This is a really good article. You'd almost want to pay for it! (ha, ha).
This is probably a consequence of the current fad of 'uprating' a nuclear plant. They jammed in a few hundred more tubes into the steam generator, and left out some stabilizing plates to make room. The thing shook itself apart.
The same thing happened at Darlington (vibration), and that's why they are going to replace everything. Maybe the new steam generators will last as long as these Callie ones. :) I don't think they can build these things as well as they used to, and I'm sure the Darlington vibration always gets worse as things loosen up.