ML16355A381

Staff Assessment of Response to 10 CFR 50.54(F) Information Request - Flood-Causing Mechanism Reevaluation (Public)

ML16355A381
Person / Time
Site:	Robinson
Issue date:	01/05/2017
From:	Juan Uribe Japan Lessons-Learned Division
To:	Glover R Duke Energy Progress
URIBE, JUAN F. NRR/JLD 415-3809
Shared Package
ML16355A365	List: ML16355A381
References
CAC MF3586
Download: ML16355A381 (33)

Text

OFFICIAL USE ONLY*SECURITY RELATED INFORMATION UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 January 5, 2017 Mr. Richard Michael Glover Site Vice President H.B. Robinson Steam Electric Plant Duke Energy 3581 West Entrance Road, RNPA01 Hartsville, SC 29550

SUBJECT:

H.B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO. 2 - STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST

- FLOOD-CAUSING MECHANISM REEVALUATION (CAC NO. MF3586)

Dear Mr. Glover:

By letter dated March 12, 2012, the U.S. Nuclear Regulatory Commission (NRC) issued a request for information pursuant to Title 1O of the Code of Federal Regulations, Section 50.54(f)

(hereafter referred to as the 50.54(f) letter). The request was issued as part of implementing lessons learned from the accident at the Fukushima Dai-ichi nuclear power plant. Enclosure 2 to the 50.54(f) letter requested that licensees reevaluate flood-causing mechanisms using present-day methodologies and guidance. By letter dated March 12, 2014 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML14086A384), Duke Energy Progress, LLC (Duke, the licensee) responded to this request for H. B. Robinson Steam Electric Plant, Unit No. 2.

By letter dated December 23, 2015 (ADAMS Accession No. ML15357A065), the NRC staff sent Duke a summary of the staff's review of the licensee's reevaluated flood-causing mechanisms.

The enclosed staff assessment provides the documentation supporting the NRC staff's conclusions summarized in the letter. As stated in the letter, the reevaluated flood hazard results for local intense precipitation, streams and rivers, failure of dams, storm surge, and seiche were not bounded by the current design-basis flood hazard. Therefore, the NRC staff anticipates that the licensee will complete additional evaluations of the unbounded flood mechanism, through a focused evaluation or integrated assessment, as discussed in COMSECY-15-0019, "Closure Plan for the Reevaluation of Flooding Hazard for Operating Nuclear Power Plants," and Japan Lessons-Learned Division (JLD) Interim Staff Guidance (ISG) JLD-ISG-2016-01, "Guidance for Activities Related to Near-Term Task Force Recommendation 2.1, Flooding Hazard Reevaluation; Focused Evaluation and Integrated Assessment." This closes out the NRC's efforts associated with CAC No. MF3586.

Enclosure 1 transmitted herewith contains Security-Related Information. When separated from Enclosure 1, this document is decontrolled.

OFFICIAL USE ONL¥=SECURIT'/ RELATED INFOFIMA'flON R. Glover If you have any questions, please contact me at (301) 415-3809 or e-mail at Juan.Uribe@nrc.gov.

Jua Uribe, Project Manager Haza s Management Branch Japan Lessons-Learned Division Office of Nuclear reactor Regulation Docket No. 50-261

Enclosures:

1. Staff Assessment of Flood Hazard Reevaluation Report (non-public, Security related information)

2. Staff Assessment of Flood Hazard Reevaluation Report (public) cc w/encl: Distribution via Listserv OFFICIAL USE ONLY SECURITY RELATED INFORMATIOH

OFFICIAL USE ONLY-SECURITY RELATED INFORMATION R. Glover H. B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO. 2 - STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST- FLOOD-CAUSING MECHANISM REEVALUATION DATED January 5, 2017.

DISTRIBUTION:

PUBLIC GBowman, NRR MShams, NRR JUribe, NRR RidsNroDsea Resource CCook, NRO SHelton, NRO BHarvey, NRO JGiacinto, NRO JThompson , NRO LQuinnWillingham, NRO SBensi, NRO RidsNrrPMRobinson RidsNrrDorllpl2-1 ADAMS Accession Nos.: PKG ML16355A365; ML16350A205(ENCL1 NON-PUBLIC);

ML16355A381 (ENCL 2 PUBLIC) *Email Concurrence OFFICE NRR/JLD/JHMB/PM NRR/JLD/LA NRO/DSENRHM1/TR NAME JUribe Slent HAhn DATE 12/14/2016 12/20/2016 11/3/2016 OFFICE NRO/DSENRHM1/BC NRR/JLD/JHMB/BC(A) NRR/JLD/JHMB/PM NAME CCook GBowman (BTitus (A)) JUribe DATE 11/3/2016 12/30/2016 1/5/2017 OFFICIAL RECORD *COPY OFFICIAL USE ONLY-SECURITY RELATED INFORMATION

OFFICIAL l::ISE ONLY SECl::IRITY RELATED INFORMATION STAFF ASSESSMENT BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE FLOODING HAZARD REEVALUATION REPORT NEAR-TERM TASK FORCE RECOMMENDATION 2.1 H. B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO. 2 DOCKET NO. 50-261

1.0 INTRODUCTION

By letter dated March 12, 2012, the U.S. Nuclear Regulatory Commission (NRC) issued a request for information to all power reactor licensees and holders of construction permits in active or deferred status, pursuant to Title 1O of the Code of Federal Regulations ( 10 CFR),

Section 50.54(f), "Conditions of Licenses" (hereafter referred to as the "50 .54(f) letter) (NRC ,

2012a) . The request was issued in connection with implementing lessons-learned from the 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant as documented in the Near-Term Task Force (NTTF) Report (NRC, 2011 a) . Recommendation 2.1 in that document recommended that the NRC staff issue orders to all licensees to reevaluate seismic and flooding hazards for their sites against current NRC requirements and guidance (NRC, 2011 a) .

Subsequent Commission Papers, SECY-11-0124 (NRC , 2011 b) and SECY-11-0137 (NRC, 2011 c) directed the NRC staff to issue requests for information to licensees pursuant to 10 CFR 50.54(f) to address this recommendation .

Enclosure 2 to the 50.54(f) letter (NRC, 2012a) requested that licensees reevaluate flood hazards for their respective sites using present-day methods and regulatory guidance used by the NRC staff when reviewing applications for early site permits (ESPs) and combined licenses (COLs). The required response section of Enclosure 2 specified that the NRC staff would provide a prioritization plan indicating the Flooding Hazard Reevaluation Report (FHRR) deadlines for each plant. On May 11, 2012, the NRC staff issued its prioritization of the FHRRs (NRC, 2012c)

By letter dated March 12, 2014, Duke Energy Progress, LLC (Duke, the licensee) provided its FHRR for H. B. Robinson Steam Electric Plant, Unit No. 2 ( Robinson) (Duke, 2014b). Duke provided a revised FHRR with a new local intense precipitation (LIP) and associated site drainage analysis that included a site-specific probable maximum precipitation (ssPMP) evaluation on August 29, 2015 (Duke, 201 Sb). The NRC staff issued requests for additional information (RAls) to the licensee on June 18, 2014 (NRC, 2014a) , March 4, 2015 (NRC, 2015a) , and December 14, 2015 (NRC, 2015c). The licensee responded to the RAls by letters dated July 9, 2014 (Duke, 2014c) , May 26 , 2015 (Duke, 2015a), and December 15, 2015 (Duke, 2015c) . The FHRR and responses to the associated RAls provide the flood hazard input necessary to complete the additional assessments consistent with the process outlined in COMSECY-15-0019 (NRC, 2015b), Japan Lessons Learned (JLD)lnterim Staff Guidance (ISG)

JLD-ISG-2012-01 , Revision 1 (NRC, 2016a) , and JLD-ISG-2016-01 , Revision 0 (NRC, 2016b) .

On December 23, 2015, the NRC issued a revised Interim Staff Response (ISR) letter to the licensee (NRC , 2015d) . The purpose of the !SR letter is to provide the flood hazard information suitable for the assessment of mitigating strategies developed in response to Order EA-12-049, "Requirements for Mitigation Strategies for Beyond-Design-Basis External Events" (NRC, 2012b) and the additional assessments associated with NTTF Recommendation 2.1 : Flooding.

OFFICIAL USli ONbY SliCURITY RELATED INFORMATION Enclosure

OFFICIAL USE ONLY SECURITY RELATED INFORMATION The ISR letter also made reference to this staff assessment, which documents the NRC staff's basis and conclusions. The flood hazard mechanism values presented in the ISR letter's enclosures match the values in this staff assessment without change or alteration.

The reevaluated flood hazard results for LIP, streams and rivers, failure of dams, storm surge and seiche flood-causing mechanisms are not bounded by the plant's current design basis (COB) hazard. Consistent with the 50.54(f) letter and amended by the process outlined in COMSECY-15-0019 and JLD-ISG-2016-01, Revision 0 (NRC, 2015b; NRC, 2016b), the NRC staff anticipates that for LIP, the licensee will perform and document a focused evaluation that assesses the impact of the LIP hazard on the site, and evaluates and implements any necessary programmatic, procedural, or plant modifications to address this hazard exceedance.

For the stream and rivers, failure of dams, storm surge, and seiche flood-causing mechanisms, the NRC staff anticipates that the licensee will submit either (1) an integrated assessment or (2) a focused evaluation confirming the capability of existing flood protection or implementing new flood protection consistent with the process outlined in COMSECY-15-0019 (NRC, 2015b) and JLD-ISG-2016-01, Revision O (NRC, 2016b). Additionally, for any reevaluated flood hazards that are not bounded by the plant's COB hazard, the licensee is expected to develop any flood event duration (FED) and associated effect (AE) parameters that have not been provided as part of the FHRR, in order to conduct the mitigating strategies assessment (MSA) and focused evaluations or integrated assessments as discussed in Appendix G of NEl-12-06, Revision 2 (NEI, 2015), JLD-ISG-2012-01, Revision 1 (NRC, 2016a), JLD-ISG-2012-05 (NRC, 2012d), and JLD-ISG-2016-01, Revision O (NRC, 2016b), respectively.

2.0 REGULATORY BACKGROUND 2.1 Applicable Regulatory Requirements As stated above, Enclosure 2 to the 50.54(f) letter (NRC, 2012a) requested that the licensee reevaluate flood hazards for their respective sites using present-day methods and regulatory guidance used by the NRC staff when reviewing applications for ESPs and COLs. This section of the staff assessment describes present-day regulatory requirements that are applicable to the FHRR.

Sections 50.34 (a)(1), (a)(3), (a)(4), (b)(1), (b)(2), and (b)(4), of 10 CFR, describe the required content of the preliminary and final safety analysis report, including a discussion of the facility site with a particular emphasis on the site evaluation factors identified in 10 CFR Part 100. The licensee should provide any pertinent information identified or developed since the submittal of the preliminary safety analysis report in the final safety analysis report.

