No one knows where Japan’s ongoing nuclear disaster will end. The current signs are ominous. We can only hope that the spent-fuel ponds can be prevented from boiling off. In the meantime, we can try to draw out some of the implications of the event. I’ve attempted that in my latest Bulletin column.
The first and simplest point is that Fukushima is both the first nuclear disaster resulting from a natural disaster and the first serious failure of multiple reactors at once. These observations are related. Every other serious event – Windscale, TMI, Chernobyl, etc. – was a one-off caused by internal failures of some sort. What’s happening in Japan isn’t the same at all; this is a natural disaster that has cascaded into a hydra-headed technological disaster.
A compound event isn’t so unusual in itself, although we sometimes don’t fully register them, preferring for some reason to emphasize the “natural” part of what are really natural-technological disasters. Just to pick one example: the inundation of a major city in 2005 gets shorthanded as “Hurricane Katrina,” and not as “The Great New Orleans Multi-Point Levy Failure.”
All Together Now
Group failures are unlikely to happen by sheer chance. Lightning rarely hits two houses next to each other on the same night. It’s more likely that a single wildfire will engulf both of them. This phenomenon is known to statisticians as “tail dependence.” This paper defines tail dependence as “the tendency of dependence between two random variables to concentrate in the extreme values… such that severe losses are more likely to happen together.”
In other words, there are situations that cause everything to crash at once. As the homely proverb has it, “It never rains but it pours.” Anyone living in the PEPCO service area will understand.
That’s a real problem for nuclear power that has perhaps not been adequately recognized. Extreme events may be rare in any given spot, but from a global and multi-decade perspective, they’re more common. Disasters will happen. As a result, some will argue that nuclear power should be abandoned; others will argue that we can live with the risks. But these are the same arguments as ever, really. Reactors will continue to be built, but existing levels of safety won’t suffice.
Previous experience may teach us something about multi-decade efforts to manage catastrophic risks. Insurance companies have known since the 19th century not to insure properties next door to each other, and not to write too many policies in any single city (back in the days before asbestos insulation, when entire cities could and did burn up). It’s reasonable to ask now whether the next generation of nuclear power plants should be quite so bunched up, where the same natural disaster can clobber several of them at once.
Update | March 25, 2011. In the Washington Post, David Nakamura and Chico Harlan describe the vain efforts of seismologist Yukinobu Okamura to get NISA and TEPCO to understand the implications of the year 869 earthquake discussed in my latest Bulletin column and in the comments here.