How does a nuclear weapon work?

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Atomic bombs work with nuclear fission. Two subcritical masses of uranium or plutonium are quickly put together to form a critical mass—that is, enough material to start a runaway fission reaction. Hydrogen bombs work with nuclear fusion. They have two stages: a primary and a secondary. The primary undergoes a fission explosion that triggers a fusion explosion in material in the secondary. Hydrogen bombs release thousands of times more energy than atomic bombs.