ML17255A524

NRC Report for the Audit of Exelon Generating Company Llc'S Flood Hazard Reevaluation Report Submittal Relating to the NTTF Recomendation 2.1--Flooding for Peach Bottom Atomic Power Station, Units 2 and 3-Public Redacted

ML17255A524
Person / Time
Site:	Peach Bottom
Issue date:	09/25/2017
From:	Gibson L K Japan Lessons-Learned Division
To:	Bryan Hanson Exelon Nuclear
Gibson L K, NRR/JLD, 415-1056
Shared Package
Ml17255A475	List: ML17255A524
References
CAC MF6598, CAC MF6599
Download: ML17255A524 (26)

Text

OFFICIAL USE ONLY SECURITY RELATED INFORMATION Mr. Bryan C. Hanson UNITED STATES NUCLEAR REGULATORY COM M ISSION WASHINGTON , D.C. 20555--0001 September 25 , 2017 President and Chief Nuclear Officer Exelon Nuclear 4300 Winfield Road Warrenville, IL 60555

SUBJECT:

NUCLEAR REGULATORY COMMISSION REPORT FOR THE AUDIT OF EXELON GENERATION COMPANY, LLC'S FLOOD HAZARD REEVALUATION REPORT SUBMITTAL RELATING TO THE NEAR-TERM TASK FORCE RECOMMENDATION 2.1-FLOODING FOR PEACH BOTTOM ATOMIC POWER STATION , UNITS 2 AND 3 (CAC NOS. MF6598 AND MF6599)

Dear Mr. Hanson:

By letter dated August 26, 2015 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML 15230A235}, the U.S. Nuclear Regulatory Commission (NRG) i nformed you of the staff's plan to conduct a regulatory audit of Exelon Generation Company , L LC's (the licensee)

Flood Hazard Reevaluation Report (FHRR) submittal related to the NearT erm Task Force Recommendation 2.1-Flooding for Peach Bottom Atomic Power Station , Units 2 and 3 (Peach Bottom). The audit was intended to support the NRG staff review of the li censee's FHRR and the subsequent issuance of a staff assessment.

T he audit meetings conducted on February 12 , 2016, March 4 , 2016 , March 11 , 2016 , and June 17, 2016, were performed consistent with NRG Office of Nuclear Reactor Regulation , Off i ce Instruction LIC-111 , "Regulatory Audits," dated December 29 , 2008 (ADAMS Access i on No. ML082900195). Therefore , the purpose of this letter is to provide you with the final audit report , wh i ch summarizes and documents the NRC's regulatory audit of the licensee's FHRR submitta l. E nclosure 1 transmitted herewith contains Security-Related Information. When separated from E nclosure 1 , this document is decontrolled. OFFICIAL USE ONLY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION B. Hanson If you have any questions, please contact me at (301) 415-1056 or e-mail at Lauren.Gibson@nrc.gov. Docket Nos. 50-277 and 50-278 Enclosures

1. Audit Report (Non-Public)

2. Audit Report (Public) cc w/encl: Distribution via Listserv Sincerely, Lauren K. Gibson, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation OFFICIAL USE ONLY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION NUCLEAR REGULATORY COMMISSION AUDIT REPORT FOR THE AUDIT OF EXELON GENERATION COMPANY, LLC'S FLOOD HAZARD REEVALUATION REPORT SUBMITTALS RELATING TO THE NEAR-TERM TASK FORCE RECOMMENDATION 2.1-FLOODING FOR PEACH BOTTOM ATOMIC POWER STATION. UNITS 2 AND 3 BACKGROUND AND AUDIT BASIS 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 1 o of the Code of Federal Regulations (10 CFR), Section 50.54(f) "Conditions of license" (hereafter referred to as the " 50.54(f) letter"). The request was issued in connection with implementing lessons-learned from the 2011 acc i dent at the Fukushima Dai-ichi nuclear power plant, as documented in The Near-Term Task Force Review of Ins i ghts from the Fukushima Da i-ichi Accident.

