Originally Posted by
SOPS
I don’t understand why they describe it as a ‘clean bomb’. WTF is ‘clean’ about it? It was polished before the exploded it?
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.