TEPCO told the Nuclear and Industrial Safety Agency four days before giant tsunami crippled the Fukushima No. 1 nuclear power plant that the facility could be pounded by tidal waves higher than calculated in the plant's safety design.
Yoshinori Moriyama, NISA deputy director-general for nuclear accident measures, told reporters at an Aug. 24 news conference that a written communication from Tokyo Electric Power Co. dated March 7 said tsunami could exceed 10 meters in height. No immediate action was taken, however. TEPCO's top management had also been informed of the prediction.
According to NISA and TEPCO, in spring 2008 the utility calculated, on a trial basis, the heights of tsunami that could hit the Fukushima No. 1 and Fukushima No. 2 nuclear power plants if a magnitude-8.3 earthquake hit.
On the basis of a 2002 appraisal by the government's Headquarters for Earthquake Research Promotion, the assumed earthquake was calculated to occur below the seabed off eastern Japan between the Sanriku coast of the Tohoku region and the Boso Peninsula in Chiba Prefecture.
The tsunami inundation heights were predicted at 10.2 meters near the No. 5 and No. 6 reactors, and between 8.4 and 9.3 meters near the No. 1 through No. 4 reactors on the seaward side of the Fukushima No. 1 plant, all in excess of the maximum 5.7 meters anticipated in the safety design. The maximum local tsunami run-up height was estimated at 15.7 meters.
At the Fukushima No. 1 plant, seawater pumps, essential for cooling the reactors, are located 4 meters above sea level, whereas reactor buildings and other facilities stand 10 meters above sea level. Tsunami triggered by the March 11 Great East Japan Earthquake reached heights of 13 meters near the seashore and between 11.5 and 15.5 meters near the reactor buildings.
TEPCO sent the trial calculation results in writing to NISA on March 7. An office director at NISA verbally told TEPCO to submit a report soon and to take appropriate measures.
In September 2009, TEPCO verbally told a NISA official that tsunami in excess of 6 meters could hit the Fukushima No. 1 plant, but the official did not tell TEPCO to take preventative measures.
"I really feel sorry for the insufficient tsunami evaluations and countermeasures," Moriyama said at the Aug. 24 news conference.
TEPCO presented the trial calculation results to its own management board in June 2008, but did not take further action. No records were available for earthquakes that occurred in the past in the hypothetical source area off Fukushima Prefecture, and therefore could serve as references. Data on earthquakes that recur off the Sanriku coast were used instead. For this reason, TEPCO decided that the predictions were merely trial calculations based on assumptions.
TEPCO did not consider publishing the predictions after the March 11 disaster either, because the government's Investigation Committee on the Accident at the Fukushima Nuclear Power Stations was to investigate the issue, an official said.
Prior to the trial calculations in question, TEPCO informed an international conference in 2006 that tsunami higher than anticipated in the safety design could arrive within 50 years. It said there was about a 10 percent chance of this happening and gave a 1 percent probability to a tsunami in excess of 10 meters hitting.
This article was written by Jin Nishikawa and Eisuke Sasaki.
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