What? Your last statement would seem to indicate you don’t have the slightest idea what is involved.
Also, this *was* the 7th biggest earthquake in recorded history.
Right after that quake and primary tsunami the plants operators were reporting that the reactor containments were intact, the building and the cooling systems they housed were intact, and there was still a source of power even thought the generators had been flooded.
There was a race to get more power either through new generators or to reconnect the plant to the grid before the batteries ran out. They failed and the batteries ran out depriving the cooling systems of power to function. Then the reactors got very hot and various gasses built up inside the buildings. Then they exploded and it has been all down hill from there.
For the Japanese not to anticipate a scenario where they would be reduced to battery power for more than eight hours smells to me of a corporation wanting to cut corners on safety.
I believe the upgrade to 9.0 made it the fifth biggest.