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Spent nuclear fuel rods are stored in pools of water at Japan's Fukushima Dai-ichi nuclear power plant. The water keeps the rods cool while protecting the environment from their radiation.
Japanese authorities have raised the crisis level at the Fukushima Daiichi nuclear plant to a seven from five under international disaster standards. That's the worst rating given and the same as the one given to the devastating Chernobyl accident in Ukraine. Sounds bad, right? Here to explain what the rating means, is Ploughshares President Joseph Cirincione.