Recommendation 2.1 i n 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. Subsequent Staff Requirements Memoranda associated with Commission Papers SECY 11-0124 and SECY 11-0137 , instructed the NRC staff to issue requests for information to licensees pursuant to 1 O CFR 50.54(f). By letter dated August 12 , 2015 (Agencywide Documents Access and Management System Access i on No. ML 15233A067), Exelon Generation Company , LLC (Exelon , the licensee) submitted its Flood Hazard Reevaluation Report (FHRR) for Peach Bottom Atomic Power Station , Units 2 and 3 (Peach Bottom). The NRC i s in the process of reviewing the aforementioned submittal and has completed a regulatory audit of the licensee to better understand the development of the submittal, identify any sim i larities/differences with past work completed , and ultimately aid in its review of the licensees' FHRR. This audit summary is being completed in accordance with the guidance set forth in NRC Office of Nuclear Reactor Regulation , Office Instruction LIC-111 , " Regulatory Audits ," dated December 29 , 2008 (ADAMS Accession No. ML082900195). AUDIT LOCATION AND DATES The audit was completed by document review via a webinar session in conjunction with the use of the licensee's established electronic reading room (ERR) and teleconference on February 12 , 2016 , March 4 , 2016 , March 11 , 2016 , and June 17 , 2016. Enclosure 2 OFFICIAL USE ONLY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION AUDIT TEAM Title Team Member Organization Team Leader , NRR/JLD Tekia Govan NRC Technical Monitor Richie Rivera-Luao NRC Technical Staff Ken See NRC Technical Deoutv Division Director Andy Camobell NRC Technical Branch Chief Christopher Cook NRC NRC Contractor Roger Kay U.S. Army Corp of Engineers (USACE) N RC Contractor Curtis Miller USACE* NRC Contractor John Quinn Argonne National Laboratory (ANL) N RC Contractor Eugene Yan ANL A list of the licensee's part icipants can be found in Attachment

2. DOCUMENTS AUDITED Attachment 1 of this report contains a list which details the documents that were reviewed by the NRC staff , in part or in whole , as part of this audit. The documents were located in an ERR during the NRC staff's review. No additional information was requested to be submitted on the docket as a result of this audit. AUDIT ACTIVITIES In general , the audit activities consisted mainly of the following actions:

• Review background information on site topography and geographical characteristics of the watershed.

• Review site physical features and plant layout.

• Understand the selection of important assumptions and parameters that would be the basis for evaluating the individual flood causing mechanisms described in the 50.54(f) letter.

• Review model input/output files to computer analyses such as Hydrologic Engineering Center -Hydrologic Modeling System (HEC-HMS) and FL0-2D to have an understanding of how modeling assumptions were programmed and executed.

• Status of the submittal for associated effects and flood event duration. Table 1 summarizes specific technical topics (and resolution) of important items that were discussed and clarified during the audit. The items discussed in Table 1 may be referenced/mentioned in the staff assessment in more detail. OFFICIAL USll!: ONbY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION EXIT MEETING/BRIEFING On June 17 , 2016 , the NRC staff closed out the discuss i on of the techn i cal topics described above. OFFICIAL USE ONLY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION Table 1: Peach Bottom, Units 2 and 3 Information Needs and Response Summary I NFORMAT I ON INFORMAT I ON NEED REQUES T RE SPONSE NEED NO. 1 All Flood-Causing Mechanisms

-Comparison o f In response to the in formation request (Exelon, 2016a), the Reeva l uated Flood Hazard with Cur r en t Des i gn licensee stated that the CLB and the CDB are synonymous , i.e. Bas i s they have the same meaning. Background

Recommendation 2.1 of the 50.54(f) letter The NRC staff reviewed the information provided by the licensee (NRC, 2012) provides instructions for the Flood Hazard and concluded it was sufficient to address the informat i on need Reevaluation Report (FHRR). Under Section 1, Hazard request. Reevaluation Report , Items c and d, licensees are requested to perform: c. Comparison of current and reevaluated flood-causing mechanisms at the site. Provide an assessment of the current design basis flood elevation to the reevaluated flood elevation for each flood-causing mechanism.

I nclude how the findings from Enclosure 4 of this letter (i.e., Recommendation 2.3 flooding walkdowns) support this determination. If the current design basis flood bounds the reevaluated hazard for all flood-causing mechanisms, include how this finding was determined. d. Interim evaluation and actions taken or planned to address any higher flooding hazards relative to the design-basis, prior to completion of the integrated assessment described below , if necessary.

