I understand that an atomic bomb works by detonating fuel (uranium or plutonium) that sparks a chain reaction of fission so every atom causes the fission of more atoms until the fuel is used up. But I don’t understand 2 parts of this process:
1. Why does the fuel (uranium atoms) blow up? I see some sources saying the atoms are being split, but other sources say atoms are being smashed into each other. Which is it? And why does performing that action cause an atom to violently explode?
2. Once the fission is happening and growing exponentially through the fuel, why doesn’t it set off a chain reaction with the atmosphere? Why exactly can’t uranium spark the fission of nitrogen? Why does the chain reaction stop when the uranium is gone?
I know other atomic bomb questions have been asked, but in my research I couldn’t find these answers. Thanks so much!
In: Chemistry
Fissile fuel (uranium or plutonium) has atoms that sometimes naturally split apart by themselves, releasing energy and neutrons. Those neutrons can make other fuel atoms also split apart, leading to a chain reaction which produces heat. This only happens if there is enough fuel in one place (this is called a critical mass), so the bomb has a mechanism to transform a subcritical mass into one that blows up. All modern bombs use regular explosives to squish a ball of plutonium so hard it blows up. The immense heat from the chain reaction turns the bomb and its surroundings into a giant incandescent ball of plasma.
Modern hydrogen bombs actually use a very small plutonium core, and use the heat from that explodig to smash hydrogen atoms together. When hydrogen atoms fuse, enormous amounts of energy is released (this is what makes the sun shine, after all) making for a much bigger explosion. The hydrogen reaction also makes neutrons, which makes more of the plutonium react, further increasing the yield. In the first atomic bombs, less than 10% of the fuel had time to react before the bomb blew apart.
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Radioactive atoms are unstable. They will spontaneously split into smaller atoms and release energy and in this case, neutrons. This is fission. You can also combine atoms up till you make iron and also release energy but this is way harder to do. This is fusion and it’s what powers the Sun.
Atom bombs use plutonium or enriched uranium. They will have the fuel in a shape that is almost critical. Sub critical means no chain reaction can happen. Critical means a self sustaining chain reaction is happening and super critical means an exponential chain reaction is happening. These are the implosion and gun type bombs. One of each was dropped on Japan.
Nuclear reactors maintain a critical state to generate power. Atom bombs use explosives to crush the fuel to make it supercritical or can shoot a pellet of fuel at the main batch to make it supercritical.
When the fuel goes supercritical, each atom split is releasing neutrons which then split more atoms. Say for every atom split they split two more atoms. This is the explosion.
This doesn’t blow up the atmosphere as neutrons can’t just split any atom. Non radioactive atoms are stable and will mostly just absorb the neutron or deflect it and take some of its kinetic energy. You’d need the power of the Sun to have enough energy to fuse the atoms in the atmosphere. An atom bomb it just too weak. Even a hydrogen or thermonuclear bomb, which uses fission and fusion, doesn’t have enough energy.
>Why exactly can’t uranium spark the fission of nitrogen?
Two reasons for this:
1. The nitrogen in air is not at all dense. The uranium chain reaction in a nuclear bomb only works because the uranium is compressed under the force of the initial explosion. Not only is air not being compressed like that, it’s a gas. If you somehow had *uranium gas* in the atmosphere, there simply wouldn’t be enough of it to cause a chain reaction.
2. Nitrogen fission doesn’t release energy, it absorbs energy. As a general rule, nuclei that are lighter than iron release energy through fusion, and making them undergo fission costs energy. Nuclei that are heavier than iron release energy through fission, and fusing them with other nuclei costs energy. It is *possible* to split a nitrogen nucleus into smaller nuclei, but you’d need to hit it with a very high speed particle, and the resultant particles would be slower, so there wouldn’t be a chain reaction.
1. When nuclear fission splits in the case of Uranium-235 or Plutonium-239, energy is released – quite a lot of energy given their mass. A neutron collision usually causes fission, but a U-235 splitting will create 3 additional neutrons to split up other nearby uranium atoms.
2. Eventually, you run out of uranium atoms to split, so the reaction terminates. That chain reaction of neutrons smashing into an cleaving uranium ends as there is no more uranium. This isn’t the mechanism through which atmospheric atoms would split. I think the Manhattan Project did have concerns about the atmosphere igniting though. But that’s not through a nuclear reaction, just a general chemical reaction.
Interestingly, in nuclear power plants, they will use control rods to collect those neutrons to prevent an uncontrolled chain reaction.
Man, I learned this stuff in Grade 4 with Microsoft Encarta. Good memories.
1. The fuel produces a fuckton of energy enclosed in the bomb, that’s how bombs works and that’s why it explodes. There are two kinds of nuclear bombs, fission and fusion. Fission is uranium or plutonium being split, and fusion is hydrogen being smashed together, so both can be true. It violently explodes because energy is being produced.
2. Fission only produces energy in elements that are heavier than iron, you need fusion to create energy for nitrogen. Nitrogen does actually fuse together in a nuclear bomb, but the rate at which heat is dissapated from the nuclear blast is much faster than the rate at which energy from fusion of nitrogen. This was calculated before the first nuclear test.
Splitting atoms is hard, the cores are unbelievable small compared to the space they take due to the electrons maintaining their distance to any other cores. Uranium is used specifically because it can be split by a neutron **and** that it releases **two more neutrons** plus a huge amount of energy i the process. The extreme vast majority of atoms cant be split this way. And even for uranium it needs to be very tightly packed and surrounded by “mirrors” that bounces the neturons back for it to work.
