Complex sentence: It is colloquially referred to as "the secret of the hydrogen bomb" because it employs hydrogen fusion to generate neutrons, though in most applications the bulk of its destructive energy comes from uranium fission, not hydrogen fusion.
Split into simpler sentences: It is colloquially referred to as "the secret of the hydrogen bomb" because it employs hydrogen fusion to generate neutrons. However, in most applications the bulk of its destructive energy comes from uranium fission, not hydrogen fusion.