How dense would the atmosphere need to be to result in a cataclysmic chain reaction during a nuclear explosion?

bluGill , 23 days ago

for fission, nothing in the atmosphere is fissable and so it can't. Fusion would be possible but that starts spontaniously via pressure.

at least that is how I read wikipedia, I await a real phyiscist to tell me how close I am.

FaceDeer , 23 days ago

Yeah, you're not going to get a self-sustaining reaction in Earth's atmosphere if it wasn't already hot and compressed enough that there would be a self-sustaining reaction happening anyway. It's just not a plausible concern. You only get self-sustaining fusion in stars, so Earth would have to be a star in this scenario.