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U.S. Nuclear Power Industry Post-Fukushima Seismic Response Initiatives

Greg Hardy1, John Richards2, Andrew Mauer3, and Robert Kassawara4

1Senior Principal, Simpson Gumpertz & Heger, US

2Project Manager, Electric Power Research Institute, US

3Senior Project Manager, Nuclear Energy Institute, US

4Project Manager, Electric Power Research Institute, US

ABSTRACT

Following the accident at the Fukushima Daiichi nuclear power plant (NPP) resulting from the 11 March 2011 Great Tohoku Earthquake and subsequent tsunami, the United States Nuclear Regulatory Commission (USNRC) established a Near Term Task Force (NTTF) to conduct a systematic review of USNRC processes and regulations and to develop a set of recommendations intended to clarify and strengthen the regulatory framework for protection against natural phenomena. The USNRC issued an information request on 12 March 2012 related to Fukushima NTTF Recommendations 2.1, 2.3, and 9.3 (USNRC, 2012a). The requested seismic information associated with Recommendation 2.1 reflects:

· Information related to the updated seismic hazards at operating NPPs.

· Information based on a seismic risk evaluation (SMA) or seismic probabilistic risk assessment

(SPRA), as applicable.

· Information that would be obtained from an evaluation of the spent fuel pool (SFP).

Subsequent to these NTTF recommendations related to seismic, the U.S. nuclear power industry has collectively developed a far reaching program that consists of key seismic research elements, seismic evaluation methodology enhancements, and both a short term and a longer term program to address the seismic safety of the U.S. nuclear power fleet. This paper provides an overview of the key elements that have been developed and a summary of the status of Post-Fukushima reviews in the United States.

U.S. NUCLEAR INDUSTRY RESPONSE TO THE FUKUSHIMA EARTHQUAKE

Key new initiatives were developed by the nuclear power industry in the United States in response to the Fukushima earthquake in 2011. Many of these seismic innovations were developed as part of the Screening Prioritization and Implementation Details (SPID) document (EPRI, 2013a) developed as a long term seismic program and the innovations within the Expedited Seismic Evaluation Program (ESEP) developed as a near-term seismic program (EPRI, 2013b). Specific seismic initiatives being used to respond to the NTTF seismic requests have included:

· Development of new seismic hazard estimates for all U.S. nuclear plants.

· Investigating the fleet risk significance of these new estimates of seismic hazard.

· Screening based on updated site-specific seismic hazard and ground motion response spectrum

(GMRS) estimates relative to the plant safe shutdown earthquake (SSE) and high confidence of low probability of failure (HCLPF) capacities.

· Conduct of key elements of an SPRA, including structure modelling and scaling of existing

seismic responses for the sites where the hazard estimates have increased significantly.

· Testing program to determine capacities to high frequency motions (reported in separate SMiRT

paper).

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SEISMIC HAZARD REASSESSMENT IN THE UNITED STATES

The first major part of Enclosure 1 of the 50.54(f) letter (USNRC, 2012a) consisted of the request to calculate seismic hazards at existing plant sites following the USNRC-endorsed guidance in the SPID (EPRI, 2013a). These seismic hazards incorporate Probabilistic Seismic Hazard Assessment methods using the recently developed Central and Eastern United States ( CEUS) Seismic Source Characterization for Nuclear Facilities (USNRC, 2012b), together with an updated ground-motion model for the CEUS (EPRI, 2013c) and site-specific site amplification calculations. CEUS plants submitted these site-specific seismic hazards by 31 March 2014, and the Western U.S. plants submitted their hazards by 31 March 2015. The GMRS values for representative CEUS sites are shown in Figure 1, along with the Seismic Qualification Utility Group Reference Spectrum established to characterize seismic capacity levels based on earthquake experience data. The new hazard for many of the CEUS sites showed increased energy in the higher frequency parts of the spectrum than previous estimates of the hazard had estimated. However, this high frequency ground motion is known to not be as damaging to typical equipment and structures as the lower frequency seismic accelerations.

Figure 1 – Representative GMRS hazard Values for CEUS Sites

In order to assess the significance of these new hazards, the Electric Power Research Institute (EPRI) performed a fleet-wide seismic risk assessment to document the overall effect of the new hazard on the U.S. fleet (NEI, 2014). EPRI’s assessment considered the changes in the seismic core damage frequency (SCDF) relative to earlier fleet-wide estimates performed by the USNRC for GI 199 (USNRC, 2010). The plant-level fragility values developed by the USNRC were used directly for the SCDF calculations in the EPRI fleet risk assessment, which allowed for a direct comparison of the SCDF estimates using the

newly developed seismic hazards and the USNRC’s SCDF estimates in 2010 using the 2008 United

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New seismic hazard analyses have been completed for all sixty-one CEUS NPP sites at the time of this study. EPRI calculated the approximate SCDFs for each of these sites using methods that the USNRC used to assess changing seismic hazard in the past. As can be seen from Figure 2, the overall distribution of SCDFs for the fleet indicates that the impact of the updated seismic hazard has been to actually reduce cumulative characterization of the fleet risk relative to estimates obtained using either the 2008 USGS or the 1994 LLNL hazard assessments.

