Steve Mirsky in Scientific American:
First came the earthquake, centered just off the east coast of Japan, near Honshu. The horror of the tsunami quickly followed. Now the world waits as emergency crews attempt to stop a core meltdown from occurring at the Fukushima Daichi nuclear reactor, already the site of an explosion of the reactor's housing structure.
At 1:30pm EST on March 12, American nuclear experts gathered for a call-in media briefing. While various participants discussed the policy ramifications of the crisis, physicist Ken Bergeron provided most of the information regarding the actual damage to the reactor.
“Reactor analysts like to categorize potential reactor accidents into groups,” said Bergeron, who did research on nuclear reactor accident simulation at Sandia National Laboratory in New Mexico. “And the type of accident that is occurring in Japan is known as a station blackout. It means loss of offsite AC power—power lines are down—and then a subsequent failure of emergency power on site—the diesel generators. It is considered to be extremely unlikely, but the station blackout has been one of the great concerns for decades.
“The probability of this occurring is hard to calculate primarily because of the possibility of what are called common-cause accidents, where the loss of offsite power and of onsite power are caused by the same thing. In this case, it was the earthquake and tsunami. So we're in uncharted territory, we're in a land where probability says we shouldn't be. And we're hoping that all of the barriers to release of radioactivity will not fail.”
Bergeron explained the basics of overheating at a nuclear fission plant.