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I happened to hear people saying that the nuclear fusion bomb tests could set the atmosphere on fire. I have some serious doubts about that - but I have no facts.
Nuclear fusion reaction requires $15*10^{6}$ kelvins to start. If we produce such temperature in "open air" would the atmosphere become a fuel for further fusion? Shouldn't the whole thing just be torn apart by its terrible pressure?

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This questions was taken somewhat seriously while the first fusion explosives were being designed and built. I believe the fear was that the explosions would ignite nitrogen burning (i.e. chemistry) in the atmosphere and that this would prove to be self sustaining. Given that fusion bombs have been detonated in the atmosphere quite a few times the experimental answer seems to be "no". –  dmckee May 28 '13 at 20:49
    
Regarding Nitrogen burning, does that release enough energy to be self-sustaining? If we could produce a bomb with enough energy to get it started would it actually chain reaction? It seems like if a fusion bomb can't start the process then there is no way enough energy could be released to sustain it. –  Brandon Enright May 28 '13 at 21:04
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@BrandonEnright - As far as I remember, kinetics of nitrogen-oxygen reactions are very interesting - many species, dependence on high (around 2000-4000K) temperature. With the explosion (again, AFAIR) the main impediment to exothermic oxidation of nitrogen is the short time scale during which the necessary conditions hold. (Hope that someone from Chemistry SE would clear up the confusion.) –  Deer Hunter May 28 '13 at 21:26
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AFAIK, the concerns early in the Manhattan Project all dealt with the initiation of a nitrogen nuclear fusion reaction, referred to figuratively as nitrogen burning. The OP seems to refer to this case; other comments seem to discuss chemical burning. –  User58220 May 29 '13 at 0:25
    
@dmckee I love that historical note. There are numerous texts from throughout the early and mid 20th century that hint at the concerns of runaway atmospheric burning (e.g. work by Akira Sakurai). I don't know if I feel relieved that there were scientists worried about that kind of thing; or concerned that it doesn't seem to have gotten that much attention... –  zhermes May 29 '13 at 1:04

3 Answers 3

up vote 7 down vote accepted

From what I have read in "American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer" Teller was the first one to express this concern before the Trinity test. Also quoting from: http://www.sciencemusings.com/2005/10/what-didnt-happen.html

Physicist Edward Teller considered another possibility. The huge temperature of a fission explosion -- tens of millions of degrees -- could fuse together nuclei of light elements, such as hydrogen, a process that also releases energy (later, this insight would be the basis for hydrogen bombs). If the temperature of a detonation was high enough, nitrogen atoms in the atmosphere would fuse, releasing energy. Ignition of atmospheric nitrogen might cause hydrogen in the oceans to fuse. The Trinity experiment might inadvertently turn the entire planet into a chain-reaction fusion bomb.

Robert Oppenheimer, chief of the American atomic scientists, took Teller's suggestion seriously. He discussed it with Arthur Compton, another leading physicist. "This would be the ultimate catastrophe," wrote Compton. "Better to accept the slavery of the Nazis than run a chance of drawing the final curtain on mankind!"

Oppenheimer asked Hans Bethe and other physicists to check their calculations of the ignition temperature of nitrogen and the cooling effects expected in the fireball of a nuclear bomb. The new calculations indicated that an atmospheric conflagration was impossible." Bethe apparently then convincingly showed that the atmosphere would not be set on fire by a nuclear bomb.

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But this reference, sciencemusings.com/2005/10/what-didnt-happen.html makes it clear that the concern was nitrogen fusion, not burning. –  User58220 May 30 '13 at 0:23
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OK fair enough, I am using a less technical term but I will edit it make it more accurate. –  physicsphile May 30 '13 at 1:44
    
+1 for a really cool story that I hadn't heard before. –  Mike May 30 '13 at 2:39
    
@physicsphile My concern was that throughout these comments, there is confusion about whether nuclear fusion of nitrogen, or chemical burning of $N_2$ and $O_2$, is the actual subject. The OP clearly is interested in fusion, as were Oppenheimer et al... –  User58220 May 31 '13 at 0:32
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I don't think anyone is suggesting chemical burning, they are just using casual/descriptive language when they say "set the atmosphere on fire". –  physicsphile May 31 '13 at 11:32

I'd simply like to add to physicsphile's answer.

The primary source for this question is

Konopinski, E. J; C. Marvin; Edward Telle, "Ignition of the Atmosphere with Nuclear Bombs", Los Alamos National Laboratory technical report #LA-602

It shows that the answer to the OP's question is "highly unlikely". It does not prove impossibility. It's an interesting read from the point of view that these were the calculations and reasonings that the whole future of life on Earth was decided with.

As a physicist, I would say the document is highly sound. Altogether acceptable for making decisions about money expenditure of any kind, even sound enough that one would OK an experiment that could risk even hundreds of lives with (although it is hard to think of a realistic example). But it's a little scary to think that the whole future of life on Earth was decided with it .....

So let's look at the experimental data. We haven't ignited the atmosphere yet. I think this experimental fact is important for your question: as I understand it, the fine details of the explosion dynamics are to a large degree found by trial and error, and such experimental data are all classified anyway. But the following comments are probably relevant. The biggest bomb to date was the Soviet Tsar Bomba, which let slip $2.4\times10^{17}J$, or $2.6{\rm kg}$ (that's right, kilograms!) of energy (57MT TNT equivalent). The fireball from this monster was eight kilometres across. At this size of bomb, you have probably reached a scale where bigger bombs are going to mean a proportionally bigger volume of space at roughly the same temperatures (of the order of $10^8{\rm K}$. Moreover, Edward Teller calculated that at yields not much higher, the effect of increased yield (as far as the atmosphere is concerned) is negligible: a big chunk of the atmosphere around the blast is accelerated to Earth escape velocity and is lost into space, so adding yield simply means that the escaping gas is going to escape faster: it's not coming back once it reaches $11{\rm km\, s^{-1}}$, so what happens to it is immaterial.

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Given the current political situation, I feel very uncomfortable reading about kilometres wide fireballs created by Russian bombs... –  Tomáš Zato Dec 19 at 10:54

Around the 60s, a treaty was signed to ban development of nuclear fusion devices with yield greater than about 50 MT (don't remember exact number), in order to prevent fusion of atmospheric hydrogen, thus the uncontrolled multiplication of the device explosive yield. That was before the Threshold Test Ban Treaty was signed in 1974 and entered into force in 1990.

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