ML23320A026

DG-1290 (RG 1.59 Rev 3) Public Comment Resolution

ML23320A026
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Issue date:	07/09/2024
From:	Joseph Kanney NRC/RES/DRA/FXHAB
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DG-1290, Rev 3
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1 Response to Public Comments on Draft Regulatory Guide (DG)-1290 Design Basis Floods for Nuclear Power Plants (RG) 1.59 On February 23, 2022 the NRC published a notice in the Federal Register (87 FR 10260) that Draft Regulatory Guide, DG-1290, proposed revision 3 of Regulatory Guide was available for public comment. The Public Comment period ended on April 11, 2022. The NRC received comments from the individuals or organizations listed below. The NRC has combined the comments and NRC staff responses in the following table.

Comments were received from the following:

Frances A. Pimentel for Nuclear Energy Institute (NEI) 1201 F Street NW, Suite 1100 Washington, DC 20004 ADAMS Accession No. ML22102A201 Peter Hastings for Kairos Power, LLC 707 West Tower Avenue, Suite A Alameda, CA 94501 ML22102A210 Scott Ferrara for Department of Energy National Laboratory Regulatory Framework Coordination and Integration Group PO. Box 1625

• 2525 North Fremont Street Idaho Falls, Idaho 83415I ML22102A207 O. Martinez (marinezo@ans.org) for the ASME/ANS Joint Committee on NuclearRisk Management ML22102A208 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition Kairos Power LLC Using an annual exceedance probability of 1 x 10-6 is inconsistent with other established NRC guidance and precedent. Design basis external hazards are described in NEI 18-04 (Reference 1) which is endorsed by Regulatory Remove content in DG-1290 that would require the design basis of a nuclear power plant to include events beyond a 1 x 10-4 per year frequency.

This guidance does not require all applicants to use an annual exceedance probability beyond 1x10-4. For example, the staff added Appendix K, Considerations for Applying Guidance to Advance Reactors and Small Modular Reactors. Appendix K does not specify a required annual exceedance probability (AEP) or design basis frequency for external flooding.

Using an annual exceedance probability of 1x10-6 as a screening target for flooding hazards is consistent with established NRC

2 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition Guide 1.233 (Reference 2).

RG 1.233 states:

NEI 18-04 describes a set of DBEHLs that will determine the design-basis seismic events and other external events that the SR SSCs will be required to withstand. When the DBEHLs are determined using NRC-approved methodologies, this approach is generally consistent with current practices and provides acceptable protection of SR SSCs. When supported by available methods, the PRA model is expected to address the full spectrum of internal events and external hazards that pose challenges to the capabilities of the plant, including external hazard levels exceeding the DBEHLs. The inclusion of external events within the BDBE category supports the overall risk-informed approach in NEI 18-04 and guidance for large light water reactors, which is the primary focus of this regulatory guide.

The introductiuon to DG-1290 notes that the guidance primarily reflects lessons learned from past reviews of large light water NPP applications as well as additional lessons learned from staff reviews of operating NPP licensees post-Fukushima flooding hazard reevaluations. This guide may be useful in the review of other types of nuclear reactors as well as non-power production or utilization facilities. However, there are no statements in DG-1290 that make its use obligatory for any type of non-LWR currently under consideration for development.

Also see staff response to NEI Comment #6.

3 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition the DID assessments described in subsequent sections. The PRA results, including consideration of external hazards, will also validate a designers initial selections of DBAs and SR SSCs protected against DBEHLs, and ensure no new DBAs are introduced by external hazards.

Consistent with RG 1.233s endorsement of NEI 18-04, design basis external hazards, like floods, are evaluated using the frequency-consequence target shown in Figure 3-1 of NEI 18-04. The figure shows that events to be included in the design basis occur at a frequency of 1 x 10-4 and greater, while defense in depth is accounted for by evaluating beyond design basis events at frequencies below 1 x 10-4.

Kairos Power also submitted a topical report, KP-TR-009 (Reference 3) which the NRC approved in a safety

4 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition evaluation (Reference 4).

The topical report and NRCs endorsement discussed treatment of external hazards, including floods, consistent with the approach outlined in RG 1.233.

The draft guidance in DG-1290 would require flooding events to be included in the design basis that would be considered beyond the design basis under RG 1.233. No discussion is provided in DG-1290 to justify why external floods would require different treatment than other external hazards or other design basis events at a nuclear power plant.

Furthermore, the NRCs quantitative safety goals, whose basis is provided in References 5 and 6, set fleetwide targets for the average core damage frequency at 1 x 10-4/yr.

Establishing an annual exceedance probability of a Design Basis flood at 1x10-6/yr, which becomes the

5 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition initiating event flooding magnitude for a conservative and stylized plant response, as opposed to a realistic assessment of the plant response, is excessively conservative as in comparison to the stated reactor safety goal.

Idaho National Laboratory Draft Regulatory Guide (DG)-

1290 represents an impressive documentation of a complex deterministic approach to assessing flooding risk at a reactor site.

However, this document only reinforces the need for the treatment of flooding phenomena within a probabilistic framework. The concept of probable maximum has helped justify a very subjective basis for establishing nuclear power plant design bases for protection against low frequency, extreme natural phenomena hazards. We are now moving to a risk-informed regulatory basis for assuring adequate protection against accidents at new nuclear power plants.

We need a probabilistic basis for assuring adequate protection against low frequency natural phenomena hazards consistent with this risk-informed approach. If DG-1290 were adopted as a revision to Regulatory Guide 1.59 at this time, it would send the wrong message to the designers of new nuclear power plants.

This comment raises some of the same points as the comment received from Kairos Power LLC (above). With respect to those points, the staff would like to reiterarate that the updates in DG-1290 primarily reflect lessons learned from past reviews of large light water NPP applications. The staff has noted that the guidance may be useful in the review of other types of nuclear reactors as well as non-power production or utilization facilities, but use of DG-1290 is not obligatory for non-LWRs.

