Fukushima was a huge anomaly.
The generators had fuel enough to run for weeks, but those tanks were washed away in the tsunami. The battery backup for the gensets worked exactly as designed and kept everything under control until the battery banks ran out of juice.
Nobody anticipated that there would not be power available for over three DAYS, let alone the weeks it took to get power there. Had the diesel tanks not washed away, we wouldn’t be having this discussion, it would have been a non-event.
Of course, I mentioned it merely for demonstration points regular folks could understand. Drama gets attention.
The design of Fukushima specified that the diesel tanks were to be mounted inside the protective wall. Some genius put them outside the wall. Flood from Tsunami washed them away!!
Nobody apparently paid attention to the ancient tsunami marker stones ...