ML24262A011

Comment (2) of Frances A. Pimentel on Design-Basis Floods for Nuclear Power Plants and Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents

ML24262A011
Person / Time
Site:	Nuclear Energy Institute
Issue date:	09/13/2024
From:	Pimentel F Nuclear Energy Institute
To:	Office of Administration
References
NRC-2024-0110, 89FR60580 00002
Download: ML24262A011 (1)

Text

PUBLIC SUBMISSION As of: 9/18/24, 7:16 AM Received: September 13, 2024 Status: Pending_Post Tracking No. m11-6l7l-9dvv Comments Due: September 13, 2024 Submission Type: Web Docket: NRC-2024-0110 Design-Basis Floods for Nuclear Power Plants and Guidance for Assessment of Flooding Hazards due to Water Control Structure Failures and Incidents Comment On: NRC-2024-0110-0001 Draft Regulatory Guides: Design-Basis Floods for Nuclear Power Plants and Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Document: NRC-2024-0110-DRAFT-0005 Comment on FR Doc # 2024-15479 Submitter Information Email:smp@nei.org Organization:Nuclear Energy Institute General Comment See attached file(s)

Attachments 09-13-24_NEI Comments on DG 1290 9/18/24, 7:18 AM blob:https://www.fdms.gov/aaf40300-3899-4427-bc98-942da8b02e79 blob:https://www.fdms.gov/aaf40300-3899-4427-bc98-942da8b02e79 1/1 SUNSI Review Complete Template=ADM-013 E-RIDS=ADM-03 ADD: Bridget Curran, Edward O'Donnell, Mary Neely Comment (2)

Publication Date:7/26/2024 Citation: 89 FR 60580

Frances Pimentel Senior Project Manager, Engineering and Risk Phone: 302.584.8114 Email: fap@nei.org September 13, 2024 Office of Administration Mail Stop: TWFN-7-A60M U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

Subject:

NEI Comments on DG-1290, Revision 1, Design-Basis Floods for Nuclear Power Plants, and DG-1417, Guidance for Assessment of Flooding Hazards due to Water Control Structure Failures and Incidents, Docket ID NRC-2024-0110 Project Number: 689 Submitted via regulations.gov

Dear Program Management,

Announcements, and Editing Staff, The Nuclear Energy Institute (NEI)1, on behalf of our members, appreciates the opportunity to provide comments on the subject Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants and DG-1417, Guidance for Assessment of Flooding Hazards due to Water Control Structure Failures and Incidents. We appreciate the opportunity to provide the attached comments on both draft guides.

The regulatory analysis associated with the revision to DG-1290 concludes that this update should enhance nuclear power plant safety by providing up-to-date guidance and information on determining the effects of severe flooding on nuclear power plants. While the proposed revision does reference updated information, the guidance on design basis flood estimation approaches lacks specific application details that would enhance the effectiveness of the Regulatory Guide (RG) as a useful tool to both end users and the NRC. Additionally, the industry is concerned that the guidance does not provide enough details for a reader to understand which concurrent flooding hazards should be considered by an applicant.

1 The Nuclear Energy Institute (NEI) is responsible for establishing unified policy on behalf of its members relating to matters affecting the nuclear energy industry, including the regulatory aspects of generic operational and technical issues. NEIs members include entities licensed to operate commercial nuclear power plants in the United States, nuclear plant designers, major architect and engineering firms, fuel cycle facilities, nuclear materials licensees, and other organizations involved in the nuclear energy industry.

Office of Administration September 13, 2024 Page 2 Nuclear Energy Institute We believe that DG-1290 should acknowledge that external floods have notable differences from other external hazards, including having warning time (hours to days) to prepare, only affecting SSCs below the level of the flood, and that strategies exist to mitigate and recover from flood events (barriers, portable pumps, etc.). A licensee or applicant should be able to credit mitigation strategies in their flooding analyses.

DG-1290, Section C.2.a acknowledges that methods and data acceptable to the NRC to provide an annual excedent probability (AEP) of 1x10-6 as specified in DG-1290 and DG-1417 do not exist. It is unclear how applicants could use qualitative methods to achieve this level of precision, and quantitative methods endorsed as acceptable to the NRC do not exist at this time. The industry strongly believes that if there are no acceptable methods for determining a flood event or dam failure with an AEP of 1x10-6, guidance should be provided for an acceptable approach using a more realistic AEP, such as 1x10-4.

Furthermore, the industry believes the expectations put forth in DG-1290 and DG-1417 are inconsistent with the NRC Commissions PRA Policy Statement which statesthat PRA should be implemented in a consistent and predictable manner which promotes regulatory stability and efficiency. This is not achieved if every application is reviewed and considered on a case-by-case basis without clear and consistent guidance.

Additionally, several sections of DG-1290 include the statement, all methods and assumptions should be clearly conservative. Requiring that all individual inputs/methods/assumptions be conservative goes beyond what should be needed to provide reasonable assurance of adequate protection and can result in unnecessary, compounded conservatisms.

The regulatory analysis associated with DG-1417 concludes that the issuance of a new RG is warranted to reflect the availability of new information and improved analysis methodologies used during the post-Fukushima flooding re-evaluations and would enhance plant safety by providing up-to-date guidance and information on the determination of design basis floods. However, much of the work and analysis described in this DG are governed by federal and state dam safety regulators and are performed by the dam owners. Typically, unless the utility is also the dam owner, licensees do not have the information to implement this DG, nor can licensees impose requirements on the dam and levee owners.

Therefore, the industry questions the use of a RG as the repository for the information contained in the Interim Staff Guidance (ISG). The industry believes that a NUREG-KM would be a better approach to incorporate JLD-ISG-2013-01 into the NRCs regulatory guidance framework. NUREG-KMs have recently been used to capture other post-Fukushima lessons learned for both flooding and seismic issues, for example in NUREG-KM-0015, Considerations for Estimating Site-Specific Probable Maximum Precipitation at Nuclear Power Plants in the United States of America, and NUREG-KM-0017, Seismic Hazard Evaluations for U.S. Nuclear Power Plants: Near-Term Task Force Recommendation 2.1 Results."

If the staff decide not to pursue incorporation of the ISG information into a NUREG-KM, then it is recommended that the guidance in the proposed DG be constrained to that which can be implemented by

Office of Administration September 13, 2024 Page 3 Nuclear Energy Institute a licensee. For example, there are two levels of screening included in DG-1417 which can be directly used by licensees (see Attachment 2, Comment #2):

1) Screening of dams to identify non-critical dams which do not require further analysis

2) Seismic screening methods which provide the options to conclude non-failure or assume failure in lieu of detailed site-specific analysis for dams that remain as potentially critical The seismic screening process listed above could involve an applicant evaluating a ground motion at an appropriate AEP, provided a sufficient seismic margin or factor of safety is demonstrated to show that dam failure is unlikely to occur. However, what is required to document sufficient seismic margin or factor of safety is not defined in DG-1417. In addition, the DG should also consider situations where the applicant does not own the dam or have access to the seismic analysis of the dam.

Attachments 1 and 2 provide additional details on the industry recommendations for these guidance documents. We appreciate the NRCs effort in developing this draft guidance and encourage consideration of all stakeholder comments prior to finalizing the guidance contained in DG-1290 and DG-1417. We trust that you will find these comments useful and informative as you finalize the draft.

Please contact Kelli Voelsing at khv@nei.org or me at fap@nei.org with any questions or comments about the content of this letter or the attached comments.

Sincerely, Frances A. Pimentel, Senior Project Manager Engineering and Risk - Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1 - Consolidated Comments on Draft Regulatory Guide DG-1417 c:

Joseph Kanney, RES/DRA/FXHAB, NRC Edward ODonnell, RES/DE/RGPMB, NRC Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-1 Number Section/

Page Comment/Basis Recommendation

1.

General The words from Section C.2.a are important in terms of the ability to implement the guidance in this DG:

Although the NRC does not currently provide specific guidance on PFHA techniques, the staff uses an average annual probability of exceedance of less than 1x10-6 as a metric to evaluate the reasonableness of combined-event flooding scenarios; this screening value may be estimated using qualitative or semiquantitative methods. The staff considers this metric a reasonable criterion to apply to design-basis flood estimates using frequency-based screening, assuming reasonable confidence limits can be established.

The staff will review probabilistic characterization of extreme floods by various mechanisms or combinations of mechanisms on a case-by-case basis.

This text acknowledges that methods and data to achieve the 1x10-6 annual exceedance probability (AEP) levels specified in DG-1290 and DG-1417 do not exist. It is unclear how applicants could use qualitative methods to achieve this level of precision, and it is clear that quantitative methods endorsed as acceptable to the NRC do not exist at this time.

The expectation put forth in this text and throughout DG-1290 is inconsistent with the NRC Commissions PRA Policy Statement (August 1995) one objective of which was that PRA should be implemented in a consistent and predictable manner which promotes regulatory stability and efficiency - which is not achieved if every application is reviewed and considered on a case-by-case basis where expectations and guidance are unclear. Depending on pre-application meetings and case-by-case reviews will lead to inefficiency and regulatory instability.

This is further inconsistent with the PRA Policy Statement which also states that the use of PRA technology should be increased in all regulatory matters to the extent supported by the state of the art in PRA methods and data - where it is clear based on the words in the DG, that the current PRA methods and data as described in the RG are not sufficient to demonstrate or meet the 1x10-6 criteria that the DG establishes as guidance.

The DGs should be revised to provide guidance consistent with the level of safety that provides reasonable assurance and in consideration of the state-of-practice methods and data available for practical implementation, such that regulatory efficiency and stability are achieved.

Because there are no acceptable methods for determining a 1x10-6 AEP flood hazard, guidance should be provided for an acceptable approach using a less stringent threshold (such as 1x10-4) with an acceptable mitigation strategy for greater floods, if warranted.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-2

2.

General DG-1290 refers to withdrawn and little used standards 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. Reference to withdrawn standards that are not maintained and were not widely used is not recommended. It is also noted that parts of these withdrawn standards are inconsistent with other guidance in these two DGs.

The withdrawn standards are cited as a basis for the 1x10-6 metric.

However, ANSI/ANS 2.8-1992 states that, technology is not available to assess precise numeric probabilities of all separate extreme events and their dependencies, and acknowledges that the combinations presented in the document are conservative postulations of probabilities for separate events and like postulations of their dependency which are assumed to meet the combined probability target of 1x10-6. Additionally, some assumptions used in the probability assessments in Appendix B of ANSI/ANS 2.8-1992 are not entirely correct in their treatment of dependencies.

Because these standards are not maintained, and because parts of these documents are inconsistent with other parts of DG-1290, these documents should not be referenced and cited as a basis in DG-1290.

The staff should include any information or methods from these withdrawn standards in this document with the appropriate technical basis and/or cite a different source that is not withdrawn or no longer current.

3.

General There is inconsistent use of acronyms (both on their own and after the first explanation) throughout the document.

Some examples of acronyms used inconsistently are:

U.S. NRC, NRC, US Nuclear Regulatory Commission Agencywide Documents Access and Management System, ADAMS nuclear power plant, NPP American Nuclear Standards Institute, ANSI American Nuclear Society, ANS American Society of Civil Engineers, ASCE U.S. Army Corps of Engineers, USACE Federal Emergency Management Agency, FEMA National Oceanic and Atmospheric Administration, NOAA light water reactors, LWRs probable max flood, PMF probable max precipitation, PMP structures, systems, and components, SSCs Hierarchical Hazard Analysis, HHA Appendix or appendix Section or section A technical edit of the document should be performed prior to publication to ensure that each acronym is defined and used consistently throughout the document.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-3

4.

General DG-1290 uses the term state-of-art (or similar phrasing) in multiple places throughout the document. A reading of these sections suggests that the term is generally being used to confer what is normally referred to as state-of-practice methods or tools, where "state-of-art" generally connotes a technology or process that is cutting-edge, but perhaps difficult to apply or accessible only to experts, and "state-of-practice" suggests that which is commonly done (practiced) in a particular business or profession.

The term state-of-practice should be used in lieu of state-of-art throughout the document when referring to methods that are commonly used.

5.

Applicable Regulations, Page 2 Reference is made to GDC 2 as the source of regulatory requirement for design bases for protection against natural phenomena. This is applicable to LWR technology, but not necessarily for non-LWRs. While PDC development is agnostically applicable to all technologies for license/permit applications, the GDCs themselves have technology-specific applicability.

Recommend adding a reference to RG 1.232 as an example of one acceptable means of adopting the requirement for design bases for protection against natural phenomena for non-LWR technology.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-4

6.

Related

Guidance, Page 2 DG-1290 states that SSCs that should be designed to withstand the effects of a design basis flood are those identified in RG 1.29 (Seismic Design Criteria). To the reader, it may be unclear why a RG in Seismic Design which does not contain the word flooding or flood is cited as the source of SSCs that must be designed to withstand the effects of a design basis flood. A footnote explaining the connection to RG 1.29 was included in RG 1.59, Revision 2, but does not appear in DG-1290. This footnote should be added back to DG-1290.

Include the following footnote citation to RG 1.29 which appeared in Rev 2 of RG 1.59 but is missing from DG-1290:

Footnote to RG 1.29: As described in Regulatory Guide 1.102, Regulatory Guide 1.29, "Seismic Design Classification," identifies structures, systems, and components of light-water-cooled nuclear power plants that should be designed to withstand the effects of the Safe Shutdown Earthquake and remain functional. These structures, systems, and components are those necessary to ensure (1) the integrity of the reactor coolant pressure boundary, (2) the capability to shut down the reactor and maintain it in a safe shutdown condition, or (3) the capability to prevent or mitigate the consequences of accidents that could result in potential offsite exposures comparable to the guideline exposures of 10 CFR Part 100. These same structures, systems, and components should also be designed to withstand conditions resulting from the design basis flood and retain capability for cold shutdown and maintenance thereof of other types of nuclear power plants. It is expected that safety-related structures, systems, and components of other types of nuclear power plants will be identified in future regulatory guides. In the interim, Regulatory Guide 1.29 should be used as guidance when identifying safety-related structures, systems, and components of other types of nuclear power plants.

Also, a reference should be made that for non-LWR technologies that do not utilize RG 1.29, an alternative approach to safety significance and classification could be utilized (e.g., NEI 18-04).

