The Fukiashi Daiichi Reactor # 3 had an explosion of it's containment building last night Oklahoma Time. It sounds very serious, however, I have since learned more about the design of the Boiling Water Reactors Used there.
As I had reported earlier, the technicians and workers had been venting hydrogen and water vapor, and that was the root cause of both explosions. However, there are actually three containments found in these reactors.
The containment that blew up was actually designed to blow up in a fashion that would not destroy the secondary containment (they have blow out panels), nor the reactor vessel. This was the outer containment and there are still two other containments protecting the reactor core, or actually protecting the Japanese from what is in the core.
There is the reactor vessel itself, then an outer layer of concrete that is 6 feet thick (2 meters for those fans of the metric system.) Another safety feature built right into the reactor is an inverted cone at the bottom of the reactor. What this does is prevents molten fuel (Uranium and Plutonium since No. 3 uses Mixed Oxide Fuel.) from accumulating in a subcritical mass. If the core does melt down, the molten fuel will accumulate in the bottom of the core in the shape of a large donut, hopefully not building up substantial heat in the process.
What about the reports of radiation? That does not mean that the reactor core has been breached. If you recall they have been venting Hydrogen and radioactive steam from the reactor, and it had accumulated in the outer containment. The explosion released some radioactive materials in the process, however not a major Chernobyl like release.
The evacuation zone has been at 20 kilometers and holding.
I would expect that most of the radioactive materials released will end up in the ocean. Nothing major has been detected here in Oklahoma city, Just background rates:
PM1703m = 6 MicroRoentgens per hour; PM1208 Geiger counter watch .10 MicroSeiverts per hour; Digilert is averaging 12 counts per minute.