Consistent with the NRCs risk-informed regulatory approach and in recognition that detailed knowledge concerning the design of new, advanced nuclear power plants with respect to external floods is not be available at this time, the staff added Appendix K to DG-1290 as a tool to assist potential applicants in better understanding the extent to which those designs may or may not be susceptible to external flood hazards. In Section K-2 (Specific Flood Hazard Considerations for Advanced Reactors and Small Modular Reactors) of DG-1290, the staff stated, in part, that:

A PRA or other appropriate analysis, if available, should be sufficient to evaluate the performance of engineering safety barriers or systems and to demonstrate that the proposed reactor design can operate under a reasonable range of site-specific conditions and in doing so meet specified radiation exposure limits.

6 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, describes the basics of a Probabilistic Flood Hazard Assessment approach: The following discussion is based on a hybrid approach similar to that recommended by the National Research Council (1988) where a numerical runoff simulation model is used in conjunction with the probabilistic representation of the statistical distribution of extreme meteorological events. The PFHA consists of two discrete components: 1.

probability distributions of meteorological input variables (such as precipitation, air temperature, relative humidity) of extreme events. derivation of probability distribution of flood hazards (such as discharge, flow depth and velocity, duration of inundation) using a numerical runoff simulation model or The staff believes that this statement promotes the risk-informed review approach advocated for in this comment.

In reference to the cited quote from NUREG/CR-7046 concerning the basics of a PFHA framework, the staff does not dispute the quoted statement. However, the staff observe that the authors of that report at page 5-8 noted the following with respect to PFHA methods:

A comprehensive Probabilistic Flood Hazard Assessment (PFHA) methodology has not yet been developed. However, discrete components of the PFHA are now available, although the overall framework still needs to be developed. This report [NUREG/CR-7046] is not a suitable place to describe and develop these concepts The staff recognizes the need for PFHA guidance and is actively working on developing such guidance. However, in the meantime, the staff does not want to delay updates based on lessons learned from the recent reviews of large LWR reactors. Consequently, until such time that PFHA guidance becomes available, the staff recommends prospective advanced reactor applicants consider the guidance provided in Appendix K.

7 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition combination of models. The probabilistic approaches for estimation of extreme value distributions have been in existence for some time. Both parametric and nonparametric approaches are now sufficiently well understood that they can be applied in practice.

Why do we need a probabilistic approach rather than the traditional probable maximum approach:

1. Ultimately, what is meant by probable maximum is a subjective judgment of very low probability rather than a maximum value that for a physical phenomenon is unlikely to exist. For example, although there are arguments about limits on maximum precipitation, in practice one can always develop exceptions to the limits.

2. Extent of flooding involves a complex interplay of phenomena: intensity of rainfall, over-topping of

8 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition dams, failure of dams, melting of snow, drainage, and rise in water height, which need to be compounded in assessing the probability of ingress of water into a building, immersion of vital equipment, and the failure of that vital equipment. The dependencies among the frequencies of events must be considered to realistically assess the associated risk.

3. Climatological changes will affect the risk of high consequence meteorological events. To assess the frequency of very low frequency events, it is typical risk assessment practice to look for evidence of paleo-data, such as pre-historic floods. In reality, paleo-data are representative of the climate of the era in which they occurred and may have no actual relevance to current conditions. In contrast, it is likely that over the lifetime

9 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition of a new nuclear plant, e.g., 80 years, the local climate of the plant will change substantially.

Although there are significant uncertainties in projecting future rainfall patterns, there are models that can be used to make those predictions based on different scenarios of greenhouse gas emissions for the locality of a plant.

These impacts, including uncertainties, should be accounted for in assessing the risk of low frequency natural phenomena events leading to the failures of vital equipment. To do that, we cannot use the compounding of probable maxima.

In 1994, with funding from the United States (U.S.) Nuclear Regulatory Commission (NRC), the Army Corp of Engineers developed the technical basis for an improved RG 1.59 for the analysis of the adverse effects of flooding. The new

10 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition approach was based on a hierarchical assessment approach (HHA) tailored for the estimation of the design basis flood hazard at a nuclear power plant site.

Although the intent was to provide a deterministic approach to the development of the design basis flood, the approach describes a probabilistic approach, Probabilistic Flood Hazard Assessment (PFHA) for potential use after an overall probabilistic framework could be adopted by the NRC. The Army Corps of Engineers has documented the approach in Manual No. 1110-2-1417.

We do not endorse the Army Corps of Engineers approach or mean to imply that an acceptable approach to assessing the risk of low frequency natural hazards (in-particular flooding risk over the lifetime of the plant) currently exists.

Nevertheless, the elements of such an approach do exist and such an approach is

11 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition feasible and worthy of development. In the interim, it would be premature to issue a revision to RG 1.59.

Nuclear Energy Institute 1 Overall message There are some places that hint at a nuclear power plant design that is not challenged by a flood event not needing to perform the detailed analyses described in this DG. This DG would benefit from explicitly stating that a purely deterministic approach that assumes the entire facility is under water should not need to provide the NRC staff with the rigorous, site-specific analyses described in this DG.

This DG would benefit from explicitly stating that a purely deterministic approach that assumes a worst-case flood condition should not need to provide the NRC staff with the rigorous, site-specific analyses described in this DG.

The staff feels the recommended statement is not needed because the guidance explicitly addresses a graded approach.

The staff finds that the hierarchcial hazard assement (HHA) approach described in this guide provides the flexibility to apply a graded or simplified approach when warranted by site conditions and plant design. This is reflects lessons learned from past reviews of large light water NPP applications and post-Fukushima flooding reevaluation submittals.

For non-LWRs, the staff addressed the issue raised in this comment by introducing the text found in Appendix K of DG-1290. The last sentence of paragraph 2 in Section K-1 acknowledges that the flood hazard evaluation can be simplified if an applicant can demonstrate that the fundamental safety functions of the power station can be maintained consistent with the requirements of GDC 2. The rigor with which the applicant makes that demonstration relies in large measure on both the hydrologic/meteorological characteristics site as well as the specific design features of the proposed power station. The review logic depicted in Figure K-1 and the supporting narrative in Section K-2 is intended to provide prospective applicants with guidance on how rigorous any compliance demonstration might need to be.

