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TOKYO (Reuters) - The magnitude 9 earthquake that struck a Japanese nuclear plant in March hit with almost 30 percent more intensity than it had been designed to withstand, raising withstand, raising the possibility that key systems were compromised even before a massive tsunami hit.
Embattled operator Tokyo Electric Power said Monday that partial data recovered from the crippled Fukushima Daiichi plant showed the ground acceleration during the quake exceeded its design specifications at three of the six reactors.
"This was clearly a larger earthquake than we had forecast," said Junichi Matsumoto, a Tepco spokesman for nuclear issues. "It would have been hard to anticipate this."
The March 11 quake and the nearly 15-meter (50-ft) tsunami that followed devastated Japan's northeastern coast and killed more than 15,000 people. Another 9,500 are still missing.
The disaster also unleashed the worst nuclear accident since Chernobyl. Some 80,000 residents around the plant some 240 km (150 miles) north of Tokyo have been evacuated because of the risk from radiation.
Japanese officials have previously said the meltdown that took place in three of the reactors at Fukushima was caused by the loss of power to cooling systems when the tsunami knocked out backup diesel generators. Tepco has repeatedly called the combined disaster an event "beyond expectations."
But new details released this week have called some of that account into question. At the same time, new data and inspections at the site have shown that the reactors suffered far more serious damage than previously thought and forced officials to abandon their initial approach to bringing the plant to a shutdown.
Officials said it was not clear whether the Fukushima plant had been damaged by the quake and said an immediate inspection was impossible because of the all-hands effort to stabilize the reactors.
But a finding that the reactors or key safety equipment were damaged by the quake itself could complicate the growing debate on the future of nuclear power in Japan at a time when Tokyo is under pressure to tighten safety standards.
Prime Minister Naoto Kan has pledged a blank-slate review of Japan's energy policy in the wake of the accident. Under pressure for his handling of the crisis, Kan has won support for a decision to close a second nuclear plant seen at risk from a major earthquake.
Surveys released Monday showed that two-thirds of voters back Kan's move to call for the Hamaoka nuclear plant in central Japan to be shut down until it can be better defended against a disaster like the quake and tsunami that hit Fukushima.
The government and Tepco are set to provide their first formal update on efforts to stabilize the Fukushima plant on Tuesday.
Officials have said they will stick with a goal of shutting down the reactors by January. At that point of "cold shutdown," the fuel in the reactors would be cool enough so that it would not be at risk of boiling off the water being pumped in as a coolant and radiation barrier.
Once workers have gained control of Fukushima, attention could shift to the effort to retrieve the fuel from the site, including the melted uranium in three of the reactors, and send it to a more permanent storage site. Experts have said that could take a decade or more.
"The most important work is to cool the reactors with water and that is working," said Goshi Hosono, a special advisor to Kan on nuclear power.
Japanese officials said this week what many outside experts had expected: that the uranium fuel in reactors Nos. 1, 2 and 3 had melted down hours after the quake. Hosono said outside experts had also told him that the fuel in the No. 1 reactor appeared to have leaked out of the steel vessel designed to contain it at the core.
The effort to shut down the plant has also been complicated by the growing pool of radioactive water backing up inside the reactors and attached buildings because of leaks, officials have said.
Japan's government has promised an independent audit of the Fukushima disaster, including whether a faster response or a quicker venting of radioactive steam could have prevented powerful hydrogen explosions and the meltdowns.
"We can certainly say that if the venting took place a little earlier, we could have prevented the situation from worsening," Nuclear Safety Commission Chairman Haruki Madarame told parliament.
In another instance of an apparent error in judgment, Tepco said that a worker may have shut down a cooling system known as the isolation condenser shortly after the earthquake when he saw that the No. 1 reactor was losing temperature quicker than the utility's guidelines allowed.
"At the time, we could not have known that the tsunami was coming and that we would lose power," Matsumoto said.
Additional reporting by Chisa Fujioka; Editing by Alex Richardson