Tsar Bomba could have theoretically yielded as much as 100 megatons, but it would have resulted in a dangerous level of nuclear fallout (approximately 25% of all fallout produced since the invention of nuclear weapons in 1945). Additionally, the delivery plane would not have had sufficient time to retreat to a safe distance. Therefore, to minimize nuclear fallout, the third stage incorporated a lead tamper instead of a uranium-238 fusion tamper. It has been speculated that the second stage used this method as well.