I don't know about these particular reactors, but I've worked at nuclear power plants in the past and the final cooling backup plan was generally a variation of

I don't know about these particular reactors, but I've worked at nuclear power plants in the past and the final cooling backup plan was generally a variation of "fill the building with sea water". The reactors I knew (I assume it's similar in Japan) each had a LOT of options for cooling them. Some options would allow regular operation (which requires very fine control of cooling), some options would harmlessly shut a reactor down (where "harmlessly" means without causing any damage to the reactor). The filling-with-sea-water option was "standard operating procedure" in the sense that it was in the manual as a standard procedure, but it's the kind of thing you only get to do once because after you've done it your reactor is permanently broken and won't be restarted.