“The thing is, no one has been able to find out the exact cause of the rising, mysterious radioactive steam because the combination of physical damage and, more importantly, lethal radiation levels have made investigating the reactor impossible. Tepco has known that a followup meltdown was a severe possibility, though. The Reactor 3 fuel storage pond is still home to about 89 tons of plutonium-based mixed-oxide fuel according to the The Ecologist, and should that fuel storage pond dry out, the highly radioactive rods will melt down with devastating results.
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Source: Mystery Steam Over Fukushima Could Be Sign of Another Meltdown
