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Operation Sandstone was a series of nuclear weapon tests in 1948. It was the third series of American tests, following Trinity in 1945 and Crossroads in 1946, and preceding Ranger. Like the Crossroads tests, the Sandstone tests were carried out at the Pacific Proving Grounds, although at Enewetak Atoll rather than Bikini Atoll. They differed from ''Crossroads'' in that they were conducted by the Atomic Energy Commission, with the armed forces having only a supporting role. The purpose of the Sandstone tests was also different: they were primarily tests of new bomb designs rather than of the effects of nuclear weapons. Three tests were carried out in April and May 1948 by Joint Task Force 7, with a work force of 10,366 personnel, of whom 9,890 were military. The successful testing of the new cores in the ''Operation Sandstone'' tests rendered every component of the old weapons obsolete. Even before the third test had been carried out, production of the old cores was halted, and all effort concentrated on the new Mark 4 nuclear bomb, which would become the first mass-produced nuclear weapon. More efficient use of fissionable material as a result of Operation Sandstone would increase the U.S. nuclear stockpile from 56 bombs in June 1948 to 169 in June 1949. ==Origins== Nuclear weapons were developed during World War II by the Manhattan Project, which created a network of production facilities, and the weapons research and design laboratory at the Los Alamos National Laboratory. Two types of bombs were developed: the Mark 1 ''Little Boy'', a gun-type fission weapon using uranium-235, and the Mark 3 ''Fat Man'', an implosion-type nuclear weapon using plutonium. These weapons were not far removed from their laboratory origins. A great deal of work remained to improve ease of assembly, safety, reliability and storage before they were ready for production. There were also many improvements to their performance that had been suggested or recommended during the war that had not been possible under the pressure of wartime development. Norris Bradbury, who replaced Robert Oppenheimer as director at Los Alamos, felt that "we had, to put it bluntly, lousy bombs." Plutonium was produced by irradiating uranium-238 in three 250 MW nuclear reactors at the Hanford site. In theory they could produce of plutonium per megawatt-day, or about per month. In practice, production never approached such a level in 1945, when only between was produced per month. A ''Fat Man'' core required about of plutonium, of which 21% fissioned. Plutonium production fell off during 1946 due to swelling of the reactors' graphite neutron moderators. This is known as the Wigner effect, after its discoverer, the Manhattan Project scientist Eugene Wigner. These reactors were also required for the production (by irradiation of bismuth-209) of polonium-210, which was used in the initiators, a critical component of the nuclear weapons. Some of bismuth-209 had to be irradiated for 100 days to produce 600 curies of polonium-210, a little over . Because polonium-210 has a half-life of only 138 days, at least one reactor had to be kept running. The oldest unit, B pile, was therefore closed down so that it would be available in the future. Investigation of the problem would take most of 1946 before a fix was found. Uranium-235 was derived from enrichment of natural uranium at the Y-12 plant and K-25 site in Oak Ridge, Tennessee. Improvements in the processes and procedures of the electromagnetic and gaseous isotope separation between October 1945 and June 1946 led to an increase in production to around of uranium-235 per month, which was only enough for one of the very wasteful ''Little Boy''s. A ''Fat Man'' was 17.5 times as efficient as a ''Little Boy'', but a ton of uranium ore could yield eight times as much uranium-235 as plutonium, and on a per-gram basis, plutonium cost somewhere between four and eight times as much to produce as uranium-235, which at this time cost around $26 per gram. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Operation Sandstone」の詳細全文を読む スポンサード リンク
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