Well, according the IAEA, the planning against, and risk evaluation of, earthquakes and tsunamis for the Fukushima Daiichi NPP were insufficient:

>> The seismic hazard and tsunami waves considered in the original design were evaluated mainly on
the basis of historical seismic records and evidence of recent tsunamis in Japan. This original
evaluation did not sufficiently consider tectonic-geological criteria and no re-evaluation using such
criteria was conducted.

Source:

https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1710-Repor..., page 48.

Stop speculating and assuming, stop thinking from first principle and do the following instead:

- read text books on basic engineering, electricity generation and grid operation

- read some basics regarding nuclear reactors (or, in all seriousness, watch Chernobyl which provides some really good basic explanations)

- read the publicly available incident reports from the IAEA on thebaccidents of your choosing

All the questions you might have about any nuclear accident are answered in those reports. Those reports are prepared and investigated by experts in their field, going painstackingly over documents, event logs, design documents, meeting minutes (like a real and thorough investigation you know). Try understand all of that, and then draw whatever conclusion you want. Because by doing that, we have a basis for discussion, one rooted in reality in facts, as opposed to the headline-talking points fairytale stuff we have now, where people without any basic knowledge how things work draw assumptions from some headlines...

Once HN was a place where people applied critical thinking to topics, and where curious to learn from people knowing more (of which you find a ton around here). Since COVID, more and more people seem to think some general smartness allows them to understand even the most complex issues better than anyone else based on headlines, opinion pieces and social media talking points. I hope we can return to how things were before, it is getting really frustrating at times.

Edit: Some general engineering advice: Those once in a life time event are actually planned for, the USSR had rules for that in the 70s as well. The approach od identifying those risks was formalized as Failure Modes and Effects Analysis, FMEA. They are done for the design, production and operation, for processes, in order to identify those risks, the likelihood, the criticality and the ability to detect them. Following that, mitigation actions are defined and put into place. And, very important, they revisited regularly.

This wasn't done neither for Chernobyl nor Fukushima (just because the formal process didn't exist back then doesn't mean engineers didn't do those exercises, and the USSR had regulations in olace basically asking for the same thing. Regulations that were ignored by everyone for Cherbobyl unit No. 4). The IAEA reports for Chernobyl and Fukushima point thaz insufficient planning out explicitely, as do the USSR reports on Chernobyl. And if you think those fuck ups only happened in the distant past, the Boeing 737 Max can basically be traced back to the same root cause. Because these failures are not as much about people as they are about organizations. Hence processes and rules to follow in safety critical industries, and other large scale organizations. The cowboy style of operating in a start-up doesn't translate well to those places.