『The minimum amout of fissionable material required to sustain a fission chain reaction is called a critical mass. The ffissionable material in the atomic bomb dropped on Hiroshima was uranium 235. This U-235 was kept in two pieces, each smaller than a critical mass. To trigger the bomb, a conventional explosive forced the two pieces together,instantly creating more than a critical mass.This was called a gun-barrel bomb. The Nagasaki bomb used plutonium 239. A neutron source was placed at the center of a divided sphere of plu-otonium. This was covered by a reflector, with explosives wrapped around theb periphery. When the explosives det-onated, the plutonium in the sphere was hurled to the cen-ter, creating more than critical mass. This was an implo-sion bomb. A chain reaction began as soon as the critical mass was exceeded, and an enormous amount of energy was re-leased within a millionth of a second. The explosion instan-taneously emitted powerful heat rayｓ and radiation. The surrounding air was expanded by this awesome power,creatibg a tremendous brast. The total energy released by the Hiroshima bomb was equivalent to approximately 15thousand tons of TNT. The Nagasaki bomb equaled 21thousand tons.』

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