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

You know how a log can be hard to make it catch on fire? Now imagine the atmosphere is a nice tinder pile of logs. Normally nothing would happen but what if someone lit a match and then made one spot hot enough that it made enough heat from burning that the spot next to it get hot enough to burn and now you get more and more of the logs on fire.

The nuclear reactions in an atomic bomb are similar but instead of a match you have a neutron. Why a neutron? Because they are heavy and are not electrically charged. So if you shoot one at an atom it’s like a heavy bullet. It can make it through the cloud of electrons (negative charged almost mass less particles) that form a shield around the tiny little core (nucleus) of the atom. If that neutron hits the core which is a mix of protons and neutrons in a tight ball, then it can shatter it into pieces and depending on how it shatters it can release some neutrons at high velocity. If more than two are ejected then they can repeat that process with two other nucleuses and now there are 4 neutrons. This can keep going and now you have a runaway reaction and if the atmosphere can do this it will burn until it can’t anymore.

Just like with the pile of tinder as anyone that’s lit a fire, sometimes the fire is not concentrated enough and the amount of heat lost to the air is enough that the fire stops and some logs don’t burn.

The same can happen with a nuclear reaction. If some of those neutrons start to miss other nucleuses because things are too far apart or because they get too slow and can’t break them or because they are too fast and don’t break them right or hit nucleuses that don’t break or break different then the reaction fizzes out and it stops.

So it looks plausible when we didn’t have enough data that a very big nuclear reaction might light up the atmosphere. In practice though they knew that the atmosphere is hit by lots of very energetic (really fast bullets) all the time from outer space and from experiments they had done with devices designed to create very fast particles so they really didn’t think it was a concern but it makes for a good story lol.