Nuclear Energy Institute 2 B. Analysis Approach (Page 7)

The document states that an acceptable framework for probabilistic assessments is not currently available nor are standards acceptable to NRC staff available to review such a probabilistic analysis.

This statement conflicts with NRCs planned endorsement of the ANS/ASME PRA standard that requires PFHA be used to deal with flooding. Since the The staff is currently considering endorsement of the ANS/ASME PRA standard, but PFHA is beyond the scope of this regulatory guide. NRC staff is currently developing guidance for PFHA, but NRC endorsement of ANS/ASME PRA standard does not require that the NRC have guidance in place for PFHA.

12 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition Accordingly, Probabilistic Flood Hazard Assessments (PFHA) will be considered only on a case-by-case basis by NRC staff.

standard requires it, NRC should be able to describe what is needed or NRC should not endorse that part of the standard calling for PFHA.

NRC staff are in the process of developing regulatory guidance for PFHA. Regulatory guides are issued to describe and make available to the public methods acceptable to the regulatory staff of implementing specific parts of the Commission's regulations, to delineate techniques used by the staff in evaluating specific problems or postulated accidents, or to provide guidance to applicants.

Regulatory guides are not substitutes for regulations and compliance with them is not required. Methods and solutions different from those set out in the guides will be acceptable if they provide a basis for the findings requisite to the issuance or continuance of a permit or license by the Commission.

Nuclear Energy Institute 3 B. Analysis Approach (Page 7)

The document notes: This guide does not provide specific guidance on probabilistic methods for developing quantitative estimates of flooding hazards, however, the DG further states The staff expects that either probabilistic or frequency-based estimates, or some combination of the two, will be needed to inform the analysis of combined events.

Furthermore, the staff notes that a framework or Standard for review of these approaches does not exist.

This is noted to be of particular importance in the low frequency tails (< 10-4 /yr) of the distribution.

NRC should consider guidance on how probabilistic approaches could be used for combined events.

The staff agrees with the intent of this recommendation and notes that at least two ANS standards (ANSI/ANS-2.12-1978 and ANSI/ANS-58.21-2007), although now withdrawn*, previously addressed how to combine probability-based estimates of natural hazards including external floods. To address the intent of this comment, the staff has added add the following to Appendix A, Section A-2 (Deterministic Versus Probabilistic Analyses) of the draft regulatory guide:

In considering the 10-6 screening criterion, the staff recognizes that the definition of some design-basis floods typically does not rely on a singular flood-causing mechanism but, potentially, on two or more contemporaneous flooding events. Guidance in ANSI/ANS-2.12-1978 and ANSI/ANS-58.21-2007 (although now withdrawn) describe methods, respectively for evaluating and combing flood hazard events for design purposes.

• ANSI/ANS 2.12 1978 was administratively withdrawn in 1988. A revision was not initiated because ANS determined that there was a low-level of interest in this standard (any standards over 10 years from an ANSI approval/reaffirmation are administratively withdrawn by ANSI for lack of maintenance). ANSI/ANS 58.21 2007 was withdrawn administratively because it was superseded by

13 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition ANSI/ASME/ANS RA-S-2008, Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications. There have been a couple of revisions of this standard. The current version is ANSI/ASME/ANS RA-S-1.1-2024, Standard for Enhanced Nuclear Risk Management Nuclear Energy Institute 4 B. Analysis Approach (Page 8)

NRC notes Appendix A to this guide further discusses PFHA.

The discussion in Appendix A provides no significant guidance as to what a PFHA should contain and expectations with regards to treatment of uncertainty. Please provide more details on how a probabilistic approach could be used for combined events.

This comment is noted but the staff has no plans for elaborating on PFHA in this RG. NRC staff is currently developing regulatory guidance for PFHA.

As noted in response to NEI comment #2, the intent of the language in question was to acknowledge that prospective applicants have the option to rely on probabilistic flood hazard analysis (PFHA) methods to demonstrate compliance with the regulations. However, detailed discussion of that topic is beyond the scope of this particular regulatory guide.The staff observes that in response to the 2012 50.54(f) flood hazard reevaluation request, based on the 2011 NTTF Recommendation 2.1, licensees performed those flood hazard reevaluations largely deterministically and the few applications of PFHA methods were evaluated on a case-by-case basis.

Nuclear Energy Institute 5 B. Flood-Causing Mechanisms.

Flooding Caused by Local Intense Precipitation (Page 8 to 9) and C.3.a Flooding Caused by Local Intense Precipitation (Page 17) and While in agreement with how a vast majority of Flood Hazard Reevaluations (FHRs) were performed for Local Intense Precipitation (LIP), criteria for the functionality of a sites drainage network/stormwater management system should be provided. Mechanisms could include regular maintenance and inspection are performed as part of a Update the document to detail whether there are any acceptable circumstances or conditions to which NRC staff would consider the site drainage or stormwater management system to be functioning or partially functioning during an LIP event.

This comment is noted but the staff does not believe that this topic requires expansion on in the regulatory guide for the reasons described below.

During the staffs review of the 2012 §50.54(f) flood hazard walkdowns based based on the 2011 NTTF Recommendation 2.3, it was revealed that operators at many project sites had failed to properly maintain their storm water management infrastructure and this deficiency was identified for resolution through some Corrective Action Program or CAP). In light of this recent review experience and planned follow-up actions by some licensees (via the CAP), the staff does not believe it is necessary for the staff to detail those circumstances or conditions in which the site drainage or stormwater management system to be functioning or partially functioning during an LIP event.

14 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition C-1. General Considerations (Page C-1 to C-2) storm preparedness operating procedures.

More importantly, the staffs understanding is that major components of most NPP stormwater drainage networks were originally designed using civil infrastructure standards (i.e., design storms with 10- to 20-year average return intervals). The estimated LIP rainfalls are much larger, and the issue is not just maintenance and inspection, but design capacity versus estimated hazard.

Nuclear Energy Institute 6 B. Flood-Causing Mechanisms.

