A thermonuclear weapon is a second generation nuclear weapon design using a secondary nuclear fusion stage consisting of implosion tamper, fusion fuel, and sparkplug which is bombarded by the energy released by the detonation of a primary fission bomb within, compressing the fuel material (tritium, deuterium or lithium deuteride) and causing a fusion reaction. Some advanced designs use fast neutrons produced by this second stage to ignite a third fast fission or fusion stage. The fission bomb and fusion fuel are placed near each other in a special radiation-reflecting container called a radiation case that is designed to contain x-rays for as long as possible. The result is greatly increased explosive power when compared to single-stage fission weapons. The device is colloquially referred to as a hydrogen bomb or, an H-bomb, because it employs the fusion of isotopes of hydrogen.
The first full scale thermonuclear test was carried out by the United States in 1952; the concept has since been employed by most of the world's nuclear powers in the design of its weapons. The modern design of all thermonuclear weapons in the United States is known as the Teller–Ulam configuration for its two chief contributors, Edward Teller and Stanislaw Ulam, who developed it in 1951 for the United States, with certain concepts developed with the contribution of John von Neumann. Similar devices were developed by the Soviet Union, United Kingdom, China, France and North Korea.
As thermonuclear weapons represent the most efficient design for weapon energy yield in weapons with yields above 50 kilotons of TNT (210 TJ), virtually all the nuclear weapons of this size deployed by the five nuclear-weapon states under the NPT today are thermonuclear weapons using the Teller–Ulam design.
The radiation implosion mechanism is a heat engine that exploits the temperature difference between the secondary stage's hot, surrounding radiation channel and its relatively cool interior. This temperature difference is briefly maintained by a massive heat barrier called the "pusher"/"tamper", which also serves as an implosion tamper, increasing and prolonging the compression of the secondary. If made of uranium, enriched uranium or plutonium, it can capture fusion neutrons produced by the fusion reaction and undergo fission itself, increasing the overall explosive yield. In addition to that, some designs also make the radiation case out of a fissile material that undergoes fission. As a result, such bombs get a third fission stage, and the majority of current Teller–Ulam are fission-fusion-fission weapons. Fission of the tamper or radiation case is the main contribution to the total yield and produces radioactive fission product fallout. Wikipedia