quoting from https://en.wikipedia.org/wiki/Tsar_Bomba about the "tsar bomb": To limit fallout, the third stage and possibly the second stage had a lead tamper 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). In the Russian Wikipedia article, it quoted Soviet scientists as saying that after two hours the level of radiation at ground zero was safe enough for a human being. Given that this was not internationally monitored, any statement like that should be taken with a grain of salt, but one of my questions is whether, given the above explanation, this is credible? My second question is that if this is feasible, then is it a cost factor alone that makes the nuclear superpowers make the standard fusion bomb "dirty"?