Word: pile
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...Germans never came very close to making a bomb, apparently. In a recent issue of Britain's Nature magazine, German Nobel-Physicist Werner Heisenberg tells how they almost achieved a successful uranium pile; but they did not even try to make a bomb...
Essentially, the tame bomb is a "pile" like the original uranium pile at the University of Chicago. But uranium needs slow-moving neutrons to make its atoms split. Thus, a uranium pile is made up of small rods of uranium embedded in a large mass of graphite. Plutonium is different: its atoms can be split by fast neutrons. So a pile made of plutonium needs no graphite or other "moderator." The "Nagasaki model" atom bomb is a plutonium pile that reacts so quickly that it blows itself (and the neighborhood) to bits in millionths of a second...
...bomb-tamers of Los Alamos had a ticklish assignment: to make their bomb explode, but gently, in slow motion. How they solved the problem has not been fully explained. Uranium piles are kept from reacting too fast by inserting cadmium rods into the graphite. The rods absorb neutrons and check the action. The more cadmium, the slower the pile percolates. Some similar method may be controlling the tame plutonium...
...plutonium pile has been used only for research, where it has been extremely useful: "The fast reactor gives a more intense source of fast neutrons than physicists heretofore have been able to obtain, except during the brief time of the test of the first atomic bomb in the New Mexico desert...
Keeping its secrets close to its chest, the A.E.G. hardly mentioned the practical possibilities. The fast reactor must be surrounded, like its predecessors, by a thick shield to protect the neighborhood from destructive radiation. This limits its use. But the comparatively small size is an obvious advantage. The new pile, further developed and allowed to run faster and hotter, may be the furnace of tomorrow's atomic power plant...