Operation Sandstone - Origins

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 I Little Boy, a gun-type fission weapon using uranium-235, and the Mark III 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 0.91 grams (0.032 oz) of plutonium per megawatt-day, or about 20 kilograms (44 lb) per month. In practice, production never approached such a level in 1945, when only between 4 and 6 kilograms (8.8 and 13 lb) was produced per month. A Fat Man core required about 6.2 kilograms (14 lb) 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 of polonium-210 by irradiation of bismuth-209, which was used in the modulated neutron initiators, a critical component of the nuclear weapons. Some 62 kilograms (140 lb) of bismuth-209 had to be irradiated for 100 days to produce 600 curies of polonium-210, a little over over 132 milligrams (2.04 gr). 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 produced from enrichment of natural uranium at the Oak Ridge National Laboratory. 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 69 kilograms (150 lb) of uranium-235 per month, which was only enough for one of the very wasteful Little Boys. 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.