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

Famously, Oppenheimer and co worked out how close a nuclear bomb test would be to causing a chain reaction of nitrogen fusion in the atmosphere. They made a lot of worst-case-scenario assumptions and still came to the conclusion that no, a nuclear bomb test wouldn’t scour the surface of the world.

But let’s say the atmosphere was twice as dense as it is. Or ten times as dense. At what point would that calculation turn very, very scary?

Obligatory xkcd

Edit: man, seriously, most of the people ‘answering’ this question didn’t even read it.

UraniumBlazer ,

It would turn scary if the atmosphere would become as dense as the core of a star. Then too, a lot of stuff depends on the type of star we're talking about. Are we talking about some wimpy red dwarf core density? Yeah, we'll fuse the hydrogen in the atmosphere/on the surface all right. U wanna fuse nitrogen? Oof. U'r gonna need a much bigger star than that.

So basically, to become scary, ur atmosphere would have to be a lot more dense than what it would be if it was fkin solid (like if the gases were literally solid). It wouldn't thus be an atmosphere.

So don't worry. Have fun blowing up ur nukes!

bluGill ,

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 ,
@FaceDeer@fedia.io avatar

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.

phcorcoran ,

I don't know what chain reaction exactly they were thinking of, but from modern fusion research, I believe we can confidently say that the atmosphere would need to be interior-of-a-large-star-level dense, and even then I'm not sure you'd get nitrogen fusing with anything without a lot of hydrogen or helium around. Nitrogen-nitrogen fusion seems extremely implausible for sure

chuckleslord ,

Fusion of two nitrogen-14 nuclei and a hydrogen nucleus. That was the feared chain reaction, since both elements are abundant.

Source

phcorcoran ,

Thank you, the page you sourced references a 2024 paper inspired by the Oppenheimer movie that was super interesting to read

errer ,

Prolly the most relevant paragraph from the linked article for this discussion:

Today, especially after the detonation of the 50 MT Tsar hydrogen bomb on Novaya Zemlya in 1961, it is also experimentally verified that the danger of atmospheric or even oceanic ignition does not exist. Also, the experimental measurements obtained by Zucker and others demonstrate that the fusion probability is much smaller than the geometric cross-section for 14N+14N assumed by Teller and coworkers, further reducing the chances for such an event. Furthermore, the atmosphere is also heated only to temperatures of a few million degrees, so that the most efficient energies of the fusing nuclei are a few 100 keV and thus well below the Coulomb barrier and very much reduced by penetrability. These temperatures are noticeably lower than those in the late hydrostatic burning stages of massive stars.

Basically the temperature of the atmosphere is over an order of magnitude too low to have any chance of ignition (need 10s of millions of K), and the reaction rate is thus several orders of magnitude lower than the threshold.

lurch ,

i think the idea is that the part that already fused creates a blast wave that could create the conditions, including preassure required for more fusion. i have no idea if it's possible though.

cm0002 ,

I have no idea, commenting so I can remember to come back later

saigot ,

Try the save post button next time

cm0002 ,

Yea but this plan worked, the notification from someone commenting that reminded me to come back LMAO

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