(4) uncontrolled fusion reaction Explanation: A thermonuclear weapon is a nuclear weapon design that uses the heat generated by a fission bomb to compress a nuclear fusion stage which indirectly results in greatly increased energy yield (i.e., bomb "power"). It is colloquially referred to as a hydrogen bomb or H-bomb because it employs hydrogen fusion, though in most applications the majority of its destructive energy comes from uranium fission, not hydrogen fusion by itself.