Why Nuclear Bombs Can’t Set The World On Fire

Before the first atomic bomb was detonated, there were some fears that a fission bomb could “ignite the atmosphere.” Yes, if you’ve just watched Oppenheimer, read about the Manhattan Project, or looked into atomic weapons at all, you’ll be familiar with the concept. Physicists determined the risk was “near zero,” proceeded ahead with the Trinity test, and the world lived to see another day.

You might be wondering what this all means. How could the very air around us be set aflame, and how did physicists figure out it wasn’t a problem? Let’s explore the common misunderstandings around this concept, and the physical reactions at play.

Not Fire, But Fusion

The main misconception is that a fission bomb could “ignite” the atmosphere  in the sense that the air itself would burn. It comes down to terminology; the word “ignite” is most familiarly used to refer to fire. Combustion is a chemical reaction, involving the breaking of molecular bonds between atoms. This results in a release of energy in the form of heat, light, and so on. However, the species typically present in our atmosphere are, by and large, not very flammable. The nitrogen that makes up the greatest proportion of air certainly doesn’t want to burn; neither does argon or carbon dioxide, for that matter. If they readily reacted in such a way with the oxygen in the air, we’d already know about it.

Instead, the concern was that the great energy output from a fission bomb could instead “ignite” a nuclear chain reaction in the atmosphere. The possibility was considered as early as 1942, several years before the successful Trinity test. Physicist Edward Teller raised the idea that the intense heat created by a fission bomb could cause hydrogen atoms in the air and water to fuse together into helium. The idea was that this could then release more energy in a runaway chain reaction that quickly consumed the entire atmosphere, functionally destroying the Earth as we know it. Concerns that a reaction could occur in the world’s oceans were also raised along the way.


The matter was studied, with Teller working with Emil Konopinski on the problem. The two published their findings in a report some six months before the Trinity test took place. The two concluded that no matter how hot any one section of the Earth’s atmosphere might become, a runaway nuclear chain reaction was unlikely to be sustained. This was due to the fact that even if any fusion reactions did occur in the open atmosphere, the energy lost to the surroundings via radiation was far in excess of the extra energy released. Thus, no self-sustaining chain reaction would occur.

The report examined a variety of potential reactions, including those concerning the potential for nitrogen to get involved, but concluded there was no risk of a chain reaction occurring. Even in the event of the detonation of a truly massive thermonuclear bomb of over 1000 cubic meters in size, energy transfers from Compton scattering would shed enough energy to prevent a runaway self-sustaining nuclear reaction in air. The numbers indicated there was a large safety factor by virtue of the unsustainability of chain reactions in the atmosphere.

“Ivy Mike” was the first detonation of a thermonuclear weapon, taking place in 1952. Despite being hundreds of times more powerful than the weapons dropped on Japan, it too was incapable of “igniting” the atmosphere. Public Domain

The matter was largely considered closed, but resurfaced in the 1970s. This was largely due to an essay published in The Bulletin of the Atomic Scientists rehashing the concerns around nuclear ignition of the atmosphere during the Manhattan Project. Physicist Hans Bethe would go on to rebut the issue once more, noting that nuclear fusion reactions are only sustained under great pressure, something not present in the atmosphere or the Earth’s oceans.

Fundamentally, nuclear weapons proved to be incapable of destroying the world in a single detonation. They remain capable of doing great harm, regardless, and questions still rage as to whether they could be the end of civilization by other mechanisms, such as nuclear winter. In any case, past decades have seen the world grow far more wary of their use, and we yet hope that such events may never again come to pass.

62 thoughts on “Why Nuclear Bombs Can’t Set The World On Fire

  1. > The two concluded that no matter how hot any one section of the Earth’s atmosphere might become, a runaway nuclear chain reaction was likely to be sustained.
    This should probably be UNLIKELY to be sustained…

    1. We had the tail end of these discussions when I was in college. I seem to remember a very old Scientific American article discussing it.

