Same with rockets. The technology is basically 1950's and they would also have plenty of samples from the 60's and 70's. These days there are plenty of examples of amateur teams sending things to space and even private companies can build better rockets than nation-states.
Also, to add, I think a lot of the perception that North Korea is technology backwards is thanks to propaganda which is often exaggerated. Having seen a computer science team from there speak about 15 years ago, I was quite impressed by their level and sophistication (it was a presentation about the kwangmyong network) https://en.m.wikipedia.org/wiki/Kwangmyong_(network) so I'm sure they have many other clever people in other branches that are competent to work on anything the state requires.
So, what restarted weapons-grade plutonium production there? The George W. Bush administration effectively abrogated the deal, claiming that the DPRK had a separate active nuclear weapons program, based on evidence about as sketchy as what they provided to argue that Saddam Hussein had one. (He didn't.) The Yongbyon plutonium-production reactor was back in operation within a month.
Clinton's deal was imperfect, but it worked a whole lot better than get-tough shit from his Republican successor.
Bill Clinton's administration allowed NK to obtain nuclear weapons while supplying them with food and fuel. Their approach didn't work at all and is how we wound up in this state.
For the record not blaming the administration (Republican administration have done things equally or more stupid so not a partisan taking point, it's just they can't be excused for the dramatic failure on this because someone prefers Democrats). Likely it seemed the best course of action, they expected at least some compliance, but NK did not abide by it's agreements at all.
https://www.armscontrol.org/factsheets/dprkchron
<edit> NK also made commitments to Putin to suspend the ICBM program in exchange for aid which they also violated. So again, Clinton is not the only one burnt by them. But saying the Clinton administration's approach was a success is not right. It was very much an unsuccess.
There is plenty of blame to go around, the Bush administration certainly handled, well just about everything it came across, very badly, but saying the Clinton approach worked is just wrong. It didn't work, NK was cheating on the deal while being fed and fueled. Certainly we shouldn't go back to paying them extortion and bribes, that is not a good idea at all.
https://www.washingtonpost.com/news/fact-checker/wp/2017/08/...