a fission reaction ie like the jap weapons were uranium based. the chain reaction was caused by using spherically shaped charge (fat man) to squeeze the uranium. a hydrogen weapon uses an atomic weapon to set off its chain reaction. our dial a bomb plutonium based systems require both plutonium and tritium. Anit it great that Clinton gave Pakistan our bomb design, worked out so well. almost as well as giving china loreal space systems guidance tech.
Fat Man was a Plutonium implosion device.
For years I was on design work for the weapons in our current stockpile as well as some that have been removed.