To add to other people’s comments: Small countries also have a limited number of scientists and engineers that they can afford to employ and maintain. So putting many of those scientists and engineers on the nuclear weapon project takes them away from other things the country may need; like roads, bridges, power plants, vehicle design and construction and everything else that a country needs in order to function.
Just for creating the bombs, it’s material and proper production factory. Though if any country tried hard enough this is not such a bad hurdle.
The big problem is deciding if it’s worth it on the global political level. Once you start developing nukes you’re gonna find out who your allies and enemies are real quick (via the most powerful countries in the world that already have nukes). Is it worth getting sanctioned and potentially taking a big economic hit? Is there a chance of sabotage from foreign powers? Etc, etc.
It’s easier to just buddy up to the bigger countries.
1st – the fuel. This is hard and other countries can figure out what you’re doing.
2nd – its not enough to make a nuke like the big fat unyeldy ones that the US dropped in japan with a big fat bomber. These days you couldn’t basically deliver it without getting shot down. You have to make one small enough to fit on a missile.
3rd – You also need a decent missile, with range, enough precision, and very reliable. You don’t want it breaking up over your country while carrying a nuke, right?
These are not trivial problems.
What *is* happening is that countries are reaching the conclusion that in order not to be regime-changed, threatened or interfered with in disregard of international laws, they should have this threat in their arsenal. That is driving demand for these weapons, and making the world far more dangerous.
Most answers are focused on personel, tech know how and acquiring equipment but those arent concerning issues at all if your country is already running a nuclear program for plants and research.
The main reason is more simple; do we want to allocate such a great budget to some bombs we wont use? Do we need / are we going to use the soft threat of nukes in diplomacy or elsewhere effectively to make it worth the cost of money and diplomatic hassle we will face?
See it like that, you see its rather niche and it depends on the circumstances of the country in question.
Hands-down: getting your hands on enough fissile material to sustain the reaction.
Natural Uranium is over 99% U-238, which is too stable to be useful in a bomb.
The isotope you need for a bomb, U-235, only makes up 0.7%.
So even if the country in question gets it’s hands on Uranium, it is highly technical, difficult, and above all *expensive* to enrich it to a high enough purity of U-235.
You have to enrich from 0.7% to 3-5% U-235 to even get to “reactor grade” suitable for powering nuclear reactors. You have to enrich to *85%* or better for weapons grade.
The other route, Plutonium, can only be obtained as a byproduct from nuclear reactors, which are also expensive and highly technical to build, on top of the complexity of enriching your Uranium.
Now try pulling all that off in the face of the kind of international resistance thrown up by established powers who want to inhibit nuclear proliferation…
The difficulty of designing and constructing the actual device (simple in concept, but still pretty technical, especially if you want to miniaturize enough for a deliverable warhead) pales in comparison.
The most difficult part is sourcing the materials, from the gases/chems used in purification/centrifuge, to the very specific strategic minerals or precision-made tools (like tubings, etc.) needed.
Anti-proliferation laws, especially on “dual use” stuff keeps a lot of stuff out of “bad hands”. It’s laws, not science or technology so much, as that’s all too easy to get, as looking into Dr.Abdul Qadeer Khan will show.
The main barrier, today, for countries getting nuclear weapons is that there are lots of treaties and agreements that are designed to stop them from doing it. All of the countries in these treaties and agreements have entered into the voluntarily. So it is chiefly a political issue, not a technical one. It is possible for countries to leave these treaties (like North Korea did), but that comes with political and economic costs, and potentially the threat of being attacked or destabilized by other nations.
But for those that might be under these agreements and want to secretly work on a program (like, say, Iran), these agreements make it hard to do so without being detected. Whether that has political implications or not (and what they are or might be) depends on the situation, but that’s the deterrent from trying to do it — getting caught, and then having to deal with whatever happens next. Additionally, these treaties and so on are meant to make it hard to do certain “risky” activities.
That being said, making credible, usable nuclear weapons is still technically challenging and very expensive. It is, however, decades-old technology, and a lot easier in some areas than it used to be.
You can think of there as being two technical challenges. One is making the warheads themselves. This involves making fissile material fuel (enriched uranium or plutonium) in large quantities, both of which require a lot of work and the development of specialized facilities like centrifuge factories or nuclear reactors. This is today quite hard to do secretly. Once you have the fuel, you then have to design and produce actual weapons, but this is not nearly as hard as it was 80 years ago. The basic science has been declassified for a long time, and the tools for designing and manufacturing these kinds of devices are much more common and powerful than they ever were in the past.
The other challenge is having a credible means of “delivery”: being able to credibly threaten to be able to get the weapon from wherever you are to wherever your imagined enemy is without them shooting it down, capturing it, having it miss, etc. So this involves things like missile programs, submarine programs, maybe even bomber programs, though in the present day, depending on who your imagined enemy is, their capabilities for neutralizing a very crude threat have increased. Making a missile that can reliably hit a target on the other side of the world is still pretty difficult unless you are a country that already has a lot of experience with missiles or rockets. Historically, this kind of work has been _much_ harder than the warheads, and cost _much_ more to develop and maintain. They aren’t as flashy and exciting as the warheads themselves, so they tend to get overlooked, but just having a warhead is not enough to have a credible nuclear threat.
Again, the issue here is not that this is stuff that is truly “secret.” But it is very specialized technical knowledge and production that you cannot just pick up off the shelf. So for a “poor” nation, you are talking about them creating an entire industry from scratch to make all of this stuff, along with training the people to work in it, all while some other “rich” nation is likely trying to stop you in various ways (whether economic sanctions, political pressure, assassinating your scientists, etc.). So that’s difficult.
If we are talking about a “rich” nation that already has some pieces of this puzzle, either because they have a robust domestic nuclear energy industry or a space program (or both), then it isn’t all that hard. A nation like Japan could “go nuclear” very quickly in a technical sense if it wanted to and nobody tried to stop it (internationally or domestically).
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