Hiroshima , two masses of uranium 235 are driven together by a chemical explosive charge (The two masses must be brought together quickly to prevent the material from blowing apart before most of the uranium undergoes fission . In the second method , used in the bomb dropped on Nagasaki , a number of high explosive charges are used to crush a hollow sphere of plutonium into a dense ball . A major difficulty in constructing a fission weapon lies in the preparation of a...











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