ML17146A200

Guidance for Determination of Appropriate Regulatory Action Based on Seismic Probabilistic Risk Assessment Submittals in Response to Near Term Task Force Recommendation 2.1: Seismic

ML17146A200
Person / Time
Issue date:	08/29/2017
From:	Cj Fong NRC/NRR/DRA/APLB
To:	Giitter J NRC/NRR/DRA
Shilp Vasavada, 301-415-1228
References
2.1
Download: ML17146A200 (10)

Text

J. Giitter UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 August 29, 2017 MEMORANDUM TO:

Joseph G. Giitter, Director Division of Risk Assessment Office of Nuclear Reactor Regulation FROM:

Christopher J. Fong, Team Leader /RA/ Mehdi Reisi-Fard Probabilistic Risk Assessment Licensing Branch B Division of Risk Assessment Office of Nuclear Reactor Regulation

SUBJECT:

GUIDANCE FOR DETERMINATION OF APPROPRIATE REGULATORY ACTION BASED ON SEISMIC PROBABILISTIC RISK ASSESSMENT SUBMITTALS IN RESPONSE TO NEAR TERM TASK FORCE RECOMMENDATION 2.1: SEISMIC The purpose of this memorandum is to provide guidance to facilitate the staffs evaluations of Seismic Probabilistic Risk Assessments (SPRAs) submitted in response to the letter issued on March 12, 2012, pursuant to the Title 10 of the Code of Federal Regulations Section 50.54(f) as part of implementing the lessons learned from the Fukushima Dai-ichi accident. The guidance intends to facilitate the development of consistent and objective recommendations for the SPRA Technical Review Board (TRB). The TRB will, in turn, provide a recommendation and corresponding rationale for the appropriate level of regulatory action to the Senior Management Review Panel (SMRP).

This guidance leverages existing regulatory bases and processes to assist the staff in identifying potential plant modifications to reduce the seismic risk and in determining if they should be pursued further. It does not supplant or supersede any existing NRC guidance; instead, it provides an objective and reasonable threshold for entering more formal regulatory processes such as Management Directive 8.4, Management of Facility-Specific Backfitting and Information Collection (Agencywide Documents Access and Management System [ADAMS]

Accession No. ML12059A460).

This guidance should be used in conjunction with the SPRA review plan (ADAMS Accession No. ML16273A022) and the Phase 2 guidance (ADAMS Accession No. ML16237A103).

The enclosed document provides details of the methodology to be followed by the reviewers to achieve the stated objectives.

Enclosure:

As stated CONTACTS: Shilp Vasavada, NRR/DRA Courtney St. Peters, NRO/DSRA 301-415-1228 301-415-5864

ML17146A200 OFFICE NRR/DRA/APLB NRO/DSRA/SPR A

NRR/DRA/APLB NRR/DPR/PRMB OGC (NLO)

NAME SVasavada CSt.Peters*

CJFong FSchofer*

SClark*

DATE 08/15/17 08/22/17 08/15/17 06/30/17 08/29/17

• via email

ENCLOSURE GUIDANCE FOR DETERMINATION OF APPROPRIATE REGULATORY ACTION BASED ON SEISMIC PROBABILISTIC RISK ASSESSMENT SUBMITTALS IN RESPONSE TO NEAR TERM TASK FORCE RECOMMENDATION 2.1: SEISMIC

1. INTRODUCTION Certain licensees will be submitting a Seismic Probabilistic Risk Assessment (SPRA) in response to the U.S. Nuclear Regulatory Commissions (NRC) Request for Information (Agencywide Documents Access and Management System [ADAMS] Accession No. ML12053A340) following the recommendation of the Near Term Task Force (NTTF) formed as part of the NRCs post-Fukushima efforts. The information submitted by the licensees will be used by the staff to determine whether the associated operating nuclear plant license should be modified, suspended, or revoked. The purpose of this guidance is to aid the staff in evaluating the SPRA results, identify potential modifications to reduce the seismic risk, and determine if these modifications should be pursued further. The aim of the guidance is to develop consistent and objective recommendations for the SPRA Technical Review Board (TRB). The TRB will, in turn, provide a recommendation and corresponding rationale for the appropriate level of regulatory action to the Senior Management Review Panel (SMRP).

