The core is placed in the pressure vessel. The pressure vessel is a thick steel vessel that operates at a pressure of about 7 MPa (~1000 psi), and is designed to withstand the high pressures that may occur during an accident, but not a full melt down.
The explosions that we see, are not the containment structures. The "core" casing-catcher (main containment) of the plant, is intact. The plant is stable. Some radiation was released when the pressure vessel was vented. Most radioactive isotopes from the activated nitrogen and noble gases have since (decayed). There is damage to the first containment (Fuel Damage-Melt Down)
Both the main containment structure and the secondary containment structure are housed in the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in.
Leaking coolant does not mean that the main containment structure has failed.
The Zircaloy fuel rod barrier did its job, and the core was shut down within seconds of the Earthquake. But because the cooling cannot be restored, the core will eventually melt (after several days), and will "likely" be contained in the containment structure, as it has been cooling down for several days now.
Yes... I said "Likely Contained"
http://mitnse.com/
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UPDATE:
A fourth explosion has rocked the Fukushima nuclear plant (Today) Tuesday at Unit 4 at the facility, the Japanese Kyodo news agency reports. The agency also reported high levels of radiation at Unit 3, which was hit by a blast on Monday.
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In light of the meltdown of Fukushima: How to protect yourself and your thyroid against radiation:
kelp, ginseng, ashwaganda (recently found to regrow nerves), chlorella, zeolites, fulvic acid, nascent iodine, reishi mushroom, sea salt (also salty miso), botanical (plant-derived) or lipo- vitamin C, magnesium (chlorophyll), selenium (brazil nuts), coconut butter (coconut products), Megahydrate, Crystal Energy. Spread the word."
Start small, but increase your dosages rapidly and dramatically if you live in Hawaii, Western Canada, and the Western USA. Start now. Fallout will descend upon North America sometime around March 22.
If you live in Japan, start taking massive doses of these foods, superfoods, superherbs, and supplements immediately. These are all safe for children.
Radioactive fallout lingers longest in medicinal mushrooms. If you live in North America, pick your mushrooms NOW. I wouldn't recommend picking any medicinal mushrooms in the fallout zones (after the fallout begins) for several years, if not a decade depending on how bad the radiation is.
If you live in North America, stock up on food NOW because fresh food may be contaminated for several months after the fallout comes down from the jet stream into the fallout zones.