Since late 2019, it seems like there has been a string of years where things just keep getting worse. 2024 decided to continue that trend by having a magnitude 7.6 earthquake in Japan on New Years Day. Understandably, people get nervous when big earthquakes hit Japan. The nuclear accident at Fukushima happened almost 13 years ago, but it is still fresh in people’s minds.
Nuclear plants are designed to withstand earthquakes and do so surprisingly well. The magnitude 9.0 earthquake that led to the Fukushima disaster caused no serious damage to the facility. Fukushima was originally designed to withstand an earthquake that caused a ground motion of 600 Gal. The maximum acceleration the plant felt was 550 Gal. This was enough that the reactors all shutdown automatically, as designed, but the earthquake did NOT cause significant damage to the facility.
(Gal is a measure of acceleration, basically how much the ground moved under the plant. 1000 Gal is roughly equivalent to 1G, or equal to the acceleration due to gravity.)
The design basis tsunami for Fukushima was 5.7m. The plant itself sat 10m above sea level. There was evidence for larger tsunamis at the site, and Tepco was evaluating this, but no action had been taken at the time of the earthquake. The tsunami that hit the site that day was over 14m high. This flooded the site, destroying backup electrical power and eventually resulting in the melting of 3 reactor cores. I’ll write a whole post on this at some point.
By Shigeru23 – Own work (ref:[1] [2]), CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14896539
One way Japanese plants ensure this is by building the plants on solid rock. Several areas near the plant with thick sediment experienced 2000 Gal. Loose soil amplifies the shaking effect. This has been seen numerous times in places like San Francisco in 1906 and Mexico City in 1985. US nuclear plants have similar requirements that plants be well anchored in bedrock. The entire world updated the seismic requirements following Fukushima.
Systems that are required to safely shutdown and cool the reactors are required to meet seismic design limits. Systems that are not required to safely shutdown the plant do not have the same limits. At least one of the nuclear plants near today’s earthquake is using its emergency diesel generators to power the site. This is due to electrical issues in their switchyards from the earthquake. Accident analysis in the US often assumed the plant will lost offsite power at the same time an accident occurs. They also assume 1 of the 2 required diesel generators per reactor breaks. You have to prove that the site will maintain the ability to shutdown and cool the core.
Only 1 US reactor has ever been shutdown due to an earthquake. What may surprise you is, it was not in California. The North Anna Nuclear Plant in Mineral, Virginia (about 40 miles NW of Richmond) experienced a magnitude 5.8 earthquake on August 23, 2011. The design acceleration at North Anna was 176 Gal. The maximum shaking experienced at the site reached 255 Gal. North Anna experienced an earthquake that was greater than what it was designed to handle. This happening so soon after Fukushima (March 11, 2011), it of course caused significant concern.
The plant declared an Alert, the second highest of 4 possible emergency classifications. This was required due to exceeding the design earthquake for the site. However, no functional damage occurred. All safety systems worked as designed. The NRC requires plants that experience shaking greater than their design to shutdown and be thoroughly inspected (see slide below). The plant cannot restart until these inspections are completed and the Director of Nuclear Reactor Regulation at the NRC has to approve it. North Anna completed these inspections in November of 2011 and has been operating safely ever since.
Nuclear plants are designed to handle almost anything the engineers can think of and keep the core safely cooled. These analyses contain significant margin, which means even if a beyond design basis event happens, the plant will likely be safe. They are not perfect. Mother nature always wins. However, we can and do take as many of the possible events into account and design the plants to handle them. (Another example is tornados. Some plants are designed for a direct hit from a tornado with winds of over 360 mph. The highest recorded wind speed in a tornado is 302 mph.)
If you have any questions, please ask in the comments.
Links:
https://www.kepco.co.jp/english/energy/nuclear_power/jishin_kihon.html
https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/fs-seismic-issues.html
https://www.nrc.gov/reading-rm/doc-collections/cfr/part100/part100-appa.html
Sleepy Neutron's Nuclear Knowledge 
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