One of the following three recommendations, derived from the SPRA review plan (ADAMS Accession No. ML16273A022), will be provided by the reviewers to the TRB based on this guidance:

1. Regulatory action not warranted (termed Group 1),

2. Regulatory action should be considered (termed Group 2),

3. More thorough analysis is needed to determine if regulatory action should be considered (termed Group 3).

The current guidance is to be used in conjunction with the SPRA review plan (ADAMS Accession No. ML16273A022) and the Phase 2 guidance (ADAMS Accession No. ML16237A103).

The guidance is based on NUREG/BR-0058, Regulatory Analysis Guidelines of the U.S.

Nuclear Regulatory Commission, (ADAMS Accession No. ML042820192), NUREG/BR-0184, Regulatory Analysis Technical Evaluation Handbook, (ADAMS Accession No.ML050190193),

NUREG-1409, Backfitting Guidelines, (ADAMS Accession No. ML032230247), Nuclear Energy Institute (NEI) 05-01, Severe Accident Mitigation Alternatives (SAMA) Analysis Guidance Document (ML060530203), and plant-specific SAMA information submitted as part of license renewal applications. This guidance is not intended to supplant or supersede any existing processes but instead provide an objective and, to the extent possible, rigorous threshold for entering more formal regulatory processes such as Management Directive 8.4,

ENCLOSURE Management of Facility-Specific Backfitting and Information Collection, (MD 8.4) (ADAMS Accession No. ML12059A460).

It should be noted that the current guidance assumes that each submitted SPRA has been reviewed in accordance with the NRC Staff Review Guidance for Seismic PRA Submittals and Technical Review Checklist (ADAMS Accession No. ML16273A022) and found to be of acceptable quality (scope, level of detail, and technical adequacy) to support risk-informed decision making.

This guidance uses action levels based on seismic probabilistic risk thresholds. The staff is expected to include consideration of risk-informed decision making criteria for plants that are classified as Group 2 or Group 3 plants to determine if further regulatory action is needed and include the same in the TRB briefing package for further consideration by management.

The primary purpose of the staffs review is to determine whether potential modifications may be appropriate. After potential modifications have been identified, the staff should consider whether those modifications are necessary for adequate protection or compliance with existing requirements, or whether they may provide safety enhancements. Although this guidance focuses on modifications not needed for adequate protection or compliance with existing requirements, if the staff identifies potential adequate protection or compliance modifications, those should be described in the TRB briefing package for further consideration by management.

2. METHOD The staff will provide its recommendation for the appropriate level of regulatory action to the TRB based on the information provided by the licensee as part of its SPRA submittal and additional requests for information in combination with the guidance provided herein. This guidance is largely based on NUREG/BR-0058 and NUREG-1409. Note that in the context of the SPRA reviews and this guidance, the seismic risk metrics from the full-power SPRA (i.e. the seismic core damage frequency [SCDF] and seismic large early release frequency [SLERF]) will be used.

The methodology for determining the staffs recommendation of the extent of further regulatory action for a particular licensee is illustrated in the simplified flowchart in Figure 1 and described in detail below. The methodology uses a phased approach as follows:

Phase A: Initial Screening Phase B: Detailed Screening Phase C: Additional Refined Screening

ENCLOSURE 2.1 Phase A: Initial Screening The submittal will NOT be evaluated further for potential modifications if:

a.

The accepted SCDF is less than or equal to 1E-5 per year, AND

b.

The accepted SLERF is less than or equal to 1E-6 per year.

If a plant screens out based on these criteria, the staff should document the screening and recommend to the TRB that the plant be classified as a Group 1 plant.

If a plant does not screen out based on the above criteria, a more refined assessment to identify potential safety enhancements, as described for Phases B and C below, should be performed.

2.2 Phase B: Detailed Screening If further evaluation to identify potential modifications is deemed necessary based on Phase A, the recommended methodology for performing such an evaluation is described below. The methodology relies on, and refers to, a separate spreadsheet1 (ADAMS Accession No. ML17146A206) developed to assist reviewers in the evaluation.

