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NEI Presentation - Sept 9 Public Meeting - Flooding DGs - Final
	ML24249A201
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Site:	Nuclear Energy Institute
Issue date:	09/09/2024
From:	Pimentel F; Nuclear Energy Institute
To:	; NRC/RES/DE
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©2024 Nuclear Energy Institute DG-1290, "Design Basis Floods for Nuclear Power Plants," and DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents September 9, 2024 NRC Public Meeting

DG-1290 "Design Basis Floods for Nuclear Power Plants (Proposed RG 1.59, Revision 3)

DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents (Proposed new RG 1.256)

Cross-functional and diverse review Flooding analysis, flood protections/design, external hazards, post-Fukushima response, licensing and regulatory affairs Utility, NSSS vendors, engineering services companies, research organizations, etc.

Feedback to be submitted Detailed written comments on both documents Will use this public meeting to discuss higher-priority issues which may benefit from discussion in helping to understand the concerns and shape resolution

Industry Review

DG-1290 "Design Basis Floods for Nuclear Power Plants (PROPOSED RG 1.59, REVISION 3)

DG-1290 Key Issues Level of Conservatism 1x10-06 Metric Withdrawn standards as a technical basis Important to safety especially in the context of ARs/SMRs (Appendix K)

Climate Change Old data

all methods and assumptions should be clearly conservative Requiring that ALL inputs/methods/assumptions be conservative goes beyond what should be needed to provide reasonable assurance of adequate protection and results in compounded conservatism Proposed approach the overall methods and assumptions should provide a demonstrably conservative result With added definition of demonstrably conservative:

demonstrably conservative - A flood hazard evaluation (deterministic or probabilistic) that is supported by observed data and/or sensitivity studies showing the adopted results are on the conservative side of realistic results. The emphasis for demonstrating conservatism is placed on the flood hazard evaluation results, not on the inputs...

Level of Conservatism

How will 1x10-06 be used by the staff?

Screening for which combined flooding hazards need to be considered?

Setting the level of design basis flood when a probabilistic approach is proposed?

As a reasonable criterion to apply to design basis flood estimates?

To better understand, the following would be helpful Reference and technical basis for 1x10-06 as a target consistent with established NRC guidance Examples of how the staff would use 1x10-06 during review 1x10-06 Questions

Limited availability of data for flooding hazard:

Historical (1x10-02)

Extension of historical data (1x10-03)

Use of paleo flood data (1x10-04)

Data may not be available at all sites

Data is not complete

Data may no longer apply to current geography Stochastic simulations of weather and flooding phenomena (calibrated to the existing data) 1x10-06 Practical Implementation Challenges Extrapolation of at least 2-4 orders of magnitude Uncertainty may be significant

Practical Approaches 1x10-06 mean value, even with large uncertainty; or If 1x10-06 cannot be achieved, then Annual Exceedance Probability (AEP) in the range of 1x10-04, with descriptions of the sites defined mitigation strategies (FLEX, SAFER, etc.) to respond to a more severe flood (e.g., 1x10-06)

External Floods have differences from other hazards May have warning time (hours to days) to prepare Only SSCs below the level of the flood are exposed Strategies exist to mitigate and recover from flooding (barriers, portable pumps, etc.)

1x10-06 Path Forward Discussion

DG-1290 - Key Issues Overview Topic Comment Level of conservatism In several places the documents state that all inputs and assumptions must be conservative. Recommend revising to state that the overall analysis must be shown to be demonstrably conservative as opposed to every individual input and assumption.

1x10-06 as a metric The technical basis and use of 1x10-06 is unclear and as is NRCs intended use of this value as a metric. Given the limited information available to inform a flooding hazard, implementation will be challenging.

Reference to withdrawn/unused standards DG-1290 refers to withdrawn standards including ANSI/ANS-2.12-1978, Guidelines for Combining Natural and External Man-Made Hazards at Power Reactor Sites, and ANSI/ANS-58.21-2007, External Events in PRA Methodology. It is not recommended to reference withdrawn standards that were largely not used and are not reviewed or maintained. It is also noted that parts of these standards are not consistent with other guidance in this document.

