Earlier this month I, along with other members of San Luis Obispo Mothers for Peace (MFP), attended a 3-day workshop about the earthquake hazards to the Diablo Canyon Power Plant (DCPP). After construction of the nuclear power plant was well under way, the Hosgri Fault was belatedly acknowledged and studied, and was found to present a greater earthquake hazard than the plant was designed to withstand. NRC rules prohibit nuclear plants from being sited near major, active earthquake faults, but the NRC ignored its own regulation and issued an operating license to unit 1 in 1984, and to unit 2 in 1985. Because the Hosgri was judged capable of producing a magnitude 7.2 earthquake, the plant was retrofitted to be able to withstand up to a 7.5 magnitude event to allow for a margin of error. Pacific Gas and Electric Company (PG&E) was also directed to continue investigating local seismology in collaboration with the United States Geological Survey (USGS). The recent workshop was part of the resulting Long Term Seismic Program (LTSP) that Pacific Gas & Electric (PG&E) began at the direction of the Nuclear Regulatory Commission (NRC) when the original operating license was issued.
In 2008, using new technology and data systems, a geophysicist from the USGS uncovered another sizeable fault, which became known as the Shoreline fault. This fault is smaller than the Hosgri, but is also much closer to the plant, passing within 600 meters from the reactors. PG&E postulated that an earthquake on this fault could produce between 0.69g (gravity) to 0.74g ground acceleration at the plant, and concluded that the new ground motion was within the safety limits set for the Hosgri Fault. But clearly more research into the seismicity of the area was critical.
In response, the Senior Seismic Hazard Analysis Committee (SSHAC) has been conducting a 3-year study of the central coast, focusing on the hazards to Diablo Canyon Nuclear Power Plant. Because views of the larger technical community are fundamental, the SSHAC process is a structured framework for incorporating data and knowledge from the scientific community. Dozens of technical experts in the fields of geology, seismology, geodesy, and paleoseismology have been gathering data and developing theories. In November of 2011 the first public meeting of the SSHAC was held in San Luis Obispo to present only the data and information gathered. Then in November of 2012 the experts met again in a second meeting to present their varying interpretations of the data, both published and soon to be published. The target audience of this workshop was not the public but a team of Technical Integrators, whose job it is to observe, question, and encourage debate among the experts. Competing scientific hypotheses were considered and uncertainties captured. Eventually they are to develop a model of seismicity that encompasses the Center, Body, and Range of technically credible interpretations. The goal of the 2013 meeting will be to present the model of the seismicity of this area best supported by the data.
The 2012 SSHAC workshop concentrated on the Hosgri, Shoreline, Los Osos and San Luis Bay seismic zones. The questions were: how big is each, how fast is the slip-rate (which can indicate build-up of pressure), what is the past history, and which faults are linked? Several faults are in segments that may or may not rupture simultaneously, creating different possible earthquake strengths and scenarios. The Hosgri, for example, could vary in rupture length from 110 km, if all parts of it rupture together, to as much as 740 km if it happens to link up with the San Simeon, San Gregorio, Bolinas, or even San Andreas faults. The seismicity of the Shoreline and Hosgri faults appear similar to each other and may well connect 2-8 km below ground. Several of the scientists indicated that the scenario of the two rupturing together should not be ruled out.
In January 2011, PG&E submitted to the NRC a Report on the Analysis of the Shoreline Fault Zone (known as the Shoreline Fault Report) that “provides new geological, geophysical, and seismological data to assess the potential seismic hazard of the Shoreline fault”. The report offered five alternative interpretations to characterize the Shoreline fault based on the available data. The NRC staff in its analysis of the report considers a 6.7 magnitude to be the upper bound for any of these models of the Shoreline fault, which is well within the range of the Hosgri fault, judged capable of a 7.2 magnitude earthquake.