Wind-Generated Associated Wave Effects (Page 12)

The document notes that wind-generated wave activity may yield a significant hazard independent or coincident with severe hydrometeorological conditions, specifically that a distant storm could yield more severe wave action than a local storm event.

While agreed that a distant storm may be the bounding scenario, it would be helpful to place some bounds or framework, at least conceptually, on the types and locations of events that must be considered as part of the wave effects analysis for coastal locations.

The staff is aware of the concern raised in this comment but does not agree with this recommendation.

The selection of potential storms and storm tracks to consider is site specific and should be performed by subject matter experts with knowledge of regional storm climatology, as well as available databases and previous hazard assessments (where available).

Nuclear Energy Institute 7 C.2.a The current value used as the annual probability of exceedance for the reasonableness of combined event flooding scenarios (1E-

6) is unreasonably low and does not have an associated justification.

This value is more conservative than even the seismic values in some ways, and there is less data available for flooding in many locations. Instead, NRC should focus on the potential for core damage (or equivalent for designs using other metrics). For example, a flood with given a mean annual frequency of E-5 The staff is aware of the concern raised in this comment and notes its reasoning concerning the justification of the 10-6 AEP for external flooding.

With regard to external flooding, reference to a 10-6 AEP was based on assurance that the frequency of core damage was below 10-6/year.

This estimate was based on previous study by Chery (1985) and was later referenced in NUREG-1407 (Chen and others 1991) focusing on the first fleet-wide evaluation of external hazards. For its part, it was subsequently recommended in ANS-2.8-1992 (although now withdrawn) that an average AEP less than 10-6 was an acceptable goal for selection of flood design bases at nuclear power stations.

15 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition per year with a CCDP of 0.1, with the CCDP crediting reliability of event mitigation, should be acceptable as this is consistent with other regulatory approaches to external hazards, e.g.,

seismic. Similar consideration of event mitigation can be included in identifying the mean annual frequency for designs that use metrics other than core damage.

While the 10-6 value may be more conservative than the AEP selected for use in the evaluation of seismic events, it is generally recognized the potential effects of external flood on nuclear power stations is less well understood than is the case for seismicity where there have been multiple design reexaminations since the 1980s.

Nuclear Energy Institute 8 C.2.b The Extreme Storm Data Compilations section (at p.

15) does not recognize the value these data sources have or how they should be included in identifying flood potential. Further, site specific characterization of flood potential should not need to be included if a reactor design is not challenged by flooding.

Recognize that when a technically accurate, publicly available data source is up to date for a region, it can be employed to characterize flooding in the region. Further, recognize that some advanced NPPs may not be challenged by flooding.

In reference to the first portion of this comment, the staff agrees with the commentor that when a technically accurate, publicly available data source is up to date for a region, it can be employed to characterize flooding in the region. To clarify the staffs position on this detail, the following has been added to the paragraph in question as a second sentence:

When a technically accurate, publicly available and up to date data source is available for a region, it can be employed to characterize flooding in the region.

When it comes to recognizing that some advanced NPPs may not be challenged by external flooding, the staff agrees with the commentors observation. As noted in response to NEI Comment #1, the applicant will need to present some argument in in its license application that the effects of external floods are inconsequential to the safe operation of the nuclear reactor.

16 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition Nuclear Energy Institute 9 C.2.c.

Nonstationary Effects (Page

16)

In discussion of sea level rise, it is stated Additional site-specific analyses will need to support less conservative estimates described in USGCRP Appendix A.

The document should further elaborate on when site-specific analysis may differ from design-basis flood analyses (which are requested to follow the USGCRP Appendix A methodology). The document should detail the basis for an acceptable less conservative sea level rise estimate.

The staff agrees with this comment generally and observes that the staffs intent as expressed in this paragraph can be improved in the manner discussed below.

The staff observes that there are multiple estimates of SLR. Some have been published by individual researchers in the refereed literature as journal articles. In the U.S., estimates have been published by the U.S. Global Change Research Program (USGCRP) in its periodic national climate assessment (NCA). The staff is also aware that the Intergovernmental Panel on Climate Change (IPCC), a global body of the United Nations, has also published periodic estimates of SLR. The the USGCRP and IPCC do not conduct original research, but produce comprehensive assessments based on reviews of the literature and current scientific research. They conduct assessments generally viewed as comprehensive, objective, open, and transparent by internationally-recognized subject matter experts.

Consequently, IPCC and USGCRP SLR assessments can be viewed as an independent peer-review of the quality of the various SLR estimates.

Based on these considerations, the staff has amended Section C.2.c.,

Nonstationary Effects on page 16 in the manner described below:

Nonstationary Effects. Design basis flood estimates should include an assessment of nonstationary processes. Relative sea level rise (SLR) is the combined effect of water level change (due to the thermal expansion of ocean water as well as contributions from melting glaciers and large continental ice sheets) and land subsidence (or uplift). SLR is a well documented process that has been ongoing in many coastal regions since the end of the last ice age (generally about 11,700 years before the present). Recent developments in climate research have shown that significant global and regional warming trends have occurred in the past

17 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition several decades and are expected to continue or even accelerate, with potential implications for SLR and hydrologic extremes. A design basis flood analysis for coastal sites should include estimates of relative SLR observed and reported by the National Oceanic and Atmospheric Administrations (NOAAs) National Ocean Service (e.g., 20th century relative SLR rate), as well as projected changes in these rates, provided by organizations such as the U.S. Global Change Research Program (USGCRP) and the Intergovernmental Panel on Climate Change. For example, Appendix 3 to USGCRPs Fifth National Climate Assessment (NCA5, Ref. 35) references reports and databases containing relative SLR observations in U.S. coastal regions. Appendix A to this RG also discusses the effects of climate change on sea levels (with supporting references) in detail and describes an acceptable approach for considering these effects in design basis flood estimates. The staff considers the most conservative SLR scenario provided in the NCA to be acceptable when considering SLR effects in the estimation of design basis flood levels. Note that the NCA is updated periodically, so applicants should use the most recent NCA version. Applicants using the less conservative estimates described in the NCA will need to support these with additional site specific analyses. Applicants should describe the basis for the selection of their preferred SLR estimates, supported by a review of the literature.