General Design Criterion 2 in Appendix A of Part 50 states that structures, systems, and components (SSCs) important to safety at nuclear power plants must be designed to withstand the effects of natural phenomena such as earthquakes, tornados, hurricanes, floods, tsunamis, and seiches without the loss of capability to perform their intended safety functions. The design bases for these SSCs are to reflect appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area. The design bases are also to have sufficient margin to account for the limited accuracy, quantity, and period of time in which the historical data have been accumulated.

Section 50.2 of 10 CFR defines the "design basis" as the information that identifies the specific functions that an SSC of a facility must perform, and the specific values or ranges of values OFFICIAL USE ONLY-SECUFUT'f RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION chosen for controlling parameters as reference bounds for design which each licensee is required to develop and maintain. These values may be (a) restraints derived from generally accepted "state of the art" practices for achieving functional goals, or (b) requirements derived from analysis (based on calculation, experiments, or both) of the effects of a postulated accident for which a SSC must meet its functional goals.

Section 54.3 of 10 CFR defines the "current licensing basis" (CLB) as: "the set of NRC requirements applicable to a specific plant and a licensee's written commitments for ensuring compliance with and operation within applicable NRC requirements and the plant-specific design basis (including all modifications and additions to such commitments over the life of the license) that are docketed and in effect." This includes 10 CFR Parts 2, 19, 20, 21, 26, 30, 40, 50, 51, 52, 54, 55, 70, 72, 73, 100 and appendices thereto; orders; license conditions; exemptions; and technical specifications, as well as the plant-specific design-basis information, as documented in the most recent final safety analysis report. The licensee's commitments made in docketed licensing correspondence that remain in effect are also considered part of the CLB.

Present-day regulations for reactor site criteria (Subpart B to 10 CFR Part 100 for applications on or after January 10, 1997) state, in part, that the physical characteristics of the site must be evaluated and site parameters established such that potential threats from such physical characteristics will pose no undue risk to the type of facility proposed to be located at the site.

Factors to be considered when evaluating sites include the nature and proximity of dams and other man-related hazards (10 CFR 100.20(b)) and the physical characteristics of the site, including the hydrology (10 CFR 100.21 (d)).

2.2 Enclosure 2 to the 50.54(f) Letter Section 50.54(f) of 10 CFR states that a licensee shall at any time before expiration of its license, upon request of the Commission, submit written statements, signed under oath or affirmation, to enable the Commission to determine whether or not the license should be modified, suspended, or revoked. The 50.54(f) letter requests that all power reactor licensees and construction permit holders reevaluate all external flood-causing mechanisms at each site (NRC, 2012a). This includes current techniques, software, and methods used in present-day standard engineering practice.

2.2.1 Flood-Causing Mechanisms to NTTF Recommendation 2.1, Flooding (Enclosure 2 of the 50.54(f) letter) discusses flood-causing mechanisms for the licensee to address in the FHRR (NRC, 2012a).

Table 2.2-1 lists the flood-causing mechanisms that the licensee should consider, and the corresponding Standard Review Plan (SAP) (NRC, 2007) sections and applicable ISG documents containing acceptance criteria and review procedures.

2.2.2 Associated Effects The licensee should incorporate and report AEs per JLD-ISG-2012-05 "Guidance for Performing the Integrated Assessment for External Flooding," (NRC, 2012d), in addition to the maximum water level associated with each flood-causing mechanism. Guidance document JLD-ISG-2012-05 (NRC, 2012d), defines "flood height and associated effects" as the maximum stillwater-surface elevation plus:

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• Wind waves and runup effects

• Hydrodynamic loading, including debris

• Effects caused by sediment deposition and erosion

• Concurrent site conditions, including adverse weather conditions

• Groundwater ingress

• Other pertinent factors 2.2.3 Combined Effects Flood The worst flooding at a site that may result from a reasonable combination of individual flooding mechanisms is sometimes referred to as a "combined effects" or "combined events" flood . Even if some or all of these individual flood-causing mechanisms are less severe than their worst-case occurrence, their combination may still exceed the most severe flooding effects from the worst-case occurrence of any single mechanism described in the 50.54(f) letter (See SRP Section 2.4.2, "Areas of Review," (NRC, 2007)). Attachment 1 of the 50.54(f) letter) described the "combined event flood" as defined in American National Standards Institute/American Nuclear Society (ANSI/ANS) 2.8-1992 (ANSI/ANS, 1992) as follows:

For flood hazard associated with combined events, American Nuclear Society (ANS) 2.8-1992 provides guidance for combination of flood causing mechanisms for flood hazard at nuclear power reactor sites. In addition to those listed in the ANS guidance, additional plausible combined events should be considered on a site specific basis and should be based on the impacts of other flood causing mechanisms and the location of the site.

If two less severe mechanisms are plausibly combined per ANSI/ANS-2.8-1992, then the NRC staff will document and report the result as part of one of the hazard sections. An example of a situation where this may occur is flooding at a riverine site located where the river enters the ocean. For this site, storm surge and river flooding should be plausibly combined .

2.2.4 Flood Event Duration "Flood event duration" as defined in JLD-ISG-2012-05 (NRC, 2012d), is the length of time during which the flood event affects the site. It begins when conditions are met for entry into a flood procedure, or with notification of an impending flood (e.g ., a flood forecast or notification of dam failure) , and includes preparation for the flood . It continues during the period of inundation, and ends when water recedes from the site and the plant reaches a safe and stable state that can be maintained indefinitely. Figure 2.2-1 illustrates flood event duration.

2.2.5 Actions Following the FHRR For the sites where a reevaluated flood elevation is not bounded by the COB flood hazard for any flood-causing mechanism , the 50.54(f) letter (NRC, 2012a) requests licensees and construction permit holders to:

• Submit an Interim Action Plan with the FHRR documenting actions planned or already taken to address the reevaluated hazard.

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OFFICIAL USE ONLY SECURITY RELATED INFORMATION

• Perform an integrated assessment to (a) evaluate the effectiveness of the COB (i.e .,

flood protection and mitigation systems); (b) identify plant-specific vulnerabilities; and (c) assess the effectiveness of existing or planned systems and procedures for protecting against and mitigating consequences of flooding for the duration of the flood event.

If the reevaluated flood hazard is bounded by the CDB flood hazard for all flood-causing mechanisms at the site, licensees are not required to perform an integrated assessment.

Guidance documents COMSECY-15-0019 (NRC, 2015b) and JLD-ISG-2016-01, Revision O (NRC, 2016b) outline a revised process for addressing cases in which the reevaluated flood hazard is not bounded by the plant's CDB. The revised process describes an approach in which licensees with LIP hazards exceeding their COB flood will not be required to complete an integrated assessment, but instead will perform a focused evaluation that assess the impact of the LIP hazard at their site and then evaluates and implements any necessary programmatic, procedural or plant modifications to address the hazard exceedance. For other flood hazard mechanisms that exceed their CDB, licensees can assess the impact of these reevaluated hazards on their site by performing either a focused evaluation or an integrated assessment (NRC, 2015b; NRC, 2016b) .

3.0 TECHNICAL EVALUATION

The NRC staff reviewed the information provided for the flood hazard reevaluation of Robinson .

The licensee conducted the hazard reevaluation using present-day methodologies and regulatory guidance used by the NRC staff in connection with ESP and COL reviews . To provide additional information in support of the summaries and conclusions in the FHRR, the licensee made calculation packages available to the NRC staff via an electronic reading room .

When the NRC staff relied directly on any of these calculation packages in its review, they or portions thereof were docketed. Certain other calculation packages were found only to expand upon and clarify the information provided on the docket, and so are not docketed or cited . The staff's review and evaluation is provided below.

3.1 Site Information Enclosure 2 of the 50.54(f) letter describes site information to be contained in the FHRR, which includes SSCs important to safety within the scope of the hazard reeval uation. The licensee included this data related to SSCs in the FHRR (Duke, 2015b). The NRC staff reviewed and summarized this information in the sections below.

All elevations in this NRC staff assessment are given with respect to the mean sea level (MSL) and are rounded to the nearest one-tenth of a foot. The FHRR notes that MSL is equivalent to National Geodetic Vertical Datum of 1929 (NGVD 29).

3.1.1 Detailed Site Information Robinson, Unit No. 2 is located approximately 3 miles (mi) (4.8 kilometers (km)) west-northwest of Hartsville, South Carolina, at an approximate distance of 88 mi (142 km) from the Atlantic Ocean and 160 mi (257 km) from the Appalachian Mountains (Duke , 2015b) . The plant is located on the shore of Lake Robinson , which is the primary surface water feature in the vicinity of the site and is created by the impoundment of Black Creek by Lake Robinson Dam. There is also a discharge canal located along the western border of Lake Robinson , which serves as a cooling water source for Unit 2. The area surrounding the plant is generally rural and lightly OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL l:ISE ONLY SEGl:IRITV RELATED INFORMATION developed. A map depicting the Robinson plant site and the area around Lake Robinson is shown in Figure 3.1-1 .

Lake Robinson normal pool elevation is 220 ft (67.06 m) MSL, which is controlled by two tainter gates and two low-level outlet valves (Duke, 2015b) . Under normal operation , water flows out of the dam through valves or the tainter gates which are normally kept in the closed position (Duke, 2015b; Duke, 2012a) . Under flood conditions, the lake level is controlled by opening the tainter gates. The gates are operated by an electric hoist motor on the top of the spillway structure (Duke, 2015b) . There is also a propane gas engine to raise the gates in case of a power failure or other failure of the electric hoist. Low-level outlet valves used for normal controlled releases in the spillway structure were assumed to be unavailable for use during the reevaluated flooding condition . These tainter gates are credited for flood protection, preventing the lake from exceeding 222 ft (67.67 m) MSL under the design-basis flood. The plant operation pool elevation of 221.5 ft (67.51 m) MSL represents a historically high reservoir level and was used as a conservative initial condition in the flood hazard reevaluations (Duke, 2015b) .

3.1.2 Design-Basis Flood Hazards The COB flood levels for Robinson , which are equivalent to the CLB flood levels (Duke, 2015a) ,

are described in Section 2.1 of the FHRR (Duke , 2015b). The COB flood hazards are summarized by flood-causing mechanism in Table 3.1-2. The NRC staff reviewed the information and determined that sufficient information was provided to be responsive to the 50.54(f) letter.

Based on information provided in the licensee's FHRR (Duke, 2015b), the plant grade at the powerblock is at elevation 225.0 feet (ft) (68.58 meters (m)) MSL. Table 3.1-1 provides the summary of controlling reevaluated flood-causing mechanisms, including associated effects, the licensee computed to be higher than the powerblock elevation.

