Word: ued
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Dates: during 1970-1979
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During the crisis, Steiner worked clearly with the rest of President Bok's whiz kids--Charles U. Daly, Stephen S.J. Halt, Walter Leonard and Hale Champion. Although--Bok's young assistants initially give the impression that they are a close-kait, long-standing coterie, Steiner said they were almost total strangers until...
Lyon approached the Administration this summer about the possibility of holding a fundraising concert for McGovern in Harvard Stadium. Charles U. Daly, vice-president for Government and Community Relations, wrote to Lyon that allowing the event could jeopardize Harvard's status as a tax-exempt institution. He said that McGovern could hold a rally in the stadium provided he did not try to raise money...
...from the Oklo mine in Gabon, Africa, they found that it contained an abnormally low proportion of uranium 235, the radioactive isotope that powered the first atomic bomb with its awesome energy. In all other known uranium deposits-including a sample brought back from the moon by Apollo astronauts-U-235 invariably makes up .72% of the uranium ore; but the samples from the Oklo mine, which was opened in 1969, contained as little as .44% of U-235. Until the French discovery, levels that low had been found only in depleted uranium fuel taken from atomic reactors in which...
...when-could this extraordinary event have occurred? Using both hard information and informed guesswork, Perrin and his associates evolved an imaginative but logical hypothesis. The scientists knew that the concentration of U-235 found in contemporary uranium deposits is too low to sustain a chain reaction. But they were also aware that radioactive U-235 decays at a known rate, and that 1.7 billion years ago-the approximate age of the Oklo deposit -U-235 made up 3% of raw uranium deposits. This is roughly the same concentration that is created in artificially enriched uranium fuels and thus is enough...
Perrin suspects that water, filtering down through the Oklo deposit, became an accessory to the chain reaction. Acting as a "moderator," subsurface water slowed down the neutrons emerging from splitting U-235 atoms enough to allow them to hit and split the nuclei of other U-235 atoms. (Without a moderator, the neutrons escape from the uranium fuel too fast to sustain the reaction.) When the heat from this process became too intense, scientists believe, the water turned to steam, the neutrons speeded up, and the chain reaction halted until the uranium cooled sufficiently for the steam to condense back...