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-5

7.

Background, Page 6 In the following sentence:

The issuance of RG 1.59, Revision 3 and RG 1.256, Revision 0, coincides with the closing of JLD-ISG-2012-06 and JLD-ISG-2013-01, respectively.

Even though these ISGs will be withdrawn it would also be useful to note here that the analyses performed and evaluated using these ISGs post-Fukushima remain available in ADAMS and may be a valuable source of information for co-located sites or can be used as examples.

(Editorial) Also, change wording from coincides with to superseded by:

Consider revising this text:

The issuance of RG 1.59, Revision 3 and RG 1.256, Revision 0, coincides with the closing of JLD-ISG-2012-06 and JLD-ISG-2013-01, respectively.

To read:

JLD-ISG-2016-06 and JLD-ISG-2013-01 are superseded by the issuance of RG 1.59, Revision 3 and RG 1.256, Revision 0, respectively. While these ISG documents are superseded by the issuance of this updated RG, the flood hazard reevaluations performed by existing NPP owner/operators following the 2011 Fukushima event and evaluated using these ISG documents may be valuable sources of information for co-located sites or to be used as examples in performing the analyses described in this RG. Those flood hazard reevaluations are publicly available in ADAMS.

8.

Background, Page 7 The following sentence, These regulations further require that the most severe flood conditions at the site not compromise the ability of the power plants SSCs to protect against natural phenomena, including external flooding effects, or to perform their credited safety functions, can be grammatically confusing and could be interpreted in many ways.

One can read this sentence as requiring an applicant to determine the most severe flood and then requiring the addition of another postulated event or external hazard to be protected against on top of the most severe flood.

Consider revising this text:

These regulations further require that the most severe flood conditions at the site not compromise the ability of the power plants SSCs to protect against natural phenomena, including external flooding effects, or to perform their credited safety functions.

To read:

These regulations further require the site to demonstrate the ability of safety-related SSCs to perform their credited safety functions in the event of the most severe flood conditions.

9.

Analysis Approaches, Page 7 The addition of discussion acknowledging the possibility and technical validity of a probabilistic approach to the determination of the design basis flood hazard is a welcomed and valued addition to DG-1290.

However, based on the current wording the reader may be left with the impression that there is little work or guidance concerning technical approaches for probabilistic flood hazard assessment. It may be possible to make minor changes in sentences like the following to convey the significant amount of work done by the NRC and other organizations in this area:

Consequently, no widely accepted framework or toolset currently available for the probabilistic assessment of all potential flood hazards at a site, nor are there any standards available that are acceptable to the NRC staff for the review of this type of analysis.

Consider revising this text:

Consequently, no widely accepted framework or toolset currently available for the probabilistic assessment of all potential flood hazards at a site, nor are there any standards available that are acceptable to the NRC staff for the review of this type of analysis.

To read:

Although there is considerable work in this area and the state-of-practice continues to develop, there is currently no comprehensive accepted framework or toolset for the probabilistic assessment of all potential flood hazards at a site, nor are there any standards available that are acceptable to the NRC staff for the review of this type of analysis.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-6

10.

Flooding Caused by LIP, Page 9 The following sentence:

The simplest and most conservative LIP analysis postulates both that the sites (passive) drainage network is nonfunctional and that designated drainage components of the sites storm water management system are blocked and nonfunctional.

while true, this sentence does not acknowledge that other more complex or more realistic models and scenarios may be used.

Consider revising this text:

The simplest and most conservative LIP analysis postulates both that the sites (passive) drainage network is nonfunctional and that designated drainage components of the sites storm water management system are blocked and nonfunctional.

To read:

The simplest and most conservative LIP analysis postulates both that the sites (passive) drainage network is nonfunctional and that designated drainage components of the sites storm water management system are blocked and nonfunctional. The applicant may utilize more realistic or more detailed analyses in evaluating the proposed site or design for LIP. If so, the applicant should justify the use and applicability of the selected models and inputs for the proposed site.

11.

Flooding Caused by Dam Failure, Page 9 The first paragraph in the section titled Flooding Caused by Dam Failures would benefit from a sentence acknowledging the challenge associated with obtaining the relevant information to assess the impacts of dam failures, given the diverse ownership and oversight associated with the 91,000 dams mentioned in this paragraph.

Consider adding the following sentences to the first paragraph under Flooding Caused by Dam Failures:

The U.S. Army Corps of Engineers estimates that there are more than 91,000 dams in the United States, a portion of which share watersheds with locations of operating NPPs (Ref. 23). Due to the diverse nature of dam ownership and oversight in the US, obtaining information on the dams, reservoirs, and levees within the watershed occupied by the NPP can be challenging. Additional guidance on assessing flooding due to dam failures and incidents and obtaining information on the dams, reservoirs, and levees within the watershed occupied by the power plant project is provided in DG-1417/RG 1.256. For convenience, this RG uses the term dam to include all water storage or water control structures whose failure may lead to external flooding.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-7

12.

Flood Protection, Page 12 The intent of the paragraph, Previous revisions of this guide addressed certain aspects of flood protection, such as conditions under which alternatives to providing hardened protection would be acceptable. The NRC staff has removed such topics from this revision because it is more logical to address them in RG 1.102, which presents guidance on design and maintenance of flood protection measures. However, if it is anticipated that safety-related SSCs will need some degree of flood protection, then it will be important to consider the rate of rise of the floodwaters and the duration of flood conditions in the design and specification of flood protection methods and procedures (e.g., to establish lead times and coping times).

is understood, in that DG-1290 is intended to address the development of the design basis flood hazard, and that Flood Protection features are addressed separately in RG 1.102.

However, the current wording in RG 1.102 refers to specific previsions in RG 1.59, Revision, 2 which have been deleted in DG-1290s proposed wording. To retain continuity between the two documents as intended, reference to the acceptability of non-hardened flood protection features should be added back to DG-1290.

Consider revising this text:

Previous revisions of this guide addressed certain aspects of flood protection, such as conditions under which alternatives to providing hardened protection would be acceptable. The NRC staff has removed such topics from this revision because it is more logical to address them in RG 1.102, which presents guidance on design and maintenance of flood protection measures. However, if it is anticipated that safety-related SSCs will need some degree of flood protection, then it will be important to consider the rate of rise of the floodwaters and the duration of flood conditions in the design and specification of flood protection methods and procedures (e.g., to establish lead times and coping times).

To read:

Credit for non-hardened* flood protection features may be acceptable as described in RG 1.102.

However, if it is anticipated that safety-related SSCs will need some degree of flood protection, then it will be important to consider the rate of rise of the floodwaters and the duration of flood conditions in the design and specification of flood protection methods and procedures (e.g., to establish lead times and coping times).

With added footnote:

Footnote: Hardened protection means structural provisions incorporated in the plant design that will protect safety-related SSCs from the static and dynamic effects of floods. In addition, each component of the protection must be passive and in place, as it is to be used for flood protection, during normal plant operation.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-8

13.

Deterministic versus Probabilistic Analysis Approaches C.2.a Page 15 (Editorial) Consider revising the sentences that contain the phrases:

a basis for a framework for evaluating PFHAs.

and the lack of widely accepted probabilistic methods for assessing the full suite of potential external flood hazards Consider revising this text:

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 technical challenges of probabilistic analysis for extreme floods and the lack widely accepted probabilistic methods for assessing the full suite of potential external flood hazards at a project site are the main reasons for the deterministic focus of this RG.

To read:

Like ANSI/ANS 2.8 2019, these requirements can serve as a framework for evaluating PFHAs, but do not provide detailed review guidance on PFHA methods.

The technical challenges of probabilistic analysis for extreme floods and the lack of comprehensive and endorsed probabilistic methods for assessing the full suite of potential external flood hazards at a project site are the main reasons for the deterministic focus of this RG.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-9

14.

Deterministic versus Probabilistic Analysis Approaches C.2.a Page 15 Regarding the following text in section C.2.a:

Although the NRC does not currently provide specific guidance on PFHA techniques, the staff uses an average annual probability of exceedance of less than 1x10-6 as a metric to evaluate the reasonableness of combined-event flooding scenarios; this screening value may be estimated using qualitative or semiquantitative methods. The staff considers this metric a reasonable criterion to apply to design-basis flood estimates using frequency-based screening, assuming reasonable confidence limits can be established.

The staff will review probabilistic characterization of extreme floods by various mechanisms or combinations of mechanisms on a case-by-case basis.

The following are not clear:

1) How can an applicant establish reasonable confidence limits at the level of 1x10-6, given the available data and the multiple orders of magnitude of extrapolation required?

2) How can qualitative methods or semi-quantitative methods be used to achieve such levels of precision?

3) What specific methods and assumptions are technically justified and acceptable to the NRC in terms of achieving the 1x10-6 AEP metric for determining which combined flooding event scenarios need to be considered?

4) What alternatives are acceptable in the event that data and methods are not available to support an AEP of 1x10-6 with reasonable confidence limits, for various flood mechanisms of combinations thereof?

The DG expresses an expectation of the use of probabilistic, frequency-based methods, at least in the context of determination of which combination of flooding hazard mechanisms need to be considered, while at the same time acknowledging that methods and data acceptable to the NRC for this analysis do not exist. There is concern that the use of 1x10-6 as a metric implies a level of precision that is not justified and should not be necessary to provide reasonable assurance of protection of the health and safety of the public in the event of an external flood. Deferring this discussion to case-by-case reviews and/or pre-application negotiations will lead to regulatory inefficiency and instability.

The DG should be revised to provide guidance consistent with the level of safety that provides reasonable assurance and in consideration of the state-of-practice methods and data available for practical implementation, such that regulatory efficiency and stability are achieved.

Because there are no acceptable methods for determining a 1x10-6 AEP flood hazard, guidance should be provided for an acceptable approach using a less stringent threshold (such as 1x10-4) with an acceptable mitigation strategy for greater floods, if warranted.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-10

15.

Deterministic versus Probabilistic Analysis Approaches C.2.a Page 15 Given that uncertainty exists whether a deterministic or probabilistic approach is used to determine the design basis flood, consider revising the following sentences:

Lastly, because measured data may be variable and sparse, and because modeling of the physical processes underlying flooding involves inherent uncertainty, the results of any PFHA are uncertain.

Therefore, in conducting a PFHA it is important to quantify the sensitivity of the results to, and the uncertainty associated with, the values of input parameters.

Consider revising this text:

Lastly, because measured data may be variable and sparse, and because modeling of the physical processes underlying flooding involves inherent uncertainty, the results of any PFHA are uncertain.

Therefore, in conducting a PFHA it is important to quantify the sensitivity of the results to, and the uncertainty associated with, the values of input parameters.

To read:

Lastly, because measured data may be variable and sparse, and because modeling of the physical processes underlying flooding involves inherent uncertainty, the results of any flood hazard estimate are uncertain. Therefore, it is important to quantify the sensitivity of the results to, and the uncertainty associated with, the values of input parameters.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-11

16.

Extreme Storm Data Compilations C.2.a Page 15 It is acknowledged that many published data sets used to evaluate historical meteorological/storm data may not have been updated in several decades. It is further agreed that an applicant needs to perform due diligence in assuring that the data in these sources remains relevant and applicable for any site being evaluated.

It is suggested that this text can be enhanced to recognize ongoing processes (even after the issuance of a license) which are in place to ensure that new information that may affect the data and assumptions used in the design basis of NPPs is considered appropriately.

Consider revising this text:

However, although these reports are valuable references, larger or more intense storms may have occurred in certain regions since their publication (many of the reports are over 30 years old and have not been updated since they were issued). Therefore, before using older reports to determine design-basis flood estimates for NPP sites, applicants should confirm that more recent storms or flood events occurring in the region of interest do not invalidate the conceptualizations, assumptions, parameterizations, or estimates in the reports. If the continuing utility of these older design aids is not confirmed, the more recent storm or flooding information should be used (along with updated methods, where appropriate).

Appendices C, D, E, and F to this RG discuss known issues with specific design aids and ways to incorporate improved data and methods.

To read:

However, although these reports are valuable references, larger or more intense storms may have occurred in certain regions since their publication (many of the reports are over 30 years old and have not been updated since they were issued). Therefore, before using older reports to determine design-basis flood estimates for NPP sites, applicants should confirm that the data remains relevant and applicable and that more recent storms or flood events occurring in the region of interest do not invalidate the conceptualizations, assumptions, parameterizations, or estimates in the reports. If the continuing utility of these older design aids is not confirmed, the more recent storm or flooding information should be used (along with updated methods, where appropriate).

Appendices C, D, E, and F to this RG discuss known issues with specific design aids and ways to incorporate improved data and methods. The NRCs Process for the Ongoing Assessment of Natural Hazard Information (POANHI)* project and the analogous work undertaken by operating NPPs to address Recommendation 2 from INPO IER L1-2013-10*, Nuclear Accident at the Fukushima Daiichi Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-12 Nuclear Power Station are existing processes which consistently screen for and evaluate new data, methods and science that should be evaluated to consider potential impact on plant safety. Such processes are leveraged to enhance the usage of historical data and to ensure that new natural hazard information is routinely aggregated and assessed for potential significance to existing site safety. These processes ensure that when historical data is used in the design and licensing of NPPs, that such information will continue to be relevant and applicable, or that appropriate evaluation and mitigative actions will be put in place to address the impact of any new information on plant safety.

• References can be added to:

Process for the Ongoing Assessment of Natural Hazard Information (POANHI) --

https://www.nrc.gov/reactors/operating/ops-experience/poanhi.html or LIC-208, NRR Office Instruction Process for the Ongoing Assessment of Natural Hazards Information and INPO EVENT REPORT L1-13-10, NUCLEAR ACCIDENT AT THE FUKUSHIMA DAIICHI NUCLEAR POWER STATION, DATED MARCH 28, 2013 (available to INPO Members only)

17.

Nonstationary Effects, 2.c Page 16 It is unclear why only the most conservative sea level rise (SLR) scenario is acceptable as stated in the following text:

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.

Consider adding detail to clarify why only the most conservative SLR scenario is acceptable or add text acknowledging that less conservative scenarios may be acceptable when appropriate justification of their applicability to the site is provided.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-13

18.

