Nuclear safety lessons from Japan's summer earthquake | Bulletin of the Atomic Scientists

On July 16, 2007, an earthquake with a magnitude of somewhere between 6.6 and 6.8 struck Japan. Its epicenter was about 16 kilometers north of the Kashiwazaki-Kariwa Nuclear Power Plant (KKNPP), the biggest such plant in the world. The known results of the earthquake include a fire and leaks of radioactivity. However, news of damage to the reactors continues to emerge, the most recent being the discovery of a jammed control rod in Unit-7. Though there was no major release of radioactivity, the many failures and unanticipated events that occurred at the reactor after the earthquake have important implications for nuclear safety worldwide.

To start, the Japanese nuclear establishment never anticipated the magnitude of the earthquake. Under Japan's old guidelines, which formed the basis of the KKNPP design, the seismic hazard for each nuclear site is defined in terms of two intensities, termed S1 and S2. (See "Status Report on Seismic Re-Evaluation" PDF.) The S1 earthquake, referred to as the "maximum design earthquake," is less intense and determined by historical events and current and past fault activity. The S2 earthquake, called the "extreme design earthquake" and supposedly an impossibility, is derived from seismo-tectonic structures and active faults. These requirements were believed to provide a "sufficient range of earthquakes to assure reactor safety for any potential earthquake shaking." (See "A Developing Risk-Informed Design Basis Earthquake Ground Motion Methodology" PDF.) But clearly the S2 design earthquake wasn't extreme enough: The peak ground acceleration of the July 16 earthquake was two-and-a-half times greater than what was assumed for the S2 earthquake.