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Japan now assumes 'possibility of a meltdown' at troubled reactors

Japanese workers raced against the clock to pump seawater into two damaged nuclear reactors. It’s a last-ditch effort to cool them enough to avert the kind of core meltdowns that happened at Chernobyl and Three Mile Island.

By Staff writer / March 13, 2011

A man holds his baby as they are scanned for levels of radiation in Koriyama, Fukushima Prefecture, Japan, Sunday, March 13. Friday's quake and tsunami damaged two nuclear reactors at a power plant in the prefecture, and at least one of them appeared to be going through a partial meltdown, raising fears of a radiation leak.

Mark Baker/AP

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Japanese workers and nuclear experts raced against the clock to pump seawater into two damaged nuclear reactors. It’s a last-ditch effort to cool them enough to avert Chernobyl- or Three Mile Island-like core meltdowns even as government officials admitted partial meltdowns are already assumed to have occurred in both units.

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Japanese authorities now list six reactors at two different nuclear power plants – Fukushima I and nearby Fukushima II – in a state of emergency following the massive earthquake and tsunami waves that hit Japan Friday. A total of 11 of the nation’s 54 reactors shut down following the quake, knocking out about 30 percent of Japan’s power.

At a Sunday morning press briefing, Chief Cabinet Secretary Yukio Edano said experts were "assuming the possibility of a meltdown" at the No. 3 reactor at the Fukushima I plant, about 150 miles north of Tokyo, as well as at its No. 1 reactor.

In the case of the No. 3 unit, though, Mr. Edano said the decision had been made far sooner to inject seawater directly into the No. 3 reactor than it had on the No. 1 reactor – a previously unprecedented step that some US experts described as a desperate measure. Despite that, coolant levels at the No. 3 reactor dropped so that part of the fuel rods became exposed, Edano said.

"Unlike the No. 1 reactor, we ventilated and injected water at an early stage," Edano said, according to wire reports of the press conference. Asked if fuel rods were partially melting in the No. 1 reactor, he acknowledged "there is that possibility.”

“We cannot confirm this because it is in the reactor,” he continued. “But we are dealing with it under that assumption. We are also dealing with the No. 3 reactor based on the assumption that it is a possibility."

A nuclear reactor core meltdown occurs when fuel rods in the reactor’s core overheat and begin to melt. The rods are filled with uranium oxide ceramic pellets wrapped in zirconium cladding. It is possible for that molten material to get so hot that it could melt through the primary steel confinement shell – and then, even through the floor of the reactor building, US experts say.

More people are being evacuated

Underscoring the seriousness of the situation, the Fukushima prefecture government Sunday widened its mandatory evacuation zone to 20 kilometers (12.4 miles) around both nuclear plants – ordering an additional 80,000 people to leave, in addition to the 62,000 residents who had already left, Japanese press reports said. Up to 450,000 residents could be evacuated overall, Kyodo News reported.

Although all three of the Fukushima I’s then-operating reactors shut down successfully – with control rods inserted into the fuel core after the quake – it takes at least 24 to 48 hours for them to cool under the best of conditions, with proper circulation of cooling water. But diesel emergency generators were knocked out within an hour after the quake, apparently by the tsunami that followed. Another battery-powered cooling system backup soon lost power.

With few alternatives, Tokyo Electric Power Company, which owns the plant, began pumping seawater into the No. 1 reactor on Saturday. Even so, an enormous explosion that morning destroyed most of a secondary containment building housing the No. 1 reactor, but apparently left intact a critical steel shell – the reactor’s primary containment. Efforts to cool that unit with seawater continued.

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