Tsar Bomba (Царь-бомба), literally "Emperor Bomb", is the Western name for the RDS-220 hydrogen bomb (codenamed "Иван" (Ivan) by its developers) — the largest, most powerful nuclear weapon ever detonated.
Developed by the Soviet Union, the bomb was originally designed to have a yield of about 100 megatons of TNT; however that was reduced by half in order to limit the amount of nuclear fallout that would result. Two bombs were built, a mock bomb and the real bomb, with the real bomb being tested on October 30, 1961, in the Novaya Zemlyaarchipelago.
Despite testing, the weapon never entered service; it was simply a demonstration of the capabilities of the Soviet Union's military technology at that time.
To limit fallout, the third stage, and possibly the second stage, had a leadtamper instead of a uranium-238 fusion tamper (which greatly amplifies the reaction by fissioning uranium atoms with fast neutrons from the fusion reaction). This eliminated fast fission by the fusion-stage neutrons, so that approximately 97% of the total energy resulted from fusion alone (as such, it was one of the "cleanest" nuclear bombs ever created, generating a very low amount of fallout relative to its yield). There was a strong incentive for this modification since most of the fallout from a test of the bomb would fall on populated Soviet territory.
