I agree with Watchfuls two points that a) we are unlikely to be struck by a nuclear ICBM given the current geostrategic balance of power and b) dirty bombs have limited efficacy in the amount of actual damage they can do although obviously the fear and terror they create can be highly disruptive. (Chemical and biological weapons, while more effective than dirty bombs, also have limited tactical value. Thats why militaries tried them out during the first half of the last century and didnt continue to utilize them in their arsenals.)
However, there are scenarios of dirty bombs that would be extremely devastating. Classically conceived, the dirty bomb is some (relatively small) amount of radioactive material dispersed by the action of a conventional explosive. Not a big deal unless it happens on your block or down the street from you. And even then, maybe youll get cancer a little sooner than you otherwise would.
But dirty bombs come in many shapes and sizes. If a terrorist used a conventional explosive to disperse the radioactive material from a nuclear power plant or the deadly chemicals of a industrial waste reprocessing facility well, youve got a situation that could take out many tens of thousands of people in an urban environment.
That being said, I dont think your average terrorist is currently planning any attacks on that scale (I wont go into all the reasons why since there are many). What is far more likely is a backpack bomb on a commuter train. I ride the NYC subway every single day. Am I worried? No, because although I believe a subway bomb attack is likely, there are four and a half million other people riding the subway too. If a subway bomb killed 100 people whats the likelihood that Im actually going to be one of those 100 out of 4.5 million people? Not too darn likely.
The simple fact of life is that you are unlikely going to be a direct victim of a terrorist attack. But that doesnt mean you arent likely to be impacted by a terrorist attack. We live in a global, interconnected world. Several times terrorists have mounted attacks against Saudi oil facilities in the last couple of years. Eventually, they may prove successful. The terrorist attack you need to prepare for is the one thats going to drive oil to several hundred dollars a barrel after Abqaiq has been taken out.
http://news.bbc.co.uk/2/hi/middle_east/4747488.stm
Imagine what your city and neighborhood is going to be like when a barrel of oil is $300-$500 thats the scenario you need to plan for.
I apologize if this is somewhat off-topic in terms of the original posting - my basic point is don't be concerned about nukes from "evil-doers". Worry about the fact that you live in a highly industrialized society that depends on an uninterrupted flow of petroleum the way you depend on an uninterrupted flow of air as you breathe. A terrorist attack in the middle of the Arabian desert could effect you far more than one in your own backyard.