Nuclear Plants on Fault Lines pt. 2

Last post we talked about the existence of fault lines, and saw that a handful nuclear plants were in some in areas of a high-eathquake risk. Now, we'll talk about what it means and how it's mitigated.

The Effects

True to the name, earthquakes produce a vibration of the ground, shaking the earth along with everything that resides on it. This means water (the ocean), buildings, people, cars, etc. The quake itself isn't typically the most damaging effect of a large-scale seismic event, instead after-effects (tsunami's, fires due to electrical line failures, floods, landslides, etc) are the biggest cause of fatalities.

Mitigation

Modern building designs take earthquakes into consideration, commensurate with the location the building is being constructed in. Typically, superstructures (skyscrapers, large bridges, dams, etc) receive the most attention and research. However, there are earthquake codes for all sizes of human-occupied buildings.

One example of a mitigation techniques involves bearings (eg rubber-lead blocks) built into the base of a structure to isolate the top  part of the of the building from the bottom. This is called a "base isolation system." Another system includes dampers built into buildings which absorb vibrations. For example, this pendulum damper built into the top of the Taipei 101 skyscraper:







Regarding Nuclear

As mentioned in previous posts, the nuclear industry is no stranger to stringent safety regulations and policies. This holds true in regards to earthquakes, especially since the 2011 Fukushima event. Specifically, nuclear power plants must by law take into account:

• The most severe natural phenomena historically reported for the site and surrounding area. The NRC then adds a margin for error to account for the limited historical data accuracy;
• Appropriate combinations of the effects of normal and accident conditions with the effects of the natural phenomena; and
• The importance of the safety functions to be performed.

- The NRC

Thus, plants design and implement earthquake mitigation features based on their location. For example, the Diablo Canyon plant in Calafornia (which as we saw had a high risk) was built to withstand 6.75 magnitude earthquakes, later upgraded to 7.5. The plant includes seismic monitoring systems which can initiate a SCRAM (reactor shutdown) in its repertoire of safety features.

Plants in less seismically active areas typically rely more on their already existing plant safety features, i.e. reactor shutdown/coolant/containment systems.

Finally, the NRC has published public documents for every plant in the U.S. detailing a safety inspection conducted after Fukushima, for public perusal at:

http://www.nrc.gov/NRR/OVERSIGHT/ASSESS/follow-up-rpts.html