Handling nuclear power plant safety in the event of an earthquake

• Japanese, and most other, nuclear plants are designed to withstand earthquakes, and in the event of major earth movement, to shut down safely. 
• In 1995, the closest nuclear power plants, some 110 km north of Kobe, were unaffected by the severe Kobe-Osaka earthquake, but in 2004, 2005, 2007, 2009 and 2011 Japanese reactors shut down automatically due to ground acceleration exceeding their trip settings. 
• In 1999, three nuclear reactors shut down automatically during the devastating Taiwan earthquake, and were restarted two days later. 
• In March 2011 eleven operating nuclear power plants shut down automatically during the major earthquake.  Three of these subsequently caused an INES Level 7 Accident due to loss of power leading to loss of cooling and subsequent radioactive releases. 

Design criteria 

Nuclear facilities are designed so that earthquakes and other external events will not jeopardise the safety of the plant. In France for instance, nuclear plants are designed to withstand an earthquake twice as strong as the 1000-year event calculated for each site. It is estimated that, worldwide, 20% of nuclear reactors are operating in areas of significant seismic activity. The International Atomic Energy Agency (IAEA) has a Safety Guide on Seismic Risks for Nuclear Power Plants. Various systems are used in planning, including Probabilistic Seismic Hazard Assessment (PSHA), which is recommended by IAEA and widely accepted.
Because of the frequency and magnitude of earthquakes in Japan, particular attention is paid to seismic issues in the siting, design and construction of nuclear power plants. The seismic design of such plants is based on criteria far more stringent than those applying to non-nuclear facilities. Power reactors are also built on hard rock foundations (not sediments) to minimise seismic shaking.

Japanese nuclear power plants are designed to withstand specified earthquake intensities evident in ground motion.  These used to be specified as S1 and S2, but now simply Ss, in Gal units.  The plants are fitted with seismic detectors.  If these register ground motions of a set level (formerly 90% of S1, but at Fukushima only 135 Gal), systems will be activated to automatically bring the plant to an immediate safe shutdown.  The logarithmic Richter magnitude scale (or more precisely the Moment Magnitude Scale more generally used today) measures the overall energy released in an earthquake, and there is not always a good correlation between that and intensity (ground motion) in a particular place.  Japan has a seismic intensity scale in shindo units 0 to 7, with weak/strong divisions at levels 5 & 6, hence ten levels. This describes the surface intensity at particular places, rather than the magnitude of the earthquake itself.

The former design basis earthquake ground motion or peak ground acceleration (PGA) level S1 was defined as the largest earthquake which can reasonably be expected to occur at the site of a nuclear power plant, based on the known seismicity of the area and local active faults. A power reactor could continue to operate safely during an S1 level earthquake, though in practice they are set to trip at lower levels. If it did shut down, a reactor would be expected to restart soon after an S1 event.

Larger earthquake ground motions in the region, considering the tectonic structures and other factors, must also be taken into account, although their probability is very low. The largest conceivable such ground motion was the upper limit design basis extreme earthquake ground motion (PGA) S2, generally assuming a magnitude 6.5 earhtquake directly under the reactor. The plant's safety systems would be effective during an S2 level earthquake to ensure safe shutdown without release of radioactivity, though extensive inspection would be required before restart.  In particular, reactor pressure vessel, control rods and drive system and reactor containment should suffer no damage at all.