A male friend of mine had served in the Air Force years ago and when we drove to Crystal River--where one of Florida's major nuclear power plants is situated--he told me a very easy way to render the plant inoperable, and thus a sitting nuclear weapon.
I am not going to say HOW since some maniac could be reading this and use the info.
Reading the other article that C.D. posted on the condition of several U.S. nuclear plants, one wonders if this technology is not the 21st century's version of Pandora's Box or some other phenomenon taken straight out of mythology:
The so-called Peaceful Atom becomes the thing that sets off the extinction event (or series of events) for humankind.