The second basic type of nuclear weapon produces a large amount of its energy through
nuclear fusion reactions. Such fusion weapons are generally referred to as
thermonuclear weapons or more colloquially as
hydrogen bombs (abbreviated as
H-bombs), as they rely on fusion reactions between isotopes of
hydrogen (
deuterium and
tritium). However, all such weapons derive a significant portion, and sometimes a majority, of their energy from fission (including fission induced by neutrons from fusion reactions). Unlike fission weapons, there are no inherent limits on the energy released by thermonuclear weapons. Only six countries—United States, Russia, United Kingdom, People's Republic of China, France and India—have conducted thermonuclear weapon tests. (Whether India has detonated a "true", multi-staged
thermonuclear weapon is controversial.)
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The basics of the
Teller–Ulam design for a hydrogen bomb: a fission bomb uses radiation to compress and heat a separate section of fusion fuel.
Thermonuclear bombs work by using the energy of a fission bomb to compress and heat fusion fuel. In the
Teller-Ulam design, which accounts for all multi-megaton yield hydrogen bombs, this is accomplished by placing a fission bomb and fusion fuel (
tritium,
deuterium, or
lithium deuteride) in proximity within a special, radiation-reflecting container. When the fission bomb is detonated,
gamma and
X-rays emitted first compress the fusion fuel, then heat it to thermonuclear temperatures. The ensuing fusion reaction creates enormous numbers of high-speed
neutrons, which can then induce fission in materials not normally prone to it, such as
depleted uranium. Each of these components is known as a "stage", with the fission bomb as the "primary" and the fusion capsule as the "secondary". In large hydrogen bombs, about half of the yield, and much of the resulting
nuclear fallout, comes from the final fissioning of depleted uranium.
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By chaining together numerous stages with increasing amounts of fusion fuel, thermonuclear weapons can be made to an almost arbitrary yield; the largest ever detonated (the
Tsar Bomba of the
USSR) released an energy equivalent of over 50 million tons (50
megatons) of TNT. Most thermonuclear weapons are considerably smaller than this, due to practical constraints arising from the space and weight requirements of missile warheads.
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