3.1.3 Flood-Related Changes to the Licensing Basis The licensee stated that there have been no significant changes to the licensing basis with respect to an external flooding event (Duke , 2015b) . The NRC staff reviewed the information provided in the FHRR and determined that sufficient information was provided to be responsive to Enclosure 2 of the 50 .54(f) letter.

3.1.4 Changes to the Watershed and Local Area The licensee stated in FHRR Section 2.3, that there have been no significant changes to the Black Creek and tributary watersheds since license issuance; however, the plant site has changed slightly (Duke , 2015b) . Temporary and/or portable buildings, permanent buildings, paved parking lots, earthen berms, and Jersey barriers have been added at the site and a new drainage channel and retention pond have been constructed since the license was issued.

These changes have been included in the reevaluation when applicable (Duke, 2015b) . The NRC staff reviewed the information provided in the FHRR and determined that sufficient information was provided to be responsive to Enclosure 2 of the 50.54(f) letter.

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OFFICIAL USE ONLY SECURITY RELATED INFORMATION 3.1.5 Current Licensing Basis Flood Protection and Pertinent Flood Mitigation Features The licensee stated in FHRR Section 2.2, that the Lake Robinson Dam tainter gates are credited flood protection features in the Robinson licensing basis (Duke, 201 Sb). The function of the tainter gates is to discharge the design-basis flood waters and to prevent the lake from exceeding elevation 222 ft (67.67 m) MSL. The spillway is comprised of seven features: the left and right abutments and the center pier are passive features and the two tainter gates, the electric hoist motors, and the propane-fueled hoist motor are active features. Operation of the gates is manually controlled by using established procedures. Personnel utilize a lake-level gauge at the intake structure and two United States Geological Survey (USGS) gauge stations to ascertain lake level and head flow in order to determine appropriate positioning of the tainter gates. The gates are controlled by electric hoist motors with a propane-fueled hoist motor as backup. The flood protection and mitigation features are not associated with a unique mode of operation of the nuclear plant. The CLB does not consider the possibility of site flooding from the lake because the site grade is above the maximum lake level which can be maintained by the dam and related structures. During the flooding walkdowns performed in support of NTIF Recommendation 2.3, the licensee confirmed that the tainter gates' material condition and functionality met the acceptance criteria, there is adequate time available for the gate operation personnel to properly position the gates during a flooding event considering weather conditions, and credited operator actions are feasible (Duke, 201 Sb). The NRG staff reviewed the information provided in the FHRR and determined that sufficient information was provided to be responsive to Enclosure 2 of the 50.54(f) letter.

3.1 .6 Additional Site Details to Assess the Flood Hazard The licensee provided electronic copies of the input files used for numerical modeling and calculation packages related to LIP, river flooding due to probable maximum precipitation (PMP) , river flooding due to dam break, and the combined effects flood (Duke, 2014c) . The licensee also provided the topographical data files for LIP modeling , which are based on Light Detection and Ranging (LiDAR) image data and USGS topographical maps (Duke , 2014c) . The NRG staff reviewed the information provided in the FHRR and determined that sufficient information was provided to be responsive to Enclosure 2 of the 50.54(f) letter.

3.1 .7 Results of the Plant Walkdown Activities The 50.54(f) letter requested that licensees plan and perform plant walkdown activities to verify that current flood protection systems are available, functional , and implementable (NRG, 2012a) . The 50.54(f) letter also asked the licensee to report any relevant information from the results of the plant walkdown activities (NRC, 2012a) .

By letter dated November 26, 2012 (Duke, 2012b) , Duke provided the flood walkdown report for Robinson . The walkdown report was supplemented by an RAI response, dated January 30 ,

2014 (Duke, 2014a) . The NRG staff issued a staff assessment report on June 27, 2014, to document its review of the Walkdown Report, which concluded that the licensee's implementation of flooding walkdown methodology met the intent of the walkdown guidance (NRG, 2014b) .

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OFFICIAL USE ONLY SECURITY RELATED INFORMATION 3.2 Local Intense Precipitation and Associated Site Drainage The licensee reported in its FHRR (Duke, 2015b) that the reevaluated flood hazard for LIP results in a stillwater-surface elevation of 229.1 ft (69.83 m) MSL near safety-related structures.

This flood-causing mechanism is not included in the licensee's COB.

The licensee revised the LIP analysis to include: (a) an ssPMP, (b) an improved simulation of drainage from the roof to the ground, and (c) update the LIP flood models to capture recent site layout configurations (Duke, 2015b) . The ssPMP analysis incorporated an updated knowledge of meteorological science , the site-specific meteorology of the Robinson site, and more than 40 years of storm data not captured in the relevant Hydrometeorological Reports (HMR) 51 and 52 .

Following an example from Appendix B of NUREG/CR-7046, the licensee developed a 6-h, 1-mi2 rainfall event (NRC, 2011 d) . The total cumulative depth of this rainfall event was 29 .2 inches (74.2 cm) (Duke, 2015b} .

The licensee incrementally arranged the rainfall to create a front-loaded temporal distribution where the most intense rainfall occurs in the first five minutes of the LIP event (Duke, 2015b) .

The licensee's analysis indicated that the licensee's assumption of a f rent-loaded temporal rainfall distribution is the most conservative scenario for warning and reaction times (Duke ,

2015a) . The staff agrees with the licensee's statement that the postulation of the LIP duration and temporal distribution follows current NRC guidance.

The NRC staff reviewed the licensee's 6-h ssPMP by first calculating the 6-h PMP based on HMR 51 and HMM 52. Based on the location of the Robinson site, the 6-h HMR-based rainfall depth is 30.1 in. (76.7 cm). The NRC staff noted that the difference in cumulative rainfall depth between the HMR PMP and the ssPMP is 3.3 percent (29.2 in. (7 4.17 cm) versus 30.2 in.

(76.71 cm}), which the staff considers an insignificant difference in a 6-h rainfall depth.

Therefore, the NRC staff concludes the licensee's 6-h, 1-mi2 ssPMP depth is reasonable for the purposes of the 50.54(f) response.

To simulate flooding depths across the site from the PMP event, the licensee applied FL0-20 Pro Build No. 13.02.04 (FL0-20, 2014). The entire Robinson site and part of Lake Robinson (to the east) were included in the model's computational domain (see Figure 3.2-1 ). A uniform 15-ft (4.57-m) grid was applied over the entire numerical model domain, with a total of 153,718 computational cells. This resolution allowed for representation of each building by at least one cell and enabled representation of roads and channels on the site. For each cell , data relating to elevation, Manning's roughness coefficient (n} , curve number (CN), rainfall, and flow blockage were provided as input. The NRC staff determined that upstream river flooding would take over 40 hours1.667 days <br />0.238 weeks <br />0.0548 months <br /> to raise Lake Robinson above elevation 225 ft (68 .58 m) MSL (Duke, 2015b} . Therefore, the licensee's reservoi r level assumption of elevation 221 .5 ft (67.51 m)

MSL as a fixed boundary condition elevation was reasonable for the LIP analysis.

The licensee used updated topographic data for the Robinson site and LiDAR data for the contributing watershed outside the site to create the FL0-20 model (Duke, 2015b). The LiDAR data obtained from the South Carolina Department of Natural Resources was used to assign Manning's n values for use in FL0-20 based on vegetation type (such as forest, grassland and urban development) (Duke, 2015b) . Powerblock cells were assigned a Manning 's n value of 0.05 for paved areas (United Research Corporation (URS) , 2014a). The NRC staff conducted a sensitivity analysis of the Manning's n value within the powerblock since 0.05 is on the high end of the range typically applied for paved surfaces. For this sensitivity analysis, the NRC staff OFFICIAL USE ONL't'-SECURl'f't' RELATED INFOAMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION applied a value of 0.02, which is on the low end of the range. The resulting water surface elevations were lower than those computed with a Manning's n value of 0.05, and NRC staff therefore concluded that the licensee's Manning's roughness coefficients are reasonable.

Based on the land use code and hydrologic soil group, CNs for each computational cell were assigned using the Natural Resource Conservation Service's (NRCS) National Engineering Handbook (NRCS, 2004). In general, the CN indicates a degree of runoff potential and ranges from 30 (low runoff) to 100 (high runoff). The NRCS handbook provides CNs for different land uses with adjustment factors for three Antecedent Runoff Conditions (ARCs); ARC I for dry, ARC II for normal, and ARC Ill for wet. The licensee used the ARC II condition to determine the CNs, but values were artificially increased to 99 over the Robinson powerblock area in order to simulate the site as being impermeable with high runoff (URS, 2014a). For locations outside of the powerblock, the licensee used CN values corresponding to the identified land use and soil group under an ARC II assumption (URS, 2014a). From a review of the LIP runoff simulations, the NRC staff also observed that flooding within the powerblock area is not influenced from the outside area, as the powerblock area is elevated from the surrounding area. Therefore, the NRC staff considers the licensee's assumptions regarding infiltration parameters applied in the numerical model to be reasonable.

The licensee's UP simulation included the following key assumptions: 1) modeling of the present-day site layout, 2) a uniformly-applied 6-h, 1-mi2 ssPMP over the modeled domain (area of about 1.2 mi 2 ), 3) limiting water conveyance to gravity-driven overland flow only (passive and active drainage components are not credited), 4) preventing on-site precipitation losses due to initial abstraction, infiltration, evaporation, and transpiration, 5) setting the initial Robinson Lake elevation to the maximum operational level of 221.5 ft (67.51 m) MSL, and 6) simulating Jersey barriers as flood barriers with gaps, where appropriate (URS, 2014a; Duke, 2015b). The licensee's UP FL0-20 model simulates 6-in (15.24-cm) tall gaps at the solid barriers to represent the vehicle Jersey barriers placed around the site (Duke, 2015b). The powerblock area, including the Jersey barriers, is shown in Figure 3.2-2. The NRC staff determined that these general assumptions used in modeling the LIP flood event are reasonable.

The FL0-20 model results included maximum water elevation, maximum flow depth, maximum velocity, impact force , and static pressure for each computational cell (Duke, 201 Sb). The discharge and water surface elevation time series were extracted at each key building on the H.B. Robinson site . In the FHRR , Table 2 provided simulation results at a variety of locations, including safety- and non-safety-related buildings (Duke, 2015b), and the LIP mon itoring points shown in Figure 3.2-3. The maximum LIP flood elevation predicted by the licensee's simulation near safety-related structures is 229 .1 ft (69 .82 m) MSL, which occurred at the Fuel Handling Building.

The NRC staff reviewed the licensee's runoff analysis model, and confirmed the results are reasonable for the purposes of the 50.54(f) letter request. The NRC staff confirms the licensee's reevaluated flood hazard for LIP results in a maximum still-water surface elevation of 229.1 ft (69.82 m) MSL near safety-related structures and confirms the licensee's conclusion that the reevaluated flood hazard is not bounded by the COB flood hazard. Therefore, the NRC staff expects that the licensee will submit a focused evaluation for LIP and associated site drainage.

OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION 3.3 Streams and Rivers The licensee reported that the reevaluated flood hazard for streams and rivers results in a stillwater-surface elevation of 231 .8 ft (70.65 m) MSL. When including wind effects, the reevaluated total water surface elevation is 233.8 ft (71.26 m) MSL (Duke, 2015c) . This scenario assumes that the downstream Lake Robinson Dam fails due to overtopping. This flood-causing mechanism is described in the licensee's COB. The COB PMF elevation for streams and rivers is 222.0 ft (67.67 m) MSL (Duke, 2015b).

The licensee evaluated flooding in streams and rivers by simulating the response of the Lake Robinson watershed to a PMP event. The licensee applied the Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) (U.S. Army Corps of Engineers (USACE), 201 Ob) for the runoff simulation of the PMF. The Hydrologic Engineering Center's River Analysis System (HEC-RAS) (USACE, 201 Oa) was used for the hydraulic simulation of the flood in the contributing river segments, Lake Robinson and the Lake Robinson Dam.

The licensee used data from a Digital Elevation Model (DEM) of the Lake Robinson watershed ,

to identify drainage paths and watershed boundaries (URS, 2014b). Initial infiltration losses were estimated using the CN methodology based on soil type and land use, and runoff was computed for a total of 28 subbasins (URS, 2014b) . The licensee estimated CNs based on land-use classification , and determined the time-of-concentration for each subbasin (URS, 2014b). Storage-discharge relationships for Lake Robinson Dam were obtained from existing drawings and reports, and the Muskingum-Cunge methodology in the HEC-HMS model (USACE, 201 Ob) was used for channel routing .

The FHRR (Duke, 2015b) stated that the watershed PMP was obtained using HMR 51 (NOAA, 1978) and HMR 52 from the National Oceanic and Atmospheric Administration (NOAA, 1982).

The total Lake Robinson Dam drainage basin is 171.5 mi 2 (444 km 2 ) (Duke, 2015b) . In accordance with guidance in ANSl/ANS-2.8-1992 (ANSI/ANS, 1992), the PMF storm event began with a 500-yr rainfall depth occurring over 72-h, followed by a 72-h dry period which, in turn, was followed by the full , 72-h PMP event.

The HEC-HMS hydrologic model was calibrated using data from Hurricane Frances, which occurred in 2004 (URS, 2014b). The calibration process resulted in an initial rainfall abstraction of 4.02 in. (10.2 cm) , increasing the time of concentration by 40 percent, decreasing the curve number by 5 percent, and increasing the Manning's n channel roughness coefficient to 0.065 (URS, 2014b) . The model was then validated using data from a 1990 tropical storm and a 1916 hurricane. The resulting model-simulated peak flows were about 21 - to 25-percent higher than the historical record values, indicating that the model was conservative and slightly over-predicted peak discharge within the watershed for storms of this intensity (URS, 2014b). For storms of greater intensity, such as the PMP event, non-linearity adjustments were warranted and were incorporated by the licensee into the model (Duke, 2015b).

Based on the hierarchical hazard assessment (HHA) process, the licensee evaluated four scenarios for streams and river flooding (URS, 2014b). Scenario A assumed loss rates based on the NRCS CN method, rainfall-to-runoff transformation based on Soil Conservation Service unit-hydrograph (SCS, 1986) with nonlinearity adjustments, and instantaneous routing in channels (URS , 2014b). Scenario Bis the same as Scenario A but does not include loss.

Scenario C is the same as Scenario B but does not include non-linearity adjustments. Scenario D is the same as Scenario C but does not include any rainfall-to-runoff transformation. The OFFICIAL USE ONLY*SECURITY RELATED INFORMATION

Of'f'ICl.O,L YSli ONLY SECURITY RELATED INFORMATION results indicate that peak inflows were 69-, 80-, 60-, and 127-percent higher than the calibrated model for Scenarios A, B, C, and D, respectively (URS, 2014b) .

Since the calibrated model was found to conservatively predict runoff for the postulated 100-yr storm and hurricane Frances, and the analysis of the four scenarios showed that recommended nonlinearity adjustments would result in higher flow predictions, the licensee selected the calibrated model with the addition of nonlinearity effects, as a suitably-conservative model (URS, 2014b) . This final HEC-HMS model (also termed the 'Final Scenario' by the licensee) used the calibrated and validated model parameters with corrections to unit hydrographs and Muskingum-Cunge routing method (URS , 2014b). This final HEC-HMS model included nonlinearity corrections as recommended in NUREG/CR-7046 (NRC , 2011d) by increasing the peak discharge by one-fifth , decreasing the time-to-peak by one-third, and adjusting the rising and falling limbs of the hydrograph to preserve the runoff volume to unit depth over the drainage area. The resulting peak flow was 202,737 cubic feet per second (ft3/s) (5,741 cubic meters per second (m 3/s)), a 7-percent increase compared to the validated model in which non-linearity effects were not included (URS, 2014b).

The HEC-RAS model included Black Creek and Lake Robinson . Approximately 8 mi (12.9 km) of river upstream and 1.5 mi (2.4 km) downstream of the Robinson site were included in the model to simulate backwater effects as well as impacts from upstream bridges. The licensee developed cross-sections from a 5-ft (1 .52-m) resolution DEM built from LiDAR data (URS, 2014b). Four upstream bridges were modeled based on field measurements, while Lake Robinson Dam was modeled completely, including the discharge canal along the western edge.

At elevations above 228.0 ft (69.49 m) MSL, water within Lake Robinson follows an alternative flow-path through the site via a low point west of the dam. In order to capture this feature , a lateral structure was created in the HEC-RAS model using the weir option. Manning's n roughness coefficients were selecte.d based on the HEC-RAS reference manual (USAGE, 2010b). with values ranging from 0.03 to 0.05 within channels and a uniform value of 0.12 for overbanks (URS, 2014b) . The licensee verified the adequacy of these values by comparing them against a Federal Emergency Management Agency (FEMA) Flood Insurance Study for Darlington County (URS, 2014b).

In simulating streams and rivers flooding, the licensee made several key assumptions , including the exclusion of precipitation losses (aside from initial abstraction and infiltration losses) ,

considering PMP values as point rainfall data occurring at the basin centroid, simulating upstream reservoirs as full at the PMP event onset and excluding attenuation effects. blocking the Lake Robinson Dam low level outlet valves, excluding stop logs from use, and preventing debris accumulation at Lake Robinson . Other assumptions included using the North Carolina coastal regression equation for runoff estimation, assuming travel times were zero for the initial HHA scenario and later using the Muskingum-Cunge routing method, and setting the Lake Robinson water level to 221.5 ft (67.51 m) MSL at the PMP event onset (URS, 2014b). The licensee stated that sensitivity analyses showed the initial reservoir level had very little impact on the resulting maximum water surface elevation (WSE) (URS, 2014b) .

Unsteady HEC-RAS simulations were performed to consider the PMF event with and without failure of the Lake Robinson earth embankment dam. In regards to the PMF with Robinson Lake dam failure scenario, an earth dam erosion model called BREACH (Fread , 1991) was used. The mode l developed dam breach parameters for use in HEC-RAS including breach bottom width , side slope, bottom elevation, breach-formation time (URS, 2014b) and breach trigger elevation (Duke, 2015c) . The corresponding HEC-RAS model simulation resulted in a OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONL.Y SECURITY Rl!bATEE> INFORMATION peak WSE (Stillwater) at the Robinson site of 231.8 ft (70.65 m) MSL under an assumed failure of Lake Robinson Dam (Duke, 2015b; Duke, 2015c).

The licensee evaluated wave runup during the PMF scenario. Calculations for wave runup resulting from the 2-yr wind speed were conducted according to the USAGE Coastal Engineering Manual (CEM) (USAGE, 2002; Duke, 2015c). The resulting peak WSE at the Robinson site is 233.8 ft (71.25 m) MSL when the wave runup is included (Duke, 2015c).

The NRC staff confirms the licensee's conclusion that the reevaluated hazard elevation for flooding from streams and rivers, assuming a downstream Lake Robinson Dam failure, is 231.8 ft (70.65 m) MSL. With the inclusion of wave runup, the maximum total water surface elevation is 233.8 ft (71.26) MSL, which is not bounded by the COB flood hazard of 222.0 ft (67.36m)

MSL. Therefore, the NRC staff expects that the licensee will submit a focused evaluation or an integrated assessment for the streams and rivers flood-causing mechanism.

3.4 Failure of Dams and Onsite Water Control/Storage Structures The licensee reported in its FHRR that the reevaluated flood hazard for failure of dams results in a stillwater-surface elevation of (( ) )) MSL, and when including waves and runup, results in an elevation of (( ) )) MSL at the Robinson site (Duke, 2015b).

This flood-causing mechanism is not included in the licensee's CDS (Duke, 2015b).

The licensee investigated the impact of upstream dams by first screening dams with heights less than 25 ft (7.62 m) dam height, dams with storage volumes less than 50 acre-ft (61,670 m3 ), and dams that met the screening criteria of inconsequential and noncritical, as described in JLD-ISG-2013-01 (NRC, 2013). The licensee calculated that if these dams failed instantaneously, the elevation of Lake Robinson would only increase by (( )).

The licensee then investigated the remaining (( )) dams upstream of Lake Robinson by applying the Volume Method described in JLD-ISG-2013-01 (NRC, 2013). This resulted in a maximum water surface elevation of (( ]), assuming Lake Robinson was initially at its maximum operational pool elevation of 221.5 ft (67.51 m) MSL which is higher than the elevation under a 500-yr flood.

The licensee then chose to add the volume of all inconsequential and non-critical upstream dams to Lake Robinson as part of what the licensee called Scenario A (discussed in Section 3.3). Given the instantaneous and combined nature of the upstream dam failures, this scenario would bound all potential seismic failures of upstream dams without having to evaluate the seismic capacity of these dams. The licensee also assumed that, for the Scenario A the Robinson Lake dam will not fail during the postulated seismic event. Although the licensee did not evaluate the seismic capacity of Lake Robinson Dam, this non-failure assumption is conservative since failure of the Lake Robinson Dam would lower the water surface elevation at the Robinson site before the volume from the upstream reservoirs could enter the lake. The maximum stillwater elevation for this scenario is ((  : )) MSL.

Following ANSl/ANS-2.8 (1992) guidance, the licensee then assumed a 2-yr wind speed coincident with a Lake Robinson elevation of (( ) )] MSL. Based on the GEM (USAGE, 2002), the 2-yr wind setup was calculated as 0.46 ft (0.14 m), and the wave runup was computed as (( )). This results in a total maximum water surface elevation of ((

) )) MSL (URS, 2014e) at the Robinson site. This water elevation is also lower OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION than any elevation that could induce a hydrologic failure of Lake Robinson Dam due to overtopping.

