Originally Posted by
VP959
It was a very low fallout design, as they didn't include a U238 tamper. Pretty much all the fissile material in the trigger was consumed, and the vast majority of the output was thermonuclear, so didn't produce any significant nuclides, at least at ground level. One of the advantages of thermonuclear (fusion) devices over earlier fission devices was the very much reduced fallout, as well as the massively greater yield.
There's still some fallout, because the fusion reaction requires a fission reaction to trigger it, but nothing like the sort of contamination created by a relatively large fission device, such as the ones detonated over Japan, as the fission trigger within a fusion device is only used as a source of X rays to trigger the fusion reaction in the main fuel. There were probably three stages in the Tsar Bomba, a conventional explosive initiator, which compressed the fission trigger, much the same as a small conventional fission bomb, and the focussed X rays from that fission reaction initiated the fusion reaction that produced almost all the power of the device. The nature of a fusion reaction consumes pretty much all the fuel, leaving little in the way of material that could contaminate the area.
To quote from Wikipedia
https://en.wikipedia.org/wiki/Tsar_Bomba
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). This eliminated fast fission by the fusion-stage neutrons so that approximately 97% of the total yield resulted from thermonuclear 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 likely have descended on populated Soviet territory.