A thermonuclear weapon is a nuclear weapon design that uses the heat generated by a fission bomb to compress a nuclear fusion stage. This indirectly results in a greatly increased energy yield—explosive "power". It is colloquially referred to as a hydrogen bomb or H-bomb because it employs hydrogen fusion, though in most applications the majority of its destructive energy comes from uranium fission, not hydrogen fusion by itself. The fusion stage in such weapons is required to efficiently cause the large quantities of fission characteristic of most thermonuclear weapons.[1]
The concept of the thermonuclear weapon was first developed and used in 1952 and has since been used in most of the world's nuclear weapons.[2] The modern design of all thermonuclear weapons in the United States is known as the Teller-Ulam design for its two chief contributors, Edward Teller and Stanislaw Ulam, who developed it in 1951 for the U.S., with certain concepts developed with the contribution of John von Neumann. The first test of this principle was the "Ivy Mike" nuclear test in 1952, conducted by the United States. In the Soviet Union, the design was independently developed and known as Andrei Sakharov's "Third Idea", first tested in 1955. Similar devices were developed by the United Kingdom, China, and France, though no specific code names are known for their designs.
As thermonuclear weapons represent the most efficient design for weapon energy yield in weapons with yields above 50 kilotons, today virtually all the nuclear weapons deployed by the five nuclear-weapon states under the NPT are thermonuclear weapons using the Teller–Ulam design.[3]
The essential features of the mature thermonuclear weapon design, which officially remained secret for nearly three decades, are: 1) separation of stages into a triggering "primary" explosive and a much more powerful "secondary" explosive, 2) compression of the secondary by X-rays coming from nuclear fission in the primary, a process called the "radiation implosion" of the secondary, and 3) heating of the secondary, after cold compression, by a second fission explosion inside the secondary.
The radiation implosion mechanism is a heat engine exploiting the temperature difference between the secondary's hot, surrounding radiation channel and its relatively cool interior. This temperature difference is briefly maintained by a massive heat barrier called the "pusher", which also serves as an implosion tamper, increasing and prolonging the compression of the secondary. If made of uranium—and it usually is—it can capture neutrons produced by the fusion reaction and undergo fission itself, increasing the overall explosive yield. In many Teller–Ulam weapons, fission of the pusher dominates the explosion and produces radioactive fission product fallout.
