Virginia's Earthquake-Triggered Nuclear Reactor Shutdown Explained


Image: USGS

After yesterday's earthquake on the US East Coast, two nuclear reactors at the North Anna Power Station in Louisa County, Virginia, were shut down. I know many people will have questions about what this mean (is it a routine precaution or something gone terribly wrong?), so I did a bit of research to better understand what happened. Below are my findings...First a Disclaimer
I'm not a physicist or a nuclear engineer. I'm just a guy who has read a few books and listened to a few podcasts by nuclear engineers about this topic. I don't know everything about nuclear power plants, but I think I have an understanding of the basics. So with that said, here's the basic outline of how a reactor core is shut down:


Image: Wikipedia, CC
Nuclear Reactors 101
The first thing to understand is that the core is where the nuclear reaction that produces enough heat to boil water and spin turbines takes place. Fuel rods made of enriched radioactive materials, usually uranium containing around 20% of the more unstable U235 (a nuclear bomb contains materials that are enriched to 90%+, which is why it's physically impossible for a nuclear power plant to explode like a nuclear weapon), are bombarded with neutrons.

Some of these neutrons will hit the U235 atoms and split them, releasing energy and more neutrons. Some of these new neutrons will in turn split other U235 atoms and so one, you've got a nuclear chain reaction and you are producing enough heat to power huge electricity-generating turbines. You also of course end up with nuclear fission byproducts Caesium-137 and Strontium-40, and these emit powerful beta and gamma rays (their half-life is 30 years -- the really long-lived radioactive stuff doesn't emit very powerful radiation by definition; it's like a candle, the brighter it shines, the faster it burns down).


Image: Wikipedia, CC
Earthquake-Triggered Reactor Shutdown Explanation
That brings us to what happened yesterday in Virginia. The Washington Post reported:

Federal officials say two nuclear reactors at the North Anna Power Station in Louisa County, Va., were automatically taken off line by safety systems around the time of the earthquake [...] and no major damage has been reported.

The Dominion-operated power plant is being run off three emergency diesel generators, which are supplying power for critical safety equipment. The NRC and Dominion are sending people to inspect the plant.

My understanding is that nuclear power plants have their own seismic monitoring equipment, and that any activity higher than a certain threshold (probably set by regulation, but I couldn't find the exact number) triggers an automatic shutdown of the core. This is done by inserting control rods into the core. These are made from materials that absorb neutrons very easily, bringing down the number of neutrons below what is required to maintain the nuclear chain reaction, thus shutting down the core.

The same thing happened in Japan at the Fukushima Daiichi plant where, it must be noted, the earthquake was over a 1000 times stronger than the one felt in the US yesterday (the Richter scale is logarithmic, so each degree is a 10x increase in magnitude -- so going from 5.8 to 9.0 makes a huuuuge difference, kind of like turbulence dropping a plane 8 inches vs. 1,000 feet). Most of the problems that resulted were caused by the tsunami that flooded the plant. The reactors did shut down, but the sea water put the backup power generators out of commission (and all the salt and gunk made them very hard to fix) and so coolant couldn't be pumped to the reactor core.

As far as we can tell, nothing of the sort happened in Virginia and the reactors are being cooled properly. What must be careful monitored is the backup generators, and the physical integrity of the buildings. A full safety review should probably be done to make sure there aren't cracks in concrete or other structural weaknesses, but it is likely that this shutdown will turn out to have been routine.

The WSJ writes:

It will be at least several days before the North Anna plant is returned to service, the company said. First, workers must make sure there was no earthquake damage. That requires a thorough inspection of many systems. Then a slow and orderly process is followed to return reactors to service.


Image: USGS
Final Words
There are hundreds of earthquakes in the 5.0-6.0 range each year, and probably thousands since the 1950s when nuclear power plants started being built, so many of those are bound to have happened close to nuclear stations. Every time, the plants shut down automatically. But in the post-Fukushima world, things that went unnoticed before are bound to make the headlines...

It's healthy to think about safety, but our reaction should stay measured and fact-based. Some will say that there's nothing at all to worry about, while others will try to cause fear and panic by exaggerating the risks. As is often the case, the truth is probably somewhere in the middle.


Image: Google Maps
See also: East Coast Earthquake Sends 2 Nuclear Plants Offline, Ceiling Tiles, Bricks to the Ground
Nuclear Reactors in Earthquake Zones in the U.S. [Map]