In the event of a large solar flare the geomagnetic storm could melt all the large electrical transformers (if the grid isn't shut down first) as well as the wiring inside of and leading from back-up generators, leaving nuclear reactors without a power supply until the geomagnetic storm is over with, concerns also lie in the necessary wiring and control panels that keep the reactor functioning properly, they may need to be replaced immediately after the geomagnetic storm; nuclear power plants should have this equipment on-site ready to install or they should think about what other steps to take http://science.nasa.gov/science-news/science-at-nasa/2008/06may_carringtonflare/ http://science.nasa.gov/science-news/science-at-nasa/2009/21jan_severespaceweather/