Withstanding a Megaquake
Basic Seismic Design / Voluntarily Reinforcing Quake Resistance
Basic Seismic Design
Hamaoka Nuclear Power Station's reactor buildings have 'a highly stable pyramid structure', which is earthquake-resistant and rigid with 'foundations built directly on bedrock'.
Highly Stable Pyramid Structure
Reactor buildings are built to remain strong in earthquakes, with thick, wide foundations, numerous thick walls arranged systematically, and a low center of gravity.
Built Directly on Bedrock
Reactor buildings are built directly on solid bedrock by excavating the ground down to a depth of approximately 20 m. It is known that tremors on hard bedrock are equivalent to about a third to half as strong as those at the surface.
Automatic Shutdown Even with Small Tremors
Seismographs located on the second basement level of the reactor buildings are set to shut down the reactors automatically at 120 Gals. Units 4 and 5, which were in operation at the time of an earthquake in Suruga Bay on August 11, 2009, shut down automatically to maintain safety. Unit 3 was stopped for periodic inspection at the time.
Voluntarily Reinforcing Quake Resistance
In view of its location that falls within the seismic source region of the possible.
Tokai Earthquake, Hamaoka Nuclear Power Station has adopted seismic design since the time of initial construction, allowing a conservative margin.
In 2005, we defined the design basis seismic motion of approx. 1,000 gals on bedrock, considering the Central Disaster Management Council's estimation of seismic ground motion in the anticipated Tokai Earthquake.
To ensure resilience against a quake of this magnitude, the power station carried out reinforcement work by 2008, including the addition of reinforcing supports to pipes inside the buildings, and the construction of a support tower surrounding the exhaust stacks. It has subsequently been confirmed that the power station has sufficient resilience to withstand the impact of the so-called triple quakes of the Tokai, Tonankai and Nankai Earthquakes.
*Note: The operation of Unit 1 and Unit 2 was terminated as of January 30, 2009, following conclusion that applying reinforcement work for operation resumption was not economically viable.