The Fukushima reactor died directly during the earth quake. 3 months before end of live ... The stability of the reactors internal self sustain energy cooling pipe was the main problem. The steel was rated for a 5.5 earth quake at the beginning of the operation but after 40 years best case is 4.5, due the neutron capture degradation. Also the vessel cracked for the same reason. So it finally was a triple kill event with the flooding. The crack of the main vessel would have been a "minor problem" if they would have had enough refill water ready.
Today Japanese reactors must withstand an 8 fold stronger shock! What is equivalent to the whole building vertically falling down by 2 meters ! But still not enough after 40 years...
I remember many claims and theory of causes back in the days. I'm not sure what was proven or not wrt burst of piping and pressure vessels, but I remembered the backup generator design, which was flooded and caused complete Loss of emergency power.
Some more info in this article