The NRG staff confirms the licensee's conclusion that the reevaluated Stillwater elevation for failure of dams is elevation (( ) )) MSL. After the inclusion of wind waves and runup, the total water surface elevation for this flood-causing mechanism is ((

)) MSL. This flooding mechanism is not bounded by the COB flood hazard. Therefore, the NRG staff expects that the licensee will submit a focused evaluation or an integrated assessment for the failure of dams flood-causing mechanism.

3.5 Storm Surge The licensee reported in its FHRR that the reevaluated flood hazard for storm surge including wave setup and runup results in an elevation of 231 .8 ft (70.65 m) MSL (Duke, 2105b). This flood-causing mechanism was discussed in the COB, but was determined to not impact the site (Duke, 2015b).

The licensee evaluated flooding from the probable maximum storm surge (PMSS) following the 4 probable maximum hurricane (PMH) methodology outlined in NOAA's Technical Report NWS 23 (NOAA, 1979; Duke, 2015b). Due to the inland location of the Robinson site, the overall storm surge intensity was reduced based on the approach outlined by Kaplan and DeMaria (1995) (Duke, 2015b). Since maximum fetch distances to the Robinson site are obtained in a north-to-south direction, the accompanying wind was assumed to blow north to south. Based on the wind speed, fetch distance, water depth, and the angle between the wind and fetch, the licensee used the Zuider Zee formula (USAGE, 1977) to compute the storm surge (Duke, 2015b) .

The licensee evaluated three different storm tracks to provide various combinations of storm track distances and directions to maximize winds (URS, 2014c). A maximum wind speed of 119.30 mi/h (191.99 km/h) was calculated for a curved storm track which provided a relatively short storm track distance with a more critical angle (URS, 2014c). Using the Zuider Zee formula, a maximum storm surge (or wind setup) of 2.89 ft (0.88 m) was computed (Duke, 2015b) for a starting elevation of 221.5 ft (67.51 m) MSL in Lake Robinson. Using GEM methods (USAGE, 2002), the licensee then evaluated flooding from PMSS with wave runup.

Based on the computed values , wave runup was found to be 7.4 ft (2.26 m), resulting in a combined wind setup and runup value of 10.3 ft (3.14 m). The peak flooding elevation due to an initial lake level of 221.5 ft (67.5 m) MSL with the PMH-induced surge, wind setup, and wave runup is 231.8 ft (70.65 m) MSL (Duke, 2015b).

The NRG staff found that the maximum fetch length in Lake Robinson of 4.6 mi (7.4 km) is conservatively long. With the wind speed of 119.3 mi/h ( 191.99 km/h) and the maximum water depth of 16.2 ft (4.93 m), the NRG staff computed wind wave and runup heights using the Zuider Zee equations (USAGE, 1977) used by the licensee, and confirmed that the licensee's estimation of the maximum storm surge elevation, including wind waves and runup, is 231.8 ft (70.65 m) MSL. Therefore, staff considers the licensee's analysis to be reasonable for the purposes of the 50.54(f) response.

The NRG staff confirms the licensee's conclusion that the reevaluated flood hazard elevation for storm surge is 231 .8 ft (70. 65 m) MSL and that this hazard mechanism is not bounded by the OFFICIAb USE ONbY SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION COB flood hazard. Therefore, the NRC staff expects that the licensee will submit a focused evaluation or an integrated assessment for the storm surge flood-causing mechanism.

3.6 Seiche The licensee reported in its FHRR that the reevaluated flood hazard for seiche results in a stillwater-surface elevation of 226.2 ft (68.95 m) MSL (Duke, 2015b). This flood-causing mechanism is not included in the licensee's COB (Duke, 2015b).

In order to simulate flooding from seiche, Lake Robinson was represented as a rectangular 4.8 mi by 0.6 mi (7.7 km by 1 km) water body of uniform depth 16.4 ft (5 m) (URS, 2014d). A stillwater lake water elevation of 221.5 ft (67.51 m) MSL was used for the calculation, and the wind was assumed to blow in a north-to-south direction (Duke, 2015b). The natural oscillation period of Lake Robinson was obtained using Marian's formula (USACE, 1977; URS, 2014d)).

The resulting wind setup in the north-south direction reaches elevation 226.2 ft (68.95 m) MSL at the shoreline between the lake and plant site, while the wind in the east-west direction results in a maximum water surface elevation of 222.3 ft (67.76 m) MSL (Duke, 2015b).

To estimate the seiche caused by potential seismic activities near Lake Robinson, the licensee noted that the long-period fundamental-mode for seiche oscillations in the lake fall outside of the period range where seismic ground motions carry the most energy (Duke, 2015b). Therefore, the licensee concluded that a seismically-induced seiche is unlikely to occur in Lake Robinson (Duke, 2015b).

The NRC staff independently checked the oscillation period calculation for Lake Robinson. The NRC staff confirmed that the licensee's estimation of lake length of 4.8 mi (7.7 km) and width of 0.6 mi (1 km) is accurate based on available maps, and the licensee's average water depth in the lake of 16.2 ft (4.94 m) is conservatively deep. Using the wind-setup equation from the Shore Protection Manual (USACE, 1984) and assuming a wind speed of 119 mi/h (191.5 km/h),

the NRC staff obtained a wind-setup height of 4.5 ft (1.37 m), which is nearly equal to the licensee's estimation. Therefore, the NRC staff determined that the licensee's wind setup height is reasonable.

The NRC staff confirms the licensee's conclusion that the reevaluated flood hazard for seiche of 226.2 ft (68. 95 m) MSL is not bounded by the COB flood hazard. Therefore, the NRC staff expects that the licensee will submit a focused evaluation or an integrated assessment for the seiche flood-causing mechanism.

3.7 Tsunami The licensee reported in its FHRR that the reevaluated hazard for tsunami is not a plausible flooding mechanism and would not inundate the plant site (Duke, 2015b). This flood-causing mechanism was not included in the licensee's COB (Duke, 2015b).

In the FHRR, Section 3.5 states that the Robinson site is located approximately 87 mi (140 km) inland from the Atlantic coast, where tsunami hazards are relatively low, and 225 ft (68.58 m) above sea level (Duke, 2015b). Therefore, the licensee concluded that the site would not be subjected to the effects of ocean tsunami flooding.

OFFICIAL USE ONLY*SECURITY RELATED INFORMATION

OFFICIAb YSE ONbV SECYRITV REbATED INFORMATION During its review of the FHRR, the NRG staff investigated the potential for flooding from a landslide-induced tsunami in Lake Robinson. The NRG staff began its review by consulting the USGS landslide hazard maps (USGS, 2015). The NRG staff noted that the Lake Robinson watershed area is classified as a "no landslide hazard" zone. The NRG staff therefore concluded that a landslide-induced tsunami on the lake area is not plausible.

The NRG staff confirms the licensee's conclusion that the reevaluated hazard from tsunami flooding is not plausible at the Robinson site and is bounded by the COB flood hazard.

Therefore, the NRG does not expect that the licensee will submit a focused evaluation or an integrated assessment for the tsunami flood-causing mechanism.

3.8 Ice-Induced Flooding The licensee reported in its FHRR that the reevaluated hazard for ice-induced flooding is not a plausible flooding mechanism for the site (Duke, 2015b). This flood-causing mechanism was not included in the licensee's (!DB (Duke, 2015b).

In the FHRR , Section 3.6 states that winters in the area are mild and there is no history of Lake Robinson freezing. Consequently, ice-induced flooding is not considered a plausible flood hazard at the Robinson site. The NRG staff found no ice jam records along the Black Creek (upstream of Lake Robinson Dam) based on a search of the USAGE ice database (USAGE, 2015).

The NRG staff reviewed and confirms the licensee's conclusion that the reevaluated hazard from ice-induced flooding is not a plausible flooding mechanism at the site, and is therefore bounded by the COB flood hazard. Therefore, the NRG staff does not expect that the licensee will submit a focused evaluation or an integrated assessment for the ice-induced flood-causing mechanism.

3.9 Channel Migrations or Diversions The licensee reported in its FHRR that the reevaluated hazard for channel migrations or diversions does not inundate the plant site (Duke, 2015b ). This flood-causing mechanism was not included in the licensee's COB (Duke, 2015b) .

Seven data sources were established and evaluated for Black Creek for the channel migration and channel diversion calculation. These data sources are:

• Aerial imageries, topograph ic maps, and satellite imageries that spanned more than 70 years (from 1941 to 2013).

• Soil characteristics for the channel, channel banks and the overbank areas.

• Channel meandering.

• Riparian vegetation cover.

• Seismic activities.

• Ice expansion.

• Shear stress (flow velocity) during the PMF event (Duke, 2015b).

The seven data sources were also supported by field reconnaissance and verifications of channel migration and diversion. Stream centerlines of Black Creek and its major tributaries were digitized and the centerlines were overlaid . Examination and comparison of the digitized OFFICIAL USE ONLV*SECUAIT'/ RELATED INFOAMATION

OFFICIAL USE ONLY SECURITY REbATED INFORMATION stream lines of Black Creek and its major tributaries did not reveal any evidence of natural channel migration or diversion (Duke, 2015b).

In its review of the FHRR, the NRC staff performed an independent confirmation of the potential for flooding due to channel migration and diversion along the Lake Robinson basin and its surrounding area using topographic maps and the USGS landslide hazard database (USGS, 2015). The NRC staff noted that the basin and surrounding areas had no significant landslide hazard (USGS, 2015). Therefore, the NRC staff concluded that channel migration and diversion within and around the Lake Robinson basin is not plausible.

The NRC staff reviewed and confirms the licensee's conclusion that the flood hazard from channel migrations or diversions is not plausible and would not inundate the site. The NRC staff also confirms the licensee's conclusion that the reevaluated hazard from flooding from channel migrations or diversions is bounded by the COB flood hazard. Therefore, the NRC staff does not expect that the licensee will submit a focused evaluation or an integrated assessment for the channel migration and diversion flood-causing mechanism.

4.0 REEVALUATED FLOOD ELEVATIONS, FLOOD EVENT DURATIONS, AND ASSOCIATED EFFECTS FOR HAZARDS NOT BOUNDED BY THE COB 4.1 Reevaluated Flood Elevation for Hazards not Bounded by the COB Section 3 of this staff assessment documents the NRC staff's review of the licensee's reevaluated flood hazard water elevation results. Table 4.1-1 contains the maximum flood elevation results, including wind waves and run-up, for flood mechanisms not bounded by the COB. The NRC staff agrees with the licensee's conclusion that LIP, streams and river, failure of dams, storm surge, and seiche are the hazard mechanisms not bounded by the COB.

