A hydrogen bomb is a thermonuclear weapon -- one that uses both nuclear fission and nuclear fusion.

A simple atomic bomb, which is what North Korea is thought to have possessed before Wednesday’s test, uses only nuclear fission. Such fission bombs use the energy that is released when the nucleus of a large atom is split. 



A thermonuclear weapon uses the energy from a nuclear fission reaction to set off a nuclear fusion reaction, in which the nuclei of small atoms are forced together, releasing more energy. The result is an explosive power that is much stronger, making a hydrogen bomb much more powerful than single-stage fission weapons.

(khnews@heraldcorp.com)