      In the article, it was pointed out that Earth is constantly bombarded by particles with a range of energies, and occasionally an energy that can only be considered “extreme”: one example measured a single particle hitting the Earth with a Joule of energy.

      If the atmosphere could be ignited to fusion from a single event, then a high energy particle would have done it by now. The conclusion was that it wouldn’t happen.

      On the subject of chemical energy, note that nitrogen and hydrogen form to make ammonia, and there’s usually hydrogen in the form of water in the atmosphere or various places on land.

      This reaction is both exothermic and entropically favored, meaning that it could be self sustaining if you could figure out a way to do it. The reaction releases a lot of energy, comparable gram-for-gram to the energy released from burning wood.

      Again, if there were a way to do this then lightning strikes during thunderstorms would ignite a fair bit of the atmosphere, leaving the planet covered in ammonia.

      Why this doesn’t happen is left as an exercise for the reader, but note that the planet has had a nitrogen-based atmosphere since before the time of the dinosaurs, so if there were a natural way to do it it would have happened by now.

      1. “one example measured a single particle hitting the Earth with a Joule of energy.”

        A *joule*? Bah! That’s nothin’. That’s 16 EeV. A few of those hit any given city every year. *Thousands* of those have been measured. The highest-energy events measured are *tens* of joules in a single particle. Above that point, the universe becomes opaque to basically all particles save neutrinos, so the flux disappears.

        That being said, I think it’s more likely that you’re thinking of an article regarding the likelihood of a miniature black hole forming from a high-energy collision (e.g. from the LHC, or prior to it, the proposed SSC) and “consuming the Earth.” Someone filed an environmental complaint about the possibility of this in Hawaii, and the common (correct) response was to bring up cosmic-ray interactions.

        It’s not a great argument against igniting a nuclear chain reaction because those high-energy particles interact pretty high up in the atmosphere, where it’s much thinner, and because they *are* such high energy, they actually only barely deposit any energy at all in each individual interaction anyway. Yes, it’s a joule of energy, but it functionally ends up spread over many, many square kilometers.

        The main argument against a nuclear chain reaction is the low pressure of the atmosphere, meaning that there aren’t a lot of reactants and energy from any one interaction is lost extremely quickly. In other words, “the Earth is not a star.”

        1. n.b. for the pedantic, the highest energy cosmic ray recorded was, in fact, *two* tens of joules (320 EeV), so I’m being a bit hyperbolic, but not that much. In all likelihood that particle’s energy was overestimated: the Universe starts becoming opaque around ~4-5 J, so anything past that point has to be very local, and our galaxy does not appear to have any accelerators capable of creating something that energetic.

          1. Oh f’crying out loud, I multiplied by 1.6e19 rather than dividing by 1.6e-19.

            fixing numbers: 1 joule = 6.2 EeV, a ludicrously common flux (tens of thousands measured)
            Cosmic ray cutoff: a little under ~10 joules (convenient)
            “Highest” (technically): 320 EeV, around 50 J
            Likely highest (by modern measures): around 150 EeV, around ~25 J.

            The “oh my god” particle (the 320 EeV event) almost certainly had its energy significantly overestimated – it was measured by a new type of detector with systematics that weren’t really understood at the time.

          2. What you said intrigued me so I looked it up on wikipedia, and got this

            “The GZK limit is derived under the assumption that ultra-high energy cosmic rays are protons. Measurements by the largest cosmic-ray observatory, the Pierre Auger Observatory, suggest that most ultra-high energy cosmic rays are heavier elements known as HZE ions.[3] In this case, the argument behind the GZK limit does not apply in the originally simple form, and there is no fundamental contradiction in observing cosmic rays with energies that violate the limit. “

          3. “The GZK limit is derived under the assumption that ultra-high energy cosmic rays are protons.”

            Wikipedia’s wrong. Look up the original paper by Gaisser – “End to the cosmic ray spectrum?” The proton case is more interesting (it’s a particle resonance) but essentially all nuclei can’t propagate large distances either because of the giant dipole resonance. Essentially, the CMB photons are sized right to just explode nuclei.

