— The Japanese government on Friday declared that the Fukushima Daiichi nuclear plant had reached a stable state known as “cold shutdown,” a benchmark for progress in the fraught effort to decommission its reactors, expected to take four decades.

But the formal status change at the plant, experts cautioned, means only that its problems have become less dire; they have not disappeared. The plant still leaks radiation into the sea. Its makeshift cooling system is vulnerable to earthquakes. And the cleanup work remains dangerous, with many flooded and debris-strewn areas of the reactor buildings difficult even for robots to access.

In normal circumstances, a reactor in cold shutdown mode is entirely stable, its fuel intact, with no chance of a chain reaction. To achieve its version of a cold shutdown at Fukushima Daiichi, site of the worst nuclear accident in 25 years, Japan had to loosen the definition. Fukushima now meets the government’s requirements because temperatures at the bottom of the three damaged reactor pressure vessels have dropped below 100 degrees Celsius (212 degrees Fahrenheit). Airborne leaks into the environment have also been almost halted, with little chance of backsliding.

“We can now maintain radiation exposure at the periphery of the plant at sufficiently low levels, even in the event of another accident,” Prime Minister Yoshihiko Noda said. “We believe the Fukushima Daiichi accident has been brought under control.”

Noda’s announcement comes more than nine months after a 9.0-magnitude earthquake and a resulting tsunami flooded the Fukushima plant and knocked out several cooling systems, triggering three meltdowns. The declaration poses new questions for many of the 80,000 people who fled towns around the plant, about 150 miles northeast of Tokyo, since the government had made the cold shutdown a precondition for even considering reopening parts of the no-go zone to residents.

The Tokyo Electric Power Co., or Tepco, operator of the Fukushima plant, had pledged one month after the disaster to stabilize the plant by January. The effort involved thousands of workers, many from subsidiary companies, who used a soccer training complex as their base camp. They battled a series of unprecedented problems using risky, trial-and-error methods. Engineers needed months to install a reliable cooling system. They shipped in temporary storage facilities for fast-accumulating radioactive water. They installed a cover blanketing the Unit 1 reactor building.

The temperatures at the three damaged reactors now range between 38.1 and 67.8 degrees Celsius (100.6 and 154.0 degrees Fahrenheit), according to data Thursday from Tepco.

The next stage of work at Fukushima — starting the long-term cleanup — comes with a fresh set of challenges. Among other things, workers will have to move spent fuel rods to more stable storage areas and seal cracks that let contaminated water escape into the environment.

Tepco’s biggest challenge might be collecting the molten fuel. Much of that fuel, according to one Tepco simulation, probably burned through the inner chambers designed to hold it and dropped into the containment vessel. At one reactor, the spilled fuel nearly bored its way through the reactor building, stopping 15 inches shy of an outer steel wall.

According to an expert panel from the Japan Atomic Energy Commission, it could be 10 years before work can begin to remove the melted fuel. And some experts say that even the 40-year decommissioning timetable set by the government is optimistic. “It is not going to be a regular decommissioning process,” said Tetsunari Iida, a former nuclear engineer who directs the Institute for Sustainable Energy Policies. “It’s going to take hundreds of years.”

Japan must also decontaminate an area around the plant of some 930 square miles, according to the Environment Ministry. That process could clear the way for some evacuees to return home, if they are willing to take the risk.

Many areas within the no-entry zonea 12-mile radius around the plant — will be uninhabitable for decades, maybe longer.

Japan is discussing a plan to categorize the off-limits areas more precisely, according to a recent report in the Mainichi Shimbun newspaper. In areas exposed to fewer than 20 millisieverts of radiation per year, the Japanese upper limit for citizen exposure, residents can make “preparations” to soon return. In areas exposed to between 20 and 50 millisieverts annually, residents will need to wait at least several years before returning. Areas exposed to more than 50 millisieverts annually will be labeled “difficult to return” zones, off-limits for decades.

In the early days after the earthquake, some Japanese officials privately feared an even more damaging scenario that could have rendered half of this island nation uninhabitable.

“Spine-chilling,” former prime minister Naoto Kan said after leaving office, reflecting on the possibility.

The Japanese government has consistently played down the severity of the accident in an attempt to maintain calm. That strategy has left many Japanese skeptical about government information on the nuclear accident. It has also turned public opinion against nuclear power, once a pillar of Japan’s energy strategy. At present, only eight of Japan’s 54 reactors are operating, with the majority shut down for maintenance and unable to restart in the face of local opposition.

Special correspondent Ayako Mie contributed to this report.