TOKYO (Reuters) - The operator of the crippled Fukushima Daiichi nuclear plant said it is studying whether the facility's reactors were damaged in the March 11 earthquake even before the massive tsunami that followed cut off power and sent the reactors into crisis.
Japanese officials have said until now that the apparent meltdown 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.
A finding that the reactors 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 from local officials to tighten safety standards.
"We want to review the data from the 40 to 50 minutes between the time of the earthquake and when the tsunami struck,"
Junichi Matsumoto, a spokesman for Tokyo Electric Power Co on nuclear issues said on Monday.
The 9.0 magnitude quake and the nearly 15-meter 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.
Kyodo news agency quoted an unnamed source at the utility on Sunday as saying that the No. 1 reactor might have suffered structural damage in the earthquake that caused a release of radiation separate from the tsunami.
Matsumoto said the utility was still studying how the No. 1 reactor was tipped into crisis. Tokyo Electric, also known as Tepco, is due to release an update on its timetable for stabilizing the Fukushima reactors on Tuesday.
Separately, Tepco has provided a new analysis of the early hours of the Fukushima crisis.
The utility said on Sunday that a review of data from March 11 suggested that the fuel rods in the No. 1 reactor were completely exposed to the air and rapidly heating five hours after the quake.
By the next morning - just 16 hours later - the uranium fuel rods in the first reactor had melted down and dropped to the bottom of the pressure vessel, the cylindrical steel container that holds the fuel at the core.
The No. 2 and No. 3 reactors are expected to have gone through a similar process and like No. 1 are leaking most of the water being pumped in a bid to keep their cores cool.
A massive pond of radioactive water has collected in the basement of the No. 1 reactor. Experts fear that the contaminated water leaking from the plant could threaten groundwater and the Pacific.
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 explosions and the uranium meltdown.
In parliament on Monday, government officials were grilled by an opposition lawmaker over their immediate response to the nuclear crisis.
"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.
Both Prime Minister Naoto Kan and Trade Minister Banri Kaieda said that they had instructed Tokyo Electric to go ahead with the venting but that the company had taken time to act.
"We had instructed them to go ahead with the vent and I think Tokyo Electric was trying to do this. Even though we asked them repeatedly to vent, it did not happen and so we decided to issue an order. All of us there, including the prime minister and myself had said it should be done as soon as possible," Kaieda said.