TOKYO (AP) -- Japan is poised to declare its crippled nuclear plant virtually stable nine months after a devastating tsunami, but the facility still leaks some radiation, remains vulnerable to earthquakes and shows no prospect for cleanup for decades.
Prime Minister Yoshihiko Noda said last week that temperatures inside the Fukushima Dai-ichi plant's three melted reactor cores are almost consistently below the boiling point and radiation leaks have significantly subsided -- two key conditions in a hoped-for "cold shutdown."
Officials say the government is expected to hold a news conference Friday to declare something close to cold shutdown, though experts caution it will be, at best, a tenuous stability. The declaration would mark a step forward for the much-maligned operator, Tokyo Electric Power Co., which has struggled to control the plant after it was damaged in a huge earthquake and tsunami March 11, unleashing the worst nuclear accident since Chernobyl.
"Up until now, this has been the biggest goal," TEPCO spokesman Masao Yamaguchi said. "It would be a milestone."
The announcement is expected to refer to cold shutdown "conditions"-- less definitive phrasing than a cold shutdown. That's partly because the operator cannot measure temperatures of melted fuel in the damaged reactors in the same way as with normally functioning ones, although the company believes they have reached a stable state.
In any case, experts caution that the progress so far at Fukushima should not be overstated, and that problems could still crop up.
"TEPCO and the government are anxious to bring a certain closure to the crisis," said Kazuhiko Kudo, a nuclear physicist at Kyushu University. "It would be a problem if the announcement gives an impression that the plant has received an official safety certificate."
The announcement would mark the end of the second phase of the government's lengthy roadmap to completely decommission the plant -- a process that could take about 30 years, authorities have said.
In the next phase, officials may start discussing whether to allow some evacuated residents who lived in areas with lesser damage from the plant to return home — but that could still be months or years away. Many of more than 100,000 residents evacuated from around the plant remain in limbo, living with relatives or in temporary housing. And a 20-kilometer (12-mile) zone around the plant is expected to remain off limits for some time.
Food safety concerns also persist.
The Fukushima plant disaster, which spewed an estimated one-fifth the amount of radiation as the 1986 accident at Chernobyl, has caused contamination of rice, vegetables and beef from around the region. Recently, even trace amounts of cesium were found in baby formula.
The complex still faces numerous concerns, including the vulnerability of the spent fuel pools, which sit on the top floor of the damaged reactor buildings, and the vast amount of contaminated water that has collected in the reactor basements and nearby storage areas. Another severe earthquake could damage the spent fuel pools, which might cause the water to leak and allow the fuel to overheat.
Unit 4's spent fuel pool, which contains the largest number of fuel rods, is the biggest concern because of structural damage to the building beneath it, although TEPCO says it has reinforced the structure. Removal and storage of those fuel rods from pools at four of the reactor units is also part of the next step toward eventual decommissioning.
Another continuing concern is containing radiation leaks.
To cool the reactors, TEPCO has been injecting water into the reactors, which is then leaking out through cracks. The radioactive water has been collected and stored in huge rooms converted into storage tanks before being decontaminated and put back into the reactors as coolant. Officials say the overall volume of contaminated water keeps growing, forcing the operator to keep searching for additional storage space.
Other recent leaks have raised questions about whether the plant really is fully under control. Last week, the utility said that about 45 tons of highly radioactive water had leaked from the plant's water processing system, some possibly leaking into the ocean.
Officials have said those are isolated incidents that are being taken care of and do not affect the overall plant status.
Normally, a nuclear reactor is considered to be in cold shutdown when its coolant system is at atmospheric pressure and the reactor cores are at a temperature below 100 Celsius (212 Fahrenheit) so that it would be impossible for a chain reaction to take place.
But meeting that strict definition is impossible at Fukushima Dai-ichi because the damaged reactors' fuel has melted and its exact whereabouts is unknown. Authorities suspect most of the fuel has fallen to the bottom of the innermost steel pressure vessels, and some most likely dribbled through to the beaker-shaped containment vessel. That makes it virtually impossible to know the exact temperature of the fuel.
Temperature gauges inside the Fukushima reactors show that the temperature at the bottom of the pressure vessel is around 70 C (158 F). TEPCO officials and nuclear experts say that indicates the reactor is in a cold, stable state. But because of the educated guesswork involved, Japanese authorities are using the phrase "cold shutdown conditions," rather than "cold shutdown."
The government has also stressed that the amount of radiation now being released around the plant precincts is at or below 1 millisievert per year — equivalent to an annual legal exposure limit for ordinary citizens before the crisis began. It also says the reactor cooling and water recycling apparatus is working and sustainable.
How to remove and dispose of the melted fuel is also an issue.
Recent TEPCO simulations showed that fuel in the worst-hit reactor No. 1 has mostly melted, breached the bottom of the core, dropping to an outer compartment and eating away into its concrete foundation and reaching within a foot of the crucial steel bottom of the primary containment chamber.
"It would make sense to let the people in and outside the country know that the work is steadily continuing," said Satoru Tanaka, a nuclear physicist at the University of Tokyo. "But achieving the (cold shutdown) status does not mean the problem is over. There are so many things that still need to be taken care of and clarified."
The Nuclear Safety Commission, which is comprised of government-appointed nuclear experts, on Monday approved TEPCO's operation and safety plans covering the next phase.
But safety commission chairman Haruki Madarame urged TEPCO and the Nuclear Industrial and Safety Agency to regularly review and evaluate the plans because "the reactors are broken and we hardly know what it really is like inside the reactors and it's difficult to predict what may occur."