POWER April 2015 - 89

NUCLEAR SAFETY
2. Seismic bracing. When a new, potentially more hazardous fault was discovered offshore
during construction of the Diablo Canyon Power Plant, the turbine building was strengthened
by the addition of external concrete buttresses, the addition and/or thickening of concrete
shear walls, reinforcement of the main columns, strengthening of floor diaphragms, and the
addition of new bracing in the exterior walls and roof. Equipment upgrades included the addition
of back-up air accumulators for various air-operated valves; strengthening of the supports
for heat exchangers, filters, pumps, tanks, and pipes; and strengthening of electrical equipment,
switchgear, relays, motor controllers, and battery racks. Courtesy: PG&E
3. Seismic survey. PG&E's seismic
research has included the use of stateof-the-art
3D PCable equipment to survey
offshore fault systems, which provides
detailed imaging that is difficult to capture
with traditional 2D multichannel data. Courtesy:
PG&E
to define 'risk.' These questions consider (1)
what can go wrong, (2) how likely it is, and
(3) what its consequences might be. These
questions allow the NRC to understand likely
outcomes, sensitivities, areas of importance,
system interactions, and areas of uncertainty,
which the staff can use to identify risk-significant
scenarios. "
The NRC website has voluminous inforThat
work included low- and high-energy
seismic research in 2012 that used equipment
onboard a research vessel that " generated
low-energy sound waves that were
directed toward the ocean bottom. An array
of sensors, or hydrophones, towed behind
the vessel captured the sound waves as they
reflected back to the surface. The data were
recorded and used to create a detailed threedimensional
map of offshore seismic faults, "
according to PG&E's website.
Seismometers were also deployed on the
sea floor as passive listening devices that
are used to detect and record seismic activity
near the plant, which is then analyzed
by experts.
In September 2014, PG&E sent to the
NRC and the California Public Utilities
Commission a report based on this research.
In the news release about the report, Senior
Vice President and Chief Nuclear Officer Ed
Halpin was quoted as saying, " This research
effort, utilizing the latest technologies, demonstrates
Diablo Canyon continues to be
seismically safe. . . . These studies provide
scientists and regulators an unprecedented
scientific analysis of the seismic characteristics
near Diablo Canyon. "
As a result of both extensive oil and gas exploration
and PG&E's unique seismic research
program (Figure 3), there is general agreement
that the seismic region around Diablo Canyon
April 2015 | POWER
is among the most studied and understood in
the nation. PG&E's Jones told POWER that the
new seismic reevaluation, filed with the NRC
on Mar. 12, " reconfirms the plant's earthquake
design basis. " In the event of a beyond-designbasis
earthquake, the company will use FLEX
equipment to provide cooling.
Regulating Safety in Response to
Seismic Risk
Nobody can guarantee that Diablo Canyon
or any other nuclear power plant will never
experience a beyond-design-basis earthquake
or tsunami, but what regulators can do is ensure
that operators adhere to all the applicable
orders and regulations.
U.S. nuclear plants were originally designed
using a " deterministic " approach accounting
for maximum earthquake potential
and local geology. The NRC defines safe
shutdown earthquake (SSE) as " the maximum
earthquake potential for which certain
structures, systems, and components,
important to safety, are designed to sustain
and remain functional. " In 1997, the NRC
published new geologic and seismic criteria
through which the SSE and uncertainties are
addressed through a probabilistic seismic
hazard analysis.
The NRC explains probabilistic risk assessment
as " A systematic method for assessing
three questions that the NRC uses
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mation about seismic risks, the regulatory and
guidance history as it relates to these risks, as
well as the most recent technical documents
created in response to the Japanese disaster.
This article must necessarily provide only
an overview; those interested in the details
should consult the NRC website.
Assessing Seismic Risk. Since the first
U.S. commercial reactors were built, the
NRC and its predecessors have required that
nuclear power plant design take into account
known hazards. Regarding seismic risks,
even plants that are not located in areas of
" extensive seismic activity " are designed for
safety in the event of such activity.
For operating plants, the NRC used a deterministic
process that requires that safety-significant
structures, systems, and components
be designed to take into account:
■ The most severe natural phenomena historically
reported for the site and surrounding
area. The NRC then adds a margin for
error to account for the limited historical

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POWER April 2015

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