            Heavier nuclei can’t propagate macroscopic (100 Gpc+) distances either, although the method that they lose energy by is different and so it changes the observations at Earth.

            It’s also worth noting that the particle content *at Earth* is not the particle content *at the source*, too, and it’s very hard to disentangle the two.

          4. To give numbers, silicon, for instance by 10^20 eV can barely get 1 Mpc before exploding.

            It should also be noted that compositional data at the highest end of the spectrum (at ~100 EeV) is essentially nonexistent because there are too few events observed (and always will be barring a new detector). You can only tell protons from heavier nuclei statistically.

      2. This is a good illustration of the difference between energy and energy density.

        It’s easiest to understand intuitively in terms of temperature; we all know you’re more likely to be scalded (or scalded more severely) by sticking your hand into a pot of boiling water than sticking it into the steam cloud above the pot, even though the individual water molecules in the steam cloud have more energy, because they’re spread much further apart.

  2. Weren’t there also some calculations about the upper limit of how big a nuclear explosion could be before it simply blew out into space?

    The Tsar Bomba is said to be big enough that anything more would be a waste of plutonium as far as weapons go.

    1. Yes, in fact it was well known back then that 10 smaller dispersed explosions (i.e., < 1 Mt) would be much more effective (destroying) than one single 10 Mt one as the effects decay rapidly with distance. The Tsar Bomba was more of a show-up than anything (notwithstanding the horror and radiation).

      1. Which is why modern ICBMs carry multiple “tactical” warheads, one part is accuracy through volume of fire and the other is the effect of dispersed explosions over a wider area.

        Given the way modern warfighting has evolved the idea of nukes seem kind of silly to me. Unless the target is an entire military port facility, carrier battle group, or sprawling industrial installation like a refinery, conventional munitions breaking up critical infrastructure would be far more effective.

        Its hard to support a major fighting force when all a country’s resources are tied up with rebuilding power, water, roads and rail lines. Not to mention patching runways and getting fuel production going again. Nukes kind of delete portions of the map, like an amputation. If you lose a leg you grab a crutch and hobble forward, if the leg is crushed and bleeding you have to stop and deal with it. Kind of like an enemy sniper wounding a soldier so two more have to carry them.

        Cheerful thoughts considering Russia, China and the U.S. these days.

        1. I don’t think we’ve seen the last of Dresden and Hiroshima, et cetera. There is more to war than breaking up critical infrastructure or surgically excising military targets. Like it or not (nobody likes it) war is about demoralization and the crippling of an entire people. Mass civilian targets will get hit again sometime in the future, the same way the allies did several times during WWII. The British realized this when they invented the first concentration camps during the Boer war.

          1. “the way the allies did during WWII” I think that wasn’t just the allies. Germany, Japan etc did a lot of massacres, bombings of civilians etc (obvious but I couldn’t let that go unsaid)

    2. Actually, I thought that the Soviets left off the U-238 jacket to “limit” the detonation to “just” 57MT. Fission-Fusion, instead of Fission-Fusion-Fission that the thing was technically capable of.

      1. It was a Fission-Fusion-Fission-Fusion device. the inert jacket, if it were U-238, would have doubled the yield. It was a ridiculous amount of energy to expend in a small area, set things on fire a long way away, on the order of 100km away. So much more efficient to sprinkle a dozen or two smaller bombs.

    3. Troposphere thickness, approximately 9 miles. Tsar Bomba ground level destruction diameter, 300 miles. I think that more than meets the definition of overkill. A very large portion of its destructive energy was wasted shoving air around to do nothing but shove the air around.

      Buuuut, according to the “Monster Hunter International” books, Tsar Bomba was actually an attempt to seal a gateway to an alternate dimension populated by Very Bad Things. ;)

  3. I never knew it was Teller that did the report on potential atmospheric fusion. It’s somewhat telling that he was thinking about fusion before the first successful fission weapons seeing as he is one of the two men most responsible for fusion weapons.

