Seawater used to cool Japan nuclear plant

BY MARI YAMAGUCHI AND JEFF DONN

The Associated Press

TOKYO -- Inside the troubled nuclear power plant that was leaking radiation Saturday and Sunday, officials knew the risks were high when they decided to vent radioactive steam from a severely overheated reactor vessel.

They knew a hydrogen explosion could occur, and it did. They said the decision still trumped the worst-case alternative -- total nuclear meltdown.

The chain of events started Friday when the earthquake and tsunami severed electricity to the Fukushima Dai-ichi nuclear complex, 170 miles from Tokyo, crippling its cooling system. Then, backup power did not kick in properly at one of its units.

Faced with the need to cool off the vessel, officials decided to vent the steam, which led to an explosion.

Now, without power, and without plant pipes and pumps that were destroyed in the explosion, authorities have resorted to drawing seawater to try to cool off the overheated uranium fuel rods.

Robert Alvarez, senior scholar at the Institute for Policy Studies in Washington, said the success of using seawater and boron to cool the reactor will depend on the volume and rate of their distribution. He said the dousing would need to continue nonstop for days.

Another key, he said, was restoring electrical power to activate normal cooling systems.

The Japanese government said radiation emanating from the plant appeared to have decreased after Saturday's blast.

Officials placed Dai-ichi Unit 1, and four other reactors, under states of emergency Friday because operators had lost the ability to cool the reactors using usual procedures. An additional reactor was added to the list early today, for a total of six.

Officials began venting radioactive steam at Fukushima Dai-ichi's Unit 1 to relieve pressure inside the reactor vessel, which houses overheated uranium fuel.

Officials were aware that the steam contained hydrogen, said Shinji Kinjo, spokesman for the government Nuclear and Industrial Safety Agency. And they were aware, he said, that they were risking an explosion by deciding to vent the steam.

Officials said they needed to vent the steam to prevent the entire structure from exploding, and thus starting down the road to a meltdown.

As they were venting the steam, operators continued to try to keep the reactor fuel cool by circulating more and more cool water on the fuel rods.

But temperature in the reactor vessel kept rising, heating the zirconium cladding that makes up the fuel rod casings. Once the zirconium reached 2,200 degrees Fahrenheit, it reacted with the water, becoming zirconium oxide and hydrogen.

When the hydrogen-filled steam was vented, the hydrogen reacted with oxygen, either in the air or water outside the vessel, and exploded.