Thermonuclear weapons or hydrogen bombs were first built in both the United States and the Soviet Union from 1953 to 1955, and a dispute as to which country developed such a weapon first soon ensued, a symptom of the Cold War competition between the two powers. Both the United States and the Soviet Union considered several designs of thermonuclear bombs that relied on nuclear fusion rather than on fission for the majority of their energy release.
One reason for confusion as to priority is the fact that in some nuclear weapondesigns that used plutonium, a small amount of the tritium isotope [V] of hydrogen was used to create a partial fusion effect, to more thoroughly fission a plutonium core. The Americans tested this method, which they called a boostedweapon, as early as 1951. Since the process of boosting was kept classified for some years, outside commentators remained unaware of the distinction between a boosted fission weapon and a true hydrogen bomb. In the latter, the majority of the energy release was due to fusion.
In the late 1940s, American weapons designers thought through and abandoned a preliminary design for a true hydrogen fusion weapon. In that design, a fission weapon would ignite a deuterium fusion fuel. The device could be known as a fission-fusion-fission weapon, with layers of fissionable material, including some 238U that would fission in the intense neutron environment. The yield of such a weapon would be the equivalent of several hundred thousand tons of TNT, or several hundred kilotons.
Instead of using that abandoned design, the United States built a cooled device using liquid deuterium, detonated in the Ivy-Mike test on October 26, 1952 on Enewetak, an island in the Pacific Ocean. Later, another design worked out by Edward Teller (1908–2003) and Stanislaw Ulam (1909–1984) was far smaller and could be transported by aircraft.
The Soviets pursued a weapon similar to the abandoned early U.S. design and called it the sloyka (layer cake) design, first conceived by Yakov Zel’dovich (1914–1987) as the “first idea” and later modified by Andrei Sakharov (1921–1989). Sakharov’s “second idea” surrounded deuterium with a 238U shell in the layers or sloyka, to increase the neutron flow during the detonation. The Soviet scientists tested the sloyka on August 12, 1953, with a yield of 400 kilotons. About 15 to 20 percent of the weapon’s force derived from fusion. The United States already had larger boosted weapons, and American analysis of the Soviet 1953 test was that it was simply a boosted fission weapon. The Soviet authorities, however, continued to maintain that the August 1953 test was the world’s first hydrogen bomb. American analysts continued to call it a boosted fission weapon.
The first test by the United States of a thermonuclear device that could be dropped as a weapon from an airplane was the Teller-Ulam design tested in 1954, with a yield estimated at 15 megatons.
The Soviet “third idea,” developed by Igor Tamm (1895–1971) and Sakharov, could produce, like the Teller-Ulam device, much higher yields than using the sloyka idea. The Soviet Union tested the “third idea” weapon on November 22, 1955, producing an estimated yield
equivalent to 1.6 million tons of TNT. American scientists claimed that the test of Ivy-Mike in 1952 was the first true thermonuclear detonation and that the Teller-Ulam weapon tested in 1954 was the world’s first deliverable thermonuclear weapon. The Soviets continued to regard their 1953 test as the first thermonuclear test.