Over the past month, Japan’s Nuclear Regulatory Authority has concluded that geologic anomalies running near and under the Tsuruga and Higashidori nuclear stations might be seismic. The panel members all agreed that the bedrock cracks and “crush zones” could possibly be seismic in origin and may have moved over the past 125,000 years (Japan’s current definition of a seismically-active zone). While the science of geologic investigation stretching that far in the past is necessarily inexact, it is significant that all members of the investigative panel agreed in both counts. It is critical that their conclusions are taken seriously by both the government and the nuclear community. However, it makes no sense to cavalierly have the two nukes in question dismantled because of the possibility itself. Here’s why.
Japan’s nukes are built to the same stringent construction levels as those constructed by General Electric and Westinghouse, the two “turn-key” sources of the technology for all existent Japanese plants. Nuclear plants are never built to merely meet the design standards dictated by where they will be located. They are necessarily built to exceed all safety and siting criteria. GE and Westinghouse have long speculated that their designs would probably survive earthquakes as severe as 9 on the Richter scale. Until March 11, 2011, this assumption had never been put to the test. The Great East Japan Earthquake of 3/11/11 unsuspectingly did just that! Three multi-unit nuclear stations on the Tohoku coast were subject to a quake of 9 on the Richter scale, which was roughly five times more energetic than the 8.3 Richter scale worst-case estimates for the subduction zone. Onagawa station, the one nearest the subduction zone epicenter, survived the massive temblor essentially unscathed. The same can safely be said for the Fukushima Daiini station, 10 kilometers south of F. Daiichi. The records kept by the control room staff and emergency management team at F. Daiichi immediately following the earthquake’s peak also show no indication of damage or integrity compromise to the reactors and their emergency cooling systems. All nukes, even those 40 or more years old, are built to exceed the design requirements because it makes no sense to merely meet them and run the risk of not being allowed to operate. We now know this widely-held construction rule was the case in Japan and it kept the three most-at-risk nuclear stations from earthquake damage on 3/11/11.
The NRA should not forget these things when making decisions as to the operational viability of a nuke station. But, there is even more that should be considered. Perhaps the most important thing has to do with the underlying geology itself. Just what is the seismic potential of discovered anomalies? It took a subduction zone displacement/rupture about 400 kilometers long and 50 kilometers wide, allowing the upper plate to shift as much as 40 meters across the fault plane, to produce the worst Japanese earthquake in 450 years.1,2 It is unlikely that any of the faults, cracks and crush zones relative to any of the nuke sites in Japan could produce such a monstrous earthquake themselves. Based on this, I will make the following suggestion.
First, the NRA should establish the severity of worst-case earthquakes that the seams and/or crush zones relative to Tsuruga and Higashidori can produce. Next, multiply the severity of the hypothetical quakes by a factor of five, for the purpose of reasonable conservatism based on the 3/11/11 earthquake experience. Set that as the site-specific seismic restart criteria for each of the nukes in question. Granted, it will take more work to set site-specific standards than creating generic, nation-wide criteria. But, in this case it makes all the sense in the world. If the in-question structures, as they currently exist, can survive these conservative seismic assumptions, then the plants should pass the restart criteria relative to possible earthquakes despite the underlying geologic anomalies. If an existing unit does not pass the new site-specific standard, then the plant operators should be given the option of either upgrading structural and/or system integrity to exceed the site-specific standards or decommissioning. Japan’s teetering electrical infrastructure and faltering economy seem to demand it.
References:
- Tohoku Earthquake & Tsunami Event Recap Report; AON Benfield Limited; London; United Kingdom; August 30, 2011. (PDF download from thoughtfulleadership.aonbenfield.com)
- Estimating insured Losses from the Tohoku, Japan Earthquake and Tsunami: RMS Special Report; Risk management Solutions, Inc.; San Francisco, California; April, 2011. www.rms.com/Publications/2011TohokuReport_041111.pdf