The following steps are used to perform the analysis using the spreadsheet:

a. Determine the Risk Reduction Worth (RRW) threshold from the SCDF based Maximum Averted Cost-Risk (MACR), using the worksheet titled Calculator SCDF. This metric is represented by RRW_CDFTH in the worksheet. In order to perform the calculation: (1) link the Plant name (cell B5) to the corresponding name in column K, (2) input the site-specific mean and 95th percentile SCDF and SLERF values from the submittal in cells F4, F5, F6, and F7, respectively, and (3) change the number of units at a site depending on the submittal in cell B4.

The metric is based on the MACR from the seismic hazard and a minimum cost of United States dollars (USD) 100,000 for a potential modification. The value of 100,000 is considered reasonable based on the reviews of SAMAs (ADAMS Accession No. ML12180A425). The metric provides the minimum value of RRW that can result in a potentially justifiable modification.2 Note that the same RRW threshold value applies to both SCDF and SLERF. The important SPRA basic events for SCDF and SLERF are expected to be different and therefore, both sets (i.e., for SCDF and SLERF) should be 1 The spreadsheet is available to the reviewers as a separate file (ADAMS Accession No. ML17146A206). All references to worksheets with particular titles refer to individual worksheets in that spreadsheet.

2 In some cases, the submittal can provide other importance measures such as the Fussell-Vesely (FV).

The RRW and FV are related and one can be easily derived from another using RRW = 1 / (1 - FV).

ENCLOSURE compared separately against the threshold. Any SPRA model element3 with RRW values less than the calculated threshold need not be considered further.

b. Examples of potential modifications from licensee submitted SAMAs have been identified in a worksheet titled SAMA Information. These examples are listed along with their potential RRW and cost of implementation, if available, from plant-specific SAMA analysis. While these examples are not comprehensive they provide valuable information about the type of modifications possible as well as actual cost estimates. All examples should be reviewed for applicability regardless of the plant from which they were derived. The review of the examples should use the list of SPRA model elements with RRW > 1.005 (from SCDF or SLERF) provided with the submittal, any insights gained during the SPRA review, and the examples provided in the worksheet to identify items with the greatest potential for reducing risk. The following information and metrics should be utilized in making such determinations:

i. The RRW threshold calculated previously (applies to both SCDF and SLERF),

ii. The target RRW, based on the mean and 95th percentile values, required to be achieved (individually or in some combination) to justify a substantial safety improvement defined as a reduction in SCDF of 1E-5 per year and/or SLERF of 1E-6 per year. This value is based on the plant-specific mean and 95th percentile SCDF and SLERF inputs and is displayed in cells I11 and I12 of the spreadsheet for the mean values and cells I14 and I15 for the 95th percentiles, iii. The dominant sequences or cutsets for SCDF and SLERF provided in the submittal, and iv. The MACR value calculated previously along with the cost of implementation identified in any pertinent examples provided in the worksheet. The estimated cost of implementation should be compared with the MACR. Realistic and defensible engineering judgement can be exercised in supplanting the estimated cost of implementation from the examples provided.

c. Identify potential improvements which were previously submitted by the licensee as having the potential to address seismic risk, but were not implemented and credited in the licensees SPRA (e.g., IPEEE, SAMA, Overall Integrated Plan, Expedited Seismic Evaluation Process). Any potential modifications based on previous licensee submittals should, to the extent feasible, be subjected to the same review as described in item (b).

3 The term PRA model element is used to stand for basic events or combinations of basic events in cases where a potential modification could address multiple basic events. If combinations of basic events are used, the RRW of each basic event will be summed together to estimate the total RRW for the potential modification as an approximation.

ENCLOSURE

d. The identified potential modifications should, individually or cumulatively, reduce the SCDF by 1E-5 per year and/or the SLERF by 1E-6 per year for further consideration. It should be noted that due to the inter-dependent nature and treatment of PRA model elements, engagement with the licensee may be necessary to determine the simultaneous impact of multiple potential modifications. Furthermore, consideration should also be given to the possibility of extended operation of plants that are currently scheduled for early closure by changing the analysis period in cell B8. The TRB should be explicitly informed if further consideration of the modifications is based on the use of the 95th percentile target RRW values and/or changing the analysis period.

