The Tokyo Electric Power Company, Incorporated, would like to make comments on subjects that are reported by the newspapers, TV or websites.
May 16, 2012
Regarding some media coverings reporting "TEPCO failed to take adequate measures to prevent all power loss caused by tsunami, a risk informed by NISA in 2006." The facts confirmed by TEPCO are as follows.
January - July, 2006, Discussions in Study Group on Flooding
NISA and Japan Nuclear Energy Safety Organization (JNES) set up "the Study Group on Flooding (SGF)", Federation of Electric Power Companies of Japan (FEPC) and each electric power company attended it as observer. Discussions were made at the SGF based on the problem of design vulnerability to internal flooding of US nuclear power station and flooded seawater pumps of nuclear power station in India caused by the Indian Ocean tsunami off Sumatra. The result of the SGF has been released in JNES annual report in 2007 as "Flooding Review Task".
October 2006, Requests from NISA
Based on the result of the SGF, at a meeting on anti-seismic back-check plan, FEPC received a verbal instruction from NISA that "the tsunami can be evaluated (safety is maintained) by the method of Japan Society of Civil Engineers(JSCE), which is evaluated on the safety side. In cases that tsunamis of which height exceeds the height calculated by the method hit the power station, emergency seawater pumps installed at a lower level may lose their function and reactor cores may be damaged. Specific measures should be considered for power stations which have lower tolerances for the tsunami (tidal waves and backrush). (NISA did not talk about the cases where the buildings were flooded)". FEPC was also told to inform the requests to the top managers of each electric power company.
Approaches of TEPCO at the time
TEPCO shared the requests from NISA with the general manager of nuclear power and plant sitting division. The tsunami height calculated by the method of JSCE was maximum 5.7 m (finalized to 6.1 m) and the installation height of emergency seawater pumps was 4 m. Therefore, the electric motors had been elevated above the level of the calculated height to secure the safety. We had also been carrying out performance survey on the bearing to improve water-tightness for emergency seawater pump motors earlier than the request. However, based on the requests from NISA, TEPCO voluntarily decided to study the actual adaption of the improved watertight emergency seawater pump motors. Nevertheless, TEPCO considered that even if emergency pumps were flooded by tsunami and lose function, Fukushima Daiichi Nuclear Power Station would not lose all powers unless the tsunami reaches the building site level, since air-cooled emergency diesel generato rs were installed at the station.
April 2007, Report to NISA
FEPC reported to NISA about the results having considered in response to the requests made by NISA in October 2006, as follows.
· Report the evaluation based on the method of JSCE at anti-seismic back-checks
· Examine further power station safety improvement against tsunami, including water-tightness for electric motors
There was no additional instruction by NISA in response to these reports.
In addition, at the SGF at that time that a study was made on an assumption that the building sites were flooded choosing some nuclear power stations in Japan.
Deliberations in the SGF
○It is assumed that the level of flood were the building site level plus 1 m continuously.
○It is resulted that water break into buildings through aperture and facilities such as power supplies are submerged and lost their functions when buildings are flooded.
○The above fact that water breaking into unexpected places causes failure was not knowledge obtained by the indications from NISA, was recognized from the beginning. The deliberations confirmed the impact only as a study without considering the possibility and probability of real tsunami. The results of the workshop were summarized in the "Results of deliberations in the study group on external flooding (August 2, 2006)".
The Results of Deliberations in the SGF and the Assumption of the Height of Tsunami
○NISA agreed upon evaluating the height of tsunami by the method of JSCE and reflecting it into anti-seismic back-checks judging the adequateness of the method.. Therefore, TEPCO evaluated the tsunami height on the safety side by the method of JSCE and considered the safety of the power station was secured. In addition, since the wave source model needed to be developed based on the opinion of Headquarters for Earthquake Research Promotion and the results of the study on Jogan Tunami, TEPCO has been carrying out such actions as studying the improvement of the watertight for emergency seawater pump generators, and requesting JSCE to revise the method of evaluation in parallel,.
○Looking at the other nuclear power stations in Japan, there are some emergency seawater pumps which have been installed in relatively low levels where the margin to the evaluated height of tsunami is small. TEPCO took that NISA made an additional request to improve safety for this reason. Therefore, the requests from NISA did not focus on the measures to prevent flooded buildings caused by tsunami like this time.
In spite of the conclusion reached that the occurrence of a tsunami the size of the one that occurred on March 11th were beyond the predictive powers of TEPCO and any other Japanese organization engaged in seismic activity research, we deeply regret and apologize that the safeguards and measures put in place consequently turned out to be insufficient. From here on out, we will do our very best to apply the lessons learned from the tragic events that unfolded on that fateful day and make every effort to ensure the utmost in preparedness should another large-scale natural disaster occur.
© Tokyo Electric Power Company Holdings, Inc.
