Most of a hydrogen bomb's power actually comes from fission, not fusion
The hydrogen bomb takes its name from the fusion of heavy hydrogen isotopes, yet in typical American designs more than 80 percent of the energy comes from splitting uranium in the casing. Weapons with under 10 percent fission, called clean designs, have also been tested.
Thermonuclear weapons are second-generation nuclear arms whose yields typically run about twenty times those of first-generation fission bombs, in smaller and lighter packages. They are the only human-made source of explosions above one megaton. The Soviet Tsar Bomba, the most powerful ever detonated at 50 megatons, had its uranium tamper swapped for lead to reduce fallout.
All known designs are thought to follow the Teller–Ulam configuration, which chains stages together: a fission primary releases X-rays that are channelled to compress a separate secondary containing fusion fuel, mainly lithium-6 deuteride. Neutrons turn lithium into tritium, which fuses with deuterium; that is why they are nicknamed H-bombs. Fast neutrons then split a natural or depleted uranium tamper, supplying most of the yield and the radioactive fallout. The outer radiation case is the only part of any such bomb whose appearance is public.
Scientists thought such weapons possible by 1941, and they received basic study during the Manhattan Project. The first Soviet nuclear test spurred an all-out American effort despite objections from many former Manhattan Project scientists. Edward Teller and Stanisław Ulam outlined their configuration in 1951, with input from John von Neumann, and after preliminary experiments in Operation Greenhouse the United States set off the first full-scale test, Ivy Mike, in 1952.
The Soviet Union followed with its own multi-stage device in 1955, the United Kingdom in 1957, China in 1966 and France in 1968, each largely independently. Unlike boosted fission bombs, which use a little fusion inside a single stage limited to around a megaton, staged designs depend on the temperature and pressure reached as the secondary is squeezed.
Source: Thermonuclear weapon