Important to safety especially in the context of ARs/SMRs (Appendix K)

To avoid future confusion for advance reactors (ARs), Small Modular Reactors (SMRs) (as has existed in the current fleet re: GL 84-01) the term important to safety should be removed from Appendix K. Recommend use of the terms safety-related, non-safety-related with special treatment, and/or non-safety-related with no special treatment as endorsed in NEI 18-04.

Climate change Although perhaps the simplest to project, sea level rise (SLR) is not the only variable potentially impacted by climate change. Recommend acknowledgement of NRC and industry processes which can assure that new climate data and better projections in the future are considered for impact to plant safety.

Old data sets In several locations, the document presents that many data sets available for use in developing the flood hazard are several decades old and have not been updated. In order to, prevent the unfamiliar reader from having the impression that only old and out of date data is being used, consider updating this text to acknowledge NRC (e.g., POAHNI) and industry processes continuously monitor for new information (data, methods, science) to assess and respond to impacts to plant safety.

DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents (PROPOSED NEW RG 1.256)

©2024 Nuclear Energy Institute 11 Regulatory Guide vs. NUREG-KM The Challenge - The work and analysis described in this DG are governed by federal and state dam safety regulators and performed/controlled by the dam owners Licensees do not have the information, methods, or controls to implement this DG*

NPPs cannot impose these requirements on dam and levee owners The Question - Is a Regulatory Guide the best document choice to capture the gaps that motivated the issuance of the ISG and the lessons learned in the post-Fukushima flooding reevaluations?

Instead, NUREG-KMs have recently been used to capture post-Fukushima lessons learned for both flooding and seismic issues If needed, a DG should specifically focus on requirements that a licensee should implement (as opposed to items that are to the responsibility of dam regulators or dam owners)

Unless utility is also the dam owner

Regulatory Guide vs. NUREG-KM

America, Captured the lessons-learned concerning the staffs 50.54(f) review experience and application of a SSPMP in anticipation of its continued use Describes the technical theory, data sources, and analysis methodology that could be used to derive a SSPMP estimate NUREG-KM-0017, Seismic Hazard Evaluations for U.S. Nuclear Power Plants: Near-Term Task Force Recommendation 2.1 Results,"

Presents a seismic hazard characterization for each NPP and compares the licensees hazard characterization and the staffs confirmatory analyses Provides a comprehensive description of the probabilistic methods used by licensees and the staff

Access to Detailed Dam Information DG-1417 describes the diverse ownership and regulatory structures associated with dams and acknowledges that gaining access to the detailed information may be a challenge Practical implementation Security concerns are associated this information being public Unless the dam is owned by the utility, access to detailed information needed to address DG-1417 may not be possible Intervention by the NRC to facilitate access to this information or assessment insights will likely be necessary -

provisions for NRC assistance were not retained from ISG Re-evaluation of spillway releases for existing dams for existing sites may conflict with methods required by other dam regulating authorities for assessing these releases Perception of conflicting conclusions on acceptability of dams could be a challenge

Seismic Failure of Dams - Example Consistency and clarity of expectations for evaluating Seismic Failure Figure 10 - Seismic Dam Failure Analysis Options C1.4.3 - Seismic Failure C4.4 - Ground Shaking C4.6 - Modeling Consequences of Seismic Dam Failure When is dam failure assumed?

What is sufficient documented margin or factor of safety?

What concurrent assumptions are required?

Are those assumptions at the dam site or the NPP site?

How is this analysis performed?

Other concurrent assumptions that are expected?

(e.g., 2-year windspeed)

How would a licensee be able to perform these evaluations?

DG-1417 states that either methods or data needed to perform some of the analysis identified in the DG-1417 are not available

DG-1417 provides a wealth of relevant information, however Technical basis for expecting assessment of dam failure probabilities with AEPs of 1x10-06 to 1x10-07 is not clear Unclear how the information can be used to achieve the stated level of precision

Therefore, the only viable pathways for implementation are:

Successfully screen out the dams; or NRC communicates that dams are acceptable (i.e., can be assumed not to fail); or Assume total dam failure with concurrent assumptions and deterministically compare with DB flood for NPP, which goes well beyond what is needed for reasonable assurance of adequate protection probabilities as specified in the document remains impractical

ML24249A201

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