Temperatures appeared to be rising in the spent fuel pools at two other reactors at the plant, No. 5 and No. 6, said Yukio Edano, the chief cabinet secretary. Meanwhile, workers continued to pump seawater into the No. 1 and No. 3 reactors, where cooling systems remained unusable.
If any of the spent fuel rods in the pools do indeed catch fire, nuclear experts say, the high heat would loft the radiation in clouds that would spread the radioactivity.
A spokesman for the Japanese company that runs the stricken reactors said in an interview on Monday that the spent fuel at the Fukushima Daiichi and Daini plants had been left uncooled since shortly after the quake.
The company, Tokyo Electric, has not been able to cool the spent fuel pools because power has been knocked out, said Johei Shiomi, the spokesman. “There may be some heating up,” he said.
Depending on the freshness of the spent fuel, Mr. Lochbaum said, the water in an uncooled pool would start to boil in anywhere from days to a week. The water would boil off to a dangerous level in another week or two.
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