Chubu Electric Power Company's Hamaoka Nuclear Power Plant was struck by a strong earthquake early in the morning of August 11. The epicenter of the magnitude 6.5 earthquake was in Suruga Bay, in Shizuoka Prefecture, 40 kilometers from the Hamaoka plant.
Hamaoka-4 and -5 shut down automatically. Unit 3 was already shut for scheduled maintenance. Units 1 and 2 were shutdown permanently in January this year.
The largest ground motion recorded at the plant was 426 Gal at Unit 5. This is less than the 600 Gal "Extreme Design Earthquake" (S2) for the plant under the old seismic guidelines, or the 800 Gal "Design Basis Earthquake Ground Motion" (Ss) under the revised guidelines. However, the fact that an earthquake 40 kilometers away from the plant could cause such a large ground motion brings into question the ability of the plant to withstand an earthquake of this magnitude immediately beneath the plant.
Chubu Electric said that an alarm indicated elevated radiation levels in the fuel exchange area within the reactor building of Unit 5, but that no radiation was released. Initial visual inspection revealed no damage, but an August 13 report by Chubu Electric showed that subsequent inspections had revealed several problems. It is possible that the plant may have to be shut down for several months while checks are completed.
The quake caused considerable damage in Shizuoka Prefecture, including to the Tomei Expressway, which was closed due to damage from the quake.