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How Large of an Earthquake Could U.S. Plants Withstand?

The unfolding emergency in Japan is a sobering reminder of the inherent danger of nuclear power. This post reviews information ranking U.S. nuclear plants in terms of the risk that they could be damaged in an earthquake, and suggests that scientists might be underestimating the risk of a severe earthquake -- and thus the risk of a similar disaster occurring here.

*Updated at 3PM Pacific, March 16, 2011*

According to the U.S. Nuclear Regulatory Commission, historical earthquake activity at the location of a proposed plant is an important part of reactor design standards.  Commission staff determine the largest “credible” earthquake that could occur at a given site, and require engineers to design the plant to withstand that force, plus an added margin of safety.

Ranking U.S. Nuclear Reactors by Earthquake Hazard

The U.S. Geological Survey has learned a lot about earthquake hazards in the last 30 years, after most U.S. nuclear reactors were designed and built. In 2008, the U.S. Geological Survey updated their assessment of seismic hazardacross the United States. The update generally resulted in increased estimates of the frequency and strength of possible earthquakes in many locations.

In 2010, the U.S. Nuclear Regulatory Commission performed a review of this new earthquake hazard data and its implications for U.S. reactors, taking into account the quality of infrastructure at the plants and the type of soil the plants were built on.

Investigative reporters at MSNBC obtained data about all 104 U.S. reactors from the U.S. Nuclear Regulatory Commission, and published a ranking of the reactors in terms of the likelihood that the reactor core could be damaged by an earthquake.

The results are somewhat surprising in that some East Coast plants are more vulnerable than those in earthquake-prone California. Moreover, the new data caused the median estimate of earthquake risk at U.S. reactors to more than triple compared to earthquake hazard data available in 1982.

  1. Indian Point 3, Buchanan, N.Y.: 1 in 10,000 chance each year.
  2. 2. Pilgrim 1, Plymouth, Mass.: 1 in 14,493.
  3. Limerick 1 and 2, Limerick, Pa.: 1 in 18,868.
  4. Sequoyah 1 and 2, Soddy-Daisy, Tenn.: 1 in 19,608.
  5. Beaver Valley 1, Shippingport, Pa.: 1 in 20,833.
  6. Saint Lucie 1 and 2, Jensen Beach, Fla.: 1 in 21,739.
  7. North Anna 1 and 2, Louisa, Va.: 1 in 22,727.
  8. Oconee 1, 2 and 3, Seneca, S.C.: 1 in 23,256.
  9. Diablo Canyon 1 and 2, Avila Beach, Calif.: 1 in 23,810.
  10. Three Mile Island, Middletown, Pa.: 1 in 25,000.

MSNBC has provided a link to the ranking of all 104 reactors in this public excel file.

What Is a “Credible” Earthquake?

In light of the unfolding disaster in Japan, it seems possible that even the latest USGS could underestimate the risk. (Not to mention whether we should consider a 1 in 10,000 risk of an earthquake damaging a reactor core to provide an adequate margin of safety.)

The Wall Street Journal reviewed the safety plans for the Fukushima Daiichi nuclear reactors, which were built towithstand an earthquake of magnitude 7.9, the largest earthquake that Tokyo Electric Power Company could imagine, even with simultaneous seismic activity along the three tectonic plates in the sea east of the plants.

The actual quake that occurred was 9.0 on the Richter scale – about 12 times stronger than the company anticipated was possible.

The New York Times published an article on March 14 featuring the opinion of Ross Stein, a geological physicist at the U.S. Geological Survey, in the aftermath of the Japan quake. The article reads:

What is perhaps most surprising about the Japan earthquake is how misleading history can be. In the past 300 years, no earthquake nearly that large — nothing larger than magnitude-eight — had struck in the Japan subduction zone. That, in turn, led to assumptions about how large a tsunami might strike the coast.

“It did them a giant disservice,” said Dr. Stein of the geological survey. That is not the first time that the earthquake potential of a fault has been underestimated. Most geophysicists did not think the Sumatra fault could generate a magnitude-9.1 earthquake, and a magnitude-7.3 earthquake in Landers, Calif., in 1992 also caught earthquake experts by surprise.

“Perhaps the message is we should re-evaluate the occurrence of superlarge earthquakes on any fault,” Dr. Stein said.

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