Worldwide Effects of Nuclear War
Note 2: Nuclear Weapons Design
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.