Nonstationary Effects C.2.c Page 16 The following sentence could be enhanced by adding discussion of ongoing processes in place to assure that new information (like climate change insights) is assessed for impact to operating NPPs, even when there is too much uncertainty to provide reasonable accurate projections at the time the plant is licensed:

Because future natural changes in precipitation amounts and storm frequencies are very uncertain, this RG gives no specific guidance on these topics.

Consider revising this text:

Because future natural changes in precipitation amounts and storm frequencies are very uncertain, this RG gives no specific guidance on these topics.

To read:

Because future natural changes in precipitation amounts and storm frequencies are very uncertain, this RG gives no specific guidance on these topics. As climate projections, depending on the variables of interest (e.g., storms, precipitation, temperature, SLR), can still have a high degree of uncertainty, given the current state-of-practice, it is appropriate for applicants to recognize the role of projects like the NRCs POANHI* process and the industry response to Recommendation 2 to from INPO IER L1-2013-10*, in monitoring and evaluating new data, information and methods during the life of an operating NPP to address potential impacts to plant safety.

• References can be added to:

Process for the Ongoing Assessment of Natural Hazard Information (POANHI) --

https://www.nrc.gov/reactors/operating/ops-experience/poanhi.html or LIC-208, NRR Office Instruction Process for the Ongoing Assessment of Natural Hazards Information and INPO EVENT REPORT L1-13-10, NUCLEAR ACCIDENT AT THE FUKUSHIMA DAIICHI NUCLEAR POWER STATION, DATED MARCH 28, 2013 (available to INPO Members only)

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-14

19.

Hydrological Setting C.3 Page 16 The following sentence could involve an extremely large quantity of information:

Any existing or proposed water control structures (e.g., dams, levees, diversions, channels), both upstream and downstream of the plant site, should be described.

It should be revised to refer to DG-1417 for the scope of information that needs to be provided on water control structures with the potential to impact the site.

Consider revising this text:

Any existing or proposed water control structures (e.g., dams, levees, diversions, channels), both upstream and downstream of the plant site, should be described.

To read:

Any existing or proposed water control structures (e.g., dams, levees, diversions, channels), both upstream and downstream of the plant site, with the ability to impact the site, as described in DG-1417 Guidance for the Assessment of Flooding Hazards due to Water Control Structure Failures and Incidents should be described.

20.

Hydrologic Setting C.3 Page 17 The following sentence should be enhanced to provide more detail about when the expectation to provide paleo flood data is applicable:

When available relevant information from paleo flood studies should be included.

Consider revising this text:

When available relevant information from paleo flood studies should be included.

To read:

When data is available from prior paleo flood studies which may have been performed for other reasons or by external parties, it should be included.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-15

21.

Stream Flooding Caused by Precipitation Events C.3.b Page 18 While it is acknowledged that an analysis to be used for design basis should be conservative, explicit text requiring that ALL methods/ inputs/

assumptions be conservative goes beyond what should be needed to provide reasonable assurance of adequate protection.

For project sites where the effects of stream/riverine-based flooding are obvious from a cursory examination of the sites topography in relation to some watercourse, a progressive, HHA-like screening approach may be used. Simplified procedures may be used; however, all methods and assumptions should be clearly conservative.

Compounding conservatism by adding conservatism on top of each value/method/assumption when each input and method must be separately justified to be conservative can result in analyses which are so overly conservative that they no longer reflect reality and may skew design and decision making. The expectation should be for the overall analysis to be demonstrated to be conservative.

Consider revising this text:

For project sites where the effects of stream/riverine-based flooding are obvious from a cursory examination of the sites topography in relation to some watercourse, a progressive, HHA-like screening approach may be used. Simplified procedures may be used; however, all methods and assumptions should be clearly conservative.

To read:

For project sites where the effects of stream/riverine-based flooding are obvious from a cursory examination of the sites topography in relation to some watercourse, a progressive, HHA-like screening approach may be used. Simplified procedures may be used; however, the overall methods and assumptions should provide a demonstrably conservative* result.

With added footnote:

Footnote *: A demonstrably conservative flood hazard evaluation (deterministic or probabilistic) is one that is supported by observed data and 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.

One approach could be conducting a sensitivity analysis for a realistic range of inputs and adopting conservative results, relative to the best estimate results. Another approach might include a Monte Carlo simulation conducted around best estimate (i.e., calibrated) input parameters to quantify the mean and variability in the resulting flood elevations at the point(s) of interest. Conservatism would be incorporated into the analysis when interpreting and applying these results (e.g., adopting flood elevations at the upper 90% confidence limit from the Monte Carlo simulations).

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-16

22.

Stream Flooding Caused by Precipitation Events C.3.b Page 19 The following sentence can be clarified to state that data from the post-Fukushima flooding reevaluations can be applicable to co-located sites or used as an example for other sites:

Another important source of information, which might be more pertinent because it is site-specific, would be the riverine-based flood analyses performed by owners and operators in response to the 2011 Fukushima event. Those flood hazard reevaluations are publicly available in the NRCs Agencywide Documents Access and Management System (ADAMS).

Consider revising this text:

Another important source of information, which might be more pertinent because it is site-specific, would be the riverine-based flood analyses performed by owners and operators in response to the 2011 Fukushima event. Those flood hazard reevaluations are publicly available in the NRCs Agencywide Documents Access and Management System (ADAMS).

To read:

The riverine-based flood hazard flood analyses performed by existing NPP owner/operators following the 2011 Fukushima event may be valuable sources of information for co-located sites or to be used as examples in performing the analyses described in this RG. Those flood hazard reevaluations are publicly available in ADAMS.

23.

Flooding Caused by Storm Surges, Seiches, and Tsunamis C.3.d Page 21 The following sentence can be clarified to state that data from the post-Fukushima flooding reevaluations can be applicable to co-located sites or used as an example for other sites:

Another important, and possibly more illustrative, source of information is the site-specific flood analyses performed by affected owners and operators in response to the 2011 Fukushima event. The flood hazard reevaluations for sites exposed to storm surge, seiches, and tsunamis are publicly available in ADAMS.

Consider revising this text:

Another important, and possibly more illustrative, source of information is the site-specific flood analyses performed by affected owners and operators in response to the 2011 Fukushima event. The flood hazard reevaluations for sites exposed to storm surge, seiches, and tsunamis are publicly available in ADAMS.

To read:

The site-specific flood analyses performed by existing NPP owner/operators for sites exposed to storm surge, seiches, and tsunami following the 2011 Fukushima event may be valuable sources of information for co-located sites or to be used as examples in performing the analyses described in this RG. Those flood hazard reevaluations are publicly available in ADAMS.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-17

24.

Combined Events C.3.f Page 22 Regarding the following text in C.3.f:

The possible combinations of processes and events to consider are site-specific and depend on the geography of the project site.

ANSI/ANS-2.8-1992 describes examples of possible scenarios to consider. It was withdrawn in 2002 and updated to a probabilistic standard (Ref 34). Nevertheless, as recently as 2012, many NRC licensees were still relying on the deterministic criteria in ANSI/ANS-2.8-1992 for their Fukushima flood hazard evaluations.

The NRC staff currently uses an average annual probability of exceedance of less than 1x10-6 as a metric to evaluate the reasonableness of combined flooding event scenarios. The staff notes that ANSI/ANS-2.12-1978, Guidelines for Combining Natural and External Man-Made Hazards at Power Reactor Sites (Ref. 42) and ANSI/ANS-58.21-2007, External Events in PRA Methodology (Ref.

43) (although now withdrawn), describe methods for evaluating and combining flood hazard events for design purposes.

The following are not clear:

1) How can an applicant establish reasonable confidence limits at the level of 1x10-6, given the available data and the multiple orders of magnitude of extrapolation required?

2) How can qualitative methods or semi-quantitative methods be used to achieve such levels of precision?

3) What specific methods and assumptions are technically justified and acceptable to the NRC in terms of achieving the 1x10-6 AEP metric for determining which combined flooding event scenarios need to be considered?

4) What alternatives are acceptable in the event that data and methods are not available to support an AEP of 1x10-6 with reasonable confidence limits, for various flood mechanisms of combinations thereof?

The DG expresses an expectation of the use of probabilistic, frequency-based methods, at least in the context of determination of which combination of flooding hazard mechanisms need to be considered, while at the same time acknowledging that methods and data acceptable to the NRC for this analysis do not exist. There is concern that the use of 1x10-6 as a metric implies a level of precision that is not justified and should not be necessary to provide reasonable assurance of protection of the health and safety of the public in the event of an external flood. Deferring The DG should be revised to provide guidance consistent with the level of safety required that provides reasonable assurance and in consideration of the state-of-practice methods and data available for practical implementation, such that regulatory efficiency and stability are achieved.

Because there are no acceptable methods for determining a 1x10-6 AEP flood hazard, guidance should be provided for an acceptable approach using a less stringent threshold (such as 1x10-4) with an acceptable mitigation strategy for greater floods, if needed.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-18 this discussion to case-by-case reviews and/or pre-application negotiations will lead to regulatory inefficiency and instability.

25.

Combined Events C.3.f Page 22 Based on the current wording the reader may be left with the impression that there is little work or guidance concerning technical approaches for probabilistic flood hazard assessment. It may be possible to make minor changes to in sentences like the following to convey the significant amount of work done by the NRC and other organizations in this area:

However, guidance on formal PFHA approaches 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.

Consider rewording the sentence:

However, guidance on formal PFHA approaches 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.

To read:

Because comprehensive and endorsed guidance on PFHA approaches addressing combined events is not yet available, the NRC will assess the reasonableness of qualitative and quantitative probability estimates for combined events based on the currently available methods and state of practice, until such time as comprehensive guidance is developed and endorsed for use by the NRC.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-19

26.

A-1 Information on Recent Storm and Flood Events Page A-1 It is acknowledged that many published data sets used to evaluate historical meteorological/storm data may not have been updated in several decades. It is further agreed that an applicant needs to perform due diligence in assuring that the data in these sources remains relevant and applicable for any site being evaluated.

It is suggested that this text can be enhanced to recognize ongoing processes (even after the issuance of a license) which are in place to ensure that the data and assumptions use in the design basis of NPPs remains applicable when new information becomes available:

Although these reports are valuable references, their estimates are derived from historical storm databases that, in many instances, are several decades old and have not been updated. In some cases, larger or more intense storms have occurred in certain regions since the publication of these design references. Before using such design references to estimate design-basis floods at nuclear power plant sites, applicants should confirm that more recent storms or flood events in the region of interest do not invalidate the conceptualizations, assumptions, parameterizations, or estimates in the references.

Consider revising this text:

Although these reports are valuable references, their estimates are derived from historical storm databases that, in many instances, are several decades old and have not been updated. In some cases, larger or more intense storms have occurred in certain regions since the publication of these design references.

Before using such design references to estimate design-basis floods at nuclear power plant sites, applicants should confirm that more recent storms or flood events in the region of interest do not invalidate the conceptualizations, assumptions, parameterizations, or estimates in the references.

To read:

Although these reports are valuable references, their estimates are derived from historical storm databases that, in many instances, are several decades old and have not been updated. In some cases, larger or more intense storms have occurred in certain regions since the publication of these design references.

Before using such design references to estimate design-basis floods at nuclear power plant sites, applicants should confirm that more recent storms or flood events in the region of interest do not invalidate the conceptualizations, assumptions, parameterizations, or estimates in the references. In addition, applicants can reference the NRCs Process for the Ongoing Assessment of Natural Hazard Information (POANHI)* project and the analogous work undertaken by operating NPPs to address Recommendation 2 from INPO IER L1-2013-10*,

Nuclear Accident at the Fukushima Daiichi Nuclear Power Station, which consistently screen for and evaluate new data, methods and science to determine if any new information challenges the design bases assumptions for each operating nuclear plant. Such processes are leveraged to enhance the usage of previous data and to ensure that new natural hazard information is routinely aggregated and assessed for potential significance to existing site safety. These processes ensure that when historical data is used in the design and licensing of NPPs, that such Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-20 information will continue to be relevant and applicable, or that appropriate evaluation and mitigative actions will be put in place to address the impact of any new information.

• References can be added to:

Process for the Ongoing Assessment of Natural Hazard Information (POANHI) --

https://www.nrc.gov/reactors/operating/ops-experience/poanhi.html or LIC-208, NRR Office Instruction Process for the Ongoing Assessment of Natural Hazards Information and INPO EVENT REPORT L1-13-10, NUCLEAR ACCIDENT AT THE FUKUSHIMA DAIICHI NUCLEAR POWER STATION, DATED MARCH 28, 2013 (available to INPO Members only)

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-21

27.

A-2 Deterministic versus Probabilistic Analyses Page A-1 and A-2 Regarding this text in A-2:

Regarding safety goals, the withdrawn National Standards Institute (ANSI)/American Nuclear Society (ANS)-2.8-1992, Determining Design Basis Flooding at Nuclear Power Plant Sites (Ref. 33),

considered an annual exceedance probability of 1x10-6 to represent a reasonable criterion for selecting the design-basis flood at a nuclear power plant site. The NRC staff uses this metric to evaluate the reasonableness of combined flooding event scenarios. Therefore, the staff considers this metric a reasonable criterion to apply to design-basis flood estimates from probabilistic methods, assuming that reasonable confidence limits can be established.

In considering the 1x10-6 screening criterion, the staff recognizes that the definitions of some design-basis floods may rely not on a single flood-causing mechanism but, potentially, on two or more contemporaneous flooding events. ANSI/ANS-2.12-1978, Guidelines for Combining Natural and External Man-Made Hazards at Power Reactor Sites (Ref. 34), and ANSI/ANS-58.21-2007, External Events in PRA Methodology (Ref. 35) (although now withdrawn), describe methods for evaluating and combining flood hazard events for design purposes.

The following are not clear:

1) How can an applicant establish reasonable confidence limits at the level of 1x10-6, given the available data and the multiple orders of magnitude of extrapolation required?

2) How can qualitative methods or semi-quantitative methods be used to achieve such levels of precision?

3) What specific methods and assumptions are technically justified and acceptable to the NRC in terms of achieving the 1x10-6 AEP metric for determining which combined flooding event scenarios need to be considered?

4) What alternatives are acceptable in the event that data and methods are not available to support an AEP of 1x10-6 with reasonable confidence limits, for various flood mechanisms of combinations thereof?