Diablo Canyon Nuclear Power Plant is surrounded by over a dozen faults, but according to the Shoreline Fault Report, none seem to exceed the Hosgri, the “controlling fault”. The current licenses require the plant to meet three separate ground motion safety standards: Design Basis Event, Hosgri Event, and Double-Design Event. A Design Basis earthquake is one where the plant is able to continue operating without undue risk to the health and safety of the public. A Hosgri Event is an earthquake of up to 7.5 magnitude on the Hosgri fault, during which the plant would be shut down in a controlled way. Shutting down a nuclear plant involves many steps in order to assure continued cooling of the highly radioactive core where fission takes place.
In the event of a Hosgri Event earthquake, an aftershock or an additional earthquake would be expected to follow, and so the current operating license requires a Double-Design Basis standard, with demands twice as rigorous as the Design Basis. Critical safety structures and equipment needed to prevent a severe radiation release must be able to function after the plant has been shut down in order to keep it cooled down. The lack of such cooling was the primary cause of the horrible Fukushima disaster.
So although it is true that the Diablo plant is expected to be able to withstand the strongest predicted earthquake from the Hosgri fault, some of the critical safety features require the more rigorous standard of the Double Design Event.
The Shoreline Fault Report includes wording that seems to ignore this more rigorous standard:
“The NRC’s conservative estimates for the potential ground motions from the Shoreline fault are at or below the ground motions for which the DCPP has been evaluated previously and demonstrated to have reasonable assurance of safety. . . .”
And on page 95, under Conclusion Number 4:
“The NRC’s conservative estimate for the potential ground motions from the Shoreline fault are bounded by the ground motions for which the DCPP has been previously analyzed and shown to have adequate safety margin (i.e., the HE and LTSP ground motion response spectra).”
Omitted is the Double Design safety margin.
PG&E’s Shoreline Fault Report, developed in early 2011 before the first SSHAC workshop, lacks consideration of scenarios and interpretations of earthquake hazards presented in the 2012 SSHAC meeting. And yet the NRC is standing behind the report, ignoring the recent findings of scientists from both PG&E and the USGS.
Because PG&E had already applied to change its licensing basis to include only two ground motion safety standards, leaving out the stricter Double Design Basis, the omission in the Shoreline Report raises a red flag.
It appears to MFP that PG&E is attempting to change the current licensing basis of the plant to make it less rigorous. PG&E is pushing to lower the standard of safety by designating the Hosgri Event as the bounding event when there are situations where the Double Design Earthquake should remain the standard.
Available online is a Memorandum (August 1, 2011) from the NRC to the Office of Nuclear Reactor Regulation called the Task Interface Agreement (TIA 2011-010) which states in the section Region IV Position, paragraph 1:
“Although the LTSP margin analysis demonstrated that the new Shoreline Fault Zone information was bounded by the Hosgri event, the licensee [PG&E] didn’t evaluate the new seismic information against the other two design basis earthquakes, the Design Earthquake and the Double Design Earthquake . . . .”
And the Conclusion states:
“New seismic information developed by the licensee is required to be evaluated against all three of the seismic design basis earthquakes and the assumptions used in the supporting safety analysis as described in the FSARU. Comparison to the LTSP by itself is not sufficient to meet this requirement.”
Evidence of PG&E’s attempt to evade the demands of new seismic information is also found in two documents on the NRC website:
ML111310608 under the heading of 1R15 Operability Evaluations (71111.15)
See pages 22 – 26 (numbered pages 19 – 23)
See pages 5 and 6.
San Luis Obispo Mothers for Peace expects the NRC, as the regulator of the nuclear industry, to protect public safety rather than corporate convenience. If the agency persists in its acceptance of PG&E’s unsupported conclusions in the Shoreline Fault Report; if it supports PG&E’s efforts to evade the operability requirements of the Double Design Earthquake, then the NRC will again receive a vote of “No Confidence” from Mothers for Peace, from others in the community, and from allied intervenor organizations across the nation.