The staff has amended the second paragraph of Section A-3 (Non-stationarity: Climate Variability, Climate Change, and Sea Level Rise) as follows:

Relative SLR is the combined effect of water level change (due to thermal expansion and glacier and ice sheet melting) and land subsidence (or uplift). SLR is a well documented process that has been ongoing in most places since the end of the last ice age.

18 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition Design basis flood analyses for coastal sites should include trends in mean sea level observed and reported by the National Oceanic and Atmospheric Administrations National Ocean Service (Ref. 40, 41). However, global climate warming has been identified as a cause of the recent acceleration of SLR, and climate change is expected to increase the rate of SLR in the future, although the magnitude of the increase is uncertain. Applicants should consult the most recent authoritative climate assessments provided by the Intergovernmental Panel on Climate Change (IPCC) and the U.S.

Global Change Research Program (USGCRP) (e.g., Ref. 37, 38).

The applicant should describe the basis for the selection of its preferred SLR estimate.

Nuclear Energy Institute 10 C.3.d Flooding Caused by Storm Surges, Seiches, and Tsunamis (Page 19-20)

The process detailed for hurricane parameter selection in areas where limited historical data is available mirrors very close a probabilistic-style evaluation, both in consideration of the interdependency of meteorological conditions and storm surge-related effects of the storms.

However, there is no detailed guidance provided about how to approach the selection other than the use of current state-of-the-art knowledge of storm phenomenology which is ambiguous.

Provide a better definition and/or framework for storm parameter selection in areas with a deficient historical record that may not be representative of worst-case situation(s).

Further elaborate on the data sampling noted for synthetic storm simulations. What suite of synthetic storms would NRC staff find acceptable for an PMH evaluation?

The staff agrees that the process described in the draft regulatory guide for hurricane parameter selection in areas where limited historical data is available mirrors very close to a probabilistic-style evaluation; the reason for the similarity relates to the introduction of the JPM-OS process. In its recent review of the FHRRs submitted in response to the staffs 2012 §50.54(f) information request, licensees for about 26 coastal sites performed a storm surge analysis. At about half of those sites, licensees elected to define some probable maximum hurricane (PMH) based on a review of the literature to identify past storm events reported to have occurred in the vicinity of the power station sites. Those events were selected from the HURDAT2 data base compiled by NOAA. The other half of the sites relied on the JPM-OS methodology to generate a cohort of synthetic storms for the purposes of defining the PMH. While the same underlying storm data base was likely used in both cases. the JPM-OS approach focuses on the parameter values themselves and dissociates the events from their parameters to generate synthetic storm events that have yet to occur but are physically possible in probability space.

19 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition Based on this recent review experience, the staff does not believe it is practicable to define a framework for storm parameter selection in areas that might have what this comment refers to as a deficient historical record. This is necessarily a site-specific consideration subject to the engineering/technical judgement of the analyst performing the storm surge analysis.

Nuclear Energy Institute 11 C.3.h Combined Events (Page

22)

The NRC staff currently uses an average annual probability of exceedance of less than 1 x 10-6 as a metric to evaluate the reasonableness of combined flooding event scenarios. However, guidance on formal PFHA approaches needed for consistent treatment of combined events is lacking.

Therefore, the NRC will assess the reasonableness of qualitative and quantitative probability estimates for combined events on a case-by-case basis.

A more complete discussion of acceptability of frequency methods should be considered in the final version of the DG.

Given the significance of the metric it would seem appropriate for NRC to provide guidance to help the analyst develop an acceptable approach and presentation and, in particular, identify pitfalls.

This comment is somewhat related to earlier NEI Comment #3. The staff recognizes the concern raised in this comment and agrees that that the paragraph in question can be improved. The staff has revised the paragraph in the manner described below (see underlined material):

The NRC staff currently uses an average annual probability of exceedance of less than 1 x 10-6 as a metric to evaluate the reasonableness of combined flooding event scenarios. The staff notes that ANSI/ANS-2.12-1978 and ANSI/ANS-58.21-2007 (although now withdrawn) describe methods for evaluating and combing flood hazard events respectively for design purposes.

However, guidance on formal PFHA approaches for consistent treatment of combined events is lacking.The NRC will assess the reasonableness of qualitative and quantitative probability estimates for combined events on a case-by-case basis.

Nuclear Energy Institute 12 A-3. Non-stationarity:

Climate Variability, Climate Change, and Sea Level Rise (Page A-2 to A-3)

In terms of coastal erosion effects, the document cites a study for sea level rise (SLR) effects on coastal erosion, both for cliff-and dune-backed coastlines. However, many East coast power plant sites do not fit into classical dune-backed coastlines as there are hardened, Please include recommendations or a methodology for erosional effects (with and without SLR) for sites that may not be able to directly use published methods for classical cliff-or dune-backed coastlines.

The staff agrees with the point raised in this comment and has added additional text and references in response.

In the matter of SLR, NOAA has prepared a web page that presents SLR trends for multiple locations along the United States coastline.

That website can be found at https://tidesandcurrents.noaa.gov/sltrends/lrmap.html and is a useful resource to consult on this topic. This web site has been added to the reference list for Section A-3 of the regulatory guide.

20 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition engineered structures between the open coast and powerblock, and sometimes no traditional dune present at the site. In these situations, what is the staffs recommendations regarding SLR and erosional effects?

Or even how to estimate erosional effects in general?

In the matter of the request to update the regulatory guide to include guidance on how to estimate erosional effects in general along the coastline, there are multiple curated references on how to estimate those effects which are typically site-specific and complex. A general methodology for performing that analysis is outlined in the current edition of the Corps of Engineers Coastal Engineering Manual.

However, the US Geological Survey (USGS) recently published a series of reports under the general title National Assessment of Shoreline Change as a series of Open File Reports. These reports examined the status of the Nations coastline and concluded that there was no widely accepted, standardized method of analyzing shoreline changes for the reason described at the beginning of this paragraph.

