Ukraine

"Can we build this nuke in 2 weeks??? YES WE CAN!!!"

~~Bob the Builder.

From scratch it's obviously not feasible, but having been a former Soviet state I'd imagine a good majority of the resources needed are floating around.

The main challenges with nuclear weapons are 1. procurement of the fissile material, 2. yield efficiency, and 3. miniaturization. Once you have the first part done, as Ukraine very likely has the nuclear fuel processing facilities to do so, the second part is less important if you just want a bomb. Just look at the fact that they were so confident the Little Boy would work they didn't even bother with a prototype, even if its yield ratio was quite low. It needed about 60kg of uranium for its 15kT yield, while Fat Man managed 21kT with only about 5kg of plutonium.

So, it's a tradeoff where if nuclear material is hard to come by and you need to get the bomb somewhere far away, making something really efficient is pretty important. However, if you have sufficient material and just want a decently big boom in the middle of a field, it's quite literally something you could feasibly manage in a home workshop.

The one other note on the importance of efficiency is in regards to fallout. Anything that isn't used in the detonation is blasted every which-way, and isn't really something you want as a normal military, since a nuclear wasteland isn't strategically very useful. But, if you're just trying to fuck up someone else's day, then its less important and you can get into really "fun" stuff like dirty and cobalt bombs.

Building a nuke is pretty easy. Just put 2 spheres of plutonium or enriched uranium at either end of a tube add a regular bomb on each end and duct tape it to the front of a regular missile.

I got a guy

All you need is a brilliant high school student with a knack for nuclear physics, a rebellious streak, and a desire to prove their intellectual superiority. As shown in the 1986 documentary, The Manhattan Project, even a resourceful teen can assemble a nuclear weapon in a matter of weeks if they manage to access the right materials. Just look for someone with the right mix of ambition, genius, and a little disregard for the rules, and you’ll have yourself a homemade nuclear device in no time!

Are you paying the overtime?

Lockheed, Raytheon, Northrop Grumman, basically any nation state with the money, materials, and knowledge to do it now that people outside Los Alamos know.

Dude a boy scout named David Hahn built a nuclear reactor and a neutron gun, it can be done.