2.2.1 Outcome of Phase B Review Upon completion of the detailed screening as described above, the staff should recommend to the TRB that the plant be classified as a Group 1 plant if all of the following apply:

The required reduction in SCDF and/or SLERF cannot be achieved by the identified potential modifications, individually or in some combination, based on importance measures, available information, and engineering judgement.

Additional consideration of containment performance, as described in NUREG/BR-0058 does not identify a modification that would result in a cost-beneficial substantial safety improvement, and The staff did not identify a potential modification necessary for adequate protection or compliance with existing requirements.

The staff should document, and include in the TRB briefing package, the basis for each of the above conditions being met.

If any one of the conditions is not met, the plant becomes a potential Group 2 or Group 3 plant and the staff should continue with the additional refined screening (Phase C) as directed by the TRB.

2.3 Phase C: Additional Refined Screening Any potential modifications remaining after Phase B need to be reviewed with further refinement to the analysis. Any refinements to the analysis as well as additional considerations should be discussed with the SPRA TRB to ensure consistency in their use.

Examples of such refinement include the use of a MACR that is calculated using the SCDF contribution from only the cutsets of interest (i.e. the cutsets including the PRA model element that the potential modification(s) address) instead of the total SCDF used for Phase B and consideration of the contribution of seismic risk to the overall (i.e, all hazards, at-power) plant risk.

ENCLOSURE A sensitivity study to capture the uncertainties in the consequence estimates should also be performed. The low and high estimates for the immediate on-site dose (cell F8) are 1,000 and 14,000, respectively. Similarly, the low and high estimates for the long term on-site dose (cell F15) are 10,000 and 30,000, respectively. The as-is values in cells F8 and F15 represent the best-estimate values for the corresponding parameters. At a minimum, a sensitivity using the high estimates for the parameters in cells F8 and F15 with the mean SCDF and SLERF values should be performed.

The staff should also incorporate risk-informed decision making criteria, such as those provided in Regulatory Guide 1.174, Revision 2 (ADAMS Accession No. ML100910006), during the refined screening stage to determine if further regulatory action is needed and include the same in the TRB briefing package for further consideration by management.

2.4 Final Decision-making on Identified Potential Modifications Any potential modifcations remaining after the phased approach described in Sections 2.1 through 2.3 should be documented for further consideration by the TRB and SMRP. The documentation should include the SPRA model element(s) of interest, the corresponding RRW, a description of the proposed modification, the impact of the proposed modification, the original and, if calculated, the refined MACR, the expected cost of implementation based on available information, and the results from consideration of risk-informed decision making criteria. If potential modification candidates are similar in nature and can be combined, only the combined potential modification will be retained.

The analysis of the potential modifications will be further refined, if necessary, based on feedback and direction from the TRB. The documentation for the potential modification candidates should be updated accordingly. The resulting information will be presented to the TRB for final approval. Upon approval, the TRB will present the results, in the form of a recommendation, to the SMRP for a decision on whether to prepare a backfit analysis. The decision on whether to prepare a backfit analysis governed by MD 8.4 rests solely with the SMRP.

If a decision is made to enter MD 8.4, a backfit analysis for the identified potential modifications will be performed by staff with expertise in that area.

ENCLOSURE Figure 1. Simplified flowchart illustrating the approach for determining potential modifications SPRA quality found to be acceptable Phase A:

Is SCDF <= 1E-5 per year AND SLERF <=1E-6 per year?

Use spreadsheet and guidance to determine potential modifications Additional information Identified potential modification(s), individually or collectively, result in reduction in SCDF >= 1E-5 per year and/or SLERF >= 1E-6 per year?

Recommend regulatory action (Group 1, 2, or 3) and any potential modification(s) to the SPRA Technical Review Board Did SPRA Technical Review Board recommend further review?

Finalize recommendation N

Y Y

N N

Y Phases B and C