Word: fissionable
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...teacup of water has enough nuclear power, potentially to drive a large steamship across the Atlantic Ocean. But physicists who have studied the problem believe that an atomic engine will be no teacup affair; the only method they have found to date for releasing nuclear energy is the fission of considerable quantities of a heavy element like uranium...
...first time since 1940, U.S. physicists could talk about atomic fission without looking over their shoulders. The subject of The Bomb itself was taboo at the annual midwest meeting of the American Physical Society in Chicago; but there was plenty more for the atomic scientists to talk about...
...typical pile is a 20-foot block of graphite (pure carbon) interlarded with lumps of fissionable uranium. The chain begins with the capture of a neutron by a uranium atom. When the atom "fishes" (splits by fission), neutrons released by the reaction fly off at more than 6,000 miles a second. To give the neutrons a maximum chance of being captured by other uranium atoms, they are slowed to "thermal" speed-roughly 3 m.p.s. Normally a neutron slows down to that speed after about 110 collisions with carbon atoms...
...controlling fission, the nuclear physicists' big problem was to calculate the probability that a given atom would capture a neutron traveling at a given speed. They found that in a certain type of pile the critical size at which a lump of enriched uranium begins to cook in a nonexplosive chain reaction is 1.5 kilograms (about 3⅓ Ibs.). Theoretically, a pile might heat up to the temperature of the sun (over 6,000°), but no known container can withstand more than 1,500°. The physicists discovered that the simplest way to throttle down a pile...
...after the German Panzers had driven through Poland, and the citizens of Hiroshima were still going quietly about their daily tasks, the little man who hates to write letters wrote a letter to Franklin Roosevelt. In it he stated his conviction that a controlled chain reaction of atomic fission (and hence the atom bomb) was now feasible, that the German Government was working on an atomic bomb, that the U.S. must begin research on the bomb at once or civilization would perish. Einstein enclosed a report by his friend, Dr. Leo Szilard, describing in more technical language...