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 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.