Radiation levels may be falling at stricken nuclear plant
The radiation levels are still high at the Fukushima Daiichi plant but appear to be coming down, says the head of the U.S. Nuclear Regulatory Commission, cautioning that it is still hard to obtain accurate data.
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And the spent fuel pool at reactor building No. 4 is thought to have boiled dry, allowing the fuel rods to heat up and become damaged, also releasing radioactivity into the environment.
The nuclear cores inside the reactors are usually covered in water, but the top halves of the cores in reactors 1, 2 and 3 were exposed to air for at least several days, according to reports from the International Atomic Energy Agency and other sources. Even if those cores are resubmerged, they may have experienced permanent damage that would make them more difficult to keep cool, Lyman said.
For instance, he said, if the exposed portions of the fuel rods have swelled due to heat, the gaps between them may now be too small to pass enough water to cool them.