Consistent with the process and guidance discussed in COMSECY-15-0019 (NRC, 2015b) and JLD-ISG-2016-01, Revision O (NRC, 2016b), the NRC staff anticipates the licensee will submit a focused evaluation for LIP. For the streams and rivers, failure of dams, storm surge and seiche flood-causing mechanisms, the NRC staff anticipates the licensee will perform additional assessments of plant response, either a focused evaluation or an integrated assessment, as discussed in COMSECY-15-0019 (NRC, 2015b) and JLD-ISG-2016-01, Revision 0 (NRC, 2016b).

4.2 Flood Event Durations for Hazards Not Bounded by the COB The staff reviewed information provided in Duke's 50.54(f) responses (Duke, 2014b; Duke, 2014c; Duke, 2015a; Duke, 2015b; Duke, 201 Sc) regarding the FED parameters needed to perform the future additional assessments of plant response for flood hazards not bounded by the COB. The FED parameters for the flood-causing mechanisms not bounded by the COB are summarized in Table 4.2-1.

For LIP and associated site drainage, the licensee determined that the initial peak flood elevation would occur approximately 1-h after the start of the storm with an approximate 16-h duration of site inundation (Duke, 2015a). These FED parameters were estimated using the 2-dimentional FL0-20 model (see Section 3.2), which is driven by a postulated 6-h site-specific PMP storm event with front-peak loading. The licensee did not provide the period of recession, OFFICIAb USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONLV-SECUFU'fV RELATED INFOAM:Al"ION however the staff reviewed the model output and found that the recession time would be minimal (Duke, 201 Sa).

For the streams and rivers flood-causing mechanism, the licensee stated that the initial peak flood elevation would occur approximately 184-h after the start of the storm with an approximate 8.67-h duration of site inundation (Duke, 2015a). These FED parameters were estimated using a combination of a basin hydrologic model and a river routing hydraulic model with a postulated 72-h PMP for the basin with a center-peak loading, as discussed in Section 3.3. The licensee did not provide the period of recession; however, the staff reviewed FHRR Figure 1 and concluded the period of recession would be minimal (less than 1-h).

For dam failure , storm surge, and seiche, the licensee chose to state that FED parameters for these flood-causing mechanisms were bounded by the FED parameters for the streams and rivers flood-causing mechanism. These FED parameters are therefore noted as "not applicable" in Table 4.2-1.

The NRC staff concludes that the licensee's methods were appropriate and the provided FED parameter results are reasonable for use in future additional assessments.

4.3 Associated Effects for Hazards Not Bounded by the COB The staff reviewed information provided in the Duke 50.54(f) response (Duke, 2014b; Duke, 2014c; Duke, 201 Sa; Duke, 201 Sb; Duke, 201 Sc) regarding AE parameters needed to perform future additional assessments of plant response for flood hazards not bounded by the COB.

The AE parameters directly related to maximum total water height, such as wind waves and run-up, are presented in Table 4.1-1 . The AE parameters not directly associated with total water height are listed in Table 4.3-1.

For LIP and associated site drainage, the licensee stated that hydrodynamic loading from stormwater would not challenge site structures, and that debris strike was insignificant based upon the low flow velocities and low flow depths of stormwater on site (Duke, 2015a). The licensee also credited the additional protection afforded by the Jersey Barriers (Duke, 201 Sa; Duke, 201 Sb) . The results of the FL0-20 simulation (see Section 3.2) indicated a maximum hydrodynamic load of 43 lb/ft (64 kg/m) near the Jersey Barriers on the switchyard side and a maximum hydrostatic load of 272 lb/ft (405 kg/m) near the Fuel Handling Building (Duke, 201 Sb). The licensee reported a maximum flow velocity of 2.1 ft/s (0.6 m/s) near the Jersey Barriers (Duke, 201 Sb). The licensee stated that the effects of erosion and sediment deposition on site were minimal due to the low velocities (Duke, 2015a). The licensee stated that the COB does not discuss groundwater ingresses, but that future assessments of plant response to flooding will investigate AE parameters posed by all flood hazards mechanisms that exceed the COB, including but not limited to, groundwater ingress (Duke, 201 Sa}. The staff determined that other pertinent factors associated with the LIP event were not applicable because the inundation depths were small and flood velocities were low and away from doors and openings of safety-related buildings. The staff determined that concurrent events associated with LIP and streams and rivers, that could potentially affect the site, were not discussed by the licensee. Therefore, the licensee should address these AE parameters in future assessments, as applicable.

For the streams and rivers flood-causing mechanism , the licensee estimated a maximum flood elevation at the shore of Lake Robinson of 233.8 ft MSL (71.26 m) with a site inundation depth of 8.8 ft (2 .68 m) (Duke, 201 Sa) , as discussed in Section 3.3. The licensee stated that the OFFICIAL USE ONLV-SECUFtlTV Ff ELATED INFOAMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION associated effects (e.g., hydrodynamic, hydrostatic, and wave loads, etc .) would be provided in future assessments (Duke, 201 Sc). In the FHRR, Section 3.2.2 stated that debris from the upstream watershed would not translate to the site due to the low flow velocities in the lake and the constricted crossing of State Road S-13-346 near the north end of the lake (Duke, 2015b) .

Based on a review of the information presented in the FHRR hydrograph (FHRR Figure 1), the staff noted hydrodynamic and debris loading from the river PMF would be less significant than those of the LIP flood . This is because (1) the approaching flood velocity to the site was as low as 0.2 fVs (0.1 m/s) (URS, 2014e), (2) the duration of inundation time was shorter (8.67-h for PMF versus 16-h for LIP), and (3) the rising rate of the PMF hydrograph was milder than that of LIP (as shown in FHRR Figure 1). However, the hydrostatic loading from the river PMF could be greater than that of the LIP event because the inundation depth was 8.8 ft (2.68 m) , as compared to 2.63 ft (0.80 m) for LIP near the southwest switchyard side.

For dam failure, storm surge, and seiche, the licensee stated that AE parameters for these flood-causing mechanisms are bounded by the AE parameters for the streams and rivers flood-causing mechanism. These AE parameters are therefore noted as "not applicable" in Table 4.3-1.

The licensee is expected to provide AE parameters noted as "Not provided" in Table 4.3-1 as part of future additional assessments of plant response to flooding events. The NRC staff concludes the licensee's methods were appropriate and the provided AE parameter results are reasonable for use in future additional assessments associated with the MSA and the focused evaluations or revised integrated assessments.

4.4 Conclusion Based upon the preceding analysis, the NRC staff confirms that the reevaluated flood hazard information discussed in Section 4 is appropriate input to the additional assessments of plant response as described in the 50.54(f) letter (NRC, 2012a), COMSECY-15-0019 (NRC, 2015b),

JLD-ISG-2012-01 , Revision 1 (NRC , 2016a), and JLD-ISG-2016-01 , Revision 0 (NRC , 2016b).

The licensee is expected to develop the remaining FED and AE parameters, as applicable, to conduct the MSA and the focused evaluations or revised integrated assessments. The staff will evaluate the missing parameters marked as "not provided" in Table 4.3-1 during its review of future additional assessments.

5.0 CONCLUSION

The NRC staff has reviewed the information provided for the reevaluated flood-causing mechanisms for H.B. Robinson Steam Electric Plant, Unit No. 2. Based on its review of the information provided in the Duke 50.54(f) response (Duke, 2014b; Duke, 2014c; Duke, 201 Sa; Duke, 201 Sb; Duke, 201 Sc) the NRC staff concludes that the licensee conducted the hazard reevaluation using present-day methodologies and regulatory guidance used by the NRC staff in connection with ESP and COL reviews.

Based upon the preceding analysis, the NRC staff confirms that the licensee responded appropriately to Enclosure 2, Required Response 2, of the 50.54(f) letter, dated March 12, 2012 .

In reaching this determination, the NRC staff confirms the licensee's conclusions that (1) the reevaluated flood hazard results for LIP, streams and rivers PMF, failure of dams, storm surge, and seiche are not bounded by the CDB flood hazard, (2) a focused evaluation of plant OFFICIAL USE ONL>f*SECURITY RELATED INl90AMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION response will be performed for LIP, and a focused evaluation or an integrated assessment will be performed for the streams and rivers PMF, failure of dams, storm surge, and seiche flood-causing mechanisms; and (3) the reevaluated flood-causing mechanism information is appropriate input to additional assessments of plant response, as described in the 50.54(f) letter (NRC, 2012a), COMSECY-15-0019 (NRC, 201 Sb) , JLD-ISG-2012-01, Revision 1 (NRC ,

2016a) , and JLD-ISG-2016-01, Revision 0 (NRC, 2016b) .

OFFICIAL USE ONLY*SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION

6.0 REFERENCES

Notes: ADAMS Accession Nos. refers to documents available through NRC's Agencywide Documents Access and Management System (ADAMS). Publicly-available ADAMS documents may be accessed through http://www.nrc.gov/reading-rm/adams.html.

U.S. Nuclear Regulatory Commission Documents and Publications NRC (U.S. Nuclear Regulatory Commission) , 2007, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition ," NUREG-0800, 2007. ADAMS stores the Standard Review Plan as multiple ADAMS documents, which are accessed through the web page http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr0800/.

NRC, 2011a, "Near-Term Report and Recommendations for Agency Actions Following the Events in Japan," Commission Paper SECY-11 -0093, July 12, 2011 , ADAMS Accession No. ML11186A950.

NRC, 2011b, "Recommended Actions to be Taken Without Delay from the Near-Term Task Force Report," Commission Paper SECY-11-0124, September 9, 2011 , ADAMS Accession No. ML11245A158.

NRC, 2011 c, "Prioritization of Recommended Actions to be Taken in Response to Fukushima Lessons Learned ," Commission Paper SECY-11-0137, October 3, 2011 , ADAMS Accession No. ML11272A111.

NRC, 2011 d, "Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United State of America," NUREG/CR-7046, November 20 11, ADAMS Accession No. ML11321A195.

NRC, 2012a, Letter from Eric J. Leeds , Director , Office of Nuclear Reactor Regulation and Michael R. Johnson, Director, Office of New Reactors, to All Power Reactor Licensees and Holders of Construction Permits in Active or Deferred Status,

Subject:

"Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights from the Fukushima Dai-lchi Accident," March 12, 2012 , ADAMS Accession No. ML12056A046.

NRC, 2012b, Letter from Eric J. Leeds, Director, Office of Nuclear Reactor Regulation Regulation and Michael R. Johnson, Director, Office of New Reactors, to All Power Reactor Licensees and Holders of Construction Permits in Active or Deferred Status,

Subject:

"Issuance of Order to Modify Licenses with Regard to Requirements for Mitigation Strategies for Beyond-Design-Basis External Events ," Order EA-12-049, March 12, 2012, ADAMS Accession No. ML12054A736.

