United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-32: Revised Seismic Hazard Estimates

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.  20555

April 29, 1994


NRC INFORMATION NOTICE 94-32:  REVISED SEISMIC HAZARD ESTIMATES


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to NUREG-1488, "Revised Livermore Seismic Hazard
Estimates for 69 Sites East of the Rocky Mountains," published in April 1994.
It is expected that recipients will review the information for applicability
to their facilities, specifically with regard to the Individual Plant
Examination of External Events (IPEEE) process.  However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.

Description of Circumstances

During the 1980s, two probabilistic seismic hazard studies were undertaken,
one by the Lawrence Livermore National Laboratory (LLNL), as documented in
NUREG/CR-5250, "Seismic Hazard Characterization of 69 Nuclear Plant Sites East
of the Rocky Mountains," dated January 1989, and another by the Seismicity
Owners Group/Electric Power Research Institute (SOG/EPRI), as documented in
EPRI NP-4726, "Seismic Hazard Methodology for the Central and Eastern United
States," dated July 1986.  These studies were performed in an effort to
improve the estimation of the seismic hazard that can be expected anywhere in
the United States east of the Rocky Mountains.  The results of these studies
produced significantly different seismic hazard predictions in terms of the
absolute values of the probability of exceeding specific levels of earthquake
induced ground motion.  However, the results of both studies were consistent
in terms of the relative seismic hazard from site to site.  The consensus of
the NRC staff and its consultants was that the differences in the results were
not driven by the difference in methodologies, but rather were a result of the
difference in the way in which the uncertainties in seismicity models and
ground motions models were propagated through the computational processes of
the two methodologies.  In 1992, LLNL conducted another study under the
sponsorship of the NRC using a revised method for elicitation of seismicity
parameters, ground motion estimates, and the uncertainties associated with
those parameters and estimates.  The results of the new study were published
for comment in draft NUREG-1488 in October 1993.


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                                        April 29, 1994
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The seismic hazard estimates calculated in the original LLNL study and in the
EPRI NP-4726 study were considered in the preparation of NUREG-1407,
"Procedural and Submittal Guidance for the Individual Plant Examination of
External Events (IPEEE) for Severe Accident Vulnerabilities," published in
June 1991.  The results of the new LLNL study are substantially different from
those of the previous LLNL study.  Also, the EPRI study has not been revised
since its publication in 1986.  Therefore, the staff considers the results of
the new LLNL study to represent the best available information on seismic
hazard estimates.

Discussion

The consensus among experts associated with the probabilistic seismic hazard
analyses (PSHA) was that the large differences between the median hazard
estimates and the mean hazard estimates in the LLNL PSHA results were
primarily caused by the large uncertainty factors that accompanied the seismic
recurrence and ground motion models.  In 1992, LLNL contacted the seismicity
and ground motion experts from the original study as well as several others to
elicit the required information using techniques that allowed the experts to
be more precise about their estimates.  Using a slightly modified version of
the original LLNL Probabilistic Seismic Hazard Methodology, LLNL recalculated
the seismic hazard estimates for all the nuclear power plant sites east of the
Rocky Mountains.  The results exhibited significant reductions in the observed
spread between the median and the mean estimates of the hazard data.
Subsequently, NRC published the revised seismic hazard estimates for all
operating nuclear power plant sites east of the Rocky Mountains as draft
NUREG-1488 for comment.  The NRC received comments from two nuclear industry
companies.  These comments neither contested nor suggested amendments to the
technical data conveyed by draft NUREG-1488.  Instead, these comments
suggested changes in the IPEEE program scope.  Therefore, NUREG-1488 has been
published with no technical changes.

The NRC staff is reviewing the new information described above to see if it is
appropriate to revise the IPEEE seismic scope.  NRC review of the scope of the
current seismic IPEEE process may identify plants whose scope of investigation
can be reduced based on the new hazard information.

Any modifications of the IPEEE process by the NRC will neither necessitate a
revision to, nor have any impact on, efforts by licensees who have completed
their seismic IPEEE or are near completion, or plan to proceed with the
current guidance of Generic Letter 88-20, Supplement 4 and NUREG-1407.
Licensees who have not completed the seismic portion of the IPEEE may continue
with their programs and submit their completed IPEEEs for staff review based
on the previous studies or submit a request to adjust their IPEEE schedule to
take the new information into account.  .
IN 94-32                                         April 29, 1994
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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

/s/'d by BKGrimes


                        Brian K. Grimes, Director
                        Division of Operating Reactor Support
                        Office of Nuclear Reactor Regulation

Technical contacts:  Gus Giese-Koch, NRR
                     (301) 504-2736

               Ron Hernan, NRR
               (301) 504-2010

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