The United States and the Soviet Union, after independently developing thermonuclear weapons, both produced some numbers of such weapons of very high yield. While most thermonuclear weapons built had yields in the range of several hundred kilotons to a few megatons, some much larger weapons were built. Weapons with yields up to 20-50 megatons were developed and deployed (one Soviet weapon of 150 mt was developed but probably never operational). Of the roughly 135,000 warheads ever built by the two superpowers, about 3% had yields over 4.5 megatons.
The United States built the greater number of multimegaton weapons, doing so in the late 1950s and 1960s mostly to equip its bomber force with a massive nuclear capability against the U.S.S.R. The U.S. largely abandoned such weapons in favor of smaller nuclear weapons, allowing more flexible delivery of larger numbers of warheads. Most of the Soviet strategic nuclear capability was in its ICBMs, but like the U.S. the Soviets deployed high-yield weapons before mostly shifting to smaller, multiple warheads.
The U.S. has now retired all of its multimegaton weapons. Disassembly of the last type removed from service, the B53, may be completed in 2006. Russia probably maintains a small number ICBMs in high-yield single warhead versions. The People's Republic of China has one type of ICBM armed with high-yield warheads. Operational multimegaton weapons in 2005 thus include:
Russia's R-36M2 Voyevoda (SS-18 Mod 6) with a 20 mt warhead (possibly 5 deployed). (The UR-100N version (SS-19 Mod 2) with a 5 mt warhead may no longer be deployed.)
PRC's DF-5A (CSS-4) with a 5 mt warhead (about 24 deployed).
Multimegaton Weapons