A thermonuclear weapon, fusion weapon or hydrogen bomb (H-bomb) is a second-generation nuclear weapon, using nuclear fusion. The most destructive weapons ever created, their yields typically exceed first-generation nuclear weapons by twenty times, with far lower mass and volume requirements. Characteristics of fusion reactions can make possible the use of non-fissile depleted uranium as the weapon's main fuel, thus allowing more efficient use of scarce fissile material. Its multi-stage design is distinct from the usage of fusion in simpler boosted fission weapons. The first full-scale thermonuclear test (Ivy Mike) was carried out by the United States in 1952, and the concept has since been employed by at least the five NPT-recognized nuclear-weapon states: the United States, Russia, the United Kingdom, China, and France. The design of all thermonuclear weapons is believed to be the Teller–Ulam configuration. This relies on radiation implosion, in which X-rays from detonation of the primary stage, a fission bomb, are channelled to compress a separate fusion secondary stage containing thermonuclear fuel, primarily lithium-6 deuteride. During detonation, neutrons convert lithium-6 to helium-4 plus tritium. The heavy isotopes of hydrogen, deuterium and tritium, then undergo a reaction that releases energy and neutrons. For this reason, thermonuclear weapons are often colloquially called hydrogen bombs or H-bombs. Additionally, most weapons use a natural or depleted uranium tamper and case. This undergoes fast fission from fast fusion neutrons and is the main contribution to the total yield and radioactive fission product fallout. Thermonuclear weapons were thought possible in 1941 and were the subject of basic research during the Manhattan Project. The first Soviet nuclear test triggered an all-out pursuit of a thermonuclear weapon in the US, despite initial opposition by many former Manhattan Project scientists. The Teller-Ulam configuration, named for its chief contributors, Edward Teller and Stanisław Ulam, was outlined in 1951, with contribution from John von Neumann. Operation Greenhouse investigated thermonuclear reactions before the full-scale Mike test. Multi-stage devices were later developed and tested, largely independently, by the Soviet Union (1955), the United Kingdom (1957), China (1966), and France (1968). There is not enough public information to determine whether India, Israel, or North Korea possess multi-stage weapons. Pakistan is not considered to have developed them. After the 1991 collapse of the Soviet Union, Ukraine, Belarus, and Kazakhstan became the first and only countries to relinquish their thermonuclear weapons, although these had never left the operational control of Russian forces. Following the 1996 Comprehensive Nuclear-Test-Ban Treaty, most countries with thermonuclear weapons maintain their stockpiles and expertise using computer simulations, hydrodynamic testing, warhead surveillance, and inertial confinement fusion experiments.
Thermonuclear weapons are the only artificial source of explosions above one megaton TNT. The Tsar Bomba was the most powerful bomb ever detonated at 50 megatons of TNT (210 PJ), despite the uranium tamper being replaced with lead to reduce radioactive fallout. As they are the most efficient design for yields above 50 kilotons of TNT (210 TJ), and with decreased relevance of tactical nuclear weapons, virtually all nuclear weapons deployed by the five recognized nuclear-weapons states today are thermonuclear. Their development dominated the Cold War's nuclear arms race. Their destructiveness and ability to miniaturize high yields, such as in MIRV warheads, defines nuclear deterrence and mutual assured destruction. Extensions of thermonuclear weapon design include clean bombs with marginal fallout and neutron bombs with enhanced penetrating radiation. Nonetheless, most thermonuclear weapons designed, including all current US and UK nuclear warheads, derive most of their energy from fast fission, causing high fallout.
Terminology The adjectives "thermonuclear", "fusion", and "hydrogen" are used mainly to describe multi-stage nuclear weapons, which allow large fusion yields. These operate on the radiation implosion principle, and are synonymous with the Teller-Ulam design, independently developed by at least five countries. "Thermonuclear" refers to thermonuclear fusion, where nuclei are fused via their high collision speeds at high temperatures. Unlike fission weapons, whose detonations are mediated via neutron transport, thermonuclear yield is more directly dependent on the temperatures and pressures achieved during compression of the secondary. These are in contrast to boosted fission devices, which employ thermonuclear fusion, but detonate a single stage design theoretically limited to around one megaton. Despite their name, the simplest and most common thermonuclear weapons derive most of their yield (>80% for US weapons) from fast fission of a natural or depleted uranium tamper. Clean thermonuclear weapons (<10% fission) have also been tested and possibly deployed.
Basic principle
Primary and secondary stages The basic principle of the Teller–Ulam configuration is the idea that different parts of a thermonuclear weapon can be chained together in stages, with the detonation of each stage providing the energy to ignite the next stage. At a minimum, this implies a primary section that consists of an implosion-type fission bomb (a "trigger"), and a secondary section that consists of fusion fuel. The energy released by the primary compresses the secondary through the process of radiation implosion, at which point it is heated and undergoes nuclear fusion. This process could be continued, with energy from the secondary igniting a third fusion stage; the Soviet Tsar Bomba and US B41 nuclear bombs are believed to have used this design. Theoretically by continuing this process thermonuclear weapons with arbitrarily high yield could be constructed. Fission weapons are limited in yield because only so much fission fuel can be amassed in one place before the danger of its accidentally becoming supercritical becomes too great; the British Orange Herald was the largest single-stage fission test ever, at 700 kilotons.
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