In late November 2011, PG&E sponsored a three-day public workshop on seismic safety issues for the Diablo Canyon nuclear power plant (DCNPP). The workshop brought together PG&E technical staff, their paid consultants and government earth scientists, all of whom are working on various aspects of PG&E's ongoing seismic hazard assessment of DCNPP, to share their research results. As a concerned San Luis Obispo resident with a geologic background, I attended a portion of the conference to get an overview of what has been accomplished to date. My goal was to distill down the diverse technical information presented into a few main takeaway points from the conference, and share these with the local community. I left the conference feeling less secure about living within 10 miles of DCNPP than I did beforehand.
The conference was divided into two sections: the seismic source section which focuses on locating active faults near DCNPP and the type of earthquake they may produce; and the ground motion section which focuses on determining how the ground will shake below DCNPP (how much and at what frequency) under various earthquake scenarios. The latter ties in to the key question: will the resulting ground motion exceed the design capacity of DCNPP and potentially damage the plant to the extent that radiation is released into the environment?
An excellent summary of the research efforts for the seismic source assessment was provided by Bill Lettis, a consultant to PG&E. Work to date indicates that the web of seismically active faults around DCNPP appears to be more dense and more interlinked than previously thought. High-tech bathymetry (LIDAR or "laser radar") studies and shallow marine seismic surveys have confirmed linkage between the Hosgri and San Simeon faults, effectively increasing the length of the Hosgri and thus its rupture length and destructive potential. A newly identified seismicity lineament in Estero Bay appears to be a westward extension of the Los Osos fault beneath the coastal sand dunes and links offshore with the San Simeon fault. Several lines of evidence show the Shoreline Fault, located only 600 yards from the plant, is clearly linked with the Hosgri fault. Furthermore, a consensus has emerged that the "old view" of recent decades is not accurate, i.e. that fault segments between junction points (such as bends, steps, links, etc.) are not likely to rupture beyond the ends of the segments. Rather it is more likely that ruptures will jump across these junction points, creating larger ruptures and more energetic, dangerous quakes. PG&E's previous effort to treat each fault segment near DCNPP as separate and thereby minimize the apparent seismic risk (short fault equals lower risk) is simply no longer defensible.
Of concern is that the seismic source group does not seem to be focusing at all on the worst case scenario of linkage between the Hosgri, San Simeon and San Gregorio fault systems. Should the entire 450 kilometer length of this zone rupture at once, this would produce a quake with magnitude (M) in the range of 7.8-8.2 (Wells & Coppersmith scaling methods). A magnitude 8.2 event would be about 11.2 times more powerful than the current design capacity of the DCNPP of a M 7.5 event on the Hosgri fault, and could be expected to lead to plant failure and radiation release.
The highly technical ground motion presentations were summarized by Norm Abrahamson. A key research objective is to simulate with computers the effects of a given earthquake on ground vibration at DCNPP to infer the potential for damage to the plant. Abrahamson highlighted the HUGE number of variables that need to be taken into account in running these computer models to achieve a realistic result. These include the ground characteristics at the plant site and the potential for amplification of ground motion; the degree of attenuation of motion away from the fault rupture; the direction of travel of seismic waves to the plant; the rupture zone shape and how it propagates along the fault plane during a quake; and fault dip angles. Unfortunately, many of these parameters are either unknown or not fully defined for the fault network near DCNPP, and it is not yet clear if further work will be able to provide sufficient certainty to improve the confidence levels of computer simulations.
Most worrisome was the fact that these current state-of-the-art computer simulations are designed to model the effects of ruptures only along a single fault plane. These are not now capable of modeling combined ruptures along branching and linked faults. As described above, branching and linkages are known characteristics of the dangerous web of active faults that surrounds DCNPP. Thus the existing computer models do not seem to be capable of adequately simulating more complicated ground motions at DCNPP resulting from a simultaneous rupture along the complex network of faults that surrounds the plant. This disconnect between the current modeling capability and the actual known fault network in the area is indeed disturbing.
PG&E's ongoing seismic source studies highlight the following worrisome conclusions: 1) with each new study completed new hazards are identified as the web of dangerous, active faults surrounding DCNPP becomes ever more complex; 2) PG&E is ignoring a realistic worst case scenario of a combined Hosgri-San Simeon-San Gregorio fault rupture; and 3) the ability of computer simulations to predict the effects of various earthquake scenarios on the plant integrity is significantly compromised by uncertainties and limitations of the method. The science of ground motion modeling is evolving and appears not yet sophisticated enough to handle the real world complexities of a system of interlinked faults such as occurs very near to DCNPP.
PG&E's seismic assessment is still a work in progress. However, PG&E's continued assertions that DCNPP is safe and that the seismic risks are acceptably low seem absurd in light of the study results presented to date. Given their massive vested interest in keeping the plant operating, it is hard to imagine that PG&E would ever reach the conclusion that the seismic risks at DCNPP are unacceptable and thus recommend that the plant be closed. The temptation to minimize the public perception of risk to justify continued plant operation is simply too great.
