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Saturday, Dec. 17, 2011

Declaration, on time, finds skeptics

Officials: Cold shutdown has been achieved

Staff writer

The government and Tokyo Electric Power Co. declared Friday the three crippled reactors at the Fukushima No. 1 plant are in cold shutdown and no longer leaking large amounts of radiation.

If the authorities are correct and cooling of the reactors is stable, it would be an important milestone in ending the world's worst nuclear accident since the 1986 Chernobyl crisis. But many skeptics believe the declaration is little more than political grandstanding, given the revised definition of what constitutes cold shutdown and the date of the declaration, which had been reported long in advance, and are concerned about the long-term stability of the plant's critical coolant system.

A cold shutdown usually means the reactor core coolant temperatures are below 100 degrees, ensuring that coolant water is not boiling and that radioactive material are not escaping from the cores in significant amounts.

But reactors 1, 2 and 3 have been damaged and much of the melted fuel is believed to have penetrated through the pressure vessels and fallen to the bottom of the outer containment vessels. Tepco has been unable to take direct measurements of the temperatures at the bottoms of the containment vessels.

Still, the government declared the reactors are in cold shutdown because temperatures at the bottom of the pressure vessels, which are inside the containment vessels but have functioning sensors in place, are well below the boiling point.

Officials said that as of Friday the temperature of the atmosphere in the lower parts of the containment vessels were only 38.9 degrees in reactor 1, 67.5 degrees in reactor 2 and 57.4 degrees in reactor 3.

This indicates the entire containment vessels, the last of line of defense to contain radioactive materials, have been kept safely cool through the injection of coolant water, according to Tepco.

"We have confirmed that the reactors are in the cold shutdown condition, and the accident . . . has been brought to a conclusion," Noda told reporters later the day.

His remark immediately drew criticism from the media and local residents, who argued the contamination outside the plant will remain for a long time, and long-term concerns remain over the vulnerability of the ad-hoc system set up to keep the reactors cool.

Noda also admitted that "conclusion" only refers to the situation inside the plant, and there are still so many issues that need to be resolved outside the plant, such as decontaminating areas with high radiation, monitoring the health of those who have been exposed to radiation, as well as compensation.

Tepco recently disclosed the result of a computer simulation the utility says shows it is unlikely that melted fuel cores penetrated through the bottom of the containment vessels.

However, radiation levels are still too high to visibly confirm the actual conditions of the molten fuel believed to be at the bottom of those vessels.

But according to Tepco, the molten fuel is no longer hot enough to penetrate the last concrete layer of the containment vessels because little carbon dioxide has been detected in the atmosphere inside the vessels.

The radiation leakage from the three reactors remains below 1 millisievert per year at the perimeter of the complex, Tepco said. This level was also cited as one of the reasons for the government to declare that cold shutdown has been achieved.

Experts are still concerned about the long-term stability of the overall coolant system, which is injecting water into the reactor cores, given the danger of another major earthquake or tsunami.

Hisashi Ninokata, a professor of reactor engineering at Tokyo Institute of Technology, said he would only say with the greatest reluctance that the condition of the reactors "can be said to be in cold shutdown."

He said the melted fuel cores are no longer generating that much heat and it is highly unlikely the plant will experience another hydrogen explosion or recriticality of nuclear fission.

But the plant still remains vulnerable to another natural disaster, which could cause thecontaminated water to spill outside the compound, Ninokata said.

Tepco just this month experienced a leakage of about 150 liters of water containing strontium and other radioactive substances into the Pacific Ocean from a water processing facility.

Tepco also originally planned to clear out all of the contaminated water leaking from the reactors and flooded basement floors of the reactor buildings by the end of this year, but the plant still had about 86,000 tons of tainted water as of Tuesday.

In September, it was found that groundwater was also flooding the basements, and the utility had to slow down the water processing to balance the water pressure.

Even so, Tepco and the government claimed they have completed "Step 2" of its road map to bring the crisis under control.

The main goals of the second phase were to achieve the cold shutdown, improve the stability of the reactor cooling system, reduce the amount of contaminated water and prevent further contamination of the sea.

Goshi Hosono, minister in charge of the crisis, said Nov. 17 the government was listening to outside, non-Tepco opinions, including from the U.S. Nuclear Regulatory Commission.

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The Japan Times

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