TOKYO-- Japan's prime minister announced Friday that the country's tsunami-damaged nuclear plant has achieved a stable state of "cold shutdown," a crucial step toward the eventual lifting of evacuation orders and closing of the plant.
Prime Minister Yoshihiko Noda's announcement was intended to reassure the nation that significant progress has been made in the nine months since the March 11 tsunami sent three reactors at the Fukushima Dai-ichi plant into meltdowns in the worst nuclear crisis since Chernobyl in 1986.
But experts say the plant 140 miles northeast of Tokyo remains vulnerable to problems and its surroundings are contaminated by radiation and closing the plant safely will take 30 or more years.
"The reactors at the Fukushima Dai-ichi nuclear power plant have reached a state of cold shutdown," Noda said. "Now that we have achieved stability in the reactors, a major concern for the nation has been resolved."
Radiation released from the plant has been significantly reduced, and additional safety measures installed at the plant ensure stability even in the event of another major disaster, he said.
Noda said he hopes conditions will improve quickly so that the people who have been displaced by the crisis can return home "even a day sooner."
"There are many issues that remain," Noda said. "Our battle is not over."
The government's official endorsement of the claim by Tokyo Electric Power Co. that the reactors have reached cold shutdown status is a necessary step toward revising evacuation zones around the plant and shifting the focus from simply stabilizing the facility to actually starting the arduous process of shutting it down.
But Noda acknowledged the assessment has some important caveats.
The government said Fukushima Dai-ichi has reached cold shutdown "conditions"-- a cautious phrasing reflecting the fact that TEPCO cannot measure the temperatures of melted fuel in the damaged reactors in the same way as with normally functioning ones.
Even so, the announcement marks the end of the second phase of the government's lengthy roadmap to completely decommission the plant.
International Atomic Energy Agency chief Yukiya Amano praised TEPCO and the government for making "significant progress" in reaching the goal.
Officials can now start discussing whether to allow some evacuees to return to less-contaminated areas -- although a 12-mile zone around the plant is expected to remain off limits for years to come. The crisis displaced some 100,000 people.
"We hope this will be a step toward allowing our residents to return home, but the road ahead is long and difficult," Fukushima Gov. Yuhei Sato told reporters.
Noda said the government will step up decontamination efforts and will ready 1 trillion yen ($12.8 billion) for urgently needed projects next year. He also said 30,000 workers will be trained.
A cold shutdown normally means a nuclear reactor's coolant system is at atmospheric pressure and its reactor core is at a temperature below 212 degrees Fahrenheit (100 degrees Celsius), making it impossible for a chain reaction to take place.
According to TEPCO, temperature gauges inside the Fukushima reactors show the pressure vessel is at around 70 C (158 F). The government also says the amount of radiation now being released around the plant is at or below 1 millisievert per year -- equivalent to the annual legal exposure limit for ordinary citizens before the crisis began.
Yet, the complex still faces numerous concerns, triggering criticism that the announcement of "cold shutdown conditions" is based on a political decision rather than science. Nobody knows exactly where and how the melted fuel ended up in each reactor, and the plant is struggling with the vast amount of radioactive water that has collected in the reactor basements and nearby storage areas. Officials, including nuclear and environment minister Goshi Hosono, admitted the possibility of mechanical glitches, water leaks or other unexpected troubles down the road, but said ample protection is now in place to avert another disaster.
TEPCO President Toshio Nishizawa apologized for the accident, and vowed to further stabilize the plant and reduce its radiation release until it is finally closed.
Akira Yamaguchi, a nuclear physicist at Osaka University, said the government's definition of cold shutdown is disputable.
"But what's most important right now is that there aren't any massive radiation leaks any more," he said.
Putting longer-term issues aside, he warned that much of the backup equipment installed at the plant since the crisis began is makeshift and may break down. He said winter cold could test their strength.