The DG expresses an expectation of the use of probabilistic, frequency-based methods, at least in the context of determination of which combination of flooding hazard mechanisms need to be considered, The DGs should be revised to provide guidance consistent with the level of safety that provides reasonable assurance and in consideration of the state-of-practice methods and data available for practical implementation, such that regulatory efficiency and stability are achieved.

Because there are no acceptable methods for determining a 1x10-6 AEP flood hazard, guidance should be provided for an acceptable approach using a less stringent threshold (such as 1x10-4) with an acceptable mitigation strategy for greater floods, if needed.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-22 while at the same time acknowledging that methods and data acceptable to the NRC for this analysis do not exist. There is concern that the use of 1x10-6 as a metric implies a level of precision that is not justified and should not be necessary to provide reasonable assurance of protection of the health and safety of the public in the event of an external flood. Deferring this discussion to case-by-case reviews and/or pre-application negotiations will lead to regulatory inefficiency and instability.

28.

A-2 Deterministic versus Probabilistic Analyses Page A-2 Based on the current wording the reader may be left with the impression that there is little work or guidance concerning technical approaches for probabilistic flood hazard assessment. It may be possible to make minor changes to in sentences like the following to convey the significant amount of work done by the NRC and other organizations in this area:

However, guidance on formal PFHA approaches 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 (see Section C, Regulatory Position 3f).

Consider rewording the sentence:

However, guidance on formal PFHA approaches 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 (see Section C, Regulatory Position 3f).

To read:

Because comprehensive and endorsed guidance on PFHA approaches addressing combined events is not yet available, the NRC will assess the reasonableness of qualitative and quantitative probability estimates for combined events based on the currently available methods and state of practice, until such time as comprehensive guidance is developed and endorsed for use by the NRC basis (see Section C, Regulatory Position 3f).

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-23

29.

A-2 Deterministic versus Probabilistic Analyses Page A-2 The following sentence could be enhanced by adding discussion of ongoing processes in place to assure that new information (like climate change insights) is assessed for impact to operating NPPs, even when there is too much uncertainty to provide reasonable accurate projections at the time the plant is licensed:

Because future natural changes in precipitation amounts and storm frequencies are very uncertain, this RG gives no specific guidance on these topics.

Consider revising this text:

Because future natural changes in precipitation amounts and storm frequencies are very uncertain, this RG gives no specific guidance on these topics.

To read:

Because future natural changes in precipitation amounts and storm frequencies are very uncertain, this RG gives no specific guidance on these topics. As climate projections, depending on the variables of interest (e.g., storms, precipitation, temperature, SLR), can still have a high degree of uncertainty, given the current state-of-practice, it is appropriate for applicants to recognize the role of projects like the NRCs POANHI* process and the industry response to Recommendation 2 to from INPO IER L1-2013-10*, in monitoring and evaluating new data, information and methods during the life of an operating NPP to address potential impacts to plant safety.

• References can be added to:

Process for the Ongoing Assessment of Natural Hazard Information (POANHI) --

https://www.nrc.gov/reactors/operating/ops-experience/poanhi.html or LIC-208, NRR Office Instruction Process for the Ongoing Assessment of Natural Hazards Information and INPO EVENT REPORT L1-13-10, NUCLEAR ACCIDENT AT THE FUKUSHIMA DAIICHI NUCLEAR POWER STATION, DATED MARCH 28, 2013 (available to INPO Members only)

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-24

30.

A-3 Non-stationarity:

Climate Variability, Climate Change, and Sea Level Rise Page A-2 The following sentence could be enhanced by adding discussion of ongoing processes in place to assure that new information (like climate change insights) is assessed for impact to operating NPPs, even when there is too much uncertainty to provide reasonable accurate projections at the time the plant is licensed:

However, recent developments in climate research have shown that significant global and regional warming trends at these time scales have occurred in the past several decades and are expected to continue or even accelerate, with implications for hydrologic extremes including tropical cyclones, precipitation, and floods (Ref. 36, 37, 38, 39), as well as sea-level rise (SLR).

Consider revising this text:

However, recent developments in climate research have shown that significant global and regional warming trends at these time scales have occurred in the past several decades and are expected to continue or even accelerate, with implications for hydrologic extremes including tropical cyclones, precipitation, and floods (Ref. 36, 37, 38, 39), as well as sea-level rise (SLR).

To read:

However, recent developments in climate research have shown that significant global and regional warming trends at these time scales have occurred in the past several decades and are expected to continue or even accelerate, with implications for hydrologic extremes including tropical cyclones, precipitation, and floods (Ref. 36, 37, 38, 39), as well as sea-level rise (SLR). As climate projections, depending on the variables of interest (storms, precipitation, temperature, SLR, etc.), can still have a high degree of uncertainty, given the current state-of-practice, it is appropriate for applicants to recognize the role of projects like the NRCs POANHI* process and the industry response to Recommendation 2 to from INPO IER L1-2013-10*, in monitoring and evaluating new data, information and methods during the life of an operating NPP to address potential impacts to plant safety.

• References can be added to:

Process for the Ongoing Assessment of Natural Hazard Information (POANHI) --

https://www.nrc.gov/reactors/operating/ops-experience/poanhi.html or LIC-208, NRR Office Instruction Process for the Ongoing Assessment of Natural Hazards Information and INPO EVENT REPORT L1-13-10, NUCLEAR ACCIDENT AT THE FUKUSHIMA DAIICHI NUCLEAR POWER STATION, DATED MARCH 28, 2013 (available to INPO Members only)

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-25

31.

A-3 Non-stationarity:

Climate Variability, Climate Change, and Sea Level Rise Page A-2 The text as written acknowledges climate impact on a number of variables which may impact the flooding hazard at a site, but only provides detailed descriptions and recommendation on addressing seal level rise (SLR):

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 text can be clarified to identify why specific recommendations are provided for SLR estimates considering climate change.

Consider revising this text:

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).

To read:

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. As SLR is one of the climate variables that is easier to project, applicants should consult the most recent authoritative climate assessments provided by organizations such as the Intergovernmental Panel on Climate Change (IPCC) and the U.S. Global Change Research Program (USGCRP) (e.g., Ref. 37, 38).

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-26

32.

A-3 Non-stationarity:

Climate Variability, Climate Change, and Sea Level Rise Page A-3 The following discussion could be enhanced by reference to ongoing processes in place to ensure that new information (like climate change insights) is assessed for impact to operating NPPs, even when there is too much uncertainty to provide reasonable accurate projections at the time the plant is licensed:

The preceding discussion highlights the difficulty in translating climate research findings into practical applications for hydrologic design problems. Nonetheless, decisions need to be made, and several State and Federal agencies have developed frameworks for assessing climate change risks for water resource applications (Ref.

57, 58, 59, 60, 61, 62), mainly for river basins in the western United States. Similar efforts have been made for major European river basins (Ref 63, 64, 65). One approach involves downscaling the output from global climate models to the regional scale by applying statistical bias correction using historical meteorological data (Ref 66, 67, 68). An alternative approach is to use a regional climate model to dynamically downscale relevant hydrometeorological data (Ref. 69).

Consider revising this text:

The preceding discussion highlights the difficulty in translating climate research findings into practical applications for hydrologic design problems.

Nonetheless, decisions need to be made, and several State and Federal agencies have developed frameworks for assessing climate change risks for water resource applications (Ref. 57, 58, 59, 60, 61, 62), mainly for river basins in the western United States. Similar efforts have been made for major European river basins (Ref 63, 64, 65). One approach involves downscaling the output from global climate models to the regional scale by applying statistical bias correction using historical meteorological data (Ref 66, 67, 68). An alternative approach is to use a regional climate model to dynamically downscale relevant hydrometeorological data (Ref. 69).

To read:

The preceding discussion highlights the difficulty in translating climate research findings into practical applications for hydrologic design problems.

Nonetheless, decisions need to be made, and several State and Federal agencies have developed frameworks for assessing climate change risks for water resource applications (Ref. 57, 58, 59, 60, 61, 62), mainly for river basins in the western United States. Similar efforts have been made for major European river basins (Ref 63, 64, 65). One approach involves downscaling the output from global climate models to the regional scale by applying statistical bias correction using historical meteorological data (Ref 66, 67, 68). An alternative approach is to use a regional climate model to dynamically downscale relevant hydrometeorological data (Ref. 69). Again, given the uncertainly in projecting the impact of climate change on some variables, it is important to recognize existing process throughout the life of the nuclear plant that assure that new methods, data, and information that does become available as a result of new science, climate change, etc. does not challenge the design basis of the NPP.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-27

33.

B-3 Hydrologic Features Page B-4 (Editorial) Consider rewording the following sentence to such that safety-related and non-safety related are not clarifiers of power plant operation.

Particular attention should be given to hydrologic features intended as surface water supply sources for both safety-related and non-safety-related power plant operations (Ref. 11).

Consider revising this paragraph:

Particular attention should be given to hydrologic features intended as surface water supply sources for both safety-related and non-safety-related power plant operations (Ref. 11).

To read:

Particular attention should be given to hydrologic features intended as surface water supply sources for power plant operations, both safety-related and non-safety-related (Ref. 11).

34.

B-3.3

Dams, Reservoirs, and Levees Page B-5 The first paragraph in section B-3.3 could benefit from a sentence acknowledging the challenge associated with obtaining the relevant information to assess the impacts of dam failures as described in DG-1417.

Consider revising this text:

The applicant should provide an inventory of the dams, reservoirs, and levees within the watershed occupied by the power plant project, as well as a description of future planned features. Ownership, history, and detailed engineering descriptions should be given for all upstream and downstream features judged to be mechanisms plausibly capable of contributing to the design-basis flood. The applicant should also identify those dams, reservoirs, and levees that would be relied on for the plants safety-related water supply (Ref. 11). Detailed guidance on assessing flooding hazards due to water control structure failures and incidents is provided in DG-1417/RG 1.256 (Ref. 12).

To read:

The applicant should identify the dams, reservoirs, and levees within the watershed occupied by the power plant project, as well as a description of future planned features. Ownership, history, and detailed engineering descriptions should be given for all upstream and downstream features judged to be mechanisms plausibly capable of contributing to the design-basis flood. The applicant should also identify those dams, reservoirs, and levees that would be relied on for the plants safety-related water supply (Ref. 11). Detailed guidance on assessing flooding hazards due to water control structure failures and incidents, and obtaining information on the dams, reservoirs, and levees within the watershed occupied by the NPP is provided in DG-1417/RG 1.256 (Ref.

12).

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-28

35.

B-4 Flood History Page B-6 The discussion on the use of paleo flood data would be enhanced by adding reference to RIL 2022-04 Technical Basis for Extending Flood Frequency Curves Beyond Current Consensus Limits as a source of guidance on use of this data.

Consider revising this text:

Useful information can also be obtained from paleo flood studies that examine botanical, geological, or geomorphological information indicating the occurrence (or nonoccurrence) of large floods that predate human observation (Ref. 21). Information from these studies, when available, can be used to estimate the probable maximum flood. Examples of relevant botanical data indicative of paleo flood stages include scars on trees, abnormal tree rings, and tipped trees along a stream or river. Geophysical evidence of paleo flood stages includes slack water deposits, scour lines, and terraces (or the absence of these features). Combined with radiocarbon or other dating techniques, botanical and geophysical data can significantly enhance the flood record in favorable environments with stable channels.

To read:

Useful information may also be obtained available from paleo flood studies that have examined botanical, geological, or geomorphological information indicating the occurrence (or nonoccurrence) of large floods that predate human observation (Ref. 21).

Information from these studies, when available, can be used to estimate the probable maximum flood.

Examples of relevant botanical data indicative of paleo flood stages include scars on trees, abnormal tree rings, and tipped trees along a stream or river.

Geophysical evidence of paleo flood stages includes slack water deposits, scour lines, and terraces (or the absence of these features). Combined with radiocarbon or other dating techniques, botanical and geophysical data can significantly enhance the flood record in favorable environments with stable channels. NRC Research Information Letter (RIL) 2022-04 Technical Basis for Extending Flood Frequency Curves Beyond Current Consensus Limits contains information in the use of paleo flood data to extend flood frequency estimates.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-29

36.

C-1 Flooding Caused by LIP Page C-1 This guideline does not provide clear guidance on how to address climate change when performing LIP analysis. The discussion can address this by acknowledging that the science is still evolving in this area and the ability to make accurate projections on future storms and precipitation is highly uncertain.

Consider revising this text:

As discussed in appendix A, the validity of PMP values provided by HMRs should be assessed in light of precipitation events that have occurred in the region since the HMRs were published. Several NPP licensees developed site-specific PMP estimates during the post-Fukushima flooding reevaluations.

Applicants estimating PMP using any source other than an NWS HMR need to explain the basis for the specific PMP value used. NUREG/KM-0015, Considerations for Estimating Site-Specific Probable Maximum Precipitation at Nuclear Power Plants in the United States of America: Final Report, issued September 2021 (Ref. 18), discusses considerations for an acceptable approach to estimating a site-specific PMP as an alternative to an HMR-based estimate.

To read:

As discussed in appendix A, the validity of PMP values provided by HMRs should be assessed in light of precipitation events that have occurred in the region since the HMRs were published. Several NPP licensees developed site-specific PMP estimates during the post-Fukushima flooding reevaluations.

Applicants estimating PMP using any source other than an NWS HMR need to explain the basis for the specific PMP value used. NUREG/KM-0015, Considerations for Estimating Site-Specific Probable Maximum Precipitation at Nuclear Power Plants in the United States of America: Final Report, issued September 2021 (Ref. 18), discusses considerations for an acceptable approach to estimating a site-specific PMP as an alternative to an HMR-based estimate. As discussed in Section C.2.c and Appendix A, future natural changes in precipitation amounts and storm frequencies are very uncertain, and this RG gives no specific guidance on these topics. As climate projections, depending on the variables of interest (e.g., storms, precipitation, temperature, SLR), can still have a high degree of uncertainty, given the current state-of-practice, it is appropriate for applicants to recognize the role of projects like the Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-30 NRCs POANHI* process and the industry response to Recommendation 2 to from INPO IER L1-2013-10*, in monitoring and evaluating new data, information and methods during the life of an operating NPP to address potential impacts to plant safety.

