That stuff is so distributed and so heterogenous that I just don't think a successful attack of that sort is likely.

Edit: Maybe the most sophisticated attack of that sort was the one against the Iranian nuclear program, and while it was successful it and seems to have set back the program it was hardly a show stopper. It also required pretty specialized knowledge of the controllers used to operate the centrifuges (for enriching the fuel). If you consider the enormous variety of controllers and ways of doing things across different plants I just don't see how it would work to shut down a large fraction of even something like power generation.