· The range of SCDFs for the U.S. NPP CEUS fleet still falls between 1E-7/year and 1E-4/year.

· For individual plants, some plant SCDF estimates have increased, but the vast majority have

decreased somewhat.

· In the case of the sites for which increases were seen, none of the SCDF values approaches

1E-4/year.

Figure 2 – Comparison of CEUS Nuclear Plant Site Cumulative Distribution of SCDFs

This fleet SCDF study was reviewed by the USNRC. The positive results from this study led the industry to propose that the NTTF 2.1 resolution be separated into an expedited effort to assess the new hazard, as well as a more detailed and longer term program. This parallel path NTTF 2.1 seismic program to respond to the Fukushima disaster is outlined in Figure 3. The two major programs involved in the resolution of NTTF 2.1 are:

· Expedited Seismic Evaluation Program (EPRI, 2013b)

· Seismic Evaluation Guidance: Screening, Prioritization and Implementation Details (SPID) for

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Figure 3 – US Industry Response Initiatives in Response to NTTF 2.1: Seismic

SCREENING, PRIORITIZATION, IMPLEMENTATION AND DETAILS PROGRAM

The SPID report outlines a process and provides guidance for investigating the significance of new estimates of seismic hazard and, where necessary, performing further seismic evaluations (EPRI, 2013a). This guidance is primarily designed for use in responding to the USNRC’s NTTF Recommendation 2.1: Seismic evaluations. The guidance includes a screening process for evaluating updated site-specific seismic hazard and GMRS estimates against the plant SSE and HCLPF capacities. It also provides a selected seismic risk evaluation criteria, as well as SFP evaluation criteria. The primary value in this SPID guidance is that it has been reviewed with the USNRC and can be applied by all plants to provide a uniform and acceptable industry response to the USNRC. However, the guidance related to seismic evaluations is of value for any seismic risk assessment worldwide.

The SPID approach and methods have been developed by EPRI, working with experts from within the nuclear industry, with the intent of identifying reasonable measures that can be employed to complete an effective seismic evaluation. This approach reflects careful consideration of the USNRC’s description of an acceptable approach for the seismic elements of Recommendation 2.1. In general, the approach described in this report is intended to conform to the structure and philosophy of the nine steps suggested by the USNRC:

1. Site-specific hazard curves (common fractiles and mean).

2. Site-specific, performance-based GMRS developed from the new site-specific seismic hazard

curves at the control point elevation(s).

3. SSE ground motion values including specification of the control point elevation(s)

4. Comparison of the GMRS and SSE.

5. Additional information, such as insights from NTTF Recommendation 2.3 seismic walkdowns

and estimates of plant seismic capacity developed from previous risk assessments to inform USNRC screening and prioritization.

6. Interim evaluation and actions taken or planned to address the higher seismic hazard relative to

the design basis, as appropriate, prior to completion of the risk evaluation described below.

7. Selected risk evaluation approach (if necessary).

8. SMA or SPRA implementation guidance and documentation.

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Figure 4 illustrates the SPID process for employing this approach; it is based on a progressive screening approach and is broken down into four major task areas:

· Seismic Hazard and Site Response Characterization

· GMRS Comparisons and Plant Screening

· Prioritization of Risk Assessments

· Seismic Risk Evaluation

SPRA vs SMA Selection Criteria

and Site Prioritization

5

2.1 Effort Complete 4

SPRA SMA

Develop SPRA 6a

Submit SPRA Results and

SFP Evaluation 7a

Develop SMA 6b

Submit SMA Results and

SFP Evaluation 7b

Submit proposed actions, if any, to evaluate seismic risk

contributors 8 Phase 2 9 S P R A /S M A Se le c ti o n & P ri o ri tiza ti o n S e is m ic R is k Ev alu a ti o n Key

Red boxes/diamonds are to be completed within 18 months of the issuance of the 50.54f Post Fukushima letter.

Blue boxes/diamonds indicate completion beyond the 18 month period.

* Optional–Not applicable to Reduced Scope plants. Focused Scope plants must upgrade to Full Scope before using this path. Note that IPEEE screening is not applicable to SFPs because SFPs were not included in the scope of IPEEE evaluations [11]. See Section 7 for SFP evaluation criteria.