NRC, 2012c, Letter from Eric J. Leeds, Director, Office of Nuclear Reactor Regulation , to All Power Reactor Licensees and Holders of Construction Permits in Active or Deferred Status,

Subject:

"Prioritization of Response Due Dates for Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Flooding Hazard Reevaluations for Recommendation 2.1 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," May 11 , 2012 , ADAMS Accession No. ML12097A510.

OFFICIAL USE ONLV*SECURITV RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION NRC, 2012d, "Guidance for Performing the Integrated Assessment for External Flooding,"

Japan Lessons-Learned Project Directorate, Interim Staff Guidance JLD-ISG-2012-05, Revision 0, November 30, 2012, ADAMS Accession No. ML12311A214.

NRC, 2013a, "Guidance for Performing a Tsunami, Surge, or Seiche Hazard Assessment,"

Japan Lessons-Learned Project Directorate, Interim Staff Guidance JLD-ISG-2012-06, Revision 0, January 4, 2013, (2013a), ADAMS Accession No. ML12314A412.

NRC, 2013b, "Guidance For Assessment of Flooding Hazards Due to Dam Failure," Japan Lessons-Learned Project Directorate, Interim Staff Guidance JLD-ISG-2013-01, Revision 0, July 29, 2013, ADAMS Accession No. ML13151A153.

NRC, 2014a, Letter from Martha Barrillas, Project Manager, Division of Operating Reactor Licensing to William R. Gideon, Duke, "H.B. Robinson Steam Electric Plant, Unit 2 - Request for Additional Information Regarding Flood Hazard Reevaluation Report," June 18, 2014, ADAMS Accession No. ML14168A050.

NRC, 2014b, Letter from Martha Barrillas Project Manager, Division of Operating Reactor Licensing to William R. Gideon, Duke "H.B. Robinson Steam Electric Plant, Unit 2 - Staff Assessment of the Flooding Walkdown Report Supporting Implementation of Near-term Task Force Recommendation 2.3 related to the Fukushima Dai-lchi Nuclear Power Plant Accident,"

June 27, 2014, ADAMS Accession No. ML14176B075 (Non-Publicly Available).

NRC, 2015a, "Request for Additional Information - H.B. Robinson 2.1 Flood Hazard Reevaluation Report," email from Victor Hall, Project Manager, Japanese Lessons-Learned Division to Steve Callis and Dean Hubbard, Duke, dated March 4, 2015, ADAMS Accession No. ML15065A085.

NRC, 2015b, "Closure Plan for the Reevaluation of Flooding Hazard for Operating Nuclear Power Plants," Commission Paper COMSECY-15-0019, June 30, 2015, ADAMS Accession No. ML15153A104.

NRC, 2015c,: "Robinson RAI regarding NTTF 2.1 Flooding," email from Juan Uribe, Project Manager, Japanese Lessons-Learned Division to Al Maysam and Paul Guill, Duke, dated December 14, 2015, ADAMS Accession No. ML15348A340.

NRC, 2015d, "H.B. Robinson Steam Electric Plant, Unit No. 2- Interim Staff Response to Reevaluated Flood Hazards Submitted in Response to 10 CFR 50.54(f) Information Request -

Flood Causing Mechanism Reevaluation, " dated December 23, 2015 , ADAMS Accession No. ML15357A065 (Non-publicly Available).

NRC, 2016a, "Compliance with Order EA-12-049 Order to Modify Licenses with Regard to Requirements for Mitigation Strategies for Beyond-Design-Basis External Events", Interim Staff Guidance JLD-ISG-2012-01, Revision 1, January 22, 2016, ADAMS Accession No. ML15357A163.

NRC, 2016b, "I nterim Staff Guidance JLD-ISG-2016-01 "Guidance for Activities Related to Near-Term Task Force Recommendation 2.1, Flooding Hazard Reevaluation; Focused Evaluation and Integrated Assessment,", Revision O, July 11, 2016, ADAMS Accession No. ML16162A301 .

OFFICIAL USE ONL'f-SECUFllTY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION Codes and Standards ANSI/ANS (American National Standards Institute/American Nuclear Society), 1992, ANSl/ANS-2.8-1992, "Determining Design Basis Flooding at Power Reactor Sites," American Nuclear Society, LaGrange Park, IL, July 1992.

Other References Duke, 2012a, "Updated Final Safety Analysis Report, Revision 24," September 14, 2012 ,

ADAMS Accession No. ML122710296. (Non-Publicly Available)

Duke, 2012b, "H. B. Robinson Steam Electric Plant, Unit No. 2 Response to Recommendation 2.3 "Flooding Walkdown" of the Near-Term Task Force Review of Insights from the Fukushima Dai-lchi Accident," November 26, 2012 , ADAMS Accession No. ML12340A067 (Non-Publicly Available}.

Duke, 2014a, Letter from William R. Gideon to the NRC, "H. B. Robinson Steam Electric Plant, Unit 2, Response to NRC 10 CFR 50.54(f) Request for Additional Information Regarding Near-Term Task Force Recommendation 2.3, Flooding Update - Review of Available Physical Margin (APM) Assessments," January 30, 2014, ADAMS Accession No. ML14037A104.

Duke, 2014b, "H.B. Robinson Steam Electric Plant, Unit No. 2 - Flood Hazard Reevaluation Report, Response to NRC 10 CFR 50.54(f) Request for Information Pursuant to Title 1 O of the Code of Federal Regulations 50.54(f) regarding Recommendations 2.1, 2.3 and 9.3 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, Dated March 12, 2012," March 12, 2014, ADAMS Accession No. ML14086A384.

Duke, 2014c, Letter from R. Michael Glover on behalf of William R. Gideon to the NRG, "Response to NRG Request for Additional Information Regarding H.B. Robinson Steam Electric Plant, Unit No. 2 Flood Hazard Reevaluation Report," July 9, 2014, ADAMS Accession No. ML14206A787 (Non-Publicly Available).

Duke, 2015a, Letter from R. Michael Glover, Duke, to the NRG, "Response to NRC Request for Additional Information Regarding H.B. Robinson Steam Electric Plant, Unit No. 2 Flood Hazard Reevaluation Report (FHHR) ," May 26 , 2015, ADAMS Accession No. ML15146A390.

Duke, 2015b, Letter from R. Michael Glover, Duke, to the NRG, "H. B. Robinson, Unit 2 -

Submittal of Revision to Flooding Hazard Reevaluation Report to Provide a Revised Site Specific Local Intense Precipitation Storm ," August 29, 2015, ADAMS Accession No. ML15243A077 (Non-Publicly Available) .

Duke, 20 15c, Letter from R. Michael Glover to the NRG,

Subject:

"H. B. Robinson, Unit 2 -

Submittal of Response to the NRG Request for Additional Information Regarding Flood Hazard Reevaluation Report, Related to Selection of the Dam Breach Trigger Elevation," December 15, 2015, ADAMS Accession No. ML15349A796 (Non-Publicly Available) .

FL0-2D, 2014. FL0-2D Pro Reference Manual, FL0-2D Software, Inc., Nutrioso, Arizona.

Fread, D. L. , 1991 , "BREACH: An Erosion Model for Earthen Dam Failures," Revision 1, August 1991 , National Oceanic and Atmospheric Adm inistration/National Weather Service , Washington ,

D.C.

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OFFICIAL USE ONLY-SECUfUfY FIELA'f!D INP'OAMATION Kaplan, J. and DeMaria, M., 1995, "A Simple Empirical Model for Predicting the Decay of Tropical Cyclone Winds after Landfall," Journal of Applied Meteorology, November 1995, vol.

34, pp. 2499-2512.

NEI (Nuclear Energy Institute), 2015, "Diverse and Flexible Coping Strategies (FLEX)

Implementation Guide," NEI 12-06 Revision 2, December 2015, ADAMS Accession No. ML16005A625.

NOAA (National Oceanic and Atmospheric Administration), 1978, "Probable Maximum Precipitation Estimates, United States, East of the 105th Meridian," NOAA Hydrometeorological Report No. 51, originally published June 1978, reprinted August 1980, Available online at http://www.nws.noaa.gov/oh/hdsc/PMP _documents/HMR51.pdf.

NOAA, 1979, "Meteorological Criteria for Standard Project Hurricane and Probable Maximum Hurricane Windfields, Gulf and East Coasts of the United States," NOAA Technical Report NWS 23, September 1979, Washington, D.C.

NOAA, 1982, "Application of Probable Maximum Precipitation Estimates, United States, East of the 1051h Meridian," NOAA Hydrometeorological Report No. 52, August 1982, Available on line at http://nws.noaa.gov/oh/hdsc/PMP _documents/HMR52.pdf.

NRCS (Natural Resources Conservation Service), 2004. "National Engineering Handbook, Part 630 Hydrology," Chapters 10 & 15. 210-Vl-NEH.

Soil Conservation Service (SCS), 1986, "Urban Hydrology for Small Watersheds," Technical Release 55, Soil Conservation Service, U.S. Department of Agriculture, originally published January 1975, revision published June 1986.

URS, 2014a, "Letter from R. Michael Glover on behalf of William R. Gideon to the NRC, "H.B.

Robinson, Unit, Response to NRC Request for Additional Information Regarding Flood Hazard Reevaluation Report," July 9, 2014, ADAMS Accession No. ML14206A787 (Non-Publicly Available).

URS, 2014b, "Flooding in Rivers and Streams Due to Probable Maximum Precipitation (PMP),"

Calculation No. 3095B-13B-12-05-200-002, Revision 2, Letter from R. Michael Glover on behalf of William R. Gideon to the NRC, Document Control Desk,

Subject:

"Response to NRC Request for Additional Information Regarding H.B. Robinson Steam Electric Plant, Unit No. 2 Flood Hazard Reevaluation Report," July 9, 2014(Non-Publicly Available).

URS, 2014c, "Probable Maximum Hurricane (PMH) Induced Probable Maximum Storm Surge (PMSS)," Calculation No. 30958-138-12-05-200-003, Revision 2, Enclosure to Letter from R.

Michael Glover on behalf of William R. Gideon to the NRC, Document Control Desk,

Subject:

"Response to NRC Request for Additional Information Regarding H.B. Robinson Steam Electric Plant, Unit No. 2 Flood Hazard Reevaluation Report," July 9, 2014(Non-Publicly Available).

URS, 2014d, "Seiche Analysis," Calculation No. 30958-138-12-05-200-004, Revision 2, Enclosure to Letter from R. Michael Glover on behalf of William R. Gideon to the NRC, Document Control Desk,

Subject:

"Response to NRC Request for Additional Information OFFICl.O.b YSi O~JbY SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SfiCURITY RfibATiD INFORMATION Regarding H.B. Robinson Steam Electric Plant, Unit No. 2 Flood Hazard Reevaluation Report,"

July 9, 2014(Non-Publicly Available).

