to his friend and contemporary, noted physicist Albert Einstein. In 1939 the two scientists drafted a letter (addressed from Einstein) warning U.S. President Franklin D. Roosevelt of the plausibility of nuclear weapons, and of German experimentation with uranium and fission. In December 1941 after the Japanese attack at Pearl Harbor and the United States's entry into the war, Roosevelt ordered a secret U.S. project to investigate the potential development of atomic weapons. The Army Corps of Engineers took over and in 1942 consolidated various atomic research projects under the intentionally misnamed Manhattan Engineering District (now commonly known as the Manhattan Project) placed under the command of Brigadier General Leslie Richard Groves.
Groves recruited American physicist J. Robert Oppenheimer to became the scientific director for the Manhattan Project. Security concerns required the development of a central laboratory for physics weapon research in Los Alamos, New Mexico. Oppenheimer's leadership attracted many top young scientists, including American physicist Richard Feynman, who joined the Manhattan Project while still graduate student. Feynman and his mentor Hans Bethe calculated the critical mass fissionable material necessary to begin a chain reaction.
Fuel for the nuclear reaction was a primary concern. At the outset, the only materials seemingly satisfactory for sustaining an explosive chain reaction were either U-235 (derived from U-238) or Pu-239 (an isotope of the yet unsynthesized element plutonium). Additional requirements included an abundant supply of heavy water (e.g., deuterium and tritium). At Oak Ridge, the process of gaseous diffusion was used to extract the U-235 isotope from the 140 times more abundant U-235 found in uranium ore. At Hanford, production of P-239 was eventually made possible by leaving plutonium-238 in a nuclear reactor for an extended period.
In 1942 Italian physicist Enrico Fermi supervised the first controlled sustained chain reaction at the University of Chicago. Underneath the university football stadium, in modified squash courts, Fermi and his team assembled a lattice of 57 layers of uranium metal and uranium oxide embedded in graphite blocks to create the first reactor pile.
The Manhattan Project eventually produced four bombs. Little Boy, the code name for the uranium bomb, utilized explosives to crash pieces of uranium together to begin an explosive chain reaction. Fat Man, the code name for the plutonium bomb, was more difficult to design. It required a neutron-emitting source to initiate a chain reaction within a series of concentric nested spheres. The outermost shell was an explosive lens system surrounding a pusher/neutron absorber shell designed to reduce the effect of Taylor waves, the rapid drop in pressure at occurs behind a detonation front that could interfere with an implosion. The next nested sphere was a uranium tamper/reflector shell containing a plutonium pit and beryllium neutron initiator. The spheres were designed to implode, causing the plutonium to fuse, reach critical mass, then start the reaction.
The simple design of the uranium bomb left scientists confident of its success, but the complicated implosion trigger required by the plutonium bomb raised engineering concerns about reliability. On July 16, 1945, a plutonium test bomb code named Gadget was detonated in a remote area near Alamogordo, New Mexico. Observed by scientists wearing welder's glasses and suntan lotion for protection, the test blast (code named Trinity) was more powerful than originally thought, roughly equivalent to 20,000 tons of TNT and caused total destruction up to one mile from the blast center.
On August 6, 1945, an American B-29 "Flying Fortress," the Enola Gay, dropped the uranium bomb over Hiroshima. Sixty thousand people were killed instantly, and another 200,000 subsequently died as a result of burn and radiation injuries. Three days later, a plutonium bomb was dropped over Nagasaki. Although it missed its actual target by over a mile, the more powerful plutonium bomb killed or injured more than 65,000 people and destroyed half of the city. Ironically, ground zero, the point under the bomb explosion, turned out to be the Mitsubishi Arms Manufacturing Plant, at one time the major military target in Nagasaki. The fourth bomb remained unused.
Many Manhattan Project scientists eventually became advocates of the peaceful use of this nuclear power and advocates for nuclear weapons control.
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