• References can be added to:

Process for the Ongoing Assessment of Natural Hazard Information (POANHI) --

https://www.nrc.gov/reactors/operating/ops-experience/poanhi.html or LIC-208, NRR Office Instruction Process for the Ongoing Assessment of Natural Hazards Information and INPO EVENT REPORT L1-13-10, NUCLEAR ACCIDENT AT THE FUKUSHIMA DAIICHI NUCLEAR POWER STATION, DATED MARCH 28, 2013 (available to INPO Members only)

37.

C-2 LIP Events Page (Editorial) In this sentence it would be more accurate to reflect that the post-Fukushima reevaluations are only directly applicable as data sources for co-located sites, but can be useful as examples for other applicants:

Several NPP licensees developed site-specific PMP estimates during the post-Fukushima flooding reevaluations.

Consider revising this text:

Several NPP licensees developed site-specific PMP estimates during the post-Fukushima flooding reevaluations.

To read:

Several NPP licensees developed site-specific PMP estimates during the post-Fukushima flooding reevaluations. These evaluations, which may be useful for co-located facilities or as examples, are publicly available in ADAMS.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-31

38.

C-3 Estimating Surface Runoff Page C-3 The first paragraph in this section presents the simplest approach of assuming that all rainfall is converted directly to runoff. The discussion can be enhanced to acknowledge that other approaches may be justified.

Consider revising this text:

To simplify calculations and ensure a conservative estimate, all rainfall should be assumed to be converted to direct runoff (i.e., no credit should be taken for losses resulting from infiltration, evaporation, or other factors). Therefore, no credit should be taken for hydraulic sinks in the runoff model. The complexity of the rainfall-runoff modeling will vary case by case.

To read:

To simplify calculations and ensure a conservative estimate, all rainfall can be assumed to be converted to direct runoff (i.e., no credit should be taken for losses resulting from infiltration, evaporation, or other factors). In this case, no credit should be taken for hydraulic sinks in the runoff model. The complexity of the rainfall-runoff modeling will vary case by case, and the applicant should justify the assumptions made in their chosen approach.

39.

C-4 Conveyance of Surface Runoff Page C-4 Given that flood protection features other than height may be acceptable for protecting SSCs from DB flood effects, consider clarifying the following sentence:

Ultimately, the hydraulic model will need to demonstrate that the design-basis flood level from an LIP event will not exceed specified elevations for SSCs that perform safety functions.

Consider revising this text:

Ultimately, the hydraulic model will need to demonstrate that the design-basis flood level from an LIP event will not exceed specified elevations for SSCs that perform safety functions.

To read:

Ultimately, the hydraulic model will need to demonstrate that the design-basis flood level from an LIP event will not impact the ability of SSCs relied on to perform safety functions in the event of a design basis flood, to perform their safety functions.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-32

40.

D-2 Simplified Analyses Page D-2 While it is acknowledged that an analysis to be used for design basis should be conservative, explicit text requiring that ALL methods/ inputs/

assumptions be conservative goes beyond what should be needed to provide reasonable assurance of adequate protection.

As discussed in NUREG/CR-7046, simplified procedures may be used; however, however, all methods and assumptions should be clearly conservative.

Compounding conservatism by adding conservatism on top of each value/method/assumption when each input and method must be separately justified to be conservative can result in analyses which are so overly conservative that they no longer reflect reality and may skew design and decision making. The expectation should be for the overall analysis to be demonstrated to be conservative.

Consider revising this text:

As discussed in NUREG/CR-7046, simplified procedures may be used; however, however, all methods and assumptions should be clearly conservative.

To read:

As discussed in NUREG/CR-7046, simplified procedures may be used; however, the overall methods and assumptions should provide a demonstrably conservative result.

41.

D-5.2 Snow and Snowmelt Page D-6 (Editorial) Consider revising the following sentence to reflect that there may be other useful data sets for this information:

Information for estimating snowpack can be obtained from the Parameter-Elevation Regressions on Independent Slopes Model (PRISM) Climate Program, which is maintained at Oregon State University (Ref. 43). The PRISM program illustrates high-resolution gridded estimates of monthly and annual means for temperature and precipitation (rain and snow) as a function of elevation.

Consider revising this text:

Information for estimating snowpack can be obtained from the Parameter-Elevation Regressions on Independent Slopes Model (PRISM) Climate Program, which is maintained at Oregon State University (Ref. 43). The PRISM program illustrates high-resolution gridded estimates of monthly and annual means for temperature and precipitation (rain and snow) as a function of elevation.

To read:

Information for estimating snowpack can be obtained from data sources such as the Parameter-Elevation Regressions on Independent Slopes Model (PRISM)

Climate Program, which is maintained at Oregon State University (Ref. 43). The PRISM program illustrates high-resolution gridded estimates of monthly and annual means for temperature and precipitation (rain and snow) as a function of elevation.

42.

E Flooding Caused by Storm Surge Page E-1 (Editorial) This sentence in the introductory section does refer to specifically to types of storm surge, but it is confusing to the reader, as the following sections and headers in Appendix E immediately use the terms Tropical Cyclones and Extra-Tropical Cyclones:

The discussions of storm surge below use the generic terms hurricane and hurricane storm surge to refer to any tropical cyclone and tropical-cyclone-induced storm surge, respectively.

Consider revising the sentence to clarify its relationship to the use of the terms including the word cyclone in these sections.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-33

43.

E Flooding Caused by Storm Surge Page E-1 While it is acknowledged that an analysis to be used for design basis should be conservative, explicit text requiring that ALL methods/ inputs/

assumptions be conservative goes beyond what should be needed to provide reasonable assurance of adequate protection.

As discussed in that report, simplified procedures may be used; however, all methods and assumptions should be clearly conservative.

Compounding conservatism by adding conservatism on top of each value/method/assumption when each input and method must be separately justified to be conservative can result in analyses which are so overly conservative that they no longer reflect reality and may skew design and decision making. The expectation should be for the overall analysis to be demonstrated to be conservative.

Consider revising this text:

As discussed in that report, simplified procedures may be used; however, all methods and assumptions should be clearly conservative.

To read:

Simplified procedures may be used; however, the overall methods and assumptions should provide a demonstrably conservative result. If it cannot be demonstrated clearly that no credible postulated storm-surge-based flooding event could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

44.

E-4.1 Simplified Hurricane Storm Surge Analysis Page E-4 Dated information is not necessarily out of date, provided it is still relevant and applicable, or bounding, for the site in question.

Consider revising this text:

Another example of a simplified storm surge analysis method is the PMH approach described in NUREG/CR-7046. However, as explained in appendix A, the National Weather Service (NWS) reports from which PMH parameters are usually determined are limited and out of date, as they do not reflect hurricane activity from the past 30 years.

To read:

Another example of a simplified storm surge analysis method is the PMH approach described in NUREG/CR-7046. However, as explained in appendix A, the National Weather Service (NWS) reports from which PMH parameters are usually determined are based on historical data which may be dated, as they do not reflect hurricane activity from the past 30 years.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-34

45.

E-4.2 Coupled Wind, Wave, and Hydrodynamic Modeling Page E-5 (Editorial) In this sentence it would be more accurate to reflect that the post-Fukushima reevaluations are only directly applicable as data sources for co-located sites, but can be useful as examples for other applicants:

As mentioned above, useful site-specific information may also be obtained from the storm surge flood analyses performed by NPP owners and operators in response to the 2011 Fukushima event, which are available in ADAMS.

Consider revising this text:

As mentioned above, useful site-specific information may also be obtained from the storm surge flood analyses performed by NPP owners and operators in response to the 2011 Fukushima event, that is available in ADAMS.

To read:

As mentioned above, useful site-specific information for co-located sites or for use as an example may also be obtained from the storm surge flood analyses performed by existing NPP owner/operators following the 2011 Fukushima event, which are available in ADAMS.

46.

E-4.5.1.1 Hurricane Parameters Page E-8 (Editorial) Since the sentence refers to only frequency information, the use of the term risk information could cause confusion in this sentence:

Note that the NWS 23 method provides no risk information (e.g.,

return period) and applies only to the deterministic storm surge analysis of hurricanes.

Consider revising this text:

Note that the NWS 23 method provides no risk information (e.g., return period) and applies only to the deterministic storm surge analysis of hurricanes.

To read:

Note that the NWS 23 method provides no frequency information (e.g., return period) and applies only to the deterministic storm surge analysis of hurricanes.,

47.

F Flooding Caused by Seiche Page F-1 While it is acknowledged that an analysis to be used for design basis should be conservative, explicit text requiring that ALL methods/ inputs/

assumptions be conservative goes beyond what should be needed to provide reasonable assurance of adequate protection.

As discussed in that report, simplified procedures may be used; however, all methods and assumptions should be clearly conservative. If it cannot be demonstrated clearly that no credible seiche-based flooding event could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

Compounding conservatism by adding conservatism on top of each value/method/assumption when each input and method must be separately justified to be conservative can result in analyses which are so overly conservative that they no longer reflect reality and may skew design and decision making. The expectation should be for the overall analysis to be demonstrated to be conservative.

Consider revising this text:

As discussed in that report, simplified procedures may be used; however, all methods and assumptions should be clearly conservative. If it cannot be demonstrated clearly that no credible seiche-based flooding event could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

To read:

Simplified procedures may be used; however, the overall methods and assumptions should provide a demonstrably conservative result. If it cannot be demonstrated clearly that a credible seiche-based flooding event is not likely to affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-35

48.

G Flooding Caused by Tsunamis Page G-1 While it is acknowledged that an analysis to be used for design basis should be conservative, explicit text requiring that ALL methods/ inputs/

assumptions be conservative goes beyond what should be needed to provide reasonable assurance of adequate protection.

As discussed in that report, simplified procedures may be used; however, all methods and assumptions should be clearly conservative. If it cannot be demonstrated clearly that no credible tsunami-based flooding event could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

Compounding conservatism by adding conservatism on top of each value/method/assumption when each input and method must be separately justified to be conservative can result in analyses which are so overly conservative that they no longer reflect reality and may skew design and decision making. The expectation should be for the overall analysis to be demonstrated to be conservative.

Consider revising this text:

As discussed in that report, simplified procedures may be used; however, all methods and assumptions should be clearly conservative. If it cannot be demonstrated clearly that no credible tsunami-based flooding event could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

To read:

Simplified procedures may be used; however, the overall methods and assumptions should provide a demonstrably conservative result. If it cannot be demonstrated clearly that a credible tsunami-based flooding event is not likely to affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

49.

G Flooding Caused by Tsunamis Page G-1 Section G can be enhanced by adding that post-Fukushima work can be leveraged to screen out tsunami flood causing mechanisms or identify credible tsunamigenic sources after this sentence:

If it cannot be demonstrated clearly that no credible tsunami-based flooding event could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

Consider revising this text:

If it cannot be demonstrated clearly that no credible tsunami-based flooding event could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

To read:

If it cannot be demonstrated clearly that a credible tsunami-based flooding event is not likely to affect the site, a detailed analysis of this external flood-causing mechanism should be performed. Useful site-specific information for co-located sites or for use as an example may also be obtained from the storm surge flood analyses performed by existing NPP owner/operators following the 2011 Fukushima event, which are available in ADAMS. This work may be leveraged to screen out tsunami flood causing mechanisms or identify credible tsunamigenic sources.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-36

50.

G-3 Tsunami Computational Modeling Tools Page G-4 (Editorial) In this section it would be more accurate to reflect that the post-Fukushima reevaluations are only directly applicable as data sources for co-located sites, but can be useful as examples for other applicants:

The site-specific tsunami flood hazard assessments performed along the domestic coastline by NPP owners and operators in the United States in response to the 2011 Fukushima event constitute another important source of. Many of these assessments use the computer programs described above. The assessments are publicly available in the NRCs Agencywide Documents Access and Management System.

As mentioned above, these assessments reveal that tsunami flood hazards at US NPPs are bounded by the storm surge flood-causing mechanism.

Consider revising this text:

The site-specific tsunami flood hazard assessments performed along the domestic coastline by NPP owners and operators in the United States in response to the 2011 Fukushima event constitute another important source of. Many of these assessments use the computer programs described above. The assessments are publicly available in the NRCs Agencywide Documents Access and Management System. As mentioned above, these assessments reveal that tsunami flood hazards at US NPPs are bounded by the storm surge flood-causing mechanism.

To read:

The site-specific tsunami flood hazard assessments performed along the domestic coastline by NPP owner/operators of existing NPPs following the 2011 Fukushima event constitute another potential source of information for co-located sites or to be used as examples. Many of these assessments use the computer programs described above. The assessments are publicly available in ADAMS. As mentioned above, these assessments reveal that tsunami flood hazards at United States NPPs are bounded by the storm surge flood-causing mechanism.

51.

G-4 Associated Flooding Effects Page G-4 Wind-wave should be deleted as an associated effect for a tsunami in the second sentence of this section.

Consider revising this text:

Associated flooding effects such as wind-wave and runup effects can, in some cases, increase both the magnitude of flood inundation and the dynamic consequences of the event.

To read:

Associated flooding effects such runup effects can, in some cases, increase both the magnitude of flood inundation and the dynamic consequences of the event.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-37

52.

H-1 General Page H-1 (Editorial) In this sentence the word aerial should be replaced with areal:

Flooding due to ice jams is usually less extensive in aerial coverage and elevation than open-water floods that occur in warm weather.

Consider revising this text:

Flooding due to ice jams is usually less extensive in aerial coverage and elevation than open-water floods that occur in warm weather.

To read:

Flooding due to ice jams is usually less extensive in areal coverage and elevation than open-water floods that occur in warm weather.

53.

H-1 General Page H-1 To accurately reflect that the operation of all SSCs is not required, the following sentence should be revised:

The weight of ice accumulation should be considered wherever it affects the operation of structures, systems, and components.

Consider revising this text:

The weight of ice accumulation should be considered wherever it affects the operation of structures, systems, and components.

To read:

The weight of ice accumulation should be considered as to wherever it affects the ability of SSCs to perform their safety-related functions.

54.

H Flooding Caused by Ice Effects Page H-1 While it is acknowledged that an analysis to be used for design basis should be conservative, explicit text requiring that ALL methods/ inputs/

assumptions be conservative goes beyond what should be needed to provide reasonable assurance of adequate protection.

