Fusion reactions power the sun and the stars. The fusion of deuterium and tritium, both heavy isotopes of hydrogen, releases energy as well as a neutron with seven times more energy than a fission neutron. Fusion’s energy output per kilogram of source material is much higher than that of fission.

Fusion can be used inside a fission explosion to improve the efficiency of the weapon (boosting), or a large amount of fusion fuel can be triggered separately (thermonuclear weapon). The fusion of deuterium and tritium is initiated by the extremely high temperatures and radiation that result from fission.

(Fusion) boosted fission weapons

When fusion takes place in a fission weapon, the high-energy neutrons are more likely to collide with fissile atoms. Those higher-energy collisions release even more neutrons than simple fission, speeding up the chain reaction. This enables more material to fission before the device blows itself apart.

Boosted weapons are typically implosion devices with deuterium and tritium gas introduced into the hollow pit in the centre of the fissile pit. As fission begins, the high temperature causes fusion, and the high-energy neutrons released by fusion accelerate the fission chain reaction. Boosting has led to a hundred-fold increase in the efficiency of fission weapons since 1945, and it plays a role in nearly every nuclear weapon deployed today. Pakistan reportedly tested a boosted fission weapon in 1998.

Thermonuclear weapons (also called hydrogen bombs)

Thermonuclear bombs yield explosions in the megaton range, that is, orders of magnitude more powerful than the atomic bombs described above. The standard “Teller -Ulam design” uses a fission primary to trigger a powerful fusion secondary. The x-rays released from the primary explosion compress and ignite the secondary. The massive release of high-energy neutrons from the fusion reactions in the secondary causes even the U-238 tamper (not fissile) to fission, allowing for massive explosive yields. Using these design principles, an arbitrary number of additional stages could theoretically be added to create a “doomsday device” that would dwarf even the Tsar Bomba. However, the main threat posed by thermonuclear weapons is their ability to pack huge amounts of explosive power into small, light-weight packages that can be delivered by missiles.

The first U.S. thermonuclear test was “Ivy Mike” in 1952, which was carried out on the Enewetak atoll in the Marshall Islands. The Soviet Union responded with “Joe 19” in 1955, a bomb which was air-dropped over the Semipalatinsk test site. Britain and France have both tested devices with yields exceeding two megatons, Britain in 1957 at Christmas Island and France in 1968 at Fangataufa Atoll. China first tested a thermonuclear weapon at its Lop Nur site in 1967, less than three years after its initial atomic test.

According to the Nuclear Threat Initiative, Israel may possess boosted and possibly even thermonuclear weapons, but it has never confirmed this nor has it, to public knowledge, tested any explosive nuclear device. India claimed that its Shakti 1 test in 1998 was a successful reduced-yield thermonuclear bomb, though many outside experts remain skeptical.

Mastering thermonuclear weapons technology makes it possible for countries to miniaturize their weapons, allowing for flexibility in delivery and yield. U.S. warheads such as the B83 can be adjusted on the battlefield for either a low sub-kiloton yield (primary only) or higher yields up to 1.2 megatons. Many of the missile-delivered warheads currently deployed by the nuclear weapon States are light-weight thermonuclear weapons that fall in the 100-300 kiloton range. Other types of nuclear weapons not believed to be currently deployed include neutron bombs; and cobalt bombs.