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The risk isn't from a nuclear explosion, it's from an explosion that scatters nuclear material which is way more likely in a rocket than a bomb.
Er, yes, that's what "fallout" means. You missed @RandomRanger's point. One rocket's worth of nuclear material in the atmosphere is barely a blip. Note that even a normal rocket is chock-full of toxic chemicals, which is why we don't launch near population centers. Most normies tend to be off by many orders of magnitude when they intuit how dangerous "nucular" things are.
I'm not convinced. One NERVA style nuclear thermal rocket engine contains hundreds of kilos of uranium. Put one as an upper stage engine on a SpaceX booster and you can lift another 100 tons of cargo to orbit - which quite frequently will be 100 tons of U-235 (or 233, since we'd probably quickly get into thorium breeding if we'd consider such a project). We want to fuel an economy the size of a solar system, after all, and earth is the only place in this economy where it would be economical to mine Uranium.
Compare this to the ~4 kg of an H-bomb primer, and vaporizing a nuke fuel truck sounds a whole lot more catastrophic that an atmospheric test.
The interesting part is the "vaporizing" here. I'm pretty sure that most failure modes of such a launch would not vaporize a significant fraction of the payload or even the engine cores. The "fallout" would quite literally be tens of thousands of 1-kg pits (and a few fuel pellets) raining down from the explosion. Compared with the alternative, that contaminates a much smaller area. Manual clean-up would be possible, economical and necessary from a proliferation (and ecological, of course) perspective.
Uranium is not the problem unless you vaporize tons of it (and I do mean vaporize, not just scatter tiny nuggets around). It's far more dangerous as a toxic heavy metal than due to radiation due to its very long half life of a billion years or more. Reactor meltdowns on earth are a problem because the reactors contain significant quantities of shorter lived and thus very strongly radioactive components, most notably Cesium-137 and Iodine-131. A reactor that has barely begun operation hasn't yet had time to accumulate significant quantities of those.
Fallout really doesn't apply here as it means small heavily radioactive particles that fall down downwind of the detonation. Those particles are generated by the neutron activation of the surrounding materials and mixing up the tiny debris with radiation products from a surface burst. For airburst the quantities are smaller and are so high in the atmosphere that they've had time to decay to safer isotopes by the time they fall down in months to years.
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Hmm, I think you're talking about two different things. One is the launch, from Earth, of a nuclear-powered rocket (e.g. NERVA). Even if it contains hundreds of kilos of uranium, it's a lot fairer to compare that to an A-bomb like Little Boy (64kg) rather than just the primer of an H-bomb. And, like you said, in an accident a lot less of it is going to vaporize than it would in a proper nuclear bomb.
But I wasn't talking about the payload at all. I guess you're thinking that you'd want to lift 100 tons of U-235 to orbit for space-based nuclear rockets? I agree that's a different kind of risk. And I'm not even sure how valuable nuclear rockets would be for long space trips (there are lots of options once you're up there).
"Current" designs (well, currently available 1960's designs) of nuclear powered rockets aren't useful for launching from the surface. While they have by far the best efficiency/specific impulse of all engines available today, they have catastrophically terrible thrust to weight ratios. Absolutely useless engines for first stage and even most second stage applications. You'd only want to use them in space - then their low thrust doesn't matter, and they use their high fuel efficiency to cut down time of a Mars transfer by a factor of
3.The vast majority of atmospheric tests where tactical warheads with a boosted fission core. Those - just like H-bomb primers - always contain subcritical amounts of plutonium (4kg) for efficiency and safety (they can only fission if explosively collapsed correctly into a critical mass) reasons. Pretty much the only devices with larger amounts of fissile material are H-bombs with second stages and tampers. But even those are much, much lighter than Little Boy, and they weren't tested all that much.
Extremely valuable! Even the most primitive and conservative designs outperform chemical rockets by several hundred percent (again, in specific impulse). More batshit designs (nuclear pulse propulsion and nuclear salt water rockets) are probably technically doable today, and offer orders of magnitude more specific impulse. Those would actually unlock the outer planets and the asteroid belt, and maybe Alpha Centauri.
Sustained fusion is already difficult enough in containment, actual fusion propulsion is probably orders of magnitude more complex than that. I have no hopes to still be alive when it arrives.
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