"it is good to bear in mind Japanese nuclear power plants are built with earthquakes of this magnitude in mind."

No, unfortunately no nuclear plants in Japan have been built to withstand an 8.9 level earthquake. They have been built to survive very strong earthquakes at the 7.0 level, but not for extreme cases, and certainly not for an 8.9 earthquake.

My understanding of what has happened is that the nuclear plant automatically shut down when the earthquake occurred, as designed. Then the electricity that powered the regular cooling pumps was cut off by the quake. However, the back-up generators, powered on site by oil or natural gas, didn't kick in as expected; I'm not sure why but they may have been flooded by the tsunami. The only other back-up power available has been batteries, and there's not enough of that power to run the pumps.

I believe that additional pumps have now been brought in by helicopter and may soon be working. They'll certainly be needed soon to avoid a melt-down and catastrophic release of radiation. A release of the radioactive gas that is building up in the containment will provide some more time to deal with the problem, but will also endanger the surrounding population with low-level radiation. Evacuation has now been ordered for a 10 km radius of the nuclear plant. It should be at least twice that radius, and more depending on weather conditions (the downwind zone is of course the most vulnerable).

It's amazing how sanguine most nuclear consultants are who are resident in London or the States when consulted by the media. They should be transported to what's left of the control room in the plant to gain some much needed perspective.

By the way it looks to me as if there are at least four reactors on this site. And, yes, as has been pointed out above they are over 40 years old and their containment vessels are more brittle than when they were new and subsequently are more susceptible to cracking under duress.

Charley Noble