The Thermonuclear Bomb

A thermonuclear hydrogen or fusion bomb uses a fission bomb to provide the tremendous temperatures and pressures necessary to compress hydrogen enough to enable a fusion reaction into helium.
The most common bomb design that employs these features is called the Teller-Ulam configuration. Parts of the weapon are chained together in stages, each stage providing energy for the next stage of the explosion to proceed. Each stage consists of a primary section containing a fission bomb and a secondary section containing hydrogen. The primary explosion compresses the secondary (radiation implosion) so that fusion can take place.

The nuclear fusion releases neutrons much faster than a fission reaction and the reaction of each stage is faster. Most energy is released from fission, but the main source of neutrons for each fission stage is from fusion.