The Peach Bottom Atomic Power Station , Units 2 and 3 (Peach Bottom) FHRR (Exelon, 201 Sa) appears in the text to inconsistently provide comparison of the OFFIC I AL USE ONL Y SE C UR ITY R E LATE D INFORMA T I O N INFORMATION NEED NO. 2 3 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST RESPONSE reevaluated flood hazards with the current design basis (COB) for each flood hazard mechanism , as requested. Request: Clarify and where necessary correct the description and/or comparison of the reevaluated flood hazard to the COB for any flood hazard mechanism throughout the report that may have been i ncorrectly compared with the current licensing basis (CLB). Please confirm that this has been verified throughout the FHRR. Local Intense Precieitatlon

-Model area The licensee stated (Exelon, 2016a) that the safety-r e l ated doors Bac.kqround: The local intense precipitat i on (LIP) i s would not be sub j ect to flow from the area outs i de th e m odel equivalent to the 1-hour (hr), 1-square mile (mi 2) domain because the doors have h i gher elevat ion s than t he ground probable maximum precipitation (PMP) as described in surface at the boundary location receiving flow from th e outs i de NUREG/CR-7046 (NRC, 2011; Exelon , 201 Sa). The drainage area. Add i tionally , the licensee listed a ll cr itic al door modeling boundary area chosen by the l i censee for the locations and the ir elevat i ons in Table 1 of the response (wh i ch LIP analysis is only 0.13 mi 2 , and doesn't capture all came from the FHRR). The table's fo otnote states that t he flood-contributing rainfall within the site area. The Turbine Build i ng (Door 111) is at a non-safety-related s tr ucture contour lines indicate a valley in the topographic map of and is not flood-protected.

After reviewing , the staff requested a Figure 6.2 of calculation package PEAS-FLOOD-03 reference indicating that the Turb i ne Building is not safety-related Rev O (Exelon, 201 Sb) clearly show that run off can structure.

For this request, the l i censee provided a n excerpt from enter the Peach Bottom site from outside the current the Updated F in a l Safety Analys i s Report's (UFSAR) Appendix model boundary through the northwest corner. C.2.5.4, which desc r ibes that the T urbi ne Bu i ld i ng is not one o f the structures included in a safe sh utd own dur in g fl oo din g. The Request: Explain how the current model boundary area NRC staff confirmed the informat ion fr om the UFSAR and no the entire dra i nage basin entering the surface further information is needed. The NRC staff concluded it was drainage system of the site , or correct the model if sufficient to address the i nformation need request. needed. Local Intense Precieitation

-Missing site structures In response to the i nformation request (Exelon, 2016a), the lic ensee stated that these struct ur es were not inclu ded i n the model because thev have been re m oved from th e si t e. OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 4 5 6 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST RESPONSE

Background:

Some structures that cou ld obstruct flow are not considered in the FL0-20 modeling (Exelon , The NRC staff reviewed the informat i on provided by the licensee 2015b). and concluded i t was sufficient to add r ess the info rm at i on need req uest. Request: Provide just ifi cations for why these structures were not represented in the model, or correct if needed. Current Licensing Basis-Flooding in Streams and In response to th i s in forma tion need re ques t (Exelo n, 2016a), the Rivers lic ensee clar i fied that the d iff erences in discharge were due to Background

The staff noticed that the discharge of accounting for flood control projects (i.e. dams) versus 1 , 750 , 000 cubic feet per second (cfs) does not unregulated flow. correspond to the discharge of 1,625,000 cfs utilized The NRC staff reviewed the i nformat i on provided by the l i censee previously by Three M i le Island (TMI) near Harrisburg and concluded i t was sufficien t to address the information need (Exe l on, 2015a). request. Request: Please clarify or explain the difference in the numbers presented. Rivers and Streams In response to th i s i nformation need request , the licensee

Background:

The NRC staff noticed that the licensee provided a table of values listing the storm orientat i on associated did not provide the storm orientations for other storms in with each storm centering (Exelon, 2016a). Table 3.2.4.2.1.1 of the FHRR (Exelon , 2015a). The NRC staff rev i ewed the information provided by the licensee and concluded it was sufficient to add r ess the informat i on need Request: Please clar ify if the orientation of all storms other than the TMI Watershed Centroid differed from re quest. the selected TMI Watershed Centroid storm. Rivers and Streams In response to th i s information need request, the l i censee clar i f i ed Background

The NRC staff noticed that the results for that the d i scha r ges were presented for informationa l pu r poses TMI such as discharge are presented on page 76 (and only , as both sets of values were derived from the same HEC-others) in the FHRR (Exelon, 2015a). HMS mode l (Exe lo n, 2016a). Upon fu rther request f o r clarification , the l i censee presented a comparison of hy drographs from HEC-HMS and HEC-RAS to confirm that both models were OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST RESPONSE Original Request: Please clarify why the results of providing rough l y equivalent rout i ng over the en tir e rang e of discharge are presented for TMI in this report instead of d i scharges considered (Exelon, 2016b). Peach Bottom's resu l ts. The NRC staff reviewed the informa ti on provided by th e licensee Supplemental Request: and concluded it was sufficien t to address the i nformation need request.

