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
> 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.
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