As discussed in that report, simplified procedures may be used; however, all methods and assumptions should be clearly conservative. If it cannot be demonstrated clearly that no credible external flooding event due to ice jams could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

Compounding conservatism by adding conservatism on top of each value/method/assumption when each input and method must be separately justified to be conservative can result in analyses which are so overly conservative that they no longer reflect reality and may skew design and decision making. The expectation should be for the overall analysis to be demonstrated to be conservative.

Consider revising this text:

As discussed in that report, simplified procedures may be used; however, all methods and assumptions should be clearly conservative. If it cannot be demonstrated clearly that no credible external flooding event due to ice jams could affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

To read:

Simplified procedures may be used; however, the overall methods and assumptions should provide a demonstrably conservative result. If it cannot be demonstrated clearly that external flooding event due to ice jams is not likely to affect the site, a detailed analysis of this external flood-causing mechanism should be performed.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-38

55.

I Flooding Caused by Combined Events Page I-1 Given that the guidance on estimation of storm surge provided in Section C.3.d and E-4.4 already addresses the impact of tidal variation on potential storm surge estimates, it should not be included as an example of a different combined event.

Consider revising this paragraph to read as follows:

This appendix discusses approaches for considering combined processes and events when estimating a design-basis flood at a nuclear power plant (NPP) site. Many hydrometeorological phenomena capable of generating an external flood can occur concurrently because they are not truly independent mechanisms.

For example, a riverine flood due to basin wide precipitation may coincide with spring snowmelt, increasing the magnitude of flooding. In coastal regions, precipitation associated with a hurricane may coincide with storm surge. Therefore, consideration of flooding resulting from a single process or event is generally not adequate to determine the design-basis flood. Reasonable sequences and combinations of processes and events based on site-specific information should be considered.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-39

56.

I-1.5 Probabilities of Combined Events Page I-2 Regarding this text in I-1.5:

ANSI/ANS-2.8-1992 (Ref. 1) considers an annual exceedance probability of 1x10-6 to represent a reasonable criterion for selecting the design-basis flood at an NPP site.

The U.S. Nuclear Regulatory Commission staff has applied the 1x10-6 criterion in previous safety reviews and considers it an appropriate goal for considering combined events. However, ANSI/ANS-2.8-1992 does not include guidance on formal probabilistic flood hazard assessment approaches needed for a consistent treatment of combined events. ANSI/ANS-2.8-2019 (Ref. 3) proposes a graded screening process for evaluating external flood-causing mechanisms but does not provide detailed review guidance. There are few consistent, well-established methods for estimating the exceedance probability of probable maximum events. (For example, Refs. 4 and 5 offer recommendations on how to treat external events, including floods, in a risk assessment.) Generally, the reasonableness of qualitative and quantitative probability estimates for combined events must be assessed case by case.

The following are not clear:

1) How can an applicant establish reasonable confidence limits at the level of 1x10-6, given the available data and the multiple orders of magnitude of extrapolation required?

2) How can qualitative methods or semi-quantitative methods be used to achieve such levels of precision?

3) What specific methods and assumptions are technically justified and acceptable to the NRC in terms of achieving the 1x10-6 AEP metric for determining which combined flooding event scenarios need to be considered?

4) What alternatives are acceptable in the event that data and methods are not available to support an AEP of 1x10-6 with reasonable confidence limits, for various flood mechanisms of combinations thereof?

The DG expresses an expectation of the use of probabilistic, frequency-based methods, at least in the context of determination of which combination of flooding hazard mechanisms need to be considered, while at the same time acknowledging that methods and data acceptable to the NRC for this analysis do not exist. There is concern that the use of 1x10-6 as a metric implies a level of precision that is not The DG should be revised to provide guidance consistent with the level of safety that provides reasonable assurance and in consideration of the state-of-the-practice methods and data available for practical implementation, such that regulatory efficiency and stability are achieved.

Because there are no acceptable methods for determining a 1x10-6 AEP flood hazard, guidance should be provided for an acceptable approach using a less stringent threshold (such as 1x10-4) with an acceptable mitigation strategy for greater floods, if needed.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-40 justified and should not be necessary to provide reasonable assurance of protection of the health and safety of the public in the event of an external flood. Deferring this discussion to case-by-case reviews and/or pre-application negotiations will lead to regulatory inefficiency and instability.

57.

I-2 References Page I-2 Appendix I is missing a references section.

A references section with the appropriate references should be added to Appendix I.

58.

J-1 Coincident Wave Heights Page J-2 The last paragraph in this section should allow the use of other suitable models besides the WAM or WAVEWATCH III models, given that these models may be revised in the future.

Consider revising this text:

Consistent with USACE CEM guidance, off-coast wave activity should be determined using either the WAM (Ref. 6) or the WAVEWATCH III model (Ref. 7).

To read:

Consistent with USACE CEM guidance, off-coast wave activity should be determined using suitable models such as WAM (Ref. 6) or the WAVEWATCH III model (Ref. 7).

59.

J-2 Wave Runup Page J-2 Grammatically, the first sentence should use either or as opposed to either and.

Consider revising this text:

Wave runup can be calculated using the lesser of the maximum wave height (1.67 x the significant wave height) and the maximum breaker height, in accordance with ANSI/ANS-2.8-1992 (Ref. 3) and the USACE CEM (Ref. 4).

To read:

Wave runup can be calculated using the lesser of the maximum wave height (1.67 x the significant wave height) or the maximum breaker height, in accordance with ANSI/ANS-2.8-1992 (Ref. 3) and the USACE CEM (Ref. 4).

60.

J-2 Wave Runup Page J-2 (Editorial) To make the text more generally applicable, delete in the past in the last sentence of this section.

Revise this text:

Other models and methods used in the past include those of Goda (Ref. 11), the Technical Advisory Committee for Water Retaining Structures (Ref. 12) and van der Meer et al. (Ref. 13).

To read:

Other models and methods that have been used include those of Goda (Ref. 11), the Technical Advisory Committee for Water Retaining Structures (Ref. 12) and van der Meer et al. (Ref. 13).

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-41

61.

J-4 Hydrostatic and Hydrodynamic Forces Page J-2 The first sentence in the second paragraph incorrectly references Ref.

14.

Update the first sentence in the second paragraph with the correct reference.

62.

J-5 Debris and Water-Borne Projectiles Page J-3 The references provided in this section do not provide this information or guidance on the types of debris to be considered in particular environments.

Consider providing specific guidance, or references to other documents that do contain this guidance, on the types of debris to be considered in a particular environment.

63.

Section J-5 Debris and Water-Borne Projectiles Page J-3 The second paragraph incorrectly references Ref.14.

Update the first sentence in the second paragraph with the correct reference.

64.

J-6 Effects of Sediment Erosion or Deposition Page J-3 (Editorial) Delete storm surge or seiche from the first sentence.

Consider revising this text:

The impact of sediment erosion and deposition should be considered when storm surge or seiche flood levels are predicted to impinge on flood protection features, SSCs important to safety, and foundation materials.

To read:

The impact of sediment erosion and deposition should be considered when flood levels are predicted to impinge on flood protection features, SSCs important to safety, and foundation materials.

65.

Section J-6 Effects of Sediment Erosion or Deposition Page J-3 (Editorial) In the second sentence of this section, replace storm surge or seiche with flood.

Consider revising this text:

Thus, current velocity and wave and wind data from storm surge or seiche models should be retained and used for a detailed analysis of the effects of sediment erosion and deposition.

To read:

Thus, current velocity and wave and wind data from flood models should be retained and used for a detailed analysis of the effects of sediment erosion and deposition.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-42

66.

K Considerations for Applying Guidance to Advanced Reactors and Small Modular Reactors Page K-1 This section should add a reference to processes described in NEI 18-04, as endorsed in RG 1.233.

Consider revising this text:

Regulatory Guide 1.247, Acceptability of Probabilistic Risk Assessment Results for Non-Light-Water Reactor Risk-Informed Activities (Ref. 3), outlines a process to evaluate non-light-water reactor design safety for both internal and external events. The staff believes that potential applicants have flexibility in how they intend to demonstrate compliance with the Commissions regulations, including in how they use probabilistic risk assessment (PRA) results. Appendix K is presented as an alternative to the guidance in Regulatory Guide 1.247.

To read:

Regulatory Guide 1.247, Acceptability of Probabilistic Risk Assessment Results for Non-Light-Water Reactor Risk-Informed Activities (Ref. 3), outlines a process to evaluate non-light-water reactor design safety for both internal and external events. For applicants who choose to use this process as described in NEI 18-04, as endorsed by RG 1.233, a set of Design Basis External Hazard Levels (DBEHLs) will be selected to form an important part of the design and licensing basis. This process will determine the design basis flooding events and other external events that the safety-related SSCs will be required to withstand. The staff believes that potential applicants have flexibility in how they intend to demonstrate compliance with the Commissions regulations, including how they use probabilistic risk assessment (PRA) results. Appendix K is presented as an alternative to the guidance in Regulatory Guide 1.247.

67.

K All parts Pages K-1 through K-5 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. The terms safety-related, non-safety-related with special treatment, and/or non-safety-related with no special treatment are those endorsed in NEI 18-04.

As described above, all uses in Appendix K of the term SSCs important to safety should be replaced by safety-related SSCs consistent with the endorsed terminology for Advanced Reactors.

Search the text in Appendix K and replace the term SSCs important to safety with safety-related SSCs.

Consolidated Comments on Draft Regulatory Guide DG-1290, Revision 1, Design Basis Floods for Nuclear Power Plants A1-43

68.

Figure K-1 Flowchart of flood hazard evaluation for advanced reactor and SMR applicants Page K-5 Evaluation of adequacy statements provided in Figure K-1 are comparable to those in the NEI 18-04 Evaluation of Defense in Depth Adequacy process. Reference could be made NEI 18-04 to strength this guidance by referring to an endorsed methodology for evaluating the adequacy of DID. Adding a reference to NEI 18-04 would also help establish appropriate guidance to be used to determine if mitigating plans in response to notice of impending flood event are considered risk or safety-significant, and to what degree that may influence their incorporation into plant EOPs, HRA, etc.

Consider revising the diamonds of the flowchart:

IDENTIFY POTENTIAL FLOODING HAZARDS AFFECTING PLANT DESIGN/PERFORMANCE AT SITE EVALUATE ADEQUACY OF ENGINEERING FEATURES (E.G., SSCs IMPORTANT TO SAFETY WITH FLOOD PROTECTION FEATURES)

EVALUATE ADEQUACY OF MITIGATING PLANS IN RESPONSE TO NOTICE OF IMPENDING FLOOD EVENT To read:

IDENTIFY POTENTIAL FLOODING HAZARDS AFFECTING PLANT DESIGN/PERFORMANCE AT SITE*

EVALUATE ADEQUACY OF ENGINEERING FEATURES* (E.G., SAFETY-RELATED SSCs WITH FLOOD PROTECTION FEATURES)

EVALUATE ADEQUACY OF MITIGATING PLANS IN RESPONSE TO NOTICE OF IMPENDING FLOOD EVENT*

And add a footnote to the Figure:

• Note: The process for establishing Defense in Depth (DID) adequacy described in NEI 18-04 as endorsed by RG 1.233 is one such acceptable process that can be used to identify and evaluate these hazards."

69.

L-2 Definitions Page L-4 A definition of demonstratively conservative should be added to Appendix L to support the addition of this terminology to other sections in the comments above.

Add the following definition to section D-2:

demonstrably conservative - A flood hazard evaluation (deterministic or probabilistic) that is supported by observed data 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 Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-1 Number Section/

Page Comment/Basis Recommendation

1.

General The words from Section C1.4 are insightful and important in terms of the ability to implement the guidance in this DG:

Probabilistic approaches for estimating the extreme rainfall and flood events of interest in this RG (e.g., 1x10-4 per year or lower annual exceedance probability) exist. However, current industry consensus standards and federal guidance provide guidelines, but not detailed guidance. The NRC has established probabilistic screening criteria for man-related hazards (e.g., between 1x10-7 and 1x10-6 annual exceedance probability) that are, in theory, applicable to dam failures. However, no widely accepted methodology exists for estimating dam failure probabilities on the order of 1x10-7 to 1x10-6 annual exceedance probability.

This text acknowledges that methods and data to achieve the 1x10-6 levels specified in DG-1290 and DG-1417 do not exist. There is considerable academic information on the data, methods, and considerations that would be used in such an analysis in the DGs, but in the end, it is not possible to achieve the metrics that the NRC has put forth as criteria in these DGs.

The expectation put forth in this text and throughout DG-1290 is inconsistent with the NRC Commissions PRA Policy Statement (August 1995) one objective of which was that PRA should be implemented in a consistent and predictable manner which promotes regulatory stability and efficiency - which is not achieved if every application is reviewed and considered on a case-by-case basis where expectations and guidance are unclear. Depending on pre-application meetings and case-by-case reviews will lead to inefficiency and regulatory instability.

This is further inconsistent with the PRA Policy Statement which also states that the use of PRA technology should be increased in all regulatory matters to the extent supported by the state of the art in PRA methods and data - where it is clear based on the words in the DG, that the current PRA methods and data as described in the RG are not sufficient to demonstrate or meet the 1x10-6 criteria that the DG establishes as guidance.

The DGs should be revised to provide guidance consistent with the level of safety that provides reasonable assurance and in consideration of the state-of-practice methods and data available for practical implementation, such that regulatory efficiency and stability are achieved.

Because there are no acceptable methods for determining 1x10-6 dam failure probabilities, and in light of the fact that dam regulators and designers/owners are the entities who have access to the information necessary to perform detailed analysis of dam failure probabilities, this DG should be revised to be consistent with the types of analyses that the NPP applicant can perform and at levels of precision consistent with the current dam regulations and state of practice.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-2

2.

General There are two levels of screening included in the document which can be directly used by NPP applicants:

1) The screening of dams to identify non-critical dams which do not require further analysis (Sections C1.2/ Figure 2, C2.2/ Figures 4/5/6/7)

2) For dams that remain as potentially critical, the seismic screening methods (C4.3/Figure 10) which provide the options to conclude non-failure or assume failure in lieu of detailed site-specific dam analysis.

