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The Teller–Ulam design is the nuclear weapon design concept used in most of the world's nuclear weapons. Colloquially referred to as "the secret of the hydrogen bomb," because it employs hydrogen fusion to generate neutrons, in most applications the bulk of its destructive energy comes from uranium fission, not hydrogen fusion. It is named for its two chief contributors, Hungarian-born physicist Edward Teller and Polish-born mathematician Stanisław Ulam, who developed it in 1951, for use by the United States. It was first used in multi-megaton-range thermonuclear weapons. However, it is also the most efficient design concept for small nuclear weapons, and today virtually all the nuclear weapons deployed by the five major nuclear-armed nations use the Teller–Ulam design.
Its essential features, 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 hot radiation channel, surrounding the secondary, and the relatively cool interior of the secondary. This temperature difference is briefly maintained by a massive heat barrier called the "pusher." The pusher is also an implosion tamper, increasing and prolonging the compression of the secondary, and, if made of uranium, which it usually is, it undergoes fission by capturing the neutrons produced by fusion. In most Teller–Ulam weapons, fission of the pusher dominates the explosion and produces radioactive fission product fallout.
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 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.