Ok, this is getting a little too complicated. All you need is an appropriate amount of pure fuel, and a trigger. Everything else is just a matter of when and how big you want your boom to be. With a pure fuel (say 235), you can theoretically set it off with a hammer, provided you don’t mind being the only victim.
Two problems with this, besides the obvious. One, it might not work when you expect it to. You might have to hit it a couple times. You might hit it, the lift the hammer again, and it goes off before you hit it again. It might go off by itself before you hit it the first time. It’s all a matter of chance.
As a result, most bomb-grade fuel isn’t pure, but is instead doped a little to prevent things like this from happening. Last thing you want to do is visit the afterlife because you happened to be the unlucky guy that loaded the bomb onto the plane. Instead, you want your missles to meet a certain requirement for purity so you know that not only will they not only be stable until you trigger them, but they also won’t bounce harmlessly off the enemy’s lawn.
The other problem is the size of the boom itself. You don’t want to set off a nuclear war only to have your ICBM’s make a cute little “poof” noise. You want them to take out cities and small countries. You want them to level as much as they possibly can if you have to use them. This doesn’t necessarily mean having a pure fuel either, because if it’s too pure, it will blow before the optimum number of nuclei have reacted. Doping helps prevent this by “spacing” the nuclei far enough apart that as many of them as possible have a chance to react in the instant it takes the neutrons to move from one to another.
The trigger part is another story. Compared to making the fuel, setting up a trigger is akin to wiring your house. Not terribly hard, but you don’t want joe shmoe doing it either. The basic idea is to wrap the fuel with standard explosives, then light them off so that the resulting explosion smashes the fuel into a small, dense area. Then the fuel reacts, and the reaction takes care of itself.
The hard part is getting it right. The older bombs are easier to explain, if you’ve seen them. They are spherical, with wires hanging out of panels all around them. Each wire has to trigger the exposion at the exact right time to ignite the panel it’s attached to. A millisecond either way, and part of the fuel wont fire, or you may not get a reaction at all. Sad as it is, if the timing is completely off, all you may end up with is a conventional explosion from the trigger itself.
Just for the record, it’s not that difficult for rogue nations to build bombs. The US gov usually likes to say that it’s hard for them to get the fuel. It’s not. The hardest part is getting the people to work on them. While I’m sure there are a few crazies out there that are more than willing and capable of building a bomb, they are surprisingly few and far between.
Remember, it’s not a one man, basement-type operation. You need some impressive equiptment to prepare the fuel. You need operators to run the equiptment. You need someone to test, or at least calculate the proper arrangement of the fuel to the trigger, as well as the delivery mechanism. You need a lot of highly skilled technical people that believe that the bomb is the best answer to whatever problem you have. These tend to be few, as most highly skilled, technical people have better things to do than risk getting bombed out of existance if your enemy discovers you’re working on the bomb and decides to slow you down a bit.
For an example of how hard it is to build a bomb if you have the right people, look at the recent example of india and pakistan. Both countries have long had the skilled people to build a weapon. It was only when tensions reached a boiling point that they did it, and ignited one. Not too long after, pakistan did the same. How? They were able to drum up enough fear of each other that both sides were able to convince enough people to work on it to protect their countries.