San Luis Obispo Mothers for Peace Comments on PG&E’s Shoreline Fault Report
By Sherry Lewis, spokesperson, SLOMFP
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)
ML113112A116.
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.