To address this particular comment, the staff made the following addition to Section A-3:

There is no widely accepted, standardized method of analyzing shoreline changes such as sediment erosion and deposition. The current edition of the Corps of Engineers Coastal Engineering Manual speaks to this topic generally. However, before undertaking a study of this topic the licensee should consult the USGS National Assessment of Shoreline Change series applicable to the site of interest as a first step in attempting to estimate the effects that SLR might have on a particular project site under consideration. Those reports are publicly available on the USGS publication website.

Nuclear Energy Institute 13 A-3. Non-stationarity:

Climate Variability, Climate Change, and Sea Level The report discusses climate change and provides some details with respect to SLR.

However, there is a general discussion where it is noted that current difficulty

[exists] in translating If the NRC staff is expecting consistent treatment of climate change impacts, the NRC staff should provide a discussion based on current The staff notes that existing text within the regulatory guide already identifies which external flood hazards would be significantly impacted by climate change as called-out in a portion of this comment. The text at page A-3, paragraph 3 specifically states that:

With respect to coastal flooding, a commonly used approach to account for potential climate change and SLR effects on flood

21 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition Rise (Page A-2 to A-3) climate research findings into practical applications for hydrologic design problems.

In spite of the difficulties, decisions need to be made, and several State and Federal agencies have developed frameworks for assessing climate change risks for water resource applications.

While a number of references are identified for specific locations, little guidance is provided in that section identifying expectations as to which approaches may be preferred (or acceptable) and which hazards other than storm surge should be evaluated.

knowledge, on which of the external flood hazards would be significantly impacted by climate change and how climate change extrapolations should be treated.

estimates is to add an SLR factor to flood estimates from other mechanisms (e.g., coastal flooding, tides, or storm surge).

[emphasis added]

Nuclear Energy Institute 14 E-3. Source of Historical Storm Information (Page E-3)

The document suggests use of synthetic storms to account for conditions more severe than those in the historical record but considered to be physically reasonable. Could the staff provide guidance on when and how synthetic storms should be developed?

Presumably, synthetic storm sets are most applicable Elaborate on the use of synthetic storm sets and how those sets should be developed to properly address the staffs concern about lack of historical storm records.

The staff agrees with this recommendation. To address this comment, the staff has made the following amendment to the text in Section E-3, pointing to Section E-4.51 which discusses storm surge modeling based on synthetic storms:

In the matter of selecting synthetic storms for the PMH analysis, Section 4.5.1 of this appendix describes techniques for the development of a synthetic PMH database.

22 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition where the historical record of extreme tropical cyclone events are relatively lacking, such as the northeast U.S.

coast, but confirmation of where this technique should be applied and what type of meteorological analysis would be deemed acceptable as physically reasonable storm parameters would help guide future evaluations.

Nuclear Energy Institute 15 E-4.2 Coupled Wind, Wave, and Hydrodynamic Modeling (Page E-4 to E-5)

Acceptable hydraulic modeling software, notably Delft3D, should be included by reference for determine the storm surge and wave parameters for a PMH event.

Include Delft3D (and other acceptable, state-of-the-art software options) by reference.

The staff agrees with this comment and has added the following Delft3D software package references to the reference list in Appendix E:

Deltares, Delft3D Computer Program, The Netherlands, Delft University of Technology/ Department of Civil Engineering, 2011.

Deltares, Delft3D-FLOW User Manual, The Netherlands, University of Technology/ Department of Civil Engineering Version 3.15, Revision 33641, 2014a.

Deltares, Delft3D-WAVE User Manual, The Netherlands, University of Technology/ Department of Civil Engineering, Version:

3.05.34160. 2014b.

Nuclear Energy Institute 16 F-1.

Discussion (Pages F-1 to F-2)

The seiche methodology acknowledges that if the natural oscillation mode(s) of a water body are not dissimilar from credible forcing presumably from meteorological events, such Address the credible forcing questions and update Section F.1 as needed based on those responses.

The staff agrees with this comment and has added additional additional text in response to the comment.

First, as discussed in this comment, the staff expects that the potential for squall-like phenomena at a candidate power station site would be addressed in the manner described in Appendix E (Flooding Caused by Storm Surge). See specifically Section E-1.3. The staff further

23 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition as multiple squall lines/derechos occurring on the water body at that natural resonance frequency, would there be other expected meteorological or non-meteorological forcing to be included in the analysis?

Presumably, the geometry/direction of expected squall lines would be included in the credible forcing evaluation. For example, squall lines may only occur in an east-west direction, so they would not need to be considered in a north-south direction.

expects that the geometry/direction of any analysis would consider multiple azimuths for these phenomena, consistent with available meteorological data.

Second, the staff agrees that there are other geophysical phenomena responsible for seiches. Aside from a water bodys natural resonant oscillations, seiche (theoretically) can be generated by tsunamis, seismic ground waves, internal waves (solitons), and jet-like currents.

However, the two most common causes responsible for initiating seiches are atmospheric processes and nonlinear interaction of wind waves or swell. The focus of Appendix F (Seiches) in DG-1290 focuses on these two causes.

Bearing these points in mind, this scope of this regulatory guide is not intended to be exhaustive when it comes to identifying potentially all flood-causing mechanisms, especially for those lesser geophysical phenomena such as those described above that might be viewed as rare forcing functions. A review of the relevant literature and local conditions should inform what geophysical factors might be potentially active at a candidate site including those other credible causes alluded to in this comment.

The staff has added the following footnote to Section F-1 (Discussion) at the end of the first paragraph to address this comment:

At certain places, seiche waves due to atmospheric forcing (atmospheric gravity waves, pressure jumps, frontal passages, squalls) can also be responsible for significant harbor oscillations.

(Such waves have been referred to as meteorological tsunamis.)

The application should include a literature review to determine have pervasive these other forcing factors might be a candidate site.

