TOKYO — Declaring Japan has turned a corner in the battle to stabilize its tsunami-damaged nuclear plant, Prime Minister Yoshihiko Noda announced Friday the facility has achieved a stable state of "cold shutdown," a crucial step toward lifting evacuation orders and closing the plant.
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 the plant 140 miles (230 kilometers) northeast of Tokyo remains vulnerable to problems, 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."
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 focusing efforts 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 says Fukushima Dai-ichi has reached cold shutdown "conditions"— a cautious phrasing reflecting the fact that TEPCO cannot measure 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.
Officials can now start discussing whether to allow some evacuees to return to less-contaminated areas — although a 12-mile (20-kilometer) zone around the plant is expected to remain off limits for years to come. The crisis displaced some 100,000 people.
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.
Akira Yamaguchi, a nuclear physicist at Osaka University, said that 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.
Associated Press writer Eric Talmadge contributed to this report.