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
> 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).
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