How were the Manhattan Project scientists able to predict the possibility of the atmosphere igniting after using an atomic bomb, and how did they come to the conclusion that the atmosphere wouldn’t ignite?

Question

Did the non-zero risk of the atmosphere igniting increase as nuclear weapon yields got larger and larger?

Obviously a result of watching Oppenheimer.

In: 789

Answer 1

The type of physical reactions that happen in a nuclear weapon (or in a star for that matter) depend on density and concentration. You have to have both enough material AND have it be pure enough AND have it all adjacent to itself (a big ball of highly radioactive material, basically). Only then will it go bang. Once those conditions are met, the constituent parts of the atoms start interacting with each other in a way that ultimately tears the atoms to pieces, but without those conditions all you have is a bit of radiation. You would not want to stand next to it for long, but that is very different from a bang.

The atmosphere is insufficiently dense for this type of chain reaction to perpetuate itself outside the actual blast radius of the bomb. The bomb goes bang, and there is a big shockwave, but every atom outside the immediate blast radius just runs away when it comes to their turn to “join” the bang. The atmosphere is too squishy, basically. In order for the atmosphere to ignite, it would have to be contained in such a way that the atoms couldn’t run away from each other such that they would be “forced” to join the reaction and I don’t think even a James Bond villain is building a big containment shell around the whole planet for that purpose. A star works because there is so much material that *gravity* holds the material in place and the chain reaction can become self-sustaining, but a planet is not a star.

Edit: later, in the 50s, we came up with ways to make bombs that use gasses present in the atmosphere in great amounts (eg. Hydrogen), but same thing applies — the hydrogen making the bang is contained in the bomb under controlled conditions. The hydrogen just floating around in the atmosphere is too “loose” and too dispersed for such a reaction to continue the reaction that happens inside the bomb when it is detonated. We basically created a miniature star, but it could only last for as long as the bomb itself could hold together — a few milliseconds, maybe. Once the bomb’s containment was breached and the initial blast dissipated, the pressure drops back to “atmospheric pressure” and the reaction can no longer happen. Again, in a star this is perpetuated by gravity, in a bomb by a containment vessel. But in normal environment on Earth those conditions are simply too weak to enable the atoms to interact in that way.