Nuclear terrorism is both very easy and very difficult at the same time.
If you have weapons-grade uranium or plutonium, constructing a weapon with a high probability of successful detonation with a significant yield is quite possible for a terrorist organization of moderate resources (millions to low tens of millions).
A terrorist group is only going to get weapons grade material by A) having it be provided by a state, which might as well provide the device too or B) stealing it.
Either of those reflect a basic reality: creating weapons-grade material requires the resources of a government.
Currently, the governments known to possess such material are: US, UK, France, China, Russia, Israel, Pakistan and India. Of those, only two are at serious risk of the possibility of theft or corruption allowing terrorist groups to procure material: Russia and Pakistan.
Which brings us to difficult hypothetical number one. Hopefully, the US knows where and how the Pakistani nuclear arsenal is kept. Pakistan is afflicted by no small amount of internal tension. Musharraf has narrowly escaped three serious assassination attempts already. If the US had not provided sophisticated EM jamming equipment for his motorcade he would be dead now. If he is killed and there is a coup attempt by extremist elements and you are the President, what will you do?. Furthermore, what will you previously have told India what you will do? If you do not have a response planned for this eventuality, what do you predict possible Indian action will be? What would the potential consequences of that be?
Is this a plausible and possible hypothetical situation? If not, why? If so, what are the possible action plans?
Russia is a problem, not from government intent, but merely from the multiple problems of the vast amount of created material, endemic poverty and corruption and Islamic elements in former Soviet states. One can make a strong case that not everything is being done on the front of securing former Soviet materials that could be. However, it is a problem that is well known, well defined and funded to varying degrees ranging from not-enough to very well. Russia is really the only wild-card source of material.
At this point, someone will have to make a very good case to convince me that Iran and North Korea are not trying very hard to manufacture HEU (Iran) and plutonium (NK). First, are an Islamic theocracy and xenophobic authoritarian government headed by a figure considered to be a human deity considered ‘rational’ regimes?
Can you prevent Iran and North Korea from obtaining enough material for multiple devices? If so, how? If not, when will they obtain them (if the answer isn’t already in the case of North Korea).
Is deterrence an applicable strategy for these countries should they obtain nuclear capability? Furthermore, can the capability of these countries be safely contained in the event of revolution or meltdown? Even if the governments do not provide weapons to terrorists for ideology or profit, there are certainly elements prone to doing so. Given unrest would they be able to capitalize on this?
These are not simple issues.