    1. Years ago as a teenage college student I saw Teller give a talk about disposing of nuclear waste. He said he was the only victim of Three Mile Island because he had a heart attack. Then he said people should not panic about leaked radiation. He said it might even be beneficial. I also knew an old engineer who used to do what he called “war work” during WW2. He said he was exposed to lots of radiation, and maybe that’s why he got liver cancer. Who knows? I took a nuclear physics course, but that does not make me an expert. I hope the experts who are in charge know what they are doing. My attitude is better safe than sorry.

    1. Nuclear winter is in fact extremely unscientific. If you look back at it, it was almost entirely made up by science populist Carl Sagan to use in a PR campaign for nuclear deproliferation (a noble lie, but a lie nonetheless).
      Any serious look at it reveals that you would to multiply the cold war arsenal by several billion to get even close to impact winter events or megavolcanoes which have caused this phenomena in nature. Even extremely large volcanoes which have happened in recorded history and reduced the global temperature for a while (alarming but definitely not catastrophic) have out-classed the entire arsenal of every single nation combined at the height of the cold war build-up.
      There is no such thing as nuclear winter, and considering that the radiation released by Chernobyl was much worse in degree than a nuclear bombardment and that place is now verdant, in fact its biome is BETTER than surrounding areas because of the lack of people there for a couple decades… I think the nuclear apocalypse stories were strategically overblown to make it less likely that we went to war with them, which is fine.

      1. I think “lack of people” is what most people are concerned about. I’ve encountered people who do not understand to this day how serious was Chernobyl, and some pass it off by disparaging Russians. Then a few years later, there was Fukushima. There’s a good Japanese-made film about Fukushima. They do not soft pedal their mistakes. They did not plan for a tsunami as huge as the one that happened. If things had got totally out of hand, much of Japan would be uninhabitable. Now there is a bit of a row over the Japanese releasing treated radioactive water into the ocean. The Chinese are not pleased.

  4. “The two concluded that no matter how hot any one section of the Earth’s atmosphere might become, a runaway nuclear chain reaction was likely to be sustained.”
    I think perhaps there’s a typo here? Shouldn’t this read ” … unlikely to be sustained.”?

  5. The 1979 paper is “Necessary conditions for the initiation and propagation of nuclear-detonation waves in plane atmospheres,” which is a great read because instead of saying “yeah, no” it actually figures out if it’s possible to actually *have* that kind of an atmosphere (or ocean, mind you!).

    And the answer is… yeah, it kindof is. For instance, “If, for instance, the terrestrial oceans contained deuterium at any atom fraction greater than 1:300 (instead of the actual value of 1:6000), the ocean could propagate an equilibrium thermonuclear-detonation wave,” although the amount of energy needed to “spark” that ignition is absurd. And similarly it’s not inconceivable (from first principle calculations) that you might be able to detonate a layer of the atmosphere of a gas giant, although you’d have to find one with an extremely high deuterium fraction.

    1. Thanks for pointing this out. The article makes it sound like 1979 paper raised the concern again, rather than simply examining the question on detail. As always, it pays to check sources. :P

      1. The “came up again in the 1970s” reference was the letter by HC Dudley in Bulletin of Atomic Scientists which is complete and total nonsense.

        The 1979 paper was basically a response to the byline of that article, which said “It’s time someone ran a computer calculation on the chances of a high order fusion explosion inducing a runaway chain reaction.”

        The reason why the Dudley paper was nonsense is that his argument was that theoretical/experimental knowledge was so bad back in the 1940s that they could’ve been wrong by a lot. Except they *knew* their knowledge was bad, so they literally took “worst possible case” assumptions for everything. (It also has a lot of weird fringe arguments like basically saying “but they didn’t take dark matter into account!”)

        The reason why Bethe specifically responded to it is because Dudley had an entire section referring to Bethe’s early work on solar nuclear reactions, with basically the argument that “well, Bethe didn’t know a lot then” and referred to Compton and Oppenheimer’s work as saying “However, as Fermi, Oppenheimer, Compton and others calculated, this probability is by no means zero.” Which is nonsense – it *was* zero. Physicists just don’t talk about it that way because truly ruling something out is really, really difficult.

