Why can't nuclear reactions burn the atmosphere

[Trollfaz]

When the US first developed nuclear weapons against the Nazis and Japan, their primary concern was whether the nuclear reactions can trigger the atmospheric nitrogen to fuse and burn the whole atmosphere.
However this claim was instantly debunked by physicists. Their reasoning is that in order to fuse nitrogen, one needs extreme temperature (100MK) and extreme pressure such as in the center of massive stars( >3SM). The extreme conditions needs to be upheld constantly and is achieved when the intense mass of the star crushes in on it's center.
But for Earth's case, even if we do manage to fuse a few nitrogen atoms, the heat released will immediately radiate away while the pressure will swiftly disperse and prevent further extensive chain reaction. Is that the explanation?

 

[Nugatory]

Is that the explanation?

Pretty much, yes.

their primary concern was…

That’s way too strong a statement. There were plenty of other concerns that ranked ahead of that one.

 

[Astronuc]

However this claim was instantly debunked by physicists. Their reasoning is that in order to fuse nitrogen, one needs extreme temperature (100MK) and extreme pressure such as in the center of massive stars( >3SM). The extreme conditions needs to be upheld constantly and is achieved when the intense mass of the star crushes in on it's center.

There were two camps, one that the atmosphere would experience a 'fusion chain reaction' and the other would not. Some were concerned until the Trinity test, where it didn't happen.

The atmosphere density is too low, and the high temperatures in the core of a nuclear weapon dissipates very rapidly. It wouldn't be N-N fusion, because that would take much greater temperature and pressures, and CNO-cycle only happens in stars with the right density (and pressures) and composition (age). The pressure maintains the density at temperature (and there is quasi-steady-state balance between radiation (force) pressure and gravitational (force) pressure).

Carl Friedrich von Weizsäcker and Hans Bethe, provided the first predictions of the carbon cycle in the 1930s.
https://ui.adsabs.harvard.edu/abs/2018PhP...20..124W/abstract

Outside of stars, one needs magnetic confinement to maintain a plasma long enough to develop fusion, with light nuclei like d and t, or ³He; intertial confinement works on smaller time scales. As the atomic number of nuclei increase, the energy required to induce fusion increases (increased Coulomb repulsion), and the energy losses due to bremsstrahlung and recombination increase.

With fission devices and fusion devices, there is a tremendous number of neutrons produced, and these neutrons will flow out of the detonation plasma into surrounding atmosphere and ground where they will likely be absorbed and transmute nuclei. That is independent of the fission products, which are released to the environment.