Explain Chernobyl then?
With all off the backup systems working, a meltdown is nearly impossible. Japan is extremely unlucky as that quake and tsunami simotaniously wiped out every backup system they had.
Actually Come to think of it, Chernobyl failed somewhat in the same way. Not from a natural disaster but due to a power fluctuation that caused their plant and all the systems to shut down. The only difference between here and Chernobyl at this point is the fact that they got the rods back in the reactor allowing at least SOMETHING to be done.
Which is why they are now pumping seawater in as a last resort. The problem is that its possible the reaction is strong and throwing off enough heat that it may be steaming off the seawater before it even has a chance to do much of anything.
I personally think they should be entombing this thing now and taking a loss on the whole system rather than slamming it with seawater and boron and hoping for the best