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?


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

The only way that the atmosphere could “ignite” (it’s not actually igniting that they were worried about) would be if the gases in the atmosphere had the right properties to undergo a self-sustained nuclear reaction that would keep propagating until the atmosphere ran out of fuel if given a triggering event.

The calculation on whether a substance is capable of self-sustained nuclear reactions upon receiving neutron inputs is fairly straightforward in concept (albeit using complicated math); basically, for a self-sustained reaction to happen, the two follow conditions must be true:

1: When an energetic neutron from a nuclear bomb strikes an atom in the atmosphere, can it induce a nuclear reaction in that atom?
2: If so, are the chances high enough and the number of energetic neutrons emitted by the induced reaction high enough to, on average, induce a reaction in at least one other atom (which would then itself trigger other reactions, and so on)

If these conditions are true, then nuclear reactions would propagate endlessly throughout the atmosphere until the atmosphere is sufficiently depleted so that each nuclear reaction causes less reactions than what caused it.

If either are false, then the reaction will peter out very quickly and not really do anything outside of the fission in the nuclear bomb. They calculated that while point 1 is sometimes true, point 2 is always far, far, far lower than what is needed to self-sustain a nuclear reaction in the context of the atmosphere.

The atmosphere is full of nitrogen which could fuse during very very high temperature. Fusion of nitrogen would release energy, which means that if the energy generation from fusion would surpass the energy lost as heat radiation then the atmosphere would basically keep fusing and creating more and more energy eventually destroying the earth.

That energy balance between energy gain from fusion and energy lost from radiation was what the manhatten scientists calculated. There’s a video going into more details about the subject: [](

If you pumped enough energy into air (or really, pretty much anything) the atoms that make up the air will fuse together to make heavier elements (Nitrogen combining to make either magnesium and helium or oxygen and carbon). This would release a lot of energy, the same mechanism that makes a hydrogen bomb work. This energy could (potentially) fuel further nitrogen-nitrogen reactions, releasing more energy and so on into an every increasing explosion. This is what they were worried about.

However reaction requires a LOT of energy. As the explosion happens, it expands outward in a sphere, and as it expands the energy spreads out so that the “local” energy in any particular spot drops quickly. Moreover, only a portion (a tiny portion typically) will actually go into making the nitrogen atoms collide and collisions would actually be pretty rare, the actual atoms in air are pretty spread out.

The Mahhatan project scientists calculated the very upper limit for the energies that a bomb would create, along with the very lower bound for when fusion would occur in the atmosphere. From those calculations, they determine that the atomic bomb would not have nearly enough energy to fuse nitrogen. Furthermore, they also calculated that even if nitrogen fusion occurred, the reaction would fizzle out and it would not be enough to fuse more nitrogen. The world was safe.

Kyle Hill just did a video on this and goes through science if you want more background on it.

Earth-like atmospheres don’t ignite, or wildfires would wreak havoc on a global scale every time they happened.

And on a nuclear-level, any nuclear explosion that could cause a chain reaction in Earth-like atmospheres would be so powerful you wouldn’t be able to use it even on your enemy on the other side of the world without direct damage to yourself (and the atmosphere would be quite a minor issue at that point).

They invented the thing using maths/physics, and to work out the possibilities they used maths/physics, like people do every single day somewhere in the world. They also tested small-scale dozens of times to confirm their numbers were correct and any major difference would have sent them back to their chalkboards.

These people were designing never-before-seen weapons. They weren’t idiots. Quite the opposite. Whether you agree with their purpose or not, they were geniuses to even work out it was possible in the first place, so running some fanciful numbers on a par with “if you move on a train over 30mph, you’ll suffocate because the oxygen will be stripped away” (another “belief” held by many people until someone did the maths/physics/engineering and then laughed at the entire principle and built a machine capable of that speed to prove it) was basically a doodle in the margin in comparison.