TOKYO – Japan's government was to declare Friday that the tsunami-devastated Fukushima Dai-ichi nuclear power plant had finally achieved a "cold shutdown," meaning it has stabilized and is no longer leaking substantial amounts of radiation.
The announcement would mark a big milestone nine months after the March 11 tsunami touched off a crisis at the plant and sent three of its reactors into meltdowns. Experts noted, however, that the facility remains vulnerable to more problems and will take decades of difficult and dangerous work to safely close down.
Prime Minister Yoshihiko Noda was to announce the government's assessment of conditions at the plant in a news conference later Friday.
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 the assessment has some important caveats.
The announcement is expected to say Fukushima has reached cold shutdown "conditions"_ a less definitive 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. So the government also attached additional conditions to be met, including minimizing radiation leaks around the plant and taking backup safety measures to ensure Fukushima's wrecked reactors are safely cooled.
Even so, the announcement would mark the end of the second phase of the government's lengthy roadmap to completely decommission the plant, which is expected to take 30 years or more.
Officials can now start discussing whether to allow some evacuated residents who lived in areas with lesser damage from the plant to return home — although a 12-mile (20-kilometer) zone around the plant is expected to remain off limits for years to come.
Some 100,000 people were displaced by the crisis.
A cold shutdown normally means a nuclear reactor's coolant system is at atmospheric pressure and the 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.