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