The Hamaoka nuclear power plant, deemed the nation's most dangerous because of its location, could maintain its state of cold shutdown even if a giant tsunami breached the defenses and swamped the facility.
The assurance was given by the Nuclear and Industrial Safety Agency on April 25.
The plant is located in an area where a major earthquake is expected to strike within 30 years.
All reactors at the Hamaoka plant were shut down last May at the request of the government, and remain offline.
A Cabinet Office panel said March 31 that a tsunami with a maximum height of 21 meters would inundate the nuclear plant in Omaezaki, Shizuoka Prefecture, if a massive earthquake were to strike beneath the Nankai Trough off the southern coast of Honshu.
The hypothetical source of a Tokai earthquake lies immediately beneath the Hamaoka nuclear plant.
The prediction of tsunami 21 meters high prompted Chubu Electric Power Co., operator of the Hamaoka nuclear plant, to evaluate the impact of such an event on the plant's facilities. It submitted a report on its finding to NISA on April 16.
NISA's assessment that the plant could withstand giant tsunami came after it examined Chubu Electric's report.
The report said that even if the tsunami inundated the complex and knocked out facilities to cool the reactors, there would still be a reprieve of about six days before nuclear fuel in the reactors and spent fuel storage pools became exposed.
It would be possible to ensure safety if, during that period, rubble was removed using heavy machinery stored at elevations of 21 meters or more, and if portable pumps were mobilized to flood affected areas, the document said.
NISA agreed with the report's conclusions by conducting its own on-site investigations and through other means.
In the meantime, the nuclear watchdog called on Chubu Electric to take additional measures.
They included creating an alternative site for the emergency command post that is located at an elevation of about 10 meters, and implementing comprehensive drills to ensure that emergency pumps and cooling facilities work.
NISA emphasized that the latest assessment pertains only to a state of cold shutdown, and that separate evaluations are necessary to assess what would happen if a tsunami pounded the plant while it was operational.
Chubu Electric is building an 18-meter-high coastal levee at the site as part of mid- to long-term measures aimed at reactivating the reactors.
The utility is considering a review of its tsunami countermeasures, including a possible addition to the levee height, because the one under construction could be breached by 21-meter tsunami.
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