Background:

Hydrolog i c routing methods performed by the Hydrologic Engineering Center -Hydrologic Modeling System (HEC-HMS) tend to be much more simplified than the equat i ons used by the unsteady Hydrologic Engineering Center -River Analysis System (HEC-RAS).

Due to these computationa l differences , HEC-HMS routings may differ significantly from HEC-RAS over an equivalent reach of a river at certain discharges, particularly if the reach conta i ns structures (such as run-of-river dams). These differences may impact the magnitude and timing of the peak, particularly over longer reaches. It is noted that the same HEC-HMS model was utilized for both TMI and Peach Bottom, although the hydraulic models used to determine peak stages at each site differ significantly in upstream length (approximately 1 mile (mi) versus 28 m i). It is anticipated that there may be some differences in results between mode l s, particularly as discharges exceed the calibrat i on range. Supplemental Request: Please provide a comparison of the HEC-HMS and HEC-RAS hydrographs where the two models overlap in routing flow to the Peach Bottom site to provide some context as to how sensitive HEC-HMS results may be to length of hydraulic reach and presence of hydraulic control structures. OFFICIAL USE ONLY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. INFORMATION NEED REQUEST RESPONSE 7 Rivers and Streams In response to this information need request, the licensee clarified that the combined model was validated against the three individual storms, while the three individual storms had been used to derive three individually calibrated models , which in turn had been combined to produce the combined model (Exelon, 2016a). 8 9

Background:

In the Calibration and Validation Results section of the FHRR (Exelon, 2015a), the staff noticed that the text indicated the HEC-HMS model was calibrated against Tropical Storm Lee, Hurricane Agnes, and Hurricane Ivan. No other storms are indicated in this section , yet the heading indicates model validation was performed.

Please clarify what storm event(s) were considered for model validation.

Rivers and Streams

Background:

In the Calibration and Validation Results section of the FHRR (Exelon, 201 Sa), the staff noticed that the text indicated that the HEC-RAS model was calibrated against Tropical Storm Lee and validated against the March 1936 Storm, Hurricane Ivan and Hurricane Agnes events. The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need request. In response to this information need request, the licensee clarified that Tropical Storm Lee was selected for calibration because it was the most recent of the large events and it best matches the used in the HEC-RAS model (Exelon, 2016a). The licensee further explained that the validated storms all matched high water marks at the plant site within 0.1 feet (ft), while not concurrently over-or under-predicting Request: Please clarify why only one storm was used The NRC staff reviewed the information provided by the licensee for calibration, and why the one storm selected was not and concluded it was sufficient to address the information need the largest event. request. Rivers and Streams

Background:

The staff noticed in Table 3.2.6.2.1 (and elsewhere in text) of the FHRR (Exelon, 201 Sa) that model calibration was indicated to be acceptable in relation to stage only. Request: Please clarify if timing of the hydrograph was of any concern in calibratina the HEC-RAS model to be In response to this information need request , the licensee provided values of mean absolute error, root mean squared error and cor:elation coefficient (R) to demonstrate the timing match by comparing values of computed and observed discharge from the same time step over the duration of the hydrograph (Exelon, 2016a). The licensee also provided graphical comparison between computed and observed hydrographs that allowed better OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 10 11 12 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST used in unsteady flow, and if so , what was acceptable hydrograph timing differences for calibration.

Ice Jams

Background:

The staff noticed that there was no discussion of certain details regarding the modeling of the ice jam release in the FHRR (Exelon, 2015a). Request: Please clarify if the hydraulic modeling of the released ice jam took into account the volume of ice released from the jam and if increased roughness was considered to account for the interaction of ice flows passing downstream.

Error/ Uncertainty

Background:

The staff noticed that the uncertainty range listed in Table 3.9.3.3.1 of the FHRR (Exelon, 2015a) was not related to any statistic such as the standard deviation Request: Please clarify if the uncertainty range presented represents one standard deviation or two standard deviations in elevation. Rivers and Streams RESPONSE comparison of the timing of the rising , falling and peak portions of the hydrographs.

