So what happens when an F5 tornado hits the backup generators?
Same as Japan, pump in seawater!!
Not-so-funny humor aside, I presume they would use fresh water that is available adjacent to the plant. Hopefully the generators are in tornado-proof enclosures.
They probably still have the hand pumps around from the last time it nearly melted down. That is what saved the region when they had a control room fire in the 70’s.
The diesels are housed within the concrete reactor building. While they could, perhaps be vulnerable to flooding, not so much for a tornado. The reactor building IS the tornado shelter at most plants and would be quite safe, thank you.
The short answer is nothing. Been there, saw that.