Nuclear emergency Japan

Japanese authorities are venting radioactive steam into the air after the earthquake on Friday critically damaged a nuclear reactor at Fukushima Daiichi plant.

The Japanese government on Friday declared a nuclear emergency at Tokyo Electric Power's Fukushima Daiichi nuclear power station after the reactor's cooling system failed. The government ordered thousands of people living within 6 miles of the plant to evacuate. Early Saturday, it declared a nuclear emergency at a second power plant where a cooling system had also failed.

"It has the potential to be catastrophic," said Robert Alvarez, a senior scholar at the Institute for Policy Studies in Washington, and a former senior policy adviser to the Energy Secretary during the Clinton administration.

When an earthquake strikes, the plants automatically shut down, but the radioactive material continues to decay and produce heat. Reactor cooling systems, which rely on electric pumps to circulate water around the nuclear core, are designed to prevent overheating and pressure buildups.

The earthquake in Fukushima caused a power outage and damaged the plant's backup diesel generator, forcing the pumps to run on battery power. Workers have been unable to restore the systems.

Japan's nuclear safety agency said pressure inside the reactor had risen to abnormal levels and radiation levels inside the facility had surged to 1,000 times more than normal.

The government said it would have to release vapor from the reactor to lower the pressure and avoid a meltdown.

Chief Cabinet Secretary Yukio Edano said the amount of radioactivity in vapor would be "very small" and would not harm people or the environment.

"With evacuation in place and the ocean-bound wind, we can ensure the safety," he said at a news conference early Saturday.

The venting may relieve some pressure and give workers more time to restore the emergency cooling systems. They have a 12- to 24-hour window, Alvarez said.

"I don't think the venting is going to result in a catastrophic release, but it's definitely an indication that all is not well there," he said.

If the cooling is not restored quickly, the core can overheat, causing the water to boil over and exposing the core to air. The interior can catch fire and cause a meltdown, releasing nuclear material into the concrete containment dome that surrounds the reactor, Alvarez says.

"Is this barrier going to be sufficient?" Alvarez said. "It's a dicey proposition. The best you can say is, stay tuned."

If they re-establish a stable power supply and restore the cooling, "We should all breathe a sigh of relief," Alvarez said. "If they can't, it's very serious."

As the day wore on, news from the power plant grew worse.

In its nuclear emergency declaration late Friday afternoon, the government noted in bold letters that radioactivity had not leaked from any nuclear facilities and urged the public to stay calm.

Prime Minister Naota Kan said late Friday afternoon that he had "no reports of any radioactive materials or otherwise affecting the surrounding areas."

But by Saturday morning, the government had declared emergencies at two reactors and planned drastic steps to relieve pressure in one. The Japanese Defense ministry said it sent troops trained for chemical attack to the plant in case of a serious radiation leak.

Eleven reactors in Japan shut down automatically when they sensed ground movement, said Cham Dallas, director for the Institute for Health Management and Mass Destruction Defense at the University of Georgia.

Dallas, who works closely with Japanese disaster management officials, said the local governments plan extensively for earthquakes and potential damage to nuclear power plants. Japan gets most of its energy from nuclear fuel.

"It's a visceral fear for them," said Dallas, who spent 10 years studying the Chernobyl nuclear disaster. "They have significantly better safety systems. It's night and day. I'm not worried."