24 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition Nuclear Energy Institute 17 G-3. Tsunami Computational Modeling Tools (Page G-3 to G-4)

Multiple modeling software options are noted, but other commercial hydraulic software, namely Delft3D is comparable to ADCIRC in solving shallow-water wave propagation and should be included by reference as an appropriate technique.

Include Delft3D by reference as an acceptable shallow-water wave propagation solver for tsunami applications.

The staff agrees with this recommendation. See staff response to NEI Comment #15 for information on how the comment has been addressed.

Nuclear Energy Institute 18 Wave Height and J-2. Wave Runup (Page J-1 to J-2)

Outside of a screening-type application for the hazard, significant wave height, maximum wave height, and wave runup are relatively inconsequential parameters.

It would be rare that the largest storm waves break at/onto powerblock structures directly as larger waves would break over shallow embankments/other site features.

Hydrodynamic/hydrostatic forces from the breaking/broken waves and overtopping volumetric rates around the power block area are of critical concern, but a further explanation of an acceptable framework outside of the brief mention of ACES/CEDAS and the USACE CEM. These effects Include further methods for assessing wave overtopping and wave forces which are more critical to plant safety than wave height or wave runup height alone.

The staff agrees with this recommendation.

The following text has been added to the ends of Sections J-1 (Coincident Wave Heights)and J-2 (Wave Runup):

Other models/methods used in the past include Goda (1985),

Technical Advisory Committee for Water Retaining Structures (2002), and van der Meer and others (2016).

The following references have been added:

Goda, Y., Random Seas and Design of Maritime Structures, Tokyo, The University of Tokyo Press, 1985.

Technical Advisory Committee for Water Retaining Structures, Wave Run-up and Wave Overtopping at Dikes, Delft, Technical Advisory Committee on Flood Defence in the Netherlands [TAW],

Technical Report, 2002.

van der Meer, J.W., N.W.H. Allsop, T. Bruce, J. De Rouck, A.

Kortenhaus, T. Pullen, H. Schüttrumpf, P. Troch, and B. Zanuttigh, Manual on Wave Overtopping of Sea Defences and Related Structures. An Overtopping Manual Largely Based on European Research, but for Worldwide Application, EurOtop [Joint FCERM

25 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition are the critical component to assess the adequacy of protection measures, so more details about what the staff would deem an acceptable method would be welcomed.

Research Programme/Environment Agency and Rijkswaterstaat -

Water, Verkeer en Leefomgeving], EurOtop 2016 Pre-release Edition, October 2016. [Available on-line at www.overtopping-manual.com.]

These additions reflect methods other than the ACES/CEDAS and the USACE CEM used in connection with the 2012 FHRRs to estimate wind/wave effects.

Nuclear Energy Institute 19 J-5. Water Borne Debris The discussion in this section seems relatively brief.

Expand the discussion to more clearly indicate the expectations for treating this phenomenon.

Expand discussion and perhaps include an example application and provide relevant references of reasonable examples of an assessment of the impact of water borne debris.

The staff believes that this appendix (J-5) in DG-1290 is sufficient in terms of detail. The commenter is reminded that the 61 flood hazard reevaluation reports (FHRRs) were submitted in response to the staffs 2012 §50.54(f) information request. For those sites where there was an exceedance of some CDB, those FHRRs included an analysis of associated flooding effects such as water-borne debris loads for each flood-causing mechanism found to exceed the CDB. In its reviews of the FHRRs, the staff found that licensees were well-acquainted with the analysis methods for performing those calculations and as a result met the staffs expectations for subsequent use by licensees when implementing some site-specific Mitigating Strategies. Therefore, the staff does not believe any modification of Appendix J at Section J-5 is necessary.

Nuclear Energy Institute 20 J-6. Effects of Sediment Erosion or Deposition (Page J-3)

The USACE CEM is referenced for consideration of erosional/depositional effects for structures (e.g.,

embankment walls, roads).

Outside of standard scour methodology, does the staff expect any other analysis to assess the adequacy of plant design and protection measures with respect to Elaborate on erosional/depositional effects analyses expected by the staff.

The staff agrees that some additional detail is warranted and has added the following sentence to the last sentence in Section J-6:

In connection with any analysis of sediment erosion and deposition, it may be necessary to perform numerical modeling or even possibly scaled wave tank modeling to better understand these effects at a proposed power plant site and design. These modeling efforts should be informed by a review of the pertinent literature.

26 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition erosional/ depositional effects?

Nuclear Energy Institute 21 J-6. Effects of Sediment Erosion or Deposition (Page J-3)

Appendix G (Flooding Caused by Tsunami) references Appendix J for related effects analyses. Is the USACE CEM an adequate framework for assessing sedimentation/

depositional effects from tsunami events? Tsunami waves are long period and behave differently than swell/storm wind waves to which the scour and other similar equations presented in the USACE CEM have been calibrated. Have those equations been calibrated for tsunami-like events?

Comment on the applicability of the USACE CEM scour and erosional/ depositional effect methods are for tsunami waves.

The staff recognizes the need to improve the guidance on this topic and has amended the last sentence in Section G-4 as follows:

The application should include consideration of associated flooding effects consistent with the guidance in Appendix J and in ASCE/SEI 7-16 (ASCE/SEI 2016). Also see Robertson (2020).

The staff have included thefollowing two new references Robertson, I.N. "Tsunami Loads and Effects: Guide to the Tsunami Design Provisions of ASCE 7-16. Reston, VA, American Society of Civil Engineers, 2020.

American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI), Minimum Design Loads for Buildings and other Structures. Reston, VA, ASCE/SEI 7-22, 2022.

Nuclear Energy Institute 22 J-6. Effects of Sediment Erosion or Deposition (Page J-3)

The guidance is unclear in how this evaluation is expected to be performed, and in particular its role in a hazard assessment as opposed to fragility assessment. In evaluating the scenario impacts of erosion for assessments of SSCs, is the concern about the short-term impact of this mechanism on SSCs (single storm/weather/flood impacts)

Expand upon what specific issues are expected to be addressed within the deposition/erosion challenge within the assessment of SSC fragility. Consider including an example in the RG as to what might be reasonable to include in the treatment of these phenomena.

