Did the non-zero risk of the atmosphere igniting increase as nuclear weapon yields got larger and larger?
Obviously a result of watching Oppenheimer.
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Here is generally what happened. I wasn’t going to say anything, but there are too many “mostly right” answers mixed with too many overly complicated ones. This is intended to be sufficiently accurate and comprehensible to be proper for this sub.
The first atomic bombs were nuclear fission. This means that they took atoms with large unstable nuclei (centers) and caused those atoms to split apart, releasing energy. Because of the way in which they did this and the atoms being used (U-235 and plutonium were both used), each of these atoms caused two other atoms of the same type to break apart. The result was a “chain reaction”, with each atom triggering two others, each of which triggered two others, and so on, roughly doubling with each breaking nucleus.
The scientists knew how much energy would be released with each splitting. They knew how many atoms of the right type (roughly) that they had in the bomb. So they could readily calculate how much energy would be released. They knew how much pressure this would create, and they knew the temperature that they would generate.
There is another type of nuclear reaction, nuclear fusion. This is what powers the Sun. Insteasd of big atoms breaking up and releasing energy, you crush small atoms together to create energy. In both cases the energy that might be released is related to how far you are from the “middle” element, iron. Breaking big atoms to create atoms closer in size to iron makes energy. Crushing small atoms together to create atoms closer in size to iron makes energy. Going in either direction from iron always uses energy, never releasing it.
To get nuclear fusion, you need enormous pressure and temperature, and the further you get from the lightest element, hydrogen, the more pressure and temperature you need. This is why fusion bombs generally use hydrogen and are thus called hydrogen bombs.
In the atmosphere, there is nowhere near enough hydrogen or helium to create a fusion reaction. However, most of the atmosphere is nitrogen, and this can be used in fusion under enough pressure and temperature.
The temperature and pressure required are enormous, far higher than any bomb ever created comes close to, even if you were to surround the bomb with densely frozen nitrogen. So much higher that the idea that the original “atom bomb” could ignite it was considered by most physicists to be ridiculous.
However, Teller took the idea seriously.
It has been suggested that some of the other physicists considered Teller to be inept at mathematics. I cannot confirm this. However, a quirk in how physicists talk about things versus most people gave him credibility. When asked about the possibility, the physicists said that the chance was virtually zero.
When physicists say that the chance is virtually zero, they mean things like the odds of taking a small pistol and firing it into the air in Oregon, USA, having the bullet be caught by freak winds, the winds carrying that bullet to Africa, and the bullet falling and going through the eye of a specific rhino into its brain and killing it. And, actually, even that isn’t good enough. “Almost zero” in physicist speak means “No way in Hell, ever, are you crazy?” in normal people language.
However, with Teller backing the idea and physicists being almost incapable of saying “impossible”, a study was commissioned that took months to investigate the idea. In the end, the study said it simply would not happen.
Then Enrico Fermi, it has been suggested just to mock Teller, went around trying to make bets with his colleagues on the likelihood of it happening during the first test.
This, unfortunately, firmly embedded in the public consciousness that scientists will risk anything, even the extinction of the human race, in order to do science.
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