It's rather fascinating what the human mind can do with money + necessity. It's a reminder that great engineers can come from anywhere (and hopefully brought to work on nice-people things).
There are a lot of things we think are out of the reach of individuals or small groups of people, but the fact is someone developed many of these technologies in more primitive times without access to modern materials or the internet. We live in an age where morons can make armed drones or cruise missles, or, apparently, counterfeit helicopters.
With scraps, in a cave!
My understanding is that building a reactor is fairly easy (possibly even in reach of a smart high-school kid). But that getting hold of the enriched uranium required to run a reactor is (thankfully) rather difficult due to trade and production being heavily controlled.
It's pretty cool. It's really cool.
Once you get enough U-235, just use beryllium to reflect the neutrons back in and add a moderator(water, graphite, etc). Just don't be anywhere near it when it finally goes critical.
I remember in Hong Kong I'd be offered branded fake watches: a good fake Rolex from the excellent ABC company, not the cheap XYZ ones...
So you'd be approached on the street by a someone who looked sketchy and offered a Rolex for USD$100. They'd open a bag and let you look into it to see the Rolex while they made a big deal about checking to see that there were no cops around.
If you declined, they'd drop the price by half. Must have been effective because they did it for years.
I don't suppose it will be long, if it hasn't happened already, before unmanned drones will be delivering most of the illicit cross-border shipments. You could cheaply build a fleet of them and overwhelm most coastal defenses.
https://www.amazon.com/Narco-Submarines-Covert-Shores-Recogn...