This is a very unusual story. The old US nuclear plants are just giant kettles. Every 18 months, they stop everything, open up the reactor vessel, and replace fuel. While they're at it, they have a couple of thousand extra workers go over the place, and spruce it up.
Totally surrounding this kettle, they have a steel containment structure. If the kettle bursts (as nearly happened at Davis-Besse), then this 'dry-well', as they like to call it, is supposed to hold in the radioactive shit hitting the fan.
Naturally, this containment is supposed to be functioning at all times. In Canada, they do regular pressure tests, and always find holes, such as valves left open. But at least they care! I don't think they pressure test in the US, or these holes might have been found earlier. Either somebody was really pissed off, and went to town with a drill, or, most likely, it was something stupid.
There is also a controversy in another plant, about the containment being rusted to rat-crap. I also find that they have given up looking at seismic issues in the US. They believe that they are perfect!