The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need request. In response to this information need request the licensee indicated that the cumulative conservatisms included in other parameters would encompass the uncertainties pertaining to ice volume and roughness (Exelon, 2016a). The licensee also demonstrated that water surface profiles in the vicinity of the Peach Bottom site were relatively insensitive to the ice parameters selected due to the backwater influences posed by The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need request. In response to this information need request, the licensee clarified that the results presented were in response to sensitivity in Conowingo gate operations , not as a statistical uncertainty analysis , as was done for topographic uncertainty (Exelon , 2016a). The licensee further clarified that topographic uncertainty was not considered simultaneously with gate sensitivity.

The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need request. In response to this information need , the licensee clarified that the OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 13 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST

Background:

The staff noticed text pertaining to -that states that the crest elevation is while the HEC-RAS model uses an invert elevation of -*I (assume this is also in NAVD88 per description in section 3.2.2.12) (Exelon , 2015a). The invert elevation of the crest impacts the routing and the discharge through the dam. Request: Please clarify which crest elevation is correct. Rivers and Streams Original Request Background

The staff noticed in Figure 3.2.2.15.1 of the FHRR (Exelon , 2015a) that some low roughness values (n=0.0143) are used in the downstream reach of the HEC-RAS model. Also, abrupt changes in roughness from 0.0143 to 0.03 to 0.0375 are included in the model (Exelon, 2015b). Low roughness values decrease the computed water surface elevation (WSE). Justification for the use of low roughness values for the calibration effort is provided in the Exelon Transmittal of Design Information (TODI) and Attachment 5 of the TODI. Request: Please clar i fy or add justificat i on to support the assumption that the low roughness values would be valid for higher discharges (i.e. probable maximum flood [PMF]). Also, please clarify if abrupt changes in roughness from 0.0143 to 0.03 to 0.0375 are supported by physical changes i n the river/floodpla in or if the changes are a result of calibration efforts. RESPONSE -and noted that the typographical error does not im pact results presented in the FHRR (Exelon, 2016a). The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the i nformat i on need request. In response to this request for inform ation, the lic ensee prov i ded discussion on the applicability of the l ower roughness values for higher discharges (Exelon, 2016a). The licensee also provided aerial imagery overlaid with the location of abrupt cha ng es in roughness values. The NRC staff reviewed the information provided by the li censee and conc lu ded it was suff i c i en t to address the in formation need request for the f i rst port i on of the request (low roughness values). The staff performed sensitiv i ty analyses on the roughness values to determine the impact to computed water surfaces. Two genera l sensitivity analyses were conducted; the first included adjusting Manning's roughness values for the entire reach while the second ad j usted Manning's roughness values for an upstream reach.The results indicated a r elatively high sensitivity of the model to the roughness coeff i c i ents. The high sens i tivity of the model to roughness value was considered ju st i fication for an add i t i ona l request for information.

The licensee's second response (Exe l on, 2016b) prov i ded further in formation on a sensitivity analysis that the lic ensee conducted on roughness values in the upstream portion of the reach. The conclusion of their sensitivit anal s i s was that h i her rou hness OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 14 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST Supplemental Request

Background:

The response to the first portion of the request (low roughness values) is acceptable.

The second portion of the request pertaining to abrupt changes in roughness is not fully addressed in the response.

Based on Figures 13.3 and 13.4 of the FHRR (Exelon, 2015), it appears that the reach just downstream from would have a higher roughness value than the reach above I but the roughness values are opposite (higher upstream, lower downstream).

Additionally, from the aerial photography available , it is not clear that the downstream reach with n=0.0143 is significantly different from the reach between -(n=0.0375) or upstream from (n=0.03).

Sensitivity analyses were conducted on the roughness values to determine the impact to computed water surfaces.

Results indicated a relatively high sensitivity of the model to the roughness coefficients.

Request: Please provide detailed clarification , or show additional data and/or aerial imagery to support the justification for the significant changes in roughness values. Rivers and Streams

Background:

The staff noticed that for-an average open rate of is assumed (Exelon, 2015a)). The rate at which gates RESPONSE values in the upstream reach do not affect peak stages at the site. A thorough investigation of what would normally be considered a very low channel roughness value was documented in the Exelon TOOi and was deemed acceptable.

The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need request. In response to this information need the licensee provided further clarification on the open rate of the gates and described the use of the 10 gate clusters (Exelon, 2016a). While the written response did not directly address the request, the staff and licensee discussed the re uest durin a conference call on Februa 12, OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 15 16 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST are opened impacts the flood routing and computed discharges/stages.