This comment raises a theme previously discussed in NEI Comment

1. 5.

The staff agrees with the point raised in this comment that there is a need to examine what effects/impacts individual or repeated weather event or flooding events might have on the performance of SSCs important to safety. Nominally, GDC #4 (Environmental and dynamic effects design bases) addresses the concern raised by this comment.

The staff believes, however, that most if not all power stations have monitoring and preventive maintenance programs intended to prevent debris and sedimentation impacts in their cooling water and intake systems as well as any other SSCs important to safety that are deemed to be vulnerable to these effects.

27 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition or the combined impact of the erosion/deposition challenge over a number of months/years due to repeated events. Earlier versions of this RG didnt address this issue.

Nuclear Energy Institute 23 Appendix K-1 There is paragraph in this section that states it recognizes some advanced reactor designs may use materials that have potential for adverse reactions. It specifically calls out sodium fast reactors for their use of sodium as a coolant.

However, every design will have unique elements that may or may not contribute to flooding susceptibility.

Every design will have unique elements that may increase or decrease susceptibility to flooding, be it composition of coolant, reliance on electrical power, etc. It is unclear why sodium is explicitly stated here, rather than a broad recognition that designs should consider susceptibilities to flooding holistically.

The staff agrees that every advanced reactor design might have unique elements that may increase or decrease susceptibility to flooding.

Sodium fast reactors are just one of several advanced reactor designs under consideration by vendors. The reference to sodium fast reactors in the draft regulatory guide (at page K-1) was intended for illustrative purposes only, noting the importance of protecting SSCs important to safety for any advanced reactor system from the potentially deleterious effects of external floods.

Also see the staff response to the comment received from Kairos Power LLC.

Nuclear Energy Institute 24 Appendix K-2, Figure K-1 The flow chart starts with Describe Site Characteristics. Some advanced nuclear power plant designs can demonstrate that there will be no radiological release no matter the size of the flood.

Thus, no site flood characteristics are necessary to demonstrate safety.

Recommend initially identifying if flooding is a safety challenge to the plant design.

The staff agrees, in part, with this comment. In response to this comment, the staff has added the following sentence to the end of the second paragraph in Section K-1:

Such a demonstration can be achieved through a probabilistic risk assessment of comparable analysis.

However, the staff believes that prospective applicants will still need to address GDC 2 with respect to the potential for external floods. As noted in response to NEI Comment #1, the applicant should present some argument in its license application that the effects of external floods are inconsequential to the safe operation of the nuclear reactor.

28 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition ANS Joint Committee on Nuclear Risk Management General Comment NRC recognizes the value of PFHAs in estimating external flood hazards. PFHAs are widely used in many non-nuclear applications, some of which result in hazard estimates down to low exceedance frequencies.

Furthermore, PFHAs and methods based on PFHA concepts have been reviewed when evaluating post-Fukushima 50.54(f) responses. Nonetheless, the current language may serve to discourage the use of probabilistic methods by creating regulatory uncertainty for future applicants regarding the viability and acceptability of PFHAs to NRC staff.

Part 8 of the forthcoming ASME/ANS Level 1/

LERF PRA Standard provides a series of high-level and supporting requirements to ensure PFHAs address a range of important technical issues while recognizing the current limitations of knowledge/practice. The Standard's PFHA requirements build off experience in assessing other hazards for nuclear power facilities while making adaptations to reflect the context and characteristics of flooding hazards. As such, the associated probabilistic risk assessment (PRA) requirements can serve as a basis for a framework for evaluating PFHAs. The use of such a framework will help to ensure a PFHA has the appropriate attributes for use in a design basis Staff agrees with the recommendation to reference (but not endorse) the ASME/ANS guidance in question and has modified the DG by adding the following text to Regulatory Position 2a Deterministic versus Probabilistic Analysis Approaches.

ASME/ANS RA-S-1.1-2024, Standard for Enhanced Nuclear Risk Management (Ref 35), recently published by the ASME/ANS Joint Committee on Nuclear Risk Management, focuses on Level 1/Large Early Release Frequency Probabilistic Risk Assessment (PRA) specifically tailored for NPP operations. The standard is primarily designed for current operating LWR power plants. ASME/ANS RA-S-1.1-2024 requires that a PFHA be performed to support an external flooding PRA and, like ANSI/ANS 2.8 2019, provides a series of high-level and supporting requirements to ensure PFHAs address a range of technical issues. Like ANSI/ANS 2.8 2019, these requirements can serve as a basis for a framework for evaluating PFHAs, but do not provide detailed review guidance on PFHA methods.

The NRC staff recognizes the value and potential application of PFHA, especially as an input to PRA. NRC staff is currently developing guidance for external flooding PFHA.

29 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition assessment. It is recommended that in the forthcoming update to RG 1.59, the NRC staff reference or endorse the forthcoming PRA Standard as providing relevant requirements for assessing flood hazards probabilistically. It is believed that the use of this framework will provide the NRC and industry with a basis for ensuring quality, technical adequacy, and consistency within the PFHAs and predictability in the licensing process while continuing to allow flexibility for review and approval of the PFHA's implementation details.

ANS Joint Committee on Nuclear Risk Management Additional notes (noted during comment ballot):

It is recommended that any differences between DG-1290 and NEI 18-04 (endorsed by RG 1.233) regarding design basis frequencies be addressed in Appendix K.

In the staffs response to the public comment from Kairos Power LLC it was stated that until the performance envelope for advanced reactors as a class is better understood, the staff added Appendix K, Considerations for Applying Guidance to Advance Reactors and Small Modular Reactors. Appendix K does not specify a required annual exceedance probability (AEP) or design basis frequency for external flooding. Appendix K notes the expectation that license applications for advanced reactors will include a PRA or comparable analysis that describes and analyzes design basis external events.

30 DG-1290 Section Comment Recommendation NRC Staff Response/Disposition It is anticipated that, given publication timelines, the forthcoming revision of ASME/ANS Level 1/LERF PRA Standard

[2] will be published prior to the publication of the updated RG 1.59.