Three reactors are affected, and two plants, Fukushima and Onagawa. I would say that an earth quake and tsunami is outside the well defined.
But at the same time they are not completely independent events. Quite the opposite. It is likely that an earthquake will be followed by a tsunami.
If they engineered it so that 2 events: quake and flood are independent, then they made a mistake. Possibly there is nothing they could do but I doubt that.
This is somewhat similar to the birds and airplane engines. It is assumed that once in a while a bird will hit an engine and possibly take it out. No big deal, there are 1,2 or 3 more engines. But the problem is that birds fly in flocks. So chances are that the plane will fly into a flock of birds and all engines will go out. Which is what happened a year or two ago. Now there is probably not much they could do but hopefully at least they didn't assume that invidual engine failure due to bird hits would be completely independent failure events.
Wrong and wrong. Some backup cooling mechanisms have failed, not all safety mechanisms.
The electric backup worked as designed. The diesel backup did not, as I understand it, because it was affected by the tsunami (which is inexcusable in my opinion). And the backup diesel generators brought in from off-site, from what I've read, could not be attached due to incompatible connectors (which if true, is also inexcusable).
No, that is the reason they where suppose to fall back on diesel based pumps. The electrical outage after the quake made the electric pumps fail. The diesel based pumps also failed as we agree upon. After that there was a mechanism simply based on preasure which supposed to push out hot steam, cool it off and feed it back as water. However this mechanism led a decreased level of water (from what I have heard). Which is why they now resort to pumping it in manually. I think it's safe to say that two major explosions at the plant (even though it wasn't the reactor it self apparently) is a sign of failed safety mechanisms.
"Nuclear fuel rods at one of the reactors may have become became fully exposed raising the risk they could melt down and cause a radioactive leak, Japanese news agency Jiji said."
http://www.reuters.com/article/2011/03/11/quake-japan-nuclea...
And then later.
http://www.reuters.com/article/2011/03/13/japan-quake-onagaw...
Which at least points to the uncertainty and conflicting messages. But it's good to know that the situation has stabilized and it now is clear that it does not face the same difficulties.