Everything is "simple" with hindsight in mind.

After SL-1 we realized that that we needed to allow a reactor to fully shut down even with the most important control rod stuck in a fully withdrawn position.

https://en.wikipedia.org/wiki/SL-1#Accident_and_response

> Everything is "simple" with hindsight in mind.

The fixes are still simple and cost little.

I used to work at Boeing on airliner design. The guiding principle is "what happens when X fails" and design for that. It is not "design so X cannot fail", as we do not know how to design things that cannot fail. For Fukushima, it is "what happens if the seawall fails", not "the seawall cannot fail".

Airliners are safe not because critical parts cannot fail, but because there is a backup plan for every critical part.

Venting explosive gas into the building seems like a complete failure to do a proper failure analysis.