Request: Please clarify whether the average open rate of takes into account moving the cranes. Also, please clarify whether using gate groups consisting of up to 10 gates influences open/close rate (i.e. does HEC-RAS open .. I gates in Rivers and Streams

Background:

The staff noticed that on page 69, Sections 3.2.3.8 and 3.2.3.9 of the FHRR (Exelon, 2105a), that cross sections extend through Unit 1 and other buildings in the vicinity.

The buildings would likely act as obstructions and would decrease conveyance through the cross sections.

Request: Please clarify whether Unit 1 and other buildings in the vicinity were accounted for in either the cross section development (i.e. blocked obstructions) or in roughness values. Dam Failure

Background:

The staff noticed that for the was modeled (Exelon, 2015b). The breach size directly impacts downstream discharges and stages. RESPONSE 2016. During the call, the staff ga in ed clarification on the order of gate openings modeled within HEC-RAS. The discuss i on noted that the group o tlm l gates does open in -However, an example of when the -group would be opened is when the model is going from to -gates. Prior to operation there would be two groups, a* -*and two groups open. The operation would include clos in g the and -groups and opening another group. The NRC staff reviewed the informat i on provided by the licensee and concluded it was sufficient to address the informati on need request. The response from the licensee prov i ded a narrative describ in g the conclusion that the inclusion of Unit 1 structures would not significantly decrease the conveyance and therefore not increase water surfaces enough to adverse ly i mpact Unit 2 and Unit 3 safety-related Structures , Systems , and Components (SSCs) (Exelon, 2016a). The NRG staff reviewed the informat i on prov i ded by the licensee and concluded it was suffic i ent to address the informatio n need request. In response to this in formation need the licensee d i rected the reviewers to a separate calculation package (PEAS-FLOOD-20) where additional information perta inin g to the selected breach parameters was contained (Exelon , 2016a). PEAS-F L OOD-20 Revision 0 provided details regarding Gee's Method for est i mating the breach width. OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 17 18 19 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST RESPONSE Request: Please clarify how the for the breach was determined. width The NRG staff reviewed the i n f ormat i on prov i ded by the licensee and concluded it was suffic i ent t o address t he in fo rmat i on need request. Dam Failure

Background:

The staff noticed that i n Tab l es 3.3.6.1 and 3.3.6.2 of the FHRR (Exelon, 2015a), that the maximum WSE for the seismically induced dam failure is than the maximum WSE for the precipitation driven dam failure but maximum discharges are not significantly different.

Request: Please clarify why the discharges are relatively similar even though the stages are much different (i.e. is it due to backwater effects?) Error/Uncertainty

Background:

The s t aff noticed in Section 3.9 of the FHRR (Exelon, 201 Sa) that the text describes a sensitivity analysis conducted to determine the impacts to stage due to gate failure. No other uncertainty is descr i bed in the text. The ranges in WSE due to both knowledge and mode l uncertainty for hydrology and hydraulics may be re l atively significant.

Request: Please c l arify why uncertainty due to hydrologic and hydraulic analyses (other than failure of Gonowingo Gates) is not described. Rivers and Streams In response to this in formation need the licensee provided additional informat i on perta inin g to the hydrographs a nd associated

_vo l ume of water for th e two scenar i os (Exe l on, 20 16 a). Further review of the hydraulic mode li ng showing th e -indicates the hi ghly va r iable d i scharge rating curve is appropr i ate ly modeled. The NRG staff reviewed the i n f orma ti on p r ov i ded by th e licensee and concluded it was sufficient t o add r ess t he information need request. In response to th i s in formation need , th e licensee clar i f i ed that because calibration and validation criteria were sat i sf i ed , further examination of uncertainty was deemed unnecessary (Exelon, 2016a). While addit i onal examination of uncerta in ty would increase know l edge about the system , the response from the licensee was suffic i ent to address the information need r equest. In response to this i n formation need , the l ic ensee d i scussed the to use interpo l ated cross se ction s to maintain t he gradually vaned unstead flow re i me which i s t he fundamental ba sis for OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 20 21 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST RESPONSE

Background:

The staff noticed that the HEC-RAS the HEC-RAS program equations (Exelon, 2016a). Wh il e using model contains 2026 cross sections; however, fewer surveyed cross sections would be preferable, the staff than 10% of the cross sections are based on topo data understands that additional data was not ava i lab l e. The -the remainder are interpolated (Exelon , 2105b). The information provided by the l i censee was sufficient to address the use of interpolated cross sections can have an effect on information need request. the discharge, hydrograph timing, and computed WSE. Request: Please clarify why so many interpolated cross sections were required and describe the i mpact on WSEs , peak discharges , and hydrograph timing within the model. Flood Parameters and Comearison with COB In response to this i nformation need request , the l icensee

