Ivy Mike, the first full-scale thermonuclear device.

Experiments with particle accelerators demonstrated that fusion reactions were possible, but the first large-scale release of fusion energy was achieved in thermonuclear weapons. Their use of rapid compression and heating of fusion fuel anticipated some of the physical principles later exploited in inertial confinement fusion.

Basic Principle

Work on thermonuclear weapons developed from earlier fission-weapon research. The enormous energy released by a fission primary is used to compress and heat a separate secondary stage containing fusion fuel. In the Teller–Ulam design, this compression is driven primarily by intense X-radiation produced by the primary explosion.

Greenhouse Item

The Greenhouse Item test, conducted at Enewetak Atoll in 1951, demonstrated the principle of fusion boosting. It was still fundamentally a fission device rather than a true two-stage thermonuclear weapon. Deuterium–tritium gas placed inside the fissile core underwent fusion during the explosion, producing high-energy neutrons that increased the efficiency of the fission reaction and increased the explosive yield.

Ivy Mike

Ivy Mike, detonated in 1952, was the first full-scale thermonuclear device and the first successful test of the Teller–Ulam design. Because it used cryogenic liquid deuterium as fusion fuel, the apparatus was enormous and not suitable as a deliverable weapon; with its cryogenic equipment, it weighed about 74 tonnes. The explosion had a yield of 10.4 Mt and destroyed the island of Elugelab, leaving a crater nearly 2 kilometres across.

Teller–Ulam Design

The Teller–Ulam design is the basis of modern staged thermonuclear weapons. It consists of a fission primary and a separate secondary containing fusion fuel. X-rays released by the primary fill the surrounding radiation case and drive the rapid compression of the secondary. A central fissile “spark plug” can then undergo fission, further heating the compressed fusion fuel until thermonuclear reactions begin. High-energy fusion neutrons may also induce additional fission in surrounding uranium, greatly increasing the total yield.

Castle Bravo

The Castle Bravo test in 1954 demonstrated the first U.S. thermonuclear device using solid lithium deuteride fuel, making compact deliverable thermonuclear weapons far more practical. The device produced a yield of about 15 Mt, far above the predicted 4–8 Mt, partly because lithium-7 underwent additional neutron reactions that had been underestimated. The unexpectedly high yield produced extensive radioactive fallout over Bikini Atoll and surrounding areas.

Tsar Bomba

The largest nuclear device ever detonated was the Soviet Tsar Bomba, tested over Novaya Zemlya on 30 October 1961. It exploded at an altitude of about 4 kilometres with a yield of approximately 50 Mt. The atmospheric pressure wave generated by the blast was detected repeatedly as it travelled around the globe. No larger nuclear explosive device has ever been tested.

Search for Peaceful Uses

During the 1950s–1970s, both the United States and the Soviet Union investigated peaceful applications of nuclear explosions, including excavation, the creation of underground cavities and stimulation of natural-gas production. Some experimental nuclear detonations were actually carried out, but the approach was never adopted commercially. Radioactive contamination, environmental and safety concerns, unfavourable economics and growing public opposition ultimately outweighed the potential benefits.