The problem isn’t so much the destructiveness.
It’s the clean-up.
Exactly. I’m not even sure that’s a big deal. It probably depends on how much and what kind of stuff they have packed into it, and how big a conventional explosive they have.
Dirty bombs are not a terrorist’s weapon of choice for good reasons. It takes more money and expertise to make one. The guys who have the expertise are not going to do it, because they are smart enough to know the risks to themselves. It’s probably as likely to poison you as your intended target. You’re more likely to get caught. And even if you succeed, the destruction is not significantly more than a conventional explosive.
I remember seeing a TV interview of an anti-nuclear activist a few years ago. She was talking about how they had done a test to see if terrorists could break into a nuclear installation. The phony terrorists broke in and were there for about an hour before they got caught. She was hyperventilating as she stated that they were there long enough to make a bomb. Yes. But she did not mention that it was only a dirty bomb. She left the impression that she was talking about a nuclear explosive. Hard to see why a terrorist would add the risk of getting caught at a nuke installation to his plan when he would not be getting much more out of it than a conventional explosive.
Not to say that it could not happen, but it’s the hysteria that the terrorists are hoping for. Why give them the satisfaction when it doesn’t really merit hysteria in the first place?
Of course, a nuclear explosive is a different matter, but you can’t make a nuclear explosive from the materials you find at a nuclear power plant.