[MertsA]

There's a couple things to note about the Fukushima disaster. They planned for a tsunami, but at the time the plant was built IIRC the scientific consensus was that a tsunami was caused by a large landslide underwater. They surveyed the underwater topology around the site, they thought they were building a sea wall high enough that any tsunami in that area would be too small to overtop it.

The theory was improved in the years since the plant was constructed but no one ever went back and realized the implications for Fukushima Daiichi. It was not a new plant, they broke ground on it before they did on Chernobyl. They thought a tsunami was inevitable and planned for it, they just didn't realize that they were operating on a flawed theory of how tsunamis form.