What is so difficult about developing nuclear weapons that makes some countries incapable of making them?
Getting enough U-235 or plutonium together to make one. A gun-type device is fairly straightforward and dumb as a rock, even if it “just” levels a moderate sized city instead of flattening a 40km circle like the fancier setups. However the centrifuges for isotope separation are very expensive and very high tech – so, they aren’t sold in the Snap-On catalog and you can’t just stick one together with washing machine parts. They are purchased from a handful of companies in the US, Russia or Europe, and such purchases tend to make all the intelligence agencies go hmmmmmmm.
Nuclear weapon development is a complex and challenging process that needs a lot of resources, knowledge, and infrastructure. To build nuclear weapons, nations must overcome a number of significant technical obstacles.
The main problem is the nuclear fuel that powers the bomb. Uranium is a fairly rare element on its own, but to make a bomb you need lots of a very rare isotope of uranium (U-235) that’s chemically identical but weighs ever so slightly less.
To separate out this rare isotope you need to turn it into a gas and spin it in a centrifuge. But this is so slow you need a gigantic factory with *thousands* of centrifuges, that consume as much electrical power as a small city.
Another fuel, plutonium, is refined differently, but it also takes a massive industrial operation to make. Either way, this is all too expensive for a small group to do, only medium and large countries can afford it.
But the even bigger problem is that all this factory infrastructure is impossible to hide. If you’re making nuclear bombs, you probably have enemies who want to stop you, and a giant factory full of delicate equipment is an easy target.
So to make a bomb, you need to be rich enough to build both a gigantic power-sucking factory *and* a military powerful enough to protect it from people who would like to stop you.
The type of uranium and/or plutonium that you need for a good explosion from a bomb is the kind that’s unstable, because although the atom’s nucleus has exactly the number of protons that make it “uranium”, it has a few “extra” neutrons in there, making it unstable.
There are many [isotopes of uranium](https://en.wikipedia.org/wiki/Isotopes_of_uranium) (uranium with different numbers of neutrons in there), and basically if you mine the uranium ore you get a mix, and you have to extract the uranium 235 isotope from the mix. Which, it’s all uranium so it reacts chemically exactly the same, can’t use chemistry to separate it. It’s slightly heavier than the other kinds of uranium (because of a couple extra neutrons) but I mean you’re talking a minuscule difference. So you can try to separate it by centrifuge but it’s a LONG process and a very complex centrifuge device.
As the others have said, it’s also very radioactive and that makes it extremely hazardous and polluting to work with, and if you finally get enough kilos to make a bomb, then you need an ICBM or long range cruise missile to deliver it, and those aren’t easy to build either.
None of this stuff is easy to build *in secret*, the facilities are rather large, the missile flight testing is conspicuous, and so on.
And then, as a smaller country let’s say you build a few, then what? There are anti-missile defense systems, look at how many missiles and drones Ukraine is shooting down each day. If you only have “a few” they won’t get past missile defense systems. You need thousands, to overwhelm an enemy country’s defenses, to have a few actually penetrate and detonate.
It’s kinda like, a small country building ONE aircraft carrier. ONE. After years and years of enormous expenses (for a small country). What could they do with it?
A big part of it is the general threat from other nations. Nothing scares quite like nuclear weapons, and there’s very few reasons to make them today. What do you think the military of other countries might do on hearing news of a new country making nuclear weapons?
As for the actual production, it requires expertise and the right equipment to refine the radioactive materials and build your bombs. It’s not something to be taken lightly, since exposure to radiation and the risk of an accident (even one that doesn’t result in a BOOM) are highly dangerous.
And finally you got the delivery system. You need a way to actually get the bomb where you want it to blow up. Having a bomb sit on your own soil isn’t what you want to do with it. So you need some kind of missile system that can travel great distances and be aimed well enough. In 1945 the bombs dropped on Japan were delivered by planes that dropped them and then ran like hell. That’s not a great strategy, so you gotta do something better. And that requires some decent sophistication from your military.