I think the plants would be fine if something happened to the grid. I would expect that both 9 Mile Point and Vermont Yankee both disconnected during the big northeast blackout a couple of years ago. That's the way the system is supposed to work - when in doubt, disconnect so you don't burn stuff up. Even the smallest of the cogen plants have to have gear to sense being out of sync or the grid being down and immediately disconnect.

Likewise, when we had a big ice storm up north a few years ago, it took out the transmission lines in northern NY, but the rest of the grid worked fine. The bad parts just disconnected themselves or were disconnected.

So to me it seems designed with protection and redundancy for physical problems, but not so much problems with the central control, ie computers.