The most widely used standard for measuring the power of nuclear weapons is "yield," expressed as the quantity of chemical explosive (TNT) that would produce the same energy release. The first atomic weapon which leveled Hiroshima in 1945, had a yield of 13 kilotons; that is, the explosive power of 13,000 tons of TNT. (The largest conventional bomb dropped in World War II contained about 10 tons of TNT.) Since Hiroshima, the yields or explosive power of nuclear weapons have vastly increased. The world's largest nuclear detonation, set off in 1962 by the Soviet Union, had a yield of 58 megatons--equivalent to 58 million tons of TNT. A modern ballistic missile may carry warhead yields up to 20 or more megatons. Even the most violent wars of recent history have been relatively limited in terms of the total destructive power of the non-nuclear weapons used. A single aircraft or ballistic missile today can carry a nuclear explosive force surpassing that of all the non-nuclear bombs used in recent wars. The number of nuclear bombs and missiles the superpowers now possess runs into the thousands. 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. Note 3: Radioactivity Most familiar natural elements like hydrogen, oxygen, gold, and lead are stable, and enduring unless acted upon by outside forces. But almost all elements can exist in unstable forms. The nuclei of these unstable "isotopes," as they are called, are "uncomfortable" with the particular mixture of nuclear particles comprising them, and they decrease this internal stress through the process of radioactive decay.