# Eli5: Before the first atom bomb was detonated, there was some speculation that the chain reaction would keep continuing and lead to burning up the atmosphere. So what actually limits the size of the explosion?

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Eli5: Before the first atom bomb was detonated, there was some speculation that the chain reaction would keep continuing and lead to burning up the atmosphere. So what actually limits the size of the explosion?

In: Physics

That’s not really the thought.

The bomb would release energy at an energy density never seen before. The Air is 3/4 nitrogen and 1/5 oxygen. At some pressures you can burn nitrogen, producing nitrous oxides. This is one form of car pollution.

If nitrogen can burn and the air is 3/4 nitrogen, the question was “Would the bomb raise the energy high enough to set the whole atmosphere on fire?”. Calculations before the NM test indicated this was very unlikely, but “very unlikely” ≠ “it couldn’t happen”.

The nuclear explosion is limited by the amount of Uranium/Plutonium in the bomb.

In theory, one of the limiting factors is trying to prevent the fuel from being blasted apart, since you need to maintain a certain density to properly achieve criticality for both fusion and fission, although I’m not sure if that’s the biggest one besides practicality

The speculation about the atmosphere was more a what if hypothetical than an actual concern, like [T. Folse](https://youtu.be/QwcHBize_xI?si=bf8LYHG1Bsp1E4Eo) jokes, in the nuclear industry 1+1=100 to be on the safe side, to make that hypothetical even possible you’d need to multiple that by multiple orders of magnitude, that it’s effective impossible

>The limits were luckily never tested, but in general, I would say, the density of the atmosphere is too low,” Wiescher responded when asked whether a powerful enough bomb to burn the Earth’s atmosphere could ever be built.

>“If one would substantially increase the atmospheric density to Venus values — 100 times denser than Earth — one would still not have the density of water, and the underwater test program did not ignite the oceans, as some people predicted,” he elaborated.

Uranium and plutonium are both dense metals. When the fission reaction is initiated the uranium atom is split and releases neutrons which then hit other atoms of uranium causing them to split.

This is the basic fission reaction.

Air is much less dense, and the atoms move much faster and are less reactive. The neutrons from the fission reaction in the uranium have a much lower chance of hitting the nitrogen atoms, but more importantly if they did the nitrogen they hit is much less reactive and more stable than the uranium.

> So what actually limits the size of the explosion?

The size of the bomb. The bombs don’t heat the surrounding atmosphere enough to produce many reactions there, and these reactions don’t release enough energy to trigger more reactions in the atmosphere. Essentially all the power of the explosion comes from the bomb.

Turning mass into energy.  Means a small amount of mass.  A limited amount of energy.   They thought it might ignite the mass within the atmosphere.

There is no limit to the size of the explosion. An H-bomb typically has two stages where the first stage is fission and the 2nd stage is fusion. X-rays from the fission ignite the fusion before the fission blast blows things apart. You can build a 3 stage bomb with two fusion stages. Or a 4 stage bomb and on and on ad infinitum. This according to “The Curve of Binding Energy” by John McPhee.

The story I’ve heard is that project manager Robert Oppenheimer and Danish physicist Niels Bohr bet \$1 on whether the test would ‘ignite the ozone layer of the atmosphere’, which would likely end life on Earth.

The ozone layer doesn’t have that much more oxygen than the rest of the atmosphere, around 20% or so. The calculations determined that the test wasn’t enough energy for that sort of ignition to occur. But it was discussed, and studied before it was ruled out.

I don’t know who won the bet. My guess would be Oppenheimer, but I’ve got no basis other than Bohr’s Danish upbringing leading to a darker sense of humor.

>So what actually limits the size of the explosion?So what actually limits the size of the explosion?

Air is a reasonable insulator, that absorbs energy from big explosions. Even if the test released energy without anything to absorb it, the energy would eventually spread, and the density would decrease, just as the Sun’s energy has a limit to its reach, even though it’s total energy output is in the order of millions (billions?) of atomic bombs *per second.*