Tsunami hitting Tokyo Electric Power Co.'s Fukushima No. 1 nuclear power plant on March 11. Five-meter-high tanks were submerged one minute after the tsunami hit. (Tokyo Electric Power Co.) Water from the March 11 tsunami reached a height of 15 meters above sea level (five meters above ground) near the central waste disposal facility at the Fukushima No. 1 nuclear power plant. (Tokyo Electric Power Co.) The tsunami poured over a 10-meter-high levee at the Fukushima No. 1 nuclear power plant. (Tokyo Electric Power Co.)
Experts are questioning whether the tsunami that hit the Fukushima No. 1 nuclear power plant was as high as the plant's operator says.
Tokyo Electric Power Co. (TEPCO) says the tsunami was a towering 14-15 meters high when it hit the plant, but some analysts are pointing out that the figure is well in excess of other measurements in the vicinity.
If the wave is found to have been smaller, it would be much harder for TEPCO to argue that it had been hit by an unforeseeable calamity and would severely weaken the company's case in trying to limit its compensation liabilities.
TEPCO released on May 19 new photos of the tsunami hitting the Fukushima No. 1 plant and said a survey of sea-facing walls at the No. 1, No. 2, No. 3 and No. 4 reactors demonstrated that water had reached 4-5 meters above ground level and 14-15 meters above sea level.
That 14-15 meter figure is much higher than the 5.7 meter tsunami height envisaged in the plant's original design, and TEPCO has been emphasizing the "unanticipated" scale of the March 11 event.
"It was nearly three times as large as (the design anticipated)," said Sakae Muto, an executive vice president at TEPCO.
The company has asked for alleviation of its compensation liabilities in the light of the unusual size of the wave.
In a letter to the government-appointed panel overseeing the Fukushima compensation issue on April 25, TEPCO said: "Given that the tsunami was so large that it reached a height of 14-15 meters, there is good room for an interpretation that it amounted to an 'extremely large-scale act of God.' "
The utility is arguing that the law on compensation for nuclear accidents gives it immunity from compensation liability in such a case.
Some experts agree with TEPCO's estimates. Yuichiro Tanioka, professor of seismology at Hokkaido University, said evidence that tanks at the facility had been submerged by the water backed up the firm's analysis.
"The tsunami height was also probably around 15 meters along the seashore," he said.
But Fumihiko Imamura, professor of tsunami engineering at Tohoku University, pointed out that the photos released by TEPCO appeared to show that the tsunami was only slightly higher than a 10-meter-high levee.
"The tsunami height was probably around 10 meters along the coastline," he said.
Imamura stressed that the height of a tsunami after it hits land can exceed its height along the coastline.
"The countermeasures should distinguish between the two heights," he said.
Yasuhiro Suzuki, professor of geomorphology at Nagoya University, analyzed satellite photos with his coworkers.
They only found traces of water reaching a height of about 10 meters on a gently sloping road several hundred meters from the reactor buildings.
"The sustained depth of water in the buildings was probably a maximum of around 10 meters," Suzuki said.
He said images that appear to show that water had reached up to 15 meters in areas of the plant may be misleading. "These are probably waves that ran up locally after clashing against buildings," he said.
Tomoya Shibayama, professor of coastal engineering at Waseda University, said the water seen in the images appeared to be moving with some speed. He thinks the photos represent the height of the water as it inundated the plant, rather than waves rebounding off walls.
Meanwhile, the 2011 Tohoku Earthquake Tsunami Joint Survey Group, an association of scientists, said the height of the tsunami was less than 10 meters in the Fukushima Prefecture area.
The scale of a tsunami can be amplified in a particular location by coastal topography, but Imamura said, "There appears to be no particular reason for the tsunami to be amplified just there."
There is no doubt that the tsunami on March 11 exceeded the scale anticipated by the designers of the Fukushima plant. But, with the continuing crisis at the Fukushima plant making access to the coastline difficult for scientists, the issue still being debated is what size of wave the Fukushima plant might have survived without the breakdown seen on March 11.
Tidal gauges installed near the coastline have not produced accurate data. More precise conclusions on the height of the wave will require detailed on-site investigations.
(This article was written by Eisuke Sasaki, Shigeko Segawa and Tsuyoshi Nagano.)