Most of a hydrogen bomb's power comes from cheap, ordinary uranium
The fusion fuel in a thermonuclear weapon is not where most of its energy comes from. Fast neutrons from fusion split the outer jacket of uranium-238, a cheap leftover of enrichment that cannot sustain a chain reaction alone. That quirk of physics made enormous arsenals affordable during the arms race.
There are three basic designs. Pure fission devices are the simplest and the only kind ever used in combat, when America dropped them on Japan during the Second World War. Boosted fission weapons add a little fusion to flood the core with fast neutrons, which can more than double the fission yield. Staged thermonuclear weapons use a first stage whose X-rays crush a second stage of fusion fuel, reaching yields hundreds or thousands of times larger. Around 80% of deployed weapons today are of this staged type, because they pack so much energy into a small, light package.
Fission releases about 180 MeV each time a uranium-235 nucleus splits, and 93% of that goes into the two charged fragments flying apart at roughly 12,000 kilometres per second. Within about a microsecond their motion has turned to heat, leaving a ball of plasma several metres across at tens of millions of degrees, which glows in X-rays that the surrounding air absorbs as fireball and blast. A modern pit holds 3.5 to 4.5 kilograms of plutonium, yet only around 0.5 kilograms actually fissions to give 5 to 10 kilotonnes.
Fusion of deuterium and tritium gives 17.6 MeV, a tenth of a fission event, but from ingredients a fiftieth as heavy, so it yields about five times more energy per unit of mass. The catch is that 80% of that energy rides away on an uncharged neutron. Wrapping the fuel in heavy material such as uranium captures those neutrons and turns each into a fission worth 180 MeV, multiplying the output tenfold.
Surprises came with testing. Castle Bravo, the first device of its kind, ran 2.5 times hotter than predicted because the common isotope lithium-7, assumed inert, also joined the reactions. Early newspapers spoke of A-bombs and H-bombs, while policy specialists prefer nuclear and thermonuclear. A French patent of May 1939 had already claimed a millionfold advantage over chemical explosives.
Source: Nuclear weapon design
