I disagree.
1. Radiological "dirty bombs" have never even been made by the nuclear powers and for good reason. They are entirely too dangerous to fool with. Chemical weapons can be sealed. Radiation penetrates shielding unless the shielding is so thick and massive that the entire thing becomes immobile.
2. Chemical are easier by far. I could name a few, but I don't want to give anybody ideas.
3. Ordinary sabotage might be used: arson, explosives, etc.
4. If we are lucky, we will intercept and prevent such an attack.
But I agree on one thing: Al Qaeda will try to pull off such an attack.