POWER November 2012 - 18

Seismic Instrumentation
at Nuclear Power Plants
When a nuclear power plant experiences
ground motion due to an earthquake, an
evaluation may be needed before allowing
the plant to continue operating or to resume
operating if it has been shut down,
as was the case after the seismic event
that shut down both units at Dominion's
North Anna Power Station on August 23,
2011. (See p. 34 for a complete analysis
of the event and successful recovery program.)
The Electric Power Research Institute
(EPRI) had previously conducted
research to develop guidance regarding
the types of evaluations and inspections
that would be necessary to ensure that an
earthquake had not caused damage that
could affect safe operation of a plant.
More recently, EPRI has formulated guidance
relating to the types of instrumentation
necessary to determine the extent of
the ground motion at a plant site.
For every nuclear plant, an operating
basis earthquake (OBE) has been established.
The OBE has been defined such
that if it can be determined that the
ground motion experienced at the plant
site did not exceed the design basis, the
plant can continue to operate (or can
return to operation if it has been shut
down). If the ground motion exceeds the
OBE, or if it cannot reliably be established
whether the OBE has been exceeded, the
plant may need to shut down and remain
shut down until it can demonstrate that
the earthquake caused no damage that
could affect safe operation.
Although the OBE is often characterized
in terms of a single parameter, peak
ground acceleration, it is actually defined
by a response spectrum. A response spectrum
relates the maximum acceleration or
velocity experienced at a particular location
to the frequency associated with the
vibrations caused by the earthquake. The
response spectrum is typically presented
in the form shown in Figure 1.
Application of the Nuclear Regulatory
Commission's (NRC's) approved OBE exceedance
criterion requires not only measurement
of the OBE parameters (that is,
the response spectrum and cumulative absolute
velocity, CAV), but also walkdown
inspections of the plant. CAV indicates
the potential for a recorded earthquake to
cause damage to nuclear plant structures.
It is the absolute area under the acceleration
vs. time plot as recorded by a timehistory
digital recorder.
If the OBE criterion is not exceeded and
the inspections yield no evidence of significant
damage, the plant can remain in
operation or be restarted. Valid instrument
data, available within 4 hours after an
earthquake, are necessary to support such
a determination. Thus, it is very important
that nuclear power plants install and
maintain appropriate seismic instrumentation
that can facilitate prompt evaluations
of earthquake data.
1. Typical earthquake response. The usual form of an earthquake response spectrum
is illustrated. The plot shows maximum acceleration, velocity, or displacement caused by the
recorded earthquake at a range of frequencies associated with the vibrations caused by the
earthquake. Source: EPRI
Requirements and Options for
Instrumentation Systems
To assess whether an earthquake has exceeded
the OBE for a nuclear power plant,
it is important for a modern, online, digital
seismic instrumentation system to be
in place. EPRI suggests three options for a
seismic instrumentation system:
■ Minimum system
■ Basic automatic system
■ Complete system that complies with
NRC Regulatory Guide 1.2
Minimum System. The minimum system
would include one or two accelerographs,
depending on how the OBE was
defined for the plant. If the OBE had been
defined in the free field, one instrument
in the free field would be sufficient. On
the other hand, if the OBE had been defined
at a building location (for example,
Key References
■ A Criterion for Determining Exceedance
of the Operating Basis Earthquake.
EPRI, Palo Alto, CA: 1988. NP-5930.
■ Final Policy Statement on Technical
Specifications for Nuclear Power Reactors.
U.S. Nuclear Regulatory Commission
(NRC), Federal Register Notice
58FR39132, July 22, 1993.
■ Guidelines for Nuclear Plant Response
to an Earthquake. EPRI, Palo Alto, CA:
1989. NP-6695.
■ " Nuclear Power Plant Instrumentation for
Earthquakes. " NRC Regulatory Guide 1.12,
1997.
■ " Pre-Earthquake Planning and Immediate
Nuclear Power Plant Operator PostEarthquake
Actions. " NRC Regulatory
Guide 1.166, 1997.
■ " Restart of a Nuclear Power Plant Shut
Down by a Seismic Event. " NRC Regulatory
Guide 1.167, 1997.
■ Seismic Instrumentation in Nuclear
Power Plants for Response to OBE Exceedance:
Guidance for Implementation.
EPRI, Palo Alto, CA: 1994.
TR-104239.
0.1
1
Frequency (Hz)
18
www.powermag.com
10
100
■ Standardization of the Cumulative
Absolute Velocity. EPRI, Palo Alto,
CA: 1991. TR-100082.
POWER | November 2012
Acceleration (g)

http://www.powermag.com

POWER November 2012

Table of Contents for the Digital Edition of POWER November 2012