Background:

The staff noticed that on page 151, provided add i t i onal background on the 1936 event and noted that Hydraulic Model CalibrationNalidation Events row that the bathymetry i n the current HEC-RAS model is like l y not the March 1936 event is listed here as a validation representative of the 1936 bathymetry (Exelon, 2016a). The event; however, Table 3.2.6.2.1 shows that the HEC-licensee also noted that the event was listed as a va lid ation event RAS model WSE is 1.91 ft lower than the reported but it did not contain enough substa nti al evidence to in corporate

' historical WSE at Peach Bottom (Exelon, 2015a). changes into the HEC-RAS mode l. Based on the d i sc u ssion Model validation reflects how well a model performs for pertaining to the event's lack of ava il able data, re li ability of the simulations other than the event for which it was available data, and potential changes i n river bathyme try since specifically calibrated.

1936, the staff acknowledges that it may be more app rop riate to remove the 1936 event from cons i deration as a validat ion event. Request: Please clarify if other validation occurred for The response was determined to be acceptable based on the lack the March 1936 event. of further storm events data. Rivers and Streams In response to this i nformation need request , the licensee

Background:

The staff noticed that in calculation discussed how cont r action/expans i on coeffic i ents are generally package "Cale PEAS-FLOOD-06'

', Section 2.1.2.3, not used w i th an unsteady flow mode l (Exelon, 2016a). sheets 13-14 (Exelon , 2015b) that Addit i onally , a sens it ivity ana l ysis was conducted to show that contraction/expansion coefficients appear to have been water surfaces increased by a maximum of just ove r 1 ft at the s it e set only on the non-interpolated cross sect i ons. with an overly conservative ass u mp tio n of using contract i on/expansion coefficients at each cross sect i on. OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 22 23 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST Contraction/expansion coefficients may have an impact on computed WSEs.

Please clarify whether contraction/expansion coeff1c1ents were set on the non-interpolated cross sections only (supplied geometry appears to not have any coefficients on interpolated cross sections) and what impact this may have on computed WSEs. Rivers and Streams Background

The staff noticed that in ca l culation package " Cale PEAS-FLOOD-06'

', on Sheet 19 , Table 2.2 that footnote 1 for the SR-472 bridge notes that the datum is unknown so mean sea level was assumed for datum. A higher or lower bridge may have an impact on computed WSEs at the site. RESPONSE The NRC staff rev i ewed the informat i on provided by the licensee and concluded it was sufficient to add r ess the informat i on need request. In response to th i s i nformation need r equest the licensee noted th at because of the minor vertica l datum change expec t ed and the distance between the bridge and the Peach Bottom s it e as we ll as the existence of-dams betwee n the bridge and th e Peach Bottom site, any i mpact from changing the bridge e l evation would be negligible (Exelon , 2016a). The NRC staff rev i ewed the in format i on provided by the licensee Request: Please clarify whether any sensitivity and concluded it was suff i cient to add r ess the i nfo rmati on need analyses were completed to evaluate effects of a higher request. or lower bridge and if so, what was the impact. Rivers and Streams

Background:

The staff noticed that in calculation package "Cale PEAS-FLOOD-06", that the text in section 2.4 notes that the rating curves for_ were not used in the HEC-RAS mooel (generic equations w1thrn HEC-RAS were used instead) (Exelon, 2015b). The rating curves directly impact computed discharges and stages. I n r esponse to thi s i nformation need r equest , the li censee noted that the effects of submergence are better accoun t ed fo r using HEC-RAS computed curves than user-specified rating curves (Exelon, 2016a). The staff agrees with this assert ion but notes that at where the d i fferences between the HEC-RAS computed and " known" curves are grea t est , submergence does not appear t o be a concern (th e maximum tailwater during the PMF appears to be roughly 40 ft below the outlets). A cursory sens i t i vity analys i s was conducted by the staff using the user-def i ned rating curves that were included i n the whicha OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 24 25 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST RESPONSE Request: Please clarify why the dam rating curves taken from the " known" rating curve. The sensitivity analysis were not directly input to the HEC-RAS model and indicates low sens i t i vity to the se l ected computation method. describe the potential impact on discharges and stages downstream, especially considering the d iff erences in The NRC staff rev i ewed the informat i on prov i ded by the licensee computed vs. known rating curves reported in Section and concluded it was sufficient to add r ess the informat i on need 7.5 of calculation package "Cale PEAS-FLOOD-06". request. Rivers and Streams In response to this i nformation need request, the l icensee noted