URS, 2014e, "Flooding in Rivers and Streams Due to Upstream Dam Break," Calculation No.

30958-138-12-05-200-006, Revision 1, Enclosure to Letter from R. Michael Glover on behalf of William R. Gideon to the NRC, Document Control Desk,

Subject:

"Response to NRC Request for Additional Information Regarding H.B. Robinson Steam Electric Plant, Unit No. 2 Flood Hazard Reevaluation Report," July 9, 2014(Non-Publicly Available}.

USACE, 1977, "Shore Protection Manual," 3rd edition , Waterways Experiment Station, U.S.

Corps of Engineers, Coastal Engineering Research Center, Department of the Army, Washington, D.C.

USACE, 1984, "Shore Protection Manual," 4th edition, Waterways Experiment Station, U.S.

Corps of Engineers, Coastal Engineering Research Center, Department of the Army, Washington, D.C.

USACE, 2002, "Coastal Engineering Manual (CEM)," Engineer Manual 1110-2-1100, U.S. Army Corps of Engineers, Washington, D.C. (6 volumes).

USACE, 2010a, HEC-RAS River Analysis System , Hydraulic Reference Manual, Report No.

CPD-69, Version 4.1, USACE Hydrologic Engineering Center, January 2010. Available online at http://www.hec.usace.army.mil/software/hec-ras/.

USACE, 201 Ob, Hydrologic Modeling System HEC-HMS, User's Manual, Version 3.5, USACE Hydrologic Engineering Center, August 2010. Available online at http://www.hec.usace.army.mil/software/hec-hms/.

• USACE, 2015, "Ice Jam Database," US Army Corps of Engineers, Ice Engineering Research Group, Cold Region Research and Engineering Laboratory, Accessed from https://rsgisias.crrel. usace.army. mil/apex/f?p=524: 1 on March 31 , 2015 USGS, 2015, "Landslide Hazard Information," accessed from http://geology.com/usgs/landslides/ on March 31 , 2015.

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OFFICIAL USE ONLY SECURITY RELATED INFORMATION Table 2.2-1. Flood-Causing Mechanisms and Corresponding Guidance SAP Section(s) 1 Flood-Causing Mechanism and JLD-ISG 2 Local Intense Precipitation and Associated SRP 2.4.2 Drainage SRP 2.4.3 SRP 2.4.2 Streams and Rivers SAP 2.4.3 Failure of Dams and Onsite Water SAP 2.4.4 Control/Storage Structures JLD-ISG-2013-01 SAP 2.4.5 Storm Surge JLD-ISG-2012-06 SAP 2.4.5 Seiche JLD-ISG-2012-06 SAP 2.4.6 Tsunami JLD-ISG-2012-06 Ice-Induced SAP 2.4.7 Channel Migrations or Diversions SAP 2.4.9 Sources: NRC, 2007; NRC, 2013a; NRC, 2013b Notes:

1 SAP is the Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition .

2 JLD-ISG-2012-06 is "Guidance for Performing a Tsunami , Surge, or Seiche Hazard Assessment"; and JLD-ISFG-2013-01 is "Guidance for Assessment of Flooding Hazards Due to Dam Failure".

Table 3.1-1. Summary of Flood-Causing Mechanisms that May Exceed the Powerblock Elevation Reevaluated Flood-Causing Mechanisms that May Exceed ELEVATION in ft (m) MSL the Powerblock Elevation 225.0 ft (68.58m) MSL 1 Local Intense Precipitation and Associated Drainage Varies with the maximum near safety-related structures of 229.1 ft (69.82 m)

Streams and Rivers 233.8 ft (71.25 m)

Failure of Dams and Onsite Water Control/Storage Structures (( 11 Storm Surge 231.8 ft (70.65 m)

Seiche 226.2 ft (68.95 m)

Source: Duke, 2014b; Duke, 2015a Notes:

1 Flood height as defined in JLD-ISG-2012-05 (NRC, 2012d).

OFFICIAL USE ONLY*SECURITY RELATED INFORMATION

OFFICl.Ab USE ONbY SECURITY RELATED INFORMATION Table 3.1-2. Current Design Basis Flood Hazards Current Design Stillwater Waves/ Basis Flood Flooding Mechanism Reference 2 Elevation 1 Run up Hazard Elevation Local Intense Precipitation Not included Not included Not included in and Associated Drainage FHRR Section 2.1 in COB in COB COB Streams and Rivers River PMF 222 .0 ft MSL Not Applicable 222.0 ft MSL FHRR Section 2.1.2 Failure of Dams and Onsite Not included Not included Not included in Water Control Storage FHRR Section 2.1 in COB in COB COB Structures No impact on No impact on No impact on Storm Surge the site the site the site FHRR Section 2.1 identified identified identified Not included Not included Not included in Seiche FHRR Table 4 in COB in COB COB Not included Not included Not included in Tsunami FHRR Section 2.1 in COB in COB COB Not included Not included Not included in Ice-Induced FHRR Section 2.1 in COB in COB COB Channel Migrations or Not included Not included Not included in FHRR Section 2.1 Diversions in COB in COB COB Source NRC, 2015d Notes:

1 Reported values are rounded to the nearest one-tenth of a foot.

2 Revised FHRR (Duke, 2015b).

OFFICIAL USE ONLY*SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION Table 4.1-1. Reevaluated Hazard Elevations for Flood-Causing Mechanisms Not Bounded by the coe 1 Reevaluated Stillwater Flood-Causing Mechanism Waves/Runup Hazard Reference 3 Elevation 2 Elevation 2 Local Intense Precipitation and Associated Drainage 229.1 ft MSL Minimal 229.1 ft MSL FHRR Table 2 Streams and Rivers River PMF 4 231.8 ft MSL 2 ft 233.8 ft MSL Duke, 2015s Failure of Dams and Onsite Water (( )) (( )) (( )) FHRR Table 4 Control/Storage MSL MSL Structures6 FHRR Table 3 Storm Surge 221 .5 ft MSL 10.3 ft 231 .8 ft MSL FHRR Table 4 FHRR Table 3.4 Seiche 221.5 ft MSL 4.7 ft 226.2 ft MSL FHRR Table 4 Source: NRG, 2015d Notes:

1 Reevaluated hazard mechanisms bounded by the COB (see Table 3.1.2) are not included in this table.

2 Reported values are rounded to the nearest one-tenth of a foot.

3 Revised FHRR (Duke, 2015b) .

4 Scenario assumes Lake Robinson Dam fails due to overtopping during the PMF event.

5 Duke, 2015c, Letter from R. Michael Glover to the NRG,

Subject:

"Submittal of Response to the NRG Request for Additional Information Regarding H. B. Robinson Steam Electric Plant, Unit 2, Flood Hazard Reevaluation Report, Related to Selection of the D am Breach Trigger Ele vation ," Dec ember 15, 2015, ADAMS Accession No. ML15349A796.

6 Scenario assumes volume from all upstream dams is placed into Lake Robinson to bound potential seismic failure of these upstream dams. Lake Rob inson Dam however does not fail , either seismically or by overtopping.

OFFICIAL USE ONLV=SECURITY RELATED INFORMATION

OFFICIAL USE ONLV*SECURITY RELATED INFOftMATION Table 4.2-1. Flood Event Duration for Flood-Causing Mechanisms Not Bounded by the COB Time Available for Flood-Causing Duration of Inundation Time for Water to Preparation for Flood Mechanism of Site Recede from Site Event Local Intense Precipitation and 1-h 1 16-h2 Minimal Associated Drainage Streams and Rivers 184-h 1 8.67-h Minimal River PMF Failure of Dams and Onsite Water Control/Storage Not applicable3 Not applicable3 Not applicable3 Structures Storm Surge Not applicable3 Not applicable3 Not applicable3 Seiche Not applicable3 Not applicable3 Not applicable3 Source: Duke, 2015a Note:

1 After the start of the storm event.

2 For a 6-h LIP storm event.

3 In lieu of calculating values , the licensee chose to state that FED parameters for these flood-causing mechanisms are bounded by the FED parameters for the streams and rivers flood-causing mechanism .

OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION Table 4.3-1. Associated Effects Not Directly Associated with Total Water Height for Flood-Causing Mechanisms Not Bounded by the COB Flood-Causing Mechanism Associated Local Intense Streams and Seismic Dam Storm Surge 1 Seiche 1 Effects Factor Precipitation Rivers Filure 1 Hydrodynamic No impact on Not provided 2 Not applicable Not applicable Not applicable loading at plant the site grade identified Debris loading at Minimal Minimal Not applicable Not applicable Not applicable plant grade Sediment loading Minimal Not provided Not applicable Not applicable Not applicable at plant grade Sediment Minimal Not provided Not applicable Not applicable Not applicable deposition and erosion Concurrent Not provided Not provided Not applicable Not applicable Not applicable conditions, including adverse weather Groundwater Not provided Not provided Not applicable Not applicable Not applicable ingress Other pertinent Not applicable Not provided Not applicable Not applicable Not applicable factors (e.g.,

waterborne projectiles)

Source: Duke, 201 Sa Note:

1 In lieu of calculating values, the licensee chose to state that AE parameters for these flood-causing mechanisms are bounded by the AE parameters for the streams and rivers flood-causing mechanism.

2 Based off NRC staff AEs could be minimal for hydrodynamic and debris loading. Hydrostatic loading could be larger than that of LIP but should be insignificant compared to the COB.

OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFflCIAb USS ONbV SSCURITV RSLATED INFORMATION flood event duration

+ site preparation

• period of + recession of

• for flood event inundation water from site Conditions are met Arrival of flood Water begins to Water completely for entry into flood waters on site recede from site receded from site procedures or and plant in safe notification of and stable state impending flood thatcan be maintained indefinitely Figure 2.2-1: Flood Event Duration (NRC, 2012d)

OFFICIAL USE ONLV-SECUFUTY AELATED INfOAMATION

OFFICIAl YSE ONlV SECYRITV RELATED INFORMATION Lake Robinson Intake Structures Figure 3.1-1. Area Surrounding H.B. Robinson Steam Electric Plant (Duke, 2012a)

OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONL'f*SECURITY RELATED INFORMATION Figure 3.2-1. FL0-20 Computational Domain (Red Lines Indicate FL0-20 Model Boundary Conditions Associated with Surface Water Features) (URS, 2014a)

Figure 3.2-2: Modeling of Jersey Barriers (Blue Lines) in the FL0-20 Model (URS, 2014a)

OFFICIAL USE ONLY-SECURITY RELATED INPOAMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION (134124)

I 0(135~ ~*a..

(132096) heel m*:~ ....... (1 37146 .

(138388)

"c~ D D i (135105)

D

• • (135104 Figure 3.2.3. Major Onsite Buildings with LIP Monitoring Points {Green Dots) {URS, 2014a)

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