High radiation readings taken in the No. 1 reactor building the night of March 11 suggest it was the quake rather than the loss of cooling that critically damaged the Fukushima No. 1 power plant, a utility source said Saturday.
The belated disclosure could trigger a review of quake-preparedness at nuclear facilities across the country. Many have been focusing on increasing defenses against tsunami, which knocked out the plant's poorly placed emergency power generators.
On March 11, the nuclear plant shut down automatically just after 2:46 p.m., when the magnitude 9 quake occurred. Within an hour, it was hit by at least two tsunami. The external power supply then shut down, stopping the emergency cooling system from injecting water into the reactor core at 4:36 p.m.