** Two special screening considerations are described in Section 3.2.1.

· GMRS comparisons and screening of plants at low seismic hazard sites, and

· Narrow band exceedances in the 1 to 10 Hz range

Notes:

Develop new seismic hazard curves and GMRS 1

Submit new seismic hazard curves, GMRS, and

interim actions 2

SSE > GMRS > 10 Hz

3b

IPEEE* is sufficient for

screening 3c

IPEEE HCLPF > GMRS 1-10 Hz

3d

IPEEE HCLPF > GMRS > 10 Hz

3e High Frequency Confirmation 3f No Yes Yes Yes No No No No Yes Yes H a za rd D e v e lo p m e n t Scr ee n in g fr o m fu rth e r re v ie w

SSE > GMRS 1-10Hz **

3a

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EXPEDITED SEISMIC EVALUATION PROGRAM

The ESEP approach was designed to constitute a specific path to focus the initial industry efforts on short term evaluations that would lead to prompt modifications to some of the most important components that could improve plant seismic safety. The ESEP was developed to focus initial resources on the review of a subset of the plant equipment that can be relied upon to protect the reactor core following beyond design basis seismic events. Figure 5 depicts the basic elements of the ESEP and includes a description of where each of the key elements is discussed within the report (EPRI, 2013b). ESEP guidance includes a screening process for performing the expedited evaluations, including the equipment selection, seismic evaluation, walkdown reviews and the modification criteria associated with the near-term evaluations.

Figure 5 – Expedited Seismic Evaluation Process for the Augmented Approach

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RESULTS AND PATH FORWARD

Tremendous advances have been made with respect to the U.S. industry response to the Fukushima accident. All U.S. plants have now completed state-of-the-art site-specific seismic hazard assessments. Those new seismic hazard characterizations have been used to categorize plants into groups for completion of a number of actions, as applicable to the specific plant, including:

· Seismic Risk Assessments

· Seismic Assessments for High Frequency Ground Motions

· Spent Fuel Pool Seismic Assessments

· Evaluations of the Seismic Adequacy of the Beyond Design Basis Plant Mitigation Systems

In addition, plants where the new seismic hazard exceed the design basis have completed initial walkdown assessments associated with NTTF 2.3 program, as well as the expedited review of the installed FLEX equipment as part of the NTTF 2.1 ESEP. Both of these near-term initiatives have provided new insights into the seismic capability of key plant systems to withstand the design basis earthquake (NTTF 2.3) and the beyond design basis earthquake (ESEP).

Currently, the industry is conducting SPRAs for the group of plants with the largest ratio of GMRS to SSE. These SPRAs will take several years to complete, as they include state-of-the-art assessments of several challenging topics, depending on the actual site and plant configurations:

· Generation of Foundation Input Response Spectra at locations of key site structures (both

horizontal and vertical earthquake components)

· Develop of new/updated plant logic model for use in the SPRA

· Development of new structure models (typically a finite element model or a detailed lumped mass

model)

· Soil-structure interaction analyses including the incoherency effects for certain structures/sites

· New walkdowns of the Seismic Equipment List items

· Fragility assessments of dominant risk contributors using the separation of variables approach

· Evaluation of seismic induced flood and fire

· Consideration of human actions following the large seismic event

· Quantification of seismic risk (CDF and LERF)

As noted in Figure 3, the SPRAs are expected to be completed in phases between mid-2017 and the end of 2020. Following the submittal of these SPRAs to the USNRC, appropriate regulatory actions will be considered for closure of the NTTF 2.1: Seismic topic.

REFERENCES

Electric Power Research Institute (2013a). Seismic Evaluation Guidance: Screening, Prioritization and Implementation Details (SPID) for the Resolution of Fukushima Near-Term Task Force Recommendation 2.1: Seismic. EPRI 1025287, Palo Alto, CA, Feb.

Electric Power Research Institute (2013b). Seismic Evaluation Guidance: Augmented Approach for the

Resolution of Fukushima Near-Term Task Force Recommendation 2.1: Seismic, EPRI 300200704, Palo Alto, CA, May.

Electric Power Research Institute (2013c). Ground-Motion Model (GMM) Review Project, 2 Volumes,

EPRI 3002000717, EPRI (2004, 2006), Palo Alto, CA, June.

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USNRC (2010). Implications of Updated Probabilistic Seismic Hazard Estimates In Central And Eastern United States On Existing Plants Generic Issue 199 (GI-199), Safety Risk Assessment, U.S. Nuclear Regulatory Commission, Washington, DC, Aug.

U.S. Nuclear Regulatory Commission, (2012a). “Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3 and 9.3 of the Near-Term Task Force Review of Insights from the Fukushima Dai-Ichi Accident,” E. Leeds and M. Johnson Letter to All Power Reactor Licensees et al., Washington, DC, 12 March.

Figure

Figure 1 – Representative GMRS hazard Values for CEUS Sites
Figure 2 – Comparison of CEUS Nuclear Plant Site Cumulative Distribution of SCDFs
Figure 3 – US Industry Response Initiatives in Response to NTTF 2.1: Seismic
Figure 4 – SPID Process for Seismic Hazard, Screening and Risk Evaluations
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References

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