What stops so many other countries from building nuclear weapons?
That's an interesting question, perhaps the best answer is simply affluence, the perception of geopolitical safety, and the perception of unwanted negative repercussions of obtaining nuclear weapons. Any developed country with a significant industrial base can build nuclear weapons. Trivially so for countries with significant local nuclear power industrial infrastructure such as fuel reprocessing or Uranium enrichment. Brazil, Mexico, Canada, Japan, South Korea, Germany, all these countries, and more, could build nuclear weapons quite rapidly and in great quantity just by deciding to do so.
For everyone else, well, one hopes that it is sufficiently difficult and costly that it's just not worth it for most countries, and difficult to hide as well, though mostly that's a lie we tell ourselves to make us feel safe. The hardest part is acquiring the fissile materials, which turns out to be slightly tricky. Plutonium requires production in reactors, but more so it requires a rather inefficient process to produce weapons grade Plutonium in reactors. Reactors are hard to hide, and if a reactor is under IAEA observation it's non-trivial to use the reactor to produce Plutonium. I say non-trivial but it is certainly far from impossible, as it's been done, but it makes it difficult to produce in large quantity while under the IAEA's eyes.
Uranium enrichment is a bit of a different story, and not a particularly heartening one either. It does require very sophisticated equipment and significant industrial resources to enrich Uranium, but it turns out that this equipment can be hidden fairly well and it actually takes much less of an investment to create enrichment facilities for making weapons versus for making reactor fuel (the enrichment level needs to be much higher, but the quantity needs to be vastly smaller). For example, to process natural Uranium into 5% enrichment takes about 75% of the work as to process it into 90% enrichment (weapons grade), and you only need perhaps 16kg or so of highly enriched Uranium to make a bomb. Getting back to observability, these facilities can be easily hidden and operate without notice since they blend in well with other industrial facilities externally. The world learned of Saddam's vast Calutron facilities only due to the first Gulf War. And the world learned of North Korea's centrifuge cascades only when that regime announced their existence.
In the last 50 years half a dozen countries have acquired nuclear weapons without sanction from existing nuclear powers and in most cases without the world being certain of the existence of those nuclear programs until they were announced (such as through weapons tests). That gives some perspective on the true difficulty of preventing nuclear weapons proliferation.
As a side note, if the entire worldwide civilian Uranium enrichment capacity was diverted from making reactor fuel to making nuclear bombs it could produce around 400 nuclear bomb cores per week.
That's an interesting question, perhaps the best answer is simply affluence, the perception of geopolitical safety, and the perception of unwanted negative repercussions of obtaining nuclear weapons. Any developed country with a significant industrial base can build nuclear weapons. Trivially so for countries with significant local nuclear power industrial infrastructure such as fuel reprocessing or Uranium enrichment. Brazil, Mexico, Canada, Japan, South Korea, Germany, all these countries, and more, could build nuclear weapons quite rapidly and in great quantity just by deciding to do so.
For everyone else, well, one hopes that it is sufficiently difficult and costly that it's just not worth it for most countries, and difficult to hide as well, though mostly that's a lie we tell ourselves to make us feel safe. The hardest part is acquiring the fissile materials, which turns out to be slightly tricky. Plutonium requires production in reactors, but more so it requires a rather inefficient process to produce weapons grade Plutonium in reactors. Reactors are hard to hide, and if a reactor is under IAEA observation it's non-trivial to use the reactor to produce Plutonium. I say non-trivial but it is certainly far from impossible, as it's been done, but it makes it difficult to produce in large quantity while under the IAEA's eyes.
Uranium enrichment is a bit of a different story, and not a particularly heartening one either. It does require very sophisticated equipment and significant industrial resources to enrich Uranium, but it turns out that this equipment can be hidden fairly well and it actually takes much less of an investment to create enrichment facilities for making weapons versus for making reactor fuel (the enrichment level needs to be much higher, but the quantity needs to be vastly smaller). For example, to process natural Uranium into 5% enrichment takes about 75% of the work as to process it into 90% enrichment (weapons grade), and you only need perhaps 16kg or so of highly enriched Uranium to make a bomb. Getting back to observability, these facilities can be easily hidden and operate without notice since they blend in well with other industrial facilities externally. The world learned of Saddam's vast Calutron facilities only due to the first Gulf War. And the world learned of North Korea's centrifuge cascades only when that regime announced their existence.
In the last 50 years half a dozen countries have acquired nuclear weapons without sanction from existing nuclear powers and in most cases without the world being certain of the existence of those nuclear programs until they were announced (such as through weapons tests). That gives some perspective on the true difficulty of preventing nuclear weapons proliferation.
As a side note, if the entire worldwide civilian Uranium enrichment capacity was diverted from making reactor fuel to making nuclear bombs it could produce around 400 nuclear bomb cores per week.