> Unfortunately for Fukushima, all generators were flooded by a single Tsunami. What are the odds of an unknown event that will affect all generators? Think of EMP, contaminated fuel, a large cloud of C)2 suffocating the engines... All seem highly unlikely, but how do you plan for the unknown unknowns?

That was sadly not an unknown at all. TEPCO was definitely aware of the possibility given that flooding in the generator rooms had happened before, and they deliberately ignored studies finding they were not prepared for a tsunami.

https://en.wikipedia.org/wiki/Fukushima_nuclear_disaster#Pri...