 There is evidence that the Tsar Bomba had several third stages rather than a single very large one.The initial three-stage design (coded A620EN, not tested) was capable of yielding approximately through fast fission, 3,000 times the size of the Hiroshima and Nagasaki bombs, but it was thought that it would have caused too much nuclear fallout and the aircraft delivering the bomb would not have had enough time to escape the explosion.To limit the amount of fallout, the third stage and possibly the second stage had a lead tamper instead of a uranium-238 fusion tamper (which greatly amplifies the fusion reaction by fissioning uranium atoms with fast neutrons from the fusion reaction).