Hydrogen Bomb

The hydrogen bomb, also called a thermonuclear weapon, is a nuclear weapon that derives most of its explosive energy from nuclear fusion reactions between hydrogen isotopes, a fusion stage triggered by the heat and pressure of a smaller fission atomic bomb acting as its primary, a two-stage design that lets a hydrogen bomb reach explosive yields many times larger than a fission-only atomic bomb of comparable size. The United States conducted the first successful test of a full-scale thermonuclear device, code-named Ivy Mike, in the Pacific in 1952, and the Soviet Union followed with its own thermonuclear weapon tests within a few years, beginning a Cold War arms race in which the two superpowers, joined later by other nuclear-armed states, developed increasingly powerful and more compact thermonuclear warheads. Because a thermonuclear weapon's yield is limited mainly by the size of its fusion fuel stage rather than the harder physical limits that constrain a pure fission weapon, the hydrogen bomb produced by far the most powerful nuclear explosions ever tested, including the Soviet Union's Tsar Bomba test of 1961.