03 February 2009
The invention: A weapon of mass destruction created during World War II that utilized nuclear fission to create explosions equivalent to thousands of tons of trinitrotoluene (TNT), The people behind the invention: J. Robert Oppenheimer (1904-1967), an American physicist Leslie Richard Groves (1896-1970), an American engineer and Army general Enrico Fermi (1900-1954), an Italian American nuclear physicist Niels Bohr (1885-1962), a Danish physicist Energy on a Large Scale The first evidence of uranium fission (the splitting of uranium atoms) was observed by German chemists Otto Hahn and Fritz Strassmann in Berlin at the end of 1938. When these scientists discovered radioactive barium impurities in neutron-irradiated uranium, they wrote to their colleague Lise Meitner in Sweden. She and her nephew, physicist Otto Robert Frisch, calculated the large release of energy that would be generated during the nuclear fission of certain elements. This result was reported to Niels Bohr in Copenhagen. Meanwhile, similar fission energies were measured by Frédéric Joliot and his associates in Paris, who demonstrated the release of up to three additional neutrons during nuclear fission. It was recognized immediately that if neutron-induced fission released enough additional neutrons to cause at least one more such fission, a selfsustaining chain reaction would result, yielding energy on a large scale. While visiting the United States from January to May of 1939, Bohr derived a theory of fission with John Wheeler of Princeton University. This theory led Bohr to predict that the common isotope uranium 238 (which constitutes 99.3 percent of naturally occurring uranium) would require fast neutrons for fission, but that the rarer uranium 235 would fission with neutrons of any energy. This meant that uranium 235 would be far more suitable for use in any sort of bomb. Uranium bombardment in a cyclotron led to the discovery of plutonium in 1940 and the discovery that plutonium 239 was fissionable— and thus potentially good bomb material. Uranium 238 was then used to “breed” (create) plutonium 239, which was then separated from the uranium by chemical methods. During 1942, the Manhattan District of the Army Corps of Engineers was formed under General Leslie Richard Groves, an engineer and Army general who contracted with E. I. Du Pont de Nemours and Company to construct three secret atomic cities at a total cost of $2 billion. At Oak Ridge, Tennessee, twenty-five thousand workers built a 1,000-kilowatt reactor as a pilot plant.Asecond city of sixty thousand inhabitants was built at Hanford, Washington, where three huge reactors and remotely controlled plutoniumextraction plants were completed in early 1945. A Sustained and Awesome Roar Studies of fast-neutron reactions for an atomic bomb were brought together in Chicago in June of 1942 under the leadership of J. Robert Oppenheimer. He soon became a personal adviser to Groves, who built for Oppenheimer a laboratory for the design and construction of the bomb at Los Alamos, New Mexico. In 1943, Oppenheimer gathered two hundred of the best scientists in what was by now being called the Manhattan Project to live and work in this third secret city. Two bomb designs were developed. A gun-type bomb called “Little Boy” used 15 kilograms of uranium 235 in a 4,500-kilogram cylinder about 2 meters long and 0.5 meter in diameter, in which a uranium bullet could be fired into three uranium target rings to form a critical mass. An implosion-type bomb called “Fat Man” had a 5-kilogram spherical core of plutonium about the size of an orange, which could be squeezed inside a 2,300-kilogram sphere about 1.5 meters in diameter by properly shaped explosives to make the mass critical in the shorter time required for the faster plutonium fission process. A flat scrub region 200 kilometers southeast of Alamogordo, called Trinity, was chosen for the test site, and observer bunkers were built about 10 kilometers from a 30-meter steel tower. On July 13, 1945, one of the plutonium bombs was assembled at the site; the next morning, it was raised to the top of the tower. Two days later, on July 16, after a short thunderstorm delay, the bomb was detonated at 5:30 a.m. The resulting implosion initiated a chain reaction of nearly 60 fission generations in about a microsecond. It produced an intense flash of light and a fireball that expanded to a diameter of about 600 meters in two seconds, rose to a height of more than 12 kilometers, and formed an ominous mushroom shape. Forty seconds later, an air blast hit the observer bunkers, followed by a sustained and awesome roar. Measurements confirmed that the explosion had the power of 18.6 kilotons of trinitrotoluene (TNT), nearly four times the predicted value. Impact On March 9, 1945, 325 American B-29 bombers dropped 2,000 tons of incendiary bombs on Tokyo, resulting in 100,000 deaths from the fire storms that swept the city. Nevertheless, the Japanese military refused to surrender, and American military plans called for an invasion of Japan, with estimates of up to a half million American casualties, plus as many as 2 million Japanese casualties. On August 6, 1945, after authorization by President Harry S. Truman, the B-29 Enola Gay dropped the uranium Little Boy bomb on Hiroshima at 8:15 a.m. On August 9, the remaining plutonium Fat Man bomb was dropped on Nagasaki. Approximately 100,000 people died at Hiroshima (out of a population of 400,000), and about 50,000 more died at Nagasaki. Japan offered to surrender on August 10, and after a brief attempt by some army officers to rebel, an official announcement by Emperor Hirohito was broadcast on August 15. The development of the thermonuclear fusion bomb, in which hydrogen isotopes could be fused together by the force of a fission explosion to produce helium nuclei and almost unlimited energy, had been proposed early in the Manhattan Project by physicist Edward Teller. Little effort was invested in the hydrogen bomb until after the surprise explosion of a Soviet atomic bomb in September, 1949, which had been built with information stolen from the Manhattan Project. After three years of development under Teller’s guidance, the first successful H-bomb was exploded on November 1, 1952, obliterating the Elugelab atoll in the Marshall Islands of the South Pacific. The arms race then accelerated until each side had stockpiles of thousands of H-bombs. The Manhattan Project opened a Pandora’s box of nuclear weapons that would plague succeeding generations, but it contributed more than merely weapons. About 19 percent of the electrical energy in the United States is generated by about 110 nuclear reactors producing more than 100,000 megawatts of power. More than 400 reactors in thirty countries provide 300,000 megawatts of the world’s power. Reactors have made possible the widespread use of radioisotopes in medical diagnosis and therapy. Many of the techniques for producing and using these isotopes were developed by the hundreds of nuclear physicists who switched to the field of radiation biophysics after the war, ensuring that the benefits of their wartime efforts would reach the public.