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A Brief Explanation About Past Nuclear Disasters, and how Japan's Situation Fits
TMI: In the late 70s, a plant suffered a malfunction. One pipe was left opened, cooling water emptied the reactor core, and the engineers were oblivious to this. When the safety systems screamed at the operators to begin emergency cooling, the engineers OVERRODE the safety mechanisms to keep the cooling from occurring. It was only AFTER they realized the instruments weren't lying to them that they took corrective steps, but it was too late.
The coolant was lost, the core was exposed to air, emergency coolant couldn't be added faster than it was draining, so the fuel heated up. It got so hot, it began melting out of the zircalloy cladding, through several feel of steel, and into the concrete, where eventually it stopped. No radiation was leaked however, due to the wonderful containment building surrounding it.
Chernobyl: A series of human errors in 1986 and a horrible management team led to an ill-advised experiment testing the feasibility of starting up a reactor after immediately shutting it down. The Russian-designed plant is FAR DIFFERENT from American designs, and when it's at low power it can gain power at an astronomical rate (very very exponential). As a result, it went from almost 0 power to WAY TOO loving MUCH POWER in a fraction of a second. The human engineers saw this, and dropped in control rods, which almost immediately caught fire. The entire core (surrounded in molten hot graphite) melted through the floor, which then crashed into the tank of cooling water.
Now I dunno if you know what happens if you let molten hot metal meet large bodies of water, so let me introduce to you the idea of a steam explosion strong enough to throw 800 tons of concrete hundreds of feet away. This was the explosion that scattered radioactive particles all around the site. The subsequent fire carried radioactive smoke all across Europe, and it was the nuclear disaster to end all others. They had no containment building, or else nobody would have known much at all about it.
Because of COUNTLESS differences in building design, plant design, safety codes, employee training, etc, it is impossible for non-Russian plants (which to my knowledge are the only ones who use graphite moderators anymore) to replicate this.
Japan's plant fits much close to the TMI model, but given lessons learned from that event, it is unlikely to reach that bad a situation unless EVERYTHING goes wrong.
It COULD BE that something has gone HORRIBLY wrong, and a leak has been discovered jetting actual irradiated water/steam (containing N-16, H-3, among other radioactive particles). That would be bad. Even that wouldn't be as bad as the simpsons portrayed it during Homer goes on Disability (and the steam kills a cornfield.)
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