SENDAI (Kyodo) Earthquake acceleration levels at Tohoku Electric Power Co.’s Onagawa nuclear plant in Miyagi Prefecture exceeded quake-proof standards when the March 11 temblor slammed Tohoku, the utility said Thursday.

Tsunami triggered by the earthquake also far exceeded the maximum level anticipated and almost reached the site of the plant. The power station, which has three reactors, safely suspended operations when the disaster hit, the utility said.

Maximum acceleration values exceeded design standards at nine points in the three reactor buildings, Tohoku Electric said.

The standards were based on a scenario of a magnitude 8.2-earthquake in waters off Miyagi, whereas the March 11 temblor reached 9.0.

The utility cautioned the data should not automatically bring into question the safety of the plant, which was designed to withstand a quake stronger than the set standards.

The recorded tsunami height of about 13 meters far exceeded the plant’s anticipated maximum level of 9.1 meters, and wave marks were found at the edges of the plant, indicating the tsunami fell just short of reaching the main buildings, Tohoku Electric said.

Exposure criteria

Kyodo News

The government is reviewing the radiation exposure level currently used to designate the evacuation zone around the Fukushima No. 1 nuclear plant, as the atomic crisis triggered by last month’s massive earthquake and tsunami continues to unfold, Chief Cabinet Secretary Yukio Edano said Wednesday.

“The current standards represent safety in the event of absorbing a huge amount of radiation in a short period,” Edano told reporters, noting some changes may be necessary as residents near the plant are at risk of being exposed to radiation over an extended period.

Currently, the government says outside radiation levels of over 50 millisieverts require evacuation, and advises residents to remain indoors when levels exceed 10 millisieverts.

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