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Nitrogen injected into unit of disabled Japan nuclear plant

Move by operator meant to prevent possible explosion

By Andrew Pollack
New York Times / April 7, 2011

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TOKYO — The operator of the disabled Fukushima Daiichi nuclear power plant has begun injecting nitrogen into a reactor containment vessel to try to prevent a possible explosion.

Japan’s nuclear regulatory agency, which confirmed early today that the injections had just started, said the operation was aimed at reducing the risk of an explosion from hydrogen gas that might be building up in the plant’s No. 1 reactor. But agency officials said the step was being taken as a precaution, not because an explosion was deemed imminent.

“We do not believe there is a lot of hydrogen in the units,’’ Hidehiko Nishiyama, deputy director general of the regulatory body, the Nuclear and Industrial Safety Agency, told reporters last night. But he added that scientists did not know for sure.

This is the first injection of nitrogen into any of the reactors. The same approach might later be tried for the No. 2 and No. 3 reactors, but the No. 1 unit was chosen first because the pressures and temperatures there are higher than in the other two.

Hydrogen explosions occurred in some of the reactors in the days after the March 11 earthquake and tsunami that crippled the nuclear plant. The explosions damaged the outer buildings around the reactors. It was thought that the hydrogen was produced when the zirconium from disintegrating fuel rods reacted with steam after cooling water was lost.

Nishiyama said a concern now was that as the reactors gradually cooled there would be less steam in the containment vessels, leaving room for oxygen to enter and react with the hydrogen and cause explosions. Injecting nitrogen can reduce the amount of hydrogen and oxygen.

Nitrogen injection was one of the steps recommended by the US Nuclear Regulatory Commission in a confidential assessment dated March 26. Nishiyama said the recommendation “substantiated and reinforced’’ an idea already being discussed by the Japanese authorities.

The US assessment cited various threats to the plant. One is that the water being pumped into the reactors to cool them could put stress on the containment vessels, leaving them vulnerable to a rupture in the event of a large aftershock from the earthquake.

Nishiyama played down that concern, saying, “Even if we had an aftershock, I don’t believe it would lead to a dangerous situation.’’

Meanwhile, the government said it was considering what would, in effect, be an expansion of what is now a 12-mile evacuation zone around the Fukushima plant. It has already encouraged people within 19 miles of the plant to leave but has not ordered them to.

Yukio Edano, the government’s chief spokesman, said the expansion was being considered not because of concern that the situation at the plant was worsening, but rather because some people just outside the evacuation zone were accumulating radiation exposure over time.

“There are areas where the accumulated radiation level is rising,’’ Edano told reporters late yesterday. How large the evacuation zone becomes will depend on what level of accumulated radiation is deemed to be of concern. “Specialists in the field are currently considering the issue,’’ he said.

The government is also considering strengthening oversight of the nuclear industry by separating the regulatory agency from the ministry in charge of promoting nuclear power. The Nuclear and Industrial Safety Agency, which is part of the Ministry of Economy, Trade and Industry, has been criticized for being too cozy with companies it is supposed to police.

Last week, Prime Minister Naoto Kan said he would consider whether to remove the conflict of interest and how to do so.

This could include moving the regulatory body under control of the Cabinet and merging it with an advisory group, according to a report yesterday in the newspaper Mainichi Shimbun.

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