  6. I’m much less worried about nuclear war now than I was before the Ukraine invasion. Like, I’m pretty sure if Russia launched an ICBM, it’d probably just run out of gas and fall in the ocean.

    1. Yeah, I mean when have to Russians ever been able to successfully launch something into space? This blasé attitude towards potential nuclear war that people have now scares me more than the Cuban missile crisis, because at least back then everyone treated the threat with the appropriate level of seriousness.

    2. I was booted from reddit after disparaging the Orks and their corroded nukes.
      I don’t know when they last had a successful test launch. Google?
      I remember nuke drills in kindergarten, Hide in the hall against the concrete wall.
      Heads down, Cover your face. Portland 1962
      My dad mentioned the possible atmosphere fire after Cronkite reported it that night.
      Welcome to the new world order.

      1. Prolly cause you allowed your disapproval of the actions of their despotic government turn into race hatred based on totally-unexamined leftover Cold War propaganda, bud.

        I assume you’re American?
        We’ve invaded more sovereign nations more times than they have, and kill foreign nationals constantly. That’s leaving aside how our entire country was stolen with wholesale genocide.

        Are we orcs…or are you allowing yourself to be inflamed by sixty-year-old xenophobia?

        1. I don’t believe I said anything about race hatred. I worked with hundreds of Russians, Asians and Indians for 25+ years at Intel and HP.
          The fear was real. And it still exists. We could snuff out our existence with a mistake.
          You seem to be the one with xenophobia and a loud dog whistle. Been trolling long?

    3. Every single time a new war starts, people fall for the same old propaganda.. That “the bad guys” are at once so evil and powerful and mean, yet simultaneously incompetent, ineffective, broke, and weak enough to ridicule. I remember people saying the same stuff about Saddam twice, about Vietnam, constantly about Iran, and of course Russia has been the big one in the background that war pigs have been slavering over for a century. I’m not a huge fan of Russia, but I’m not a fan of the war pigs either. Do they bother to trot out these kinds of rhetoric and narratives when they are bombing some place like Somalia? No, they don’t even tell you that they are bombing Somalia. Because in that case it would be true, and thus unnecessary propaganda which could only bring blowback. Who knew we are—as of 2023—currently bombing Somalia? Raise your hands please.

      Do you know what Russia has that makes them special at all? Why a country like theirs is on the world stage in the first place? Lots of nukes, a cold war hangover. The nukes are the only thing stopping outer powers from carving up the territory into a lot of micro-states, then going in to agitate, start color revolutions, and exploit natural resources, as they have done in several other regions post-WWII.

      Now think. Imagine you’re in charge of that country. Do you know the very last thing that they will let fall into disrepair? The nukes. Even if nothing else in that whole place functions correctly, you can bet your ass those nukes are still being immaculately and expensively cared for.

      1. Having nukes does not guarantee victory or survival. For example, South Africa had a small nuclear stockpile. The Afrikaners could have used nukes, but against which targets and at what price? They decided to dismantle their warheads and accept the communist ANC takeover, which was supported by the USA, UK, and other global powers.

        The Zionists are said to have many nukes. What good are they against the Palestinian resistance as well as internal dissent? Concerning Iran, it only needs to sit back and wait for what they view as the inevitable collapse.

        Ukraine gave up its nukes. But what if it had them and used them? Russia would have overwhelming superiority in nukes and could, if it wanted to, quickly destroy every city and town in Ukraine.

        No one has used nukes in the post WW2 era. Even when the USA was the only nation that had nukes, and could have used them to set up one world government from Washington D.C, no doubt with the full cooperation of the UK, Canada, Australia, and New Zealand, the USA instead allowed the USSR to develop its nuclear arsenal. That decision changed history forever, since none of us who were born since that time have known what it is like to live without the fear of nuclear war in the back of our minds.

