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Partial Meltdown Feared As Japan Struggles To Cool Nuclear Reactors

Japan is struggling to cope with nuclear power plants whose cooling systems lost power, posing a dire threat to towns crippled by Friday's earthquake and tsunami. As more news comes from Japan, you can track it on NPR's main news story.

Update At 1:20 p.m. ET: Emergency At Second Plant, Trouble At Third

Japan's nuclear safety agency has declared a low-level state of emergency at its Onagawa nuclear power plant, after levels of radioactivity climbed above what is allowed. Officials told the U.N.'s atomic agency that all three reactors at the Onagawa power plant are under control.

And officials now say that a third nuclear power plant, Tokai No. 2, is having technical troubles, as well. A cooling system pump at the plant 75 miles north of Tokyo had shut down, according to Kyodo News.

But Reuters quotes the Tokai plant's operator as saying that the cooling process is still working.

Update At 10:28 a.m. ET: Second Explosion Feared.

A hydrogen explosion similar to the one that blasted away part of the exterior of a building housing a reactor Saturday could possibly occur at Unit 3 of the Fukushima No. 1 nuclear complex, officials said Sunday.

"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said. "If there is an explosion, however, there would be no significant impact on human health."

Update At 10:10 a.m. ET: More Background.

Reuters cites scientist Marco Ricotti, nuclear plant specialist and professor at the Polytechnic Institute of Milan, who says Fukushima will be no Chernobyl:

"At Chernobyl the reactor had reached a huge increase inpower, there was no safety container and there was not enoughtime to evacuate people," he told Italian newspaper Corriere Della Sera.

"At Fukushima, the reactor was turned off, there was a safety container and there was enough time to move away the surrounding population."

Our Original Post:

Nuclear fuel at Japan's Fukushima No. 1 nuclear plant has very likely suffered a partial meltdown, according to Japans' nuclear safety agency. Plant operators have been struggling to cool its reactors after losing power due to Friday's 8.9 magnitude earthquake.

Chief Cabinet Secretary Yukio Edano, says there is a "significant chance" that a partial meltdown has occurred, The Financial Times reports. He added, "I am trying to be careful with words ... This is not a situation where the whole core suffers a meltdown."

Anthony Kuhn filed this report for Newscast:

The agency says that some radioactive iodine and cesium were released from the plant after an explosion blew the roof off a building housing a reactor. Around 210,000 people have been evacuated from the area. Chief Cabinet Secretary Yukio Edano pledged that residents who wanted to could be screened for radiation.

Edano told reporters this morning that another reactor at the Fukushima No. 1 plant was also in danger of a partial meltdown. He said seawater was pumped in to cool that reactor, and some radioactive vapors had been released into the atmosphere.

We discussed the troubles at the nuclear plant yesterday — including this explanation from Jon Hamilton, of NPR's Science Desk:

A spokesman... said the reactor's core would be flooded with sea water and boric acid. The boric acid helps to suppress any lingering nuclear reaction. The spokesman also explained an explosion that destroyed a building at the Fukushima plant early [Saturday] morning. He said efforts to add cooling water to the reactor core had resulted in the production of hydrogen gas. The gas built up inside the building and then exploded.

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