As for why they blow up. The two isotopes we have left after the split has less total bound up energy than the uranium had to start with so the leftover energy is released.
> I see some sources saying the atoms are being split, but other sources say atoms are being smashed into each other. Which is it?
It’s both, sort of. An atom is “split” by being hit with a neutron, which causes it to release neutrons of its own, which then “smash into” nearby atoms, causing those atoms to split, which releases more neutrons, which smash into more atoms, making those atoms split, and so on, in a chain reaction.
>Why does the fuel (uranium atoms) blow up?
It blows up because of this chain reaction of exponentially more atoms being split. The reason this chain reaction *starts* is something called critical mass. Basically, when you get enough atoms of the correct fuel packed closely enough together, they start the chain reaction.
Imagine thousands of people standing in an open field. Every so often, one of them will throw a tennis ball in a random direction. If that tennis ball hits another person, that person will throw two tennis balls in two random directions. If those hit other people, those people throw their tennis balls, etc.
If these people are spread far very apart, then the odds of a tennis ball hitting another person are very low. And even if one ball does end up hitting someone, everyone is still too far apart for it to be guaranteed to start a chain reaction. You need to pack the people very closely together, such that any ball thrown HAS to hit another person and start and continue the chain reaction. When the people are packed that tightly, it’s called critical mass.
So you start the chain reaction by getting the uranium or plutonium to critical mass. Usually, this is done by using a conventional (non-nuclear) explosive to compress the fuel. Like the shockwave of an explosion pushing the people closer together.
>Once the fission is happening and growing exponentially through the fuel, why doesn’t it set off a chain reaction with the atmosphere?
For one, the atoms in the air are not fissile the same way the fuel atoms are; that is, when they get hit by a neutron, they don’t shoot out neutrons themselves in order to sustain a chain reaction. In our analogy of people throwing tennis balls, the atoms in the atmosphere are people who don’t have any tennis balls to throw. So even if they get hit by a ball, they won’t throw a ball of their own and continue the chain reaction.
For another, the atoms in the atmosphere are not nearly densely packed enough. Remember, the people throwing tennis balls have to be REALLY close together. The “people” (atoms) of the atmospheric air simply are not packed that close.
>Why does the chain reaction stop when the uranium is gone?
Because all the people have thrown all their tennis balls, so no one has any balls left to throw.
This is all *very* simplified and not 100% accurate, but it should give you a good general idea of how the concept works.
> Why does the fuel (uranium atoms) blow up? I see some sources saying the atoms are being split, but other sources say atoms are being smashed into each other. Which is it? And why does performing that action cause an atom to violently explode?
There are two ways to release nuclear energy. Heavy atoms like uranium and plutonium can be split. Light atoms like hydrogen and lithium can be fused together. In some bombs it is just splitting (fission); in other bombs, the energy from the splitting is used to induce fusion.
Explaining why these release energy is difficult to do in a simple way. A very simplified version would be to say that these atoms are in a somewhat stable state before the reaction, but the reaction makes them far less stable, and when trying to “resolve” that instability, they end up expressing some of that as a release in energy. Which is pretty vague. In the case of fission, a neutron is absorbed by a heavy nucleus and makes it violently unstable; in the case of fusion, two nuclei become close-enough to fuse (under extreme conditions) and the newly created nucleus is violently unstable. In both cases the violent instability results in energy release (in the form of the nucleus splitting into two high-energy fragments in the case of fission, or a high-energy particle being ejected in the case of fusion).
It is easier to just point to E=mc^2 and say, “that is happening!”, and that is what most people do, but that doesn’t really answer the question in a very satisfying way, anymore than pointing at an equation for gravity explains how rockets work.
> Once the fission is happening and growing exponentially through the fuel, why doesn’t it set off a chain reaction with the atmosphere? Why exactly can’t uranium spark the fission of nitrogen? Why does the chain reaction stop when the uranium is gone?
It takes a lot of energy to fuse nitrogen or anything else. Even inside a hydrogen bomb, where fusion can take place, it takes more than just the heat of the fission bomb to start the reactions and, importantly, sustain the reaction. One or two atoms fusing or fissioning is not a big deal. You need conditions that will make trillions of trillions of atoms fission or fuse all at once. These conditions happen to be fairly specific and hard to manufacture. A nuclear weapon is basically a machine that can temporarily create those conditions.
For fission reactions, the fuel needs to have a very specific property: it needs to be _fissile_, which means that it can be very easily fissioned by neutrons. Almost no isotopes have this property. So that reaction does not spread beyond its fuel (and indeed, fission bombs don’t even burn all of their fuel, because they destroy themselves before the reaction is complete).
For fusion reactions, the requirements to sustain the fusion reactions are extreme heat and pressure. Once you are no longer inside the center of an exploding bomb, the heat and pressure are insufficient to keep the reaction going.
Could you imagine a world in which a very large bomb could create those conditions, turning it into one big bomb? Yes — but that world would have properties very different than Earth (it would need a lot more of isotopes that are very easy to fuse), and the bomb would have to be much larger than any weapon that has ever been contemplated being built (to provide more extreme and more sustained heat than any bomb we’ve made could provide).
This question was asked when Oppenheimer was developing the A-bomb. “Will this bomb set the atmosphere on fire?” It had enough traction that they ran it through some physicists/mathematicians who calculated that the probability of it happening was less than 1 in 10000, a risk that was deemed acceptable.
Others have answered more on the technical side, but recognize that the question has enough validity that it was also asked by those who had a good understanding of nuclear physics.
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