Word: fissioned
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Dates: during 1940-1949
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...deadly serious historical fact. MARCH OF TIME, with the cooperation of 20-odd scientists, who appear in the picture, has retraced and re-enacted the main publishable stages in its cause and towards its possible cure. The motion in charts and animation makes newly graphic the basic principles of fission; shots heretofore unreleased to the screen suggest some of the effects, including, as one emblem or symbol more grim than any in Pompeii, the shadow of a human body, fire-stenciled into the pavement of Hiroshima...
...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...
...would also need equipment for: 1) removing the pile's "ash" (fission products), which slows down the chain reaction and eventually stops it altogether; 2) periodic repurifying of the uranium in the pile; 3) making repairs by remote control in case the battleship's engine broke down...
...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...