Monday, April 4, 2011

Japan's Earthquake Regulations for Nuclear Power

The commonly story about the role of the earthquake in the Fukushima Daiichi disaster is that the quake did some damage, but of course, the tsunami was the overriding cause of the event. While true, I want to provide some historical perspective.

In Japan earthquakes are a part of life. Earthquakes are evaluated by using a sensor network to place the epicenter's location and depth, then the reported magnitude is a measure of the pressure wave sent out from that point. The depth, magnitude, and distance (plus geology factors) all affect the type and severity of shaking a point on the ground experiences. The stress that buildings are subjected to are a function of this shaking. For this reason, regulations pertaining to the ability of a nuclear plant to withstand a nuclear plant are formed on the basis of horizontal ground shaking, in the Japanese regulations, the Design Basis Earthquake Ground Motion (DBEGM).

There have been repeated concerns raised about the nuclear plants in Japan with regard to earthquake, but I will argue that that the experience of the Kashiwazaki-Kariwa Nuclear Plant in 2007 had the greatest impact on the unfolding of events after March 11th this year. To begin with, the main epicenter of the 2011 Tohoku quake was over 100 km away from the plant, and it wasn't even the closest. In the case of the 2007 Niigata earthquake, the quake was on a previously unknown fault just 15 km from the plant, and this ignited a huge industry effort to revisit the seismic qualifications of all plants in Japan. So, my observation is this: in this year's quake, shaking matched the nuclear plant's rated values because of the 2007 Niigata quake.


Note: Units of "gals" are generally used for this application, which are cm/s^2 and, of course, 1/100th of a m/s^2. It's not hard to convert but I'm sticking with g values just so I don't have to explain this.

It is true that the design values for shaking were exceeded some amount at the Fukushima Daiichi plant, but as is obvious, the plants in Japan have withstood much worse in the past, and because of those experiences, they have changed their standards.

It used to be the case that the nuclear regulation in Japan required two design basis for this, which were the S1 and S2 design basis (higher of which used in the image). This was changed to only one value, the Ss basis. Of course, a risk informed approach would be preferable, which many researchers have worked on. For more information, the World Nuclear Association has a great and up-to-date article on Nuclear Power Plants and Earthquakes. There is much more to be said about the tsunami, of course.

It's important to realize that engineering safety is a process that ultimately needs a feedback. Whenever an event happens, the lessons are applied to other units and safety analysis. Also for that reason, no analysis is ever perfect and the next accident will be different from the last. One thing that can be easily said about Fukushima Daiichi is that it is certainly extremely different from past nuclear power disasters, but while saying that, let's not forget about the fact that previous mishaps in Japan's own industry had a significant impact on what the plant was prepared for.

Radiation Monitoring Links Update

It turns out that there is a publicly streaming national network for detectors in many nations. I believe this was not the case for the U.S. before this site was put online not long ago. I'm not sure about the other ones.
There is also occasional information coming from independent actors. I think I've noted before a professor in Tokyo who has been streaming his own Geiger counter. People in the U.S. are at work publishing papers on what they've managed to detect. A new bizarre development is that Greenpeace has taken their own measurements. I have to applaud their effort here. My favorite headline is "Greenpeace: Japanese Government’s Radiation Data is Accurate". Anyone reading the major news sources should understand by now just how sensationalist most reporting is, and Greenpeace has the opposite effect. Good job guys.

Symposiums on the Crisis

MIT has held a follow-up to their last forum to tackle the new information available. Even so, the lead speaker encourages people to think of it as "Rev zero".