Nuclear weapons depend on two fundamentally different types of nuclear
reactions, each of which releases energy:
Fission, which involves the splitting of heavy elements (e.g. uranium);
and fusion, which involves the combining of light elements (e.g.
hydrogen).
Fission requires that a minimum amount of material or "critical mass"
be brought together in contact for the nuclear explosion to take place.
The more efficient fission weapons tend to fall in the yield range of
tens of kilotons. Higher explosive yields become increasingly complex
and impractical.
Nuclear fusion permits the design of weapons of virtually limitless
power. In fusion, according to nuclear theory, when the nuclei of light
atoms like hydrogen are joined, the mass of the fused nucleus is
lighter than the two original nuclei; the loss is expressed as energy.
By the 1930's, physicists had concluded that this was the process which
powered the sun and stars; but the nuclear fusion process remained only
of theoretical interest until it was discovered that an atomic fission
bomb might be used as a "trigger" to produce, within one- or
two-millionths of a second, the intense pressure and temperature
necessary to set off the fusion reaction.
Fusion permits the design of weapons of almost limitless power, using
materials that are far less costly.