The configuration of the nuclear pile is such that an attack on the plant's best hope is to somehow breach the containment building and let the fissiable material escape and contanminate the region around the plant.
Thankfully it's quite hard to get a nuclear material to blow up.
BUT. "Thankfully it's quite hard to get a nuclear material to blow up."
it depends on your definition of "blow up". Enough TNT, or C4 and you can make anything "blow up". Now, causing a nuclear REACTION that makes a BOOM - is a different story.
My point...? any of our nuclear power plants can be blown up... nuclear material can be blown up. And although only a limited number of people would die from the blast - nuclear material would be dispersed over a great area and many people would suffer greatly. Not to mention the pure "mental" effect such an attack would have (which, after all, is what terrorism is all about).