Here is another perspective on this, from Scientific American. See if you can line up the facts from the two different articles.
Critters:
I just got Karl Grossman on the phone — one of the leading anti-nuclear authors and professor of journalism at SUNY-Westbury. He has been following the situation at the Fukushima nuclear power plant described in the post below. Here is my understanding of the situation after our conversation.
When the quake happened this morning, both main and backup power went out at the Fukushima plant. The power runs pumps that circulate water to keep the reactor core cool. Whether the plant is in operation or not, the core must be kept cool. Once water stops circulating, operators have between 15 and 30 seconds before a meltdown begins — even if they get the reactor under control. That core has to be cooled at all times. So problem one is that the batteries have just eight hours of time, and that was a while ago.
A meltdown would inevitably cause a massive radiation release. Hence, I learned today that a light water reactor (the most common kind used commercially) is always between 15 and 30 seconds away from a meltdown.
Next problem is that there is a radiation leak inside the containment structure. He does not know the source of the leak, but it could be from the primary or secondary loop (a light water reactor has three loops that cool the core; the first and second are radioactive; the third is not supposed to be).