As I understand it, the problem isn’t simply meltdown, the problem is re-criticality. That is, there is apparently enough spent fuel rods in the (former) cooling pools that if they were to lose their protective sleeves (I believe they are still in zirconium sleeves), they would achieve critical mass and you’d have a criticality incident—far far worse than a meltdown, though not an explosion per se. Of the very little info TEPCO is releasing, their official last night (their time) said that the possibility of criticality was no longer zero. While there have been several exposed criticality incidents before, none have been of this scale under these conditions. It would more thank likely leapfrog this incident beyond Chernobyl and possibly create a deadzone for approximately 50 miles radius. But, of immediate importance, it would also prevent any further efforts at the reactors, allowing them to meltdown, and so on. A cascade failure.
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