When you look at the terrorist groups today I see very little real threat of any Islamic extremist group or the like carrying out a nuclear attack. Getting a nuclear weapon and deploying it isn't as easy as it looks in the movies. You don't exactly go to Ukraine and buy a nuke for $10.000 from some drunk guarding an abandoned missile silo. How are you going to transport it anywhere? Even showing any potential interest in acquiring a nuclear weapon would instantly make you a high profile target for any of the countless services and agencies all over the world.

Building a nuke from scratch is, realistically speaking, impossible for any extremist group. Who has enough resources to do it? Maybe a villain from an old James Bond flick. Not to mention that even just deploying a nuke takes some very highly trained, educated specialists. They don't exactly grow on trees. A "dirty bomb" might sound scary but in effect it is far less destructive than even a small, tactical nuke though it might cause a lot of panic. But even a relatively simple device like that still takes a lot more brain and resources than the average extremist can muster.

From the terrorist point of view, why bother with such complex technology with so much risk involved when you can get almost guaranteed results with a simpler improvised or conventional attack? Blow up a major public building and many people will die. A single motivated, somewhat trained guy with an AK, a few hand grenades and some ammo (which all together costs a few hundred $ on the black market and is available anywhere in the world) can easily kill dozens of civilians in a busy street before being overwhelmed by the police. Take a few such individuals, organize them and you can do Mumbai as often as you like.

Maybe preparing for something big such as a nuclear attack is good for the media and panic mongers but I for one would rather try to be realistic about it. YMMV.