Hugh,
As an electrician with over 20 years experience in power generation, I can tell you that there is no way if a massive grid down event takes place, the nuclear reactors in this country will be safe. A failure of the electrical system will not be able to be repaired quickly. You will find one or two electricians working on nights and weekends; the rest are going to be home. Do you think they will leave their families? Most locations have more than one reactor that will be down. And I understand, you have about 4 hours to get the cooling system running. One or two men will be overwhelmed. Of all the possible catastrophes I could think of, this is the one I told my wife about 4 years ago. I had no solution then. I have no solution now. We have painted ourselves into a corner. Unless the grid is hardened to withstand an EMP or CME, we are doomed. – Master Electrician