From what I have read, it appears that it was their "safety rules" which caused the disaster.
My understanding is that the reactor melted down because it takes approximately 3-4 days to shut it down once the decision has been made to shut it down. During this time, the Reactor is still very "hot" in terms of both thermal output and radiation, and must be cooled continuously for these three or four days to prevent meltdown.
Trouble was, they had no electrical power to run the cooling system, and their backup diesel generators were rendered inoperable by the floodwaters of the Tsunami.
Had they only had electricity at the plant, they could have ran the cooling systems and safely shut down the reactors. Problem was, they *SHUT DOWN* the reactors, and could therefore no longer produce electricity from them. Had they kept the reactors producing electricity, they could have powered the cooling system themselves.
Why did they shut down the reactors? Because the "Safety Rules" in effect in Japan at the time, *REQUIRES* nuclear reactors to be shut down in the event of a big earthquake or Tsunami, even if such an event does not actually damage the reactors.
So basically it was their "safety rules" that caused the cascade failure of the whole disaster, and made things far worse than they would have been had they simply kept the reactors going.
That's my understanding of the situation. If it is in error, someone please set me straight.
No. the 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors. All three cores largely melted in the first three days.
http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Fukushima-Accident/