Although a nuke plant seems like a juicy target for terrorists, the chances of anyone getting inside and actually being able to make a nuclear power plant do something spectacular like melt down is incredibly, incredibly remote. Accidents and human error are far more likely to lead to something bad happening, than a terrorist attack, and even that risk is very, very small.

The greatest impact of a terrorist attack on a nuke plant, like a dirty bomb, is going to be psychological. Just look at the fear that people still have of the Three Mile Island incident. There was no explosion, no "China Syndrome", no escape of radioactive materials except a tiny bit of radioactive steam. I read that the residents surrounding TMI got more radiation from Chernobyl from the radioactive dust particles that travelled on the winds around the world, than they did from TMI, just down the road. It's unfortunate that the truth of TMI isn't well known. Everyone just remembers all the hysteria and conjecture while it was happening, but no one knew what the heck was actually going on inside the reactor until much later, and by then, the public wasn't interested anymore.

An old, decommissioned nuke plant might have low grade radioactive materials that might be a target, but nothing that dangerous to you. I mean, perhaps they could steal a truckload of barrels of radioactive cooling water or something like that, but the getaway truck would have to drive by your house and overturn on your front lawn for it to pose any risk to you. I'm being facetious, of course, but the risk is super tiny. I'm sure that the most radioactive components, like spent fuel rods, have been removed. There's nothing there that is going to melt down and explode or something like that.