Much of the fallout of a nuclear bomb comes from the dirt becoming so neutron irradiated that they become unstable isotopes. But also the daughter particles of a nuclear bomb are also unstable even if the u235/238 is consumed. If you explode a bomb in atmosphere the fallout concerns are much less because you aren't kicking as much irradiated dirt into the atmosphere.
Much of the fallout of a nuclear bomb comes from the dirt becoming so neutron irradiated that they become unstable isotopes. But also the daughter particles of a nuclear bomb are also unstable even if the u235/238 is consumed. If you explode a bomb in atmosphere the fallout concerns are much less because you aren't kicking as much irradiated dirt into the atmosphere.
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