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A general name given to any weapon in which the explosion results from the energy released by a reaction involving atomic nuclei, either by fission—of uranium or plutonium; or, fusion—of a heavier nucleus with two lighter hydrogen ones. Thus, the A-for atomic bomb, and the H, for hydrogen bomb are both nuclear weapons. In the history of the modern world as we know it, the atomic bomb was only used once to kill human beings: it was dropped on the Japanese people in Hiroshima and Nagasaki in 1945.