Aside from the processes above, most of the DG document includes knowledge base information about various factors that should be considered and evaluated in detailed analysis of dams. In practical terms, this information cannot be implemented or addressed by the NPP applicant because the information required to perform such an analysis is controlled and owned by 3rd parties (unless the utility owns the dam in question) and the NPP applicant is unlikely to gain access to these details.

Consider relocation of the technical details and considerations for dam analysis identified through the development and application of JLD-ISG-2013-01 information into a NUREG-KM to facilitation knowledge transfer and retention.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-3

3.

B1.

Reason for Issuance Page 7 Section B1 can be enhanced by referencing the Fukushima flooding reanalysis as potential sources of information or example for performing the types of analyses described in this DG.

Add the following at the end of the second of this section, Another important source of information is the site-specific flood analyses performed by affected owners and operators in response to the 2011 Fukushima event. These flood hazard reevaluations are publicly available in ADAMS.

Consider revising this text:

This RG is being issued to formally incorporate interim staff guidance (ISG), Guidance for Assessment of Flooding Hazards Due to Dam Failure, dated July 29, 2013 (JLD-ISG-2013-01, NRC 2013) into the NRCs regulatory framework.

JLD-ISG-2013-01 was prepared to aid completion of flood hazard reevaluations performed by NPP licensees in response to the NRCs 10 CFR 50.54(f) information request issued following the 2011 Fukushima Dai-ichi accident (NRC, 2012). The flood hazard reevaluations were to be performed using analysis methods current in 2012 and guidance used by the NRC staff for reviewing external flooding analyses for early site permit and combined license applications submitted under 10 CFR Part 52. ISGs were prepared to clarify or address issues not fully discussed in the NRCs Standard Review Plan (NUREG-0800), which was to be used as the basic reference for reviewing the post-Fukushima flood hazard reevaluations. ISGs should be withdrawn after their immediate purpose has been fulfilled or else integrated formally into the NRCs regulatory guidance framework.

To read:

This RG is being issued to formally incorporate interim staff guidance (ISG), Guidance for Assessment of Flooding Hazards Due to Dam Failure, dated July 29, 2013 (JLD-ISG-2013-01, NRC 2013) into the NRCs regulatory framework.

JLD-ISG-2013-01 was prepared to aid completion of flood hazard reevaluations performed by NPP licensees in response to the NRCs 10 CFR 50.54(f) information request issued following the 2011 Fukushima Dai-ichi accident (NRC, 2012). The flood Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-4 hazard reevaluations were to be performed using analysis methods current in 2012 and guidance used by the NRC staff for reviewing external flooding analyses for early site permit and combined license applications submitted under 10 CFR Part 52. These flooding reevaluations performed by NPP owner/operators following the Fukushima event are publicly available in ADAMS and may be a valuable source of information or example for analyses as described in this DG. At the time, ISGs were prepared to clarify or address issues not fully discussed in the NRCs Standard Review Plan (NUREG-0800), which was to be used as the basic reference for reviewing the post-Fukushima flood hazard reevaluations. ISGs should be withdrawn after their immediate purpose has been fulfilled or else integrated formally into the NRCs regulatory guidance framework.

4.

B2.

Background

Page 8 (Editorial) The following sentence should be revised to more accurately reflect an upstream dam could pose a flooding risk to downstream NPPs:

In general, failure of any dam upstream from the plant site is a potential flooding mechanism.

Consider revising this text:

In general, failure of any dam upstream from the plant site is a potential flooding mechanism.

To read:

In general, failure of a dam upstream from the plant site could be a potential flooding mechanism.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-5

5.

C1.3.

Detailed Analysis Page 13 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent it is obtainable to the first sentence of the second paragraph as indicated in the recommendation.

Consider revising this text:

Dam failure flood hazard estimation will require collecting data on the dam(s) to be analyzed (e.g.,

design documents, construction records, maintenance and inspection program, and planned modifications), as well as hydrometeorological and hydrologic data (e.g., design storms, topography, rainfall-runoff characteristics) on the river basin(s) in question.

To read:

Dam failure flood hazard estimation will require collecting data, to the extent it is obtainable, on the dam(s) to be analyzed (e.g., design documents, construction records, maintenance and inspection program, and planned modifications), as well as hydrometeorological and hydrologic data (e.g.,

design storms, topography, rainfall-runoff characteristics) on the river basin(s) in question.

6.

C1.3.

Detailed Analysis Page 15 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent it is obtainable to the first step as indicated in the recommendation.

Consider revising this text:

In summary, the detailed dam failure flood hazard analysis for potentially critical dams will comprise the following steps (see also Figure 3):

1. Data collection To read:

In summary, the detailed dam failure flood hazard analysis for potentially critical dams will comprise the following steps (see also Figure 3):

1. Data collection, to the extent it is obtainable

7.

C1.4.2.

Hydrologic Failure Page 17 The wording of the second Key Point is unclear. A recommendation for re-wording based on the assumed intent is provided.

Consider revising this text:

When considering hydrologic failure due to large floods, extreme caution should be used in estimating the probability of deterministic estimates such as the PMP or PMF.

To read:

When considering hydrologic failure due to floods it should not be assumed that deterministic estimates such as PMP or PMF equate directly to a specific AEP.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-6

8.

C1.4.3.

Seismic Failure Page 18 In Key Point 1 in this section (as well as Figure 10 and other locations in the document), the guidance allows that if the applicant can document sufficient seismic margin/factor of safety, then a 1x10-4 ground motion can be used. Specifically, what is required to document sufficient seismic margin/factor of safety should be described in the document. This explanation should specifically consider situations where the applicant does not own the dam or have access to the seismic analysis of the dam, such that it is clear how the applicant can justify sufficient seismic margin to use 1x10-4 ground motion.

Add explanation of how applicants can justify sufficient seismic margin/factor of safety to use the 1x10-4 seismic ground motion. The explanation should take into account situations where applicants do not own the dam and are not responsible for the seismic analysis of the dam.

9.

C1.4.3 Seismic Failure Page 18 This section appears to not be consistent with other parts of the document concerning the coincident loads that need to be considered when assessing potential seismic failure of dams and/or consequences of dam failures. For example, sections C1.4.3, C4, Figure 10, C4.4, and C4.6 all contain different variations and guidance relative to combining specific ground motions with certain flood frequencies and/or PMPs, and wind speed assumptions. What the applicant is specifically expected to evaluate is not clear. Additionally, the technical basis for this guidance is not understood.

Specific words of interest in this section include:

To account for this level of margin before failure, applicants may use the 1x10-4 annual frequency ground motions, at spectral frequencies important to the dam, for seismic evaluation of dams, instead of 1x10-6, as discussed above. However, appropriate engineering justification should be provided to show that the dam has sufficient seismic margin. Otherwise, the 1x10-6 ground motions should be used.

A dam should be assumed to fail if it cannot withstand the relevant seismic hazards (e.g., vibratory ground motion at spectral frequencies of importance, fault displacement, loss of strength) with an annual exceedance probability of 1x10-4 per year.

In addition, if the seismic capacity of the dam is considerably less than what is required to withstand the 1x10-4 seismic hazard, then the possibility of large (though not extreme) floods should be considered. Therefore, the dam should be assumed to fail due to seismic hazard if it cannot withstand the more severe of the following combinations:

4 annual exceedance seismic hazard combined with a 25-year flood, or

- half of the 10-4 ground motion, combined with a 500-year flood.

Revise sections C1.4.3, C4, Figure 10, C4.4, and C4.6 to make guidance consistent, as applicable. Please provide technical basis for the guidance/requirements included in the DG.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-7

10.

C1.4.4.

Sunny Day Failure Page 18 The following text:

When data are not available to confirm that the NRC staff objective has been met, a higher calculated event threshold (1x10-6 per year) is acceptable when combined with reasonable qualitative evidence that the best estimate is lower than 1x10-6. This exception is made since data are often not available to enable the accurate calculation of probabilities because of the low probabilities associated with the events under consideration.

identifies two important and relevant challenges with being able to perform the analysis and meet many of the criteria specific in this DG:

1) Data are often not available to enable the accurate calculation of probabilities at the levels that NRC has identified in this DG and DG-1290; and

2) It is unclear how qualitative evaluations can be used to justify that a best estimate is lower than 1x10-6.

The DG should be modified to use realistic thresholds that can be met in practical application by an NPP applicant based on the data and methods available.

11.

C1.4.5.

Interfacing with Dam and Levee Owners and Regulators Page 18 The title of this section suggests that it addresses interfacing with dam and levee owners and regulators; however, it does not provide this information. The section identifies the diverse group of dam owners and regulators, but there is no information on interfacing with them to achieve access to the types of information that would be required to perform the analysis in this DG. It is noted that information that was included in the ISG (Comment 12) on the role of NRC in helping to facilitate this interface and obtain the necessary information or results has been deleted from the DG. This role is likely necessary if there is to be a successful interface.

Add detail addressing specifically interfacing with the various dam owners and regulators to obtain access to the required information or to be able to obtain confirmation of acceptable results necessary to perform the evaluations described in this DG. Or retitle this section since it does not address Interfacing.

12.

C1.4.8.

Obtaining Information on Dams and Levees Page 21 This section should acknowledge that there is a security concern related to sharing the types of detailed design information on dams that would be required to perform or document the analyses identified in this DG. This is another compounding factor that makes access to the information needed to perform any of the analysis in this DG challenging.

Add a description of the security concerns related to sharing of detailed dam information as one of the reasons obtaining detailed information to support the dam flood hazard evaluation may be challenging.

13.

C1.4.8.

Obtaining Information on Dams and Levees Page 21 Staff positions in the ISG indicated support from NRC as an interface between agencies and licensees. The way the key points were re-written in the DG lessened the NRC involvement in helping to obtain information. (See Page 1-11 in ISG)

Include the original staff positions contained on Page 1-11 in the ISG in this DG. This level of interface and support is likely necessary if there is to be a successful interface to obtain the required information our outcome.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-8

14.

C2 Screening and Simplified Modeling Approaches for Watersheds with Many Dams Page 21 The Figure 1 referenced in the following sentence:

Section C1.2 and Figure 1 provided an overview of a screening approach that reduces the analysis burden for watersheds with many dams.

is not the same as the Figure 1 in the ISG. Figure 1 from the ISG was not included in the proposed draft regulatory guide. The Figure 1 from the ISG provides a useful overview of screening approaches intended to reduce the analysis burden for watersheds with many dams and should be included in the proposed regulatory guide.

The Figure 1 that this text currently refers to in DG is Reservoir Water Levels and Corresponding Storage Volumes and is not the correct figure to refer to here.

Include the correct Figure 1 from the ISG in the DG.

15.

C2 Screening and Simplified Modeling Approaches for Watersheds with Many Dams Page 21 In this section, the ISG referred to a Figure 11 Screening approach for watersheds with many dams; however the proposed DG has omitted this figure.

Include Figure 11 from the ISG in the DG.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-9

16.

C2.2.

Simplified Modeling Approaches Page 23 It is not clear what is meant by the term hierarchical-hazard-assessment-type gradation of conservatism in the following sentence:

The methods are presented in a hierarchical-hazard-assessment (HHA) type gradation of conservatism (see NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America (NRC, 2011)), and are applicable to all initiating events (hydrologic, seismic, and sunny day).

The following methods may be applied sequentially in a hierarchical-hazard-assessment type gradation of conservatism Is it the same or different from the general use of the term hierarchical-hazard-assessment (HHA)?

Consider revising this text:

The methods are presented in a hierarchical-hazard-assessment (HHA) type gradation of conservatism (see NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America (NRC, 2011)), and are applicable to all initiating events (hydrologic, seismic, and sunny day).

The following methods may be applied sequentially in a hierarchical-hazard-assessment type gradation of conservatism.

To read:

The methods are presented in a hierarchical-hazard-assessment (HHA) approach (see NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America (NRC, 2011)), and are applicable to all initiating events (hydrologic, seismic, and sunny day).

The following methods may be applied sequentially in an HHA-like.

17.

C2.3.

Representing Clusters of Dams It is not clear what the difference is between black dams and white dams means in Figure 8. This makes it difficult to understand the differences between the far right and far left drawings in Figure 8.

Add a legend explaining the coloring of the dams in Figure 8 Dam Clustering Examples.

18.

C3.1.4.

Levees Page 32 The number of significant figures describing water surface elevation in meters in the following sentence is not appropriate:

A frequently loaded levee is one that experiences a water surface elevation of 1 foot (0.3048m) or higher above the elevation of the landside levee toe at least once a day for more than 36 days per year on average (CADWR, 2012).

Consider revising this text:

A frequently loaded levee is one that experiences a water surface elevation of 1 foot (0.3048m) or higher above the elevation of the landside levee toe at least once a day for more than 36 days per year on average (CADWR, 2012).

To read:

A frequently loaded levee is one that experiences a water surface elevation of 1 foot (0.3m) or higher above the elevation of the landside levee toe at least once a day for more than 36 days per year on average (CADWR, 2012).

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-10

19.

C3.2.1 Internal Pressure Page 33 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent it is obtainable to the last key point in this section as shown in the recommendation.

Consider revising this text:

Evaluation should generally include reviewing the dam design to ensure that appropriate filters, drains, and monitoring points are included. Monitoring records from piezometers, observation wells, or other observation methods can be used to infer the absence of unremediated deficiencies.

To read:

Evaluation should generally include reviewing the dam design to ensure that appropriate filters, drains, and monitoring points are included. To the extent that it is obtainable, monitoring records from piezometers, observation wells, or other observation methods can be used to infer the absence of unremediated deficiencies.

20.

C3.2.3.

Reservoir Capacity Page 34 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent that they are obtainable to the key point in this section as shown in the recommendation.

Consider revising this text:

The potential for reductions in reservoir capacity due to sedimentation over the life of an NPP should be considered. Records from periodic bathymetric surveys of the reservoir, records of sediment production in upstream reaches, or estimates of sediment production rates for the upstream watershed can be used to support modeling assumptions.

To read:

The potential for reductions in reservoir capacity due to sedimentation over the life of an NPP should be considered. To the extent that they are obtainable, records from periodic bathymetric surveys of the reservoir, records of sediment production in upstream reaches, or estimates of sediment production rates for the upstream watershed can be used to support modeling assumptions.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-11

21.

C3.2.4.

Starting Reservoir Elevation Page 35 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent it is obtainable to the key point in this section as shown in the recommendation.

