http://www.3news.co.nz/Burying-failed-nuclear-reactor-could-make-things-worse/tabid/1160/articleID/203275/Default.aspxBurying failed nuclear reactor could make things worse
Mon, 21 Mar 2011 10:12a.m.
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"When you drop tons of material from hundreds of feet in a helicopter, you're going to do some damage," said Alex Sich, a nuclear engineer at Franciscan University in Ohio. "It could be a bad idea. ... I would ask them to stop and think three times before they do any dumping of heavy materials."
Sich, who has lived in Chernobyl and published research on the disaster there, noted that Russian authorities dumped some 5,000 tons of sand, clay and other materials from helicopters in an attempt to smother that dangerous reactor.
The Japanese situation is different, he said. The Japanese reactors are surrounded by multiple barriers designed to contain radiation from the reactor cores. If a heavy dumping cracked the inner vessels and exposed the reactor cores, "that would be absurdity," he said.
Other risks focus on the spent fuel rods, which are a key source of concern. While pouring tons of sand on the rods would block radiation from escaping, it would also insulate them and make them heat up faster. The heat could decompose the concrete floor, allowing the rods to fall through, which could complicate efforts to contain the radiation, said Elmer Lewis of Northwestern University.
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Please read the entire article at the link.
This is the best explanation I've seen yet of why they aren't already preparing to bury the reactors. This is something they'd prefer not to risk until the reactors are cooler, which could be years from now.