Comment by mpweiher

Yes. And in Fukushima, they apparently actually lowered an existing natural barrier.

https://oilprice.com/Latest-Energy-News/World-News/Tepco-Rem...

In addition, they didn't have hydrogen recombinators, which for example are/were standard in all German plants. Those plants also had special requirements for bunkers for the Diesel backup generators so they couldn't be knocked out by water.