Why is it so hard to replicate the bulding of a nuclear bomb and why only a small amount of countires have them?
In: Engineering
For a *simple* bomb the hardest part is getting the right material.
Not all atoms of an element have to be the same, they can have heavier and lighter atoms. Natural uranium has two types: Uranium-238 is by far the most common, but only the rare uranium-235 is useful for a nuclear bomb (at least for the initial explosion). You need to get rid of almost all the uranium-238 in many tonnes of uranium to get a few kilograms of relatively pure uranium-235 (“enrichment”).
If you have a mixture of different elements then chemistry has many ways to separate them, but here this isn’t going to help. Both types are uranium. You can make a gas out of uranium and spin that really fast in centrifuges. Uranium-238 is slightly heavier, so it’s going to be a bit more common at the outer edge while uranium-235 is slightly more common in the center. A single centrifuge is only providing a tiny bit of separation for a small amount of material. You need a huge amount of these centrifuges and many steps to get material that can be used in a bomb.
You can make bombs out of plutonium, too, but that doesn’t occur naturally. You need to produce it in a nuclear reactor, and running that is a lot of effort on its own (in most reactors you want to have more uranium-235, too)
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If you have managed to produce something like 50 kilograms of mostly (~90%) uranium-235 you can assemble it in two pieces that are shot together for an explosion – that’s what the US did with the Hiroshima bomb. The design is so simple that they never tested it (with uranium) before. It’s pretty inefficient, however.
If you want to make more efficient bombs then you need to make a spherical design and surround it with explosives that squeeze it together. That needs to be extremely uniform and all at the same time. It’s like trying to squeeze together a balloon. Have a tiny gap and it’ll break out there and you don’t get a nuclear explosion.
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And finally there is the political aspect. Countries like Germany and Japan have the infrastructure for uranium enrichment and they could probably build a bomb quickly. They have decided not to, they have signed treaties and let other countries check that they follow these treaties (and check the other countries in return). Other countries might be interested in it but couldn’t do secretly, so they don’t.
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