Consider revising this text:

In view of the uncertainties involved in estimating reservoir levels that might reasonably be expected to prevail at the beginning of a flooding event, the default starting water surface elevation used in flood routings for evaluation of overtopping should be the maximum normal pool elevation (i.e., the top of the active storage pool). Other starting water surface elevations may be used, with appropriate justification. Justification should be based on the operating rules and operating history of the reservoir. For example, if the flood being considered is associated with a distinct season and the operation of the dam has seasonal variations that are codified and have historically been followed, then it may be reasonable to select a starting reservoir elevation consistent with the operating rules and history. The operating history used should be of sufficient length to support any conclusions (e.g., 20 years or more). However, consideration should be given to possible instances in which the operating history or rules have been influenced by anomalous conditions such as drought.

To read:

In view of the uncertainties involved in estimating reservoir levels that might reasonably be expected to prevail at the beginning of a flooding event, the default starting water surface elevation used in flood routings for evaluation of overtopping should be the maximum normal pool elevation (i.e., the top of the active storage pool). Other starting water surface elevations may be used, with appropriate justification. Justification should be based on the operating rules and operating history, if available, of the reservoir. For example, if the flood being considered is associated with a distinct season and the operation of the dam has seasonal variations that are codified and have historically been followed, then it may be reasonable to select a starting reservoir elevation consistent with the operating rules and history. To the extent that it is obtainable, Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-12 the operating history used should be of sufficient length to support any conclusions (e.g., 20 years or more). However, consideration should be given to possible instances in which the operating history or rules have been influenced by anomalous conditions such as drought.

22.

C3.2.7.

Potential for Debris to Block Reservoir Spillway Page 36 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent it is obtainable as shown in the recommendation.

Consider revising this text:

Historical information on debris production in the watershed (or similar watersheds) can be used to gauge the potential for debris blockage. Dam owners, dam regulators, or river basin commissions often perform periodic debris studies.

To read:

To the extent that it is obtainable, historical information on debris production in the watershed (or similar watersheds) can be used to gauge the potential for debris blockage. Dam owners, dam regulators, or river basin commissions often perform periodic debris studies.

23.

C3.2.7.

Potential for Debris to Block Reservoir Spillway Page 37 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent that they are obtainable to the key point as shown in the recommendation.

Consider revising this text:

For dams that lack debris management, greater capacity reductions should be considered. The appropriate capacity reduction will vary on a case-by-case basis. The reduction used should be justified (e.g., by debris studies, for the watershed or similar watersheds).

To read:

For dams that lack debris management, greater capacity reductions should be considered. The appropriate capacity reduction will vary on a case-by-case basis. The reduction used should be justified (e.g., by debris studies, for the watershed or similar watersheds to the extent that they are obtainable).

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-13

24.

C4 Seismic Dam Failure Page 45 This section appears to not be consistent with other parts of the document concerning the coincident loads that need to be considered when assessing potential seismic failure of dams and/or consequences of dam failures. For example, sections C1.4.3, C4, Figure 10, C4.4, and C4.6 all contain different variations and guidance relative to combining specific ground motions with certain flood frequencies and/or PMPs, and wind speed assumptions. What the applicant is specifically expected to evaluate is not clear. Additionally, the technical basis for this guidance is not understood.

Specific words of interest in this section include:

Note that the seismic dam failure scenario is one where load combinations come into play (e.g., a more frequent earthquake combined with a flood event), as discussed in section C4.6. In some instances (e.g., when downstream consequences are likely to be small), the licensee may elect to simply assume that the dam fails seismically in lieu of conducting a seismic analysis. In this case, the question arises of what flood event to assume, since no frequency is assigned to the seismic event. In section C4.6, the 500-year flood is used in conjunction with the lower of the two seismic hazard levels. Therefore, a lesser flood would not be appropriate in this case.

If seismic failure is simply assumed without analysis, the seismic failure should be assumed to occur under 500-year flood conditions (or 1/2 PMF, whichever is less).

Revise sections C1.4.3, C4, Figure 10, C4.4, and C4.6 to make guidance consistent, as applicable. Please provide technical basis for the guidance/requirements included in the DG.

25.

C4.1.1.1 Use of USGS National Seismic Hazar Maps Page 47 In addition to the seismic source characterization models referenced in this section, the relevant attenuation models should also be included.

Add reference to the relevant seismic attenuation models. For example, NGA-East.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-14

26.

C4.3 Analysis of Seismic Hazards Using Readily Available Tools and Information Page 50 Section C4.3 acknowledges, that:

Since there will generally be insufficient time and resources to perform detailed seismic analyses for all dams upstream from the NPP site, the following approach may be applied.

Given this, along with the challenge of access to information controlled by the dam owners, it is unclear how any NPP applicant would be able to implement this DG beyond use of the screening level approaches.

Thus, despite the many sections of detail on the types of dams and factors to consider, it is not clear that an NPP applicant would be able to perform any of the detailed analysis described in the DG.

Consider relocation of the technical details and considerations for dam analysis identified through the development and application of JLD-ISG-2013-01 information into a NUREG-KM to facilitation knowledge transfer and retention.

27.

C4.3 Figure 10 Page 51 Central to the process outlined in Figure 10 is justification of whether the dam meets the criteria for sufficient Documented Margin/Factor of Safety. However, what is sufficient seismic margin/factor of safety is not defined, nor is it explained what is required to document this justification.

Explanation of what is required for sufficient seismic margin/factor of safety and how this is justified should be added to this section.

28.

C4.3 Figure 10 Page 51 This section appears to not be consistent with other parts of the document concerning the coincident loads that need to be considered when assessing potential seismic failure of dams and/or consequences of dam failures. For example, sections C1.4.3, C4, Figure 10, C4.2.4, C4.4, and C4.6 all contain different variations and guidance relative to combining specific ground motions with certain flood frequencies and/or PMPs, and wind speed assumptions. What the applicant is specifically expected to evaluate is not clear. Additionally, the technical basis for this guidance is not understood.

The specific area of interest in Figure 10 are the Option Assume Failure and the Non-Failure boxes which are possible outcomes with comparing either the 1x10-4 or the 1x10-6 seismic demand versus the capacity. These boxes nor these sections reference the various combinations of additional flooding, wind, PMP, etc. which are added to these criteria in various other sections of the documents.

Revise sections C1.4.3, C4, Figure 10, C4.2.4, C4.4, and C4.6 to make guidance consistent, as applicable.

Please provide technical basis for the guidance/requirements included in the DG.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-15

29.

C4.3.1 Ground Shaking Page 52 (Editorial) EPRI now has changed their name to be EPRI as opposed to an acronym for Electric Power Research Institute in their communications.

Revise this text:

A site amplification analysis should be performed to obtain site amplification functions. Methods developed by the Electric Power Research Institute to perform a site response analysis (EPRI, 1989) as described in NUREG/CR-6728, Technical Basis for Revision of Regulatory Guidance on Design Ground Motions: Hazard-and Risk-Consistent Ground Motion Spectra Guidelines (NRC, 2001) are acceptable.

To read:

A site amplification analysis should be performed to obtain site amplification functions. Methods developed by EPRI to perform a site response analysis (EPRI, 1989) as described in NUREG/CR-6728, Technical Basis for Revision of Regulatory Guidance on Design Ground Motions: Hazard-and Risk-Consistent Ground Motion Spectra Guidelines (NRC, 2001) are acceptable.

30.

C4.3.3 Liquefaction Page 53 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent that they are obtainable to the items typically included for initial assessment.

Consider revising this text:

An initial assessment of the potential for earthquake-induced ground failure typically includes:

(1) geomorphology of the site,.

To read:

An initial assessment of the potential for earthquake-induced ground failure typically includes, to the extent that they are obtainable:

(1) geomorphology of the site,.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-16

31.

C4.4 Assessment of Seismic Performance Using Existing Studies Page 54 This section appears to not be consistent with other parts of the document concerning the coincident loads that need to be considered when assessing potential seismic failure of dams and/or consequences of dam failures. For example, sections C1.4.3, C4, Figure 10, C4.4, and C4.6 all contain different variations and guidance relative to combining specific ground motions with certain flood frequencies and/or PMPs, and wind speed assumptions. What the applicant is specifically expected to evaluate is not clear. Additionally, the technical basis for this guidance is not understood.

Specific words of interest in this section include:

studies used should ideally consider seismic capacity for both the maximum normal operating pool level (i.e., top active storage) and average pool level (i.e., 50 percent exceedance duration pool level calculated using average daily water levels for the period of record).

The average nonflood tailwater level should be used with both headwater conditions above.

Revise sections C1.4.3, C4, Figure 10, C4.4, and C4.6 to make guidance consistent, as applicable. Please provide technical basis for the guidance/requirements included in the DG.

32.

C4.4.

Assessment of Seismic Performance Using Existing Studies Page 54 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent that they are obtainable to the items that can be used to demonstrate seismic capacity of a dam.

Consider revising this text:

In lieu of performing a new seismic hazard evaluation of dam performance, applicants may use existing studies or design documentation to demonstrate the seismic capability of a dam.

To read:

In lieu of performing a new seismic hazard evaluation of dam performance, applicants may use existing studies or design documentation, to the extent that they are available, to demonstrate the seismic capability of a dam.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-17

33.

C4.4.2.

Fault Displacement Page 55 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent that they are obtainable to the key point.

Consider revising this text:

Existing studies or data on dam or foundation materials can be used to assess performance of the dam with respect to surface displacement, in light of the seismic hazard defined for the site, with appropriate justification of their applicability and with appropriate conservatism to account for uncertainties.

To read:

To the extent that they are obtainable, existing studies or data on dam or foundation materials can be used to assess performance of the dam with respect to surface displacement, in light of the seismic hazard defined for the site, with appropriate justification of their applicability and with appropriate conservatism to account for uncertainties.

34.

C4.4.3.

Liquefaction Page 56 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent that they are obtainable to the key point.

Consider revising this text:

Existing studies or data on dam or foundation soils can be used to assess performance of the dam with respect to liquefaction or loss of strength, in light of the seismic hazard defined for the site, with appropriate justification of their applicability and with appropriate conservatism to account for uncertainties.

To read:

To the extent that they are obtainable, existing studies or data on dam or foundation soils can be used to assess performance of the dam with respect to liquefaction or loss of strength, in light of the seismic hazard defined for the site, with appropriate justification of their applicability and with appropriate conservatism to account for uncertainties.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-18

35.

C4.6 Modeling Consequences of Seismic Dam Failure Page 45 This section appears to not be consistent with other parts of the document concerning the coincident loads that need to be considered when assessing potential seismic failure of dams and/or consequences of dam failures. For example, sections C1.4.3, C4, Figure 10, C4.4, and C4.6 all contain different variations and guidance relative to combining specific ground motions with certain flood frequencies and/or PMPs, and wind speed assumptions. What the applicant is specifically expected to evaluate is not clear. Additionally, the technical basis for this guidance is not understood.

Specific words of interest in this section include:

If the dam failed under the 1x10-4 annual exceedance probability seismic hazard (ground motion), assume that failure coincides with the peak water level from a 25-year flood in the watershed above the dam.

If the dam failed at less than the 1x10-4 annual exceedance probability seismic hazard (ground motion), assume that failure coincides with the peak water level from a 500-year flood (or 1/2 PMF, whichever is less) in the watershed above the dam.

Water level estimates at the site should include effects of a 2-year windspeed from the critical direction.

Revise sections C1.4.3, C4, Figure 10, C4.2.4, C4.4, and C4.6 to make guidance consistent, as applicable.

Please provide technical basis for the guidance/requirements included in the DG.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-19

36.

C5.2.

Analysis of Sunny Day Failures Page 64 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent that they are obtainable to the last paragraph.

Consider revising this text:

Existing studies or data on dam or foundation soils can be used to assess performance of the dam with respect to liquefaction or loss of strength, in light of the seismic hazard defined for the site, with appropriate justification of their applicability and with appropriate conservatism to account for uncertainties. Base flow conditions for a sunny day failure are typically ignored because of the small discharge and volume compared to that of a dam breach. As a general guidance, base flow can be ignored if the dam breach flow is two times greater than the base flow. When base flow is considered, the discharge is typically estimated based on reported base flows through the dams outlet works or from stream gauge records. Additional inflow (e.g., from a storm event) is not required when analyzing a sunny day breach.

To read:

Existing studies or data on dam or foundation soils can be used to assess performance of the dam with respect to liquefaction or loss of strength, in light of the seismic hazard defined for the site, with appropriate justification of their applicability and with appropriate conservatism to account for uncertainties. Base flow conditions for a sunny day failure are typically ignored because of the small discharge and volume compared to that of a dam breach. As a general guidance, base flow can be ignored if the dam breach flow is two times greater than the base flow. When base flow is considered, the discharge is typically estimated based on reported base flows through the dams outlet works or from stream gauge records to the extent that they are obtainable. Additional inflow (e.g., from a storm event) is not required when analyzing a sunny day breach.

Consolidated Comments on Draft Regulatory Guide DG-1417, Guidance for Assessment of Flooding Hazards Due to Water Control Structure Failures and Incidents Page A2-20

37.

C5.2.1.

Sunny Day Failure Modes Page 64 Unless the dam or levee is owned by the NPP Site Owner/Operator, it may be difficult for the applicant to obtain access to the information described in the DG. Add to the extent that they are obtainable to this section.

Consider revising this text:

An essential element in evaluating the potential for sunny day failure is assessment of credible failure modes. Section C5.1 discusses common sunny day failure modes for various dam types. That discussion is fairly comprehensive, but it is not meant to be exhaustive. The purpose of the discussion is to inform the process of identifying potential failure modes. In general, identifying potential failure modes requires reviewing all relevant background information on a dam, including geology, design, analysis, construction, operations, dam safety evaluations, and performance monitoring documentation.

To read:

An essential element in evaluating the potential for sunny day failure is assessment of credible failure modes. Section C5.1 discusses common sunny day failure modes for various dam types. That discussion is fairly comprehensive, but it is not meant to be exhaustive. The purpose of the discussion is to inform the process of identifying potential failure modes. In general, identifying potential failure modes requires reviewing all relevant background information on a dam to the extent that it is obtainable, including geology, design, analysis, construction, operations, dam safety evaluations, and performance monitoring documentation.