They're not. The power of nuclear weapons is like 1% the fuel, 99% complex electronics you need to have them actually detonate with meaningful yield.

I'm not quite sure what you're driving at but the bomb that destroyed Hiroshima (the "gun" design, Little Boy) did not rely on complex electronics.

It also required quite a lot of fissile material and wasn't all that powerful. Dropped on lower Manhattan, it wouldn't even reach midtown Manhattan. See for yourself at http://nuclearsecrecy.com/nukemap/. I was actually quite surprised to realize that a "city" in 1940s was something much smaller than what we call a city today.

The point I'm aiming at is that nuclear weapons are (still) something expensive, hard to make, and useful only for nation states with large military budgets. Going for dirty bombs would be a better strategy for terrorists.

The second point I'm aiming at is that nuclear is not as scary as Hollywood seems to have made us believe.

I'd venture to guess that most people would regard the quarter of a million people dead and billions in property damage from an airburst over Manhattan (what that site indicates would happen with a Little Boy-style device) as quite significant. Losing the NYSE might just possibly have a small effect on the overall economy, as well.

> The point I'm aiming at is that nuclear weapons are (still) something expensive, hard to make, and useful only for nation states with large military budgets.

The fissile material is quite expensive, and requires a state-level actor's resources or cooperation to create or obtain. You're not going to take fuel directly out of a reactor and use it in a proper bomb. If that's the point you were trying to make, okay.

On the other hand, a simple bomb is not difficult to make given the necessary, weapons-grade material. I cannot even imagine why you think a terrorist organization would not find such a device useful.

You don't necessarily need to convert the nuclear fuel to weapons, you just need to blow the whole thing up, which ISIS is more than capable of doing, and then see how the winds carry the radio-active clouds 3,000 km away. For example see this map: http://i.imgur.com/zlRGSs3.jpg , more especially Austria.