        I’m not worried about nukes setting the sky on fire. I’m worried about my city being vaporized along with many other cities because some lunatic decided it was time to unleash hell on earth. The real danger comes from world leaders. They can turn what could be a beautiful garden paradise for all humans into a living hell. They have done so throughout history with with or without nukes. But nukes can accelerate the process way beyond conventional weapons.

    1. The Russian “Poseidon” long-endurance, nuclear-powered, nuclear-armed automated “torpedo” allegedly has a large “enhanced radioactivity” warhead designed not only to take out enemy naval installations and assets, but also commonly adjacent industrial and shipping facilities by functioning as an area-denial weapon due to the “enhanced” radioactivity.

      Which makes sense, strategically. Our lack of naval service facilities can’t even keep up with the maintenance schedule of our Navy in the current peacetime, much less deal with the repair needs of vessels during sustained warfare.

      1. There is pretty much no chance of ‘sustained warfare’.

        The Ruskys or Chinese couldn’t make a fight of it conventionally. Nukes are done quick.

        Maybe if the Ruskys fought the Chinese. That would be even better than Iraq fighting Iran.

        1. I don’t know about that. The Afghani’s seemed to be able to keep it up against a modernized force for twenty years. They fought the US, successfully, from a cave!

          Granted, they were technically on the defensive, but still…

    2. Ironically, our fear of all things nuclear is probably going to be the worst thing for humanity and the biosphere in the long run.
      Everyone likes to point out all the horrors committed by people in the religious dark ages, but do they want to listen when it is factually pointed out that the enlightenment led to the industrial revolution, which enabled the worst wars in all of history and which is on track to cause worldwide cataclysm in only a few short generations? Noooo.
      At least anti-rationality was steady state and sustainable for hundreds of thousands of years.

    3. Yea we decided the slow roast was a good replacement for setting the atmosphere on fire. I dont get how people can still say climate change isn’t affecting us, new and crazy shit is happening every year now. And really, the excuse is that “its going to cost us too much money if you are wrong”? What about the large number of dead people if we are not wrong. The potential consequences of being wrong are so heavily weighted in favor of “climate change is real” that I truly do not understand how people are still complaining about the costs of trying to limit how much damage will be done to us.

    1. The article here covers it poorly, but it’s a theoretical discussion of what would be required for that to happen, not an open question about whether it could.

      Is your spite for the Bulletin based on your qualifications to peer review it as a nuclear scientist yourself?

      1. That organization was founded on political agendas more so than objective research. Just because one has a diploma does not make their assertions more “pure” or embedded in reality.

      2. “The article here covers it poorly, but it’s a theoretical discussion of what would be required for that to happen,”

        No, the Bulletin of Atomic Scientists letter was different. That letter was nonsense, and yes, the disdain mentioned for it is well-earned.

  7. I often think about weapon stockpiling. Agreements are in place yes but it occurs to me that our overlords just do what they please and arms races will exist perpetually. Evolution from just fists.

  8. And we still don’t have a coherent theory of nuclei stability. All our theories are more or less empirical (e.g., the infamous droplet theory, which, regardless, predicted quite a few later synthesised stable isotopes), so it’s not surprising that in the early days there were such fears. People really had no clue how stable or unstable all the common elements can be if an extra neutron is added.

  9. You wrote “The two published their findings in a report *some six months before* the Trinity test took place.” —Could you tell us the source for this? I personally find it interesting that your description does not seem to be consistent with the following facts:
    1. The date of publication of the Konopinski, Marvin, and Teller report (LA-602) you linked is August 14, 1946.
    2. In his memoirs, Teller stated that he was approached by Fermi to revisit the problem “several weeks before” the test. (Teller and Shoolery (2001) p.209)
    3. Richard Hamming wrote that he was asked “[s]hortly before the first field test” to check some arithmetic about the problem which a man (probably, Teller or Konopinski) did, (Hamming (1998) American Mathematical Monthly 105: 640-)

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