Crossposted with TheGreenGrok.com, the blog by the dean of Duke's Nicholas School
What does the quake mean for nukes?
In hindsight this week's event was kinda kewl. East Coasters from Maine to Georgia and beyond felt the earth shake beneath their feet for a few moments just before 2 p.m.
With a magnitude of 5.8 and a depth of 3.7 miles, the quake, which hit near the heretofore little known Mineral, Virginia, about 38 miles from Richmond, was the most powerful in the eastern United States in about 70 years. The last of a similar size hit New York state in 1944.
The North Anna Power Station in Virginia lost grid power during Tuesday's quake and automatically shut down. Though safety features kicked in, preventing problems, questions of how our East Coast nuclear power plants can withstand quakes naturally arise. (NRC)
That makes it unusual, but not outside the envelope of what's expected for the region. In fact, in 1886, South Carolina had a pretty devastating earthquake with an estimated magnitude of 7.3.
Amazingly, because of the Eastern Seaboard's relatively rigid geology and its old, cold rocks, the shock waves from the quake were felt as far north as Ontario, Canada, and as far south as Alabama. (Did you feel it? If so, tell the U.S. Geological Society.)
Here in North Carolina, most of us felt a tremor -- like a wave passing underneath causing a rocking kind of motion. In Virginia and the D.C. area, folks tell me the experience was more intense and lasted longer with windows shattering, walls falling, stuff knocked off tables, bookshelves, and cupboards and, worse, bricks falling from buildings. Office buildings were evacuated in the nation's capital as well as in Manhattan.
That said, there were some notable if minor problems: the quake apparently put a crack in the Washington Monument, which is now shut down indefinitely along with several other monuments pending inspection, and the National Cathedral suffered what's been called "mind-boggling" damage from the earthquake, including the toppling of some 3,000 pounds of concrete from the church's top.
Structures in the nation's capital, including the White House, noticeably shaken in Tuesday's quake, prompted evacuations of many a building and shutdowns of several tourist destinations like the Washington Monument.
Given the disaster that befell Japan's Fukushima nuclear plants following the earthquake-induced tsunami last March, questions of how our East Coast nuclear power plants withstood Tuesday's shakes naturally arise and how they will in future events. (See related post.)
According to officials from the industry -- no damage, no danger, no prob. Two nuclear reactors operated by Dominion Resources at the North Anna Power Station in Louisa, Virginia, seven miles north of the epicenter and 80 miles from D.C., lost grid power and automatically shut down, as was the case in Fukushima. But unlike Fukushima, on-site diesel generators kicked in after the plant went offline, keeping waters circulating to cool the radioactive cores. (Regulators from the U.S. Nuclear Regulatory Commission maintain that all four diesel generators worked properly, while the Reuters news agency reported that one failed. Dominion's press release does not clarify.)
In an effort to reassure the public, representatives of the nuclear industry have been noting that nuclear power plants in the East are designed to withstand quakes more powerful than Tuesday's. For example, Jim Norvell, a spokesperson for Dominion Resources, noted that the North Anna plants are designed to withstand an earthquake of 6.2.
An earthquake design standard in the range of the low 6's is pretty typical of East Coast nuclear plants. For example, the Indian Point nuclear power plant, located just 24 miles north of New York City, appears to have been designed to withstand an earthquake of 6.1. MSNBC estimates that the risk of an earthquake-induced disaster at the
plant is only 1 in 10,000 -- essentially equivalent to having one event
each 10,000 years.
According to MSNBC, by the NRC's own calculations, the odds of an earthquake causing catastrophic failure to a nuclear plant anywhere on U.S. soil in a given year are 1 in 74,176. That's a factor of seven less than the "1 in 10,000" claimed by MSNBC for Indian Point. But then again, the NRC and MSNBC don't see eye to eye on a lot when it comes to quakes and nukes.
The NRC says design values for nuclear plants are determined by requiring all facilities to be able to withstand the "most severe natural phenomena historically reported for the site and surrounding area" with an additional margin of safety. The design features needed to withstand that magnitude earthquake are based on seismic calculations that determine the amount of ground acceleration or movement that would result from the quake. That movement in turn is affected by the local geology (e.g., the strength and composition of the sediments and rock beneath the plant) as well as the quake intensity and depth. (More info here, here and here [pdf].)
Margin of safety? We just had an East Coast quake of 5.8 and these plants are designed to withstand quakes of a mere few tenths magnitude larger. Is that a safe margin?
The reality may not be as bad as those numbers might imply. Because quake magnitude scales that characterize ground motion are logarithmic, small differences in numbers are a lot bigger than they might imply.
"Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value." (U.S. Geological Survey)
So when you think of the difference between a quake of magnitude 5 and 6, you're talking about the latter having 10 times as much ground movement and about 30 times more energy as the former. (For more info, see this "how much bigger" magnitude calculator and explanation.)
A 6.2-magnitude quake, the design value at North Anna, would more than double the ground motion and release four times the energy of a 5.8-magnitude quake, like what occurred on Tuesday.
The likelihood of a quake also changes significantly with a relatively small change in quake intensity. For example, the algorithm available from this USGS site indicates that there is a 20 percent probability of a 5.0 quake every 100 years near the Indian Point plant. That's a pretty high probability, but the intensity is well below the reactors' design value. By comparison, a 6.0 quake -- that's within 0.1 of the reactors' design value -- has a 2 percent probability of occurring every 100 years. A 6.5 quake, which would presumably place Indian Point in danger, has a probability of 0.7 percent every 100 years (i.e., less than a 1-in-10,000 chance each year).
Are you reassured? Is the coast clear for East Coaster's nukes? Or is the whole nuke safety issue shaky and weak?
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