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Sunday, Sep. 11, 2011

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Plugging leaks will end crisis, not cold shutdown: analysts

Evacuees' health said at risk if they return home after 'Step 2' achieved

Staff writer

Ever since the nuclear crisis erupted six months ago, the public has been clamoring to know when the damaged reactors at the Fu ku shi ma No. 1 power plant will be brought under control and when the nightmare will end.

The government and Tokyo Electric Power Co., which runs the crippled plant, are working to bring the three reactors into cold shutdown by mid-January.

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Cold shutdown means the temperature at the bottom of the pressure vessel, which holds the core, has been lowered to less than 100 degrees.

This critical milestone, known as "Step 2" in Tepco's road map for containing the crisis, would limit the release of radioactive materials from the plant to less than 1 millisievert per year, a level that poses no health risks.

Since work at the plant is proceeding relatively smoothly, it appears likely the mid-January target will be met.

But Fukushima No. 1 will still have a long way to go before the flooded plant's reactors are stable enough to be considered safe, experts warn. The main reason is the abundance of highly radioactive water.

"There are about 110,00 tons of contaminated water (in the plant) and the situation is still not completely under control because coolant water is leaking from the containment vessels.

There is no guarantee that the irradiated water won't leak from the plant (and contaminate the environment)" if another natural disaster strikes, said Hisashi Ninokata, a professor of reactor engineering at the Tokyo Institute of Technology.

After achieving cold shutdowns of reactors 1, 2 and 3, the government may declare parts of the 20-km no-go zone around the plant safe. It may even let the evacuees return, as long as the area is decontaminated and crucial infrastructure restored.

But the longer the tainted water leaks, the more the radioactive waste will grow, leaving the Fukushima plant vulnerable to further disasters, Ninokata said.

Before the Fukushima crisis can be said contained, the holes and cracks from which the water and fuel are escaping must be located and sealed. But this extremely difficult task could take years because the radiation near the reactors is simply too high to let workers get near them.

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Desolation row: The buildings of the four troubled reactors at the Fukushima No. 1 plant are seen from the air in August. KYODO PHOTO

"It'll be too early to say that the situation has reached a stable phase even after Step 2 is completed," said Chihiro Kamisawa, a researcher at Citizens' Nuclear Information Center, a nonprofit group of scientists and activists opposed to nuclear power.

When a reactor is in cold shutdown, the water cooling its fuel is still hot but no longer boiling, which significantly reduces the amount of radioactive emissions.

In late July, the temperature in reactor No. 1's pressure vessel fell below 100 degrees. On Monday, the same thing was achieved in reactor 3 after Tepco activated a system that pumps water deep into the containment vessel. But on Friday, reactor No. 2 was still boiling away with a reading of 112.6.

"Efforts seem to be making smooth progress, and I think Step 2 is likely to be achieved by mid-January," said Shinichi Morooka, a Waseda University professor and reactor expert.

Another reason for optimism is the progress being made with the water decontamination system. The cleaning rate has greatly improved in the past few weeks and exceeded 90 percent of capacity last week.

If the decontamination system ever reaches its full potential, it will allow Tepco to inject coolant at a higher rate and bring the melted cores to lower and stabler temperatures.

The government also plans to start decontaminating soil in various hot spots so the evacuees can return once the second step is completed.

But some experts are questioning whether residents should be allowed to return so soon. The cracks and holes in the leaking reactors haven't even been pinpointed yet, let alone fixed, they say.

"As an engineer, I am worried (about the plan to let residents return) when it is still unclear what is really going on inside the reactors," said Morooka.

For the time being, Tepco can only guess where the water is leaking from and which parts need repair, because radiation has prevented workers from fully exploring the buildings.

Spokesman Junichi Matsumoto said that since no extensive damage to the reactors was found during inspections of the first and second floors of the buildings, any holes or cracks are probably at the basement level.

But with the basement floors flooded, Tepco's top priority is just to get the water out. Plans to fix the reactors aren't even being discussed yet, Matsumoto said.

Asked if the containment vessels can take another quake, the Tokyo Institute of Technology's Ninokata said he believes the impact would likely be distributed evenly through the structure without widening existing cracks or holes.

But if the impact somehow focuses on parts damaged by the March 11 disasters, there could be further damage, he said.

"The containment vessel is what really ensures the safety of a nuclear reactor," Ninokata said, warning that if radioactive materials are still leaking out, allowing residents to return would risk harming their health.

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