Ichiro Takekuro, executive vice president and chief nuclear officer at Tokyo Electric Power Co (Tepco) said that the 6.8 on the Richter scale quake had its hypocenter 17 km underground. The maximum acceleration observed at Unit 1 was 680 gal (cm/s2), two and a half times the maximum design value of 273 gal(cm/s2). So far, no serious damage has been confirmed to reactor or turbine building structures or to components of high safety significance.
Some 63 incidents were confirmed, including a fire that broke out in the house transformer of Unit 3 which lasted two hours. The base of the transformer’s main unit is staked down to the rock foundation, and the turbine building on the supply side is also fixed to the rock bed. However, the support structure for the secondary connecting bus duct linking the two facilities did not have rock-bed reinforcement. There is a possibility that when the quake caused the ground underneath the support structure to sink (by some 30 cm), the conductor came in contact with the duct, triggering a short circuit and igniting leaked oil. As previously reported, 1.2 m3 of low radioactivity water was discharged to the sea from Unit 6 resulting from the overspill of the spent fuel pool. In addition, radioactive iodine and other particulate matters were discharged through the main stack at Unit 7. Regulatory authorities METI / NISA have ordered Tepco to analyse the seismic data obtained and verify the seismic safety of the facilities that are of high safety importance and conduct a thorough investigation into not only this quake but unknown faults to the extent possible and review safety.
The company must also obtain the consent of the local communities regarding the safety of the plants prior to restart of the reactors. Tepco must also investigate the cause of delay in reporting the leakage of radioactive wate and their response to the Unit 3 transformer fire.
Of the seven BWR and ABWR reactors on the site with a combined capacity of 8,212 MWe, Units 1, 5 and 6 were not in operation for annual outages while Units 3, 4 and 7 were in operation, and Unit 2 was in the process of starting operation.
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