Background:

The staff noticed that in ca l culation the distance of the bridges from the s i te would resu l t in negligible package "Cale PEAS-FLOOD-06, Section 7.1.3, sheet effects on the computed WSE at Peach Bottom (Exelon , 2016a). 51 that the energy method was used to evaluate high The NRC staff reviewed the information prov i ded by the licensee flows through the US-30 bridge and the SR-462 bridge and concluded it was suffic i ent to add r ess the information need (Exelon, 2015b). The selected high flow computation request. method within HEC-RAS can have an impact on computed WSE. Request: Please clarify whether a sensitivity analysis was conducted to eva l uate use of pressure/weir flow computations for high flow at the US-30 bridge and SR-462 bridge, and if so, what were the results. Rivers and Streams In response to this information need r equest , the licensee

Background:

The staff noticed that in calculation provided further c lari fication on the acceptance criter i a , package "Cale PEAS-FLOOD-06", Sect i ons. sheet 95 , specifically that the +/-0.1 ft criterion only applies at one location the first (#1) acceptance criteria notes that the (Exelon, 2016a). While typical calibration methods wou l d use a calibration event is within +/-0.1 ft of observed value; global value for calibration acceptance rather than at one point to however, Table 7.4 on sheet 72 shows only one point ensure the entire model is reasonab l y replicat i ng an event rather out of four at which the modeled WSE i s within 0.1 ft of than at one location only. the observed (Exelon , 201 Sb). Also, Table 8.1 only The NRC staff reviewed the information provided by the licensee includes two calibration locations for Lee vs. the four shown i n Table 7.4. The calibrated mode l was used to and concluded it was suffic i ent to add r ess the informat i on need simulate the PMF and has a direct impact on the request. computed WSEs. OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. 26 27 28 OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST RESPONSE Request: Please clarify the apparent contradiction between Sections 7.4.1 .2 and 8.0. Rivers and Streams In response to this i nformation need request the licensee noted

Background:

The staff noticed that in calculation (and provided graphical supporting documentation) that the package "Cale PEAS-FLOOD-06", HEC-RAS model oscillations downstream of Conowingo Dam do not influence the results that footnotes in Tables 7.5 and 7.7 indicate conditions upstream (Exelon, 2016a). some oscillation of the WSE around certain values The NRC staff reviewed the information provided by the licensee (Exelon, 2015b). Oscillations in the WSE may reflect other model inconsistencies.

and concluded i t was sufficient to add r ess the in formation need request. Request: _Please clarify whether large oscillations

(>5ft minutes) downstream of Conowingo Dam are 1ust1f1ed and whether or not they impact water surfaces upstream.

Rivers and Streams In response to this i nformat i on need request , the licensee

Background:

The staff noticed in calculation package acknowledged that the large init i al ad ju stments a re a model i ssue , "Cale PEAS-FLOOD-06", in the HEC-RAS model initial but noted that the ini tial adjustments in the water surface are conditions/results section, the results of the HEC-RAS frequently encountered in models dur i ng the i r " spin-up" period model sho_w large init i al adjustments in stage at certain (Exelon, 2016a). The WSE stab ili zes after 14 hours0.583 days <br />0.0833 weeks <br />0.0192 months <br /> a n d is not cross sections (e.g. for Lee simulation, stage drops expected to influence the peak PMF discharge or WSE. nearly 20 ft at RS 124639.1 over the first 14 hours0.583 days <br />0.0833 weeks <br />0.0192 months <br />) The NRC staff reviewed the informat i on provided by the licensee (Exelon , 201 Sb). Large adjustments at the start of the simulation may reflect inconsistent initial conditions and concluded it was sufficient to address the information need within the HEC-RAS model. request. Request: Please clarify if this is based on observed data or is a model issue. Rivers and Steams In response to this in formation need request , the licensee d i scussed model s ta bilitv and the need for interoolated cross OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED NO. Sources: OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION NEED REQUEST RESPONSE

Background:

The staff noticed that in Section 7.1.1 of sections.

While using surveyed cross sections is preferable to "Cale PEAS-FLOOD-06", sheet 49, second to last interpolated.

sentence of second to last paragraph , notes that cross The NRC staff rev i ewed the information provided by the licensee sections were limited to every 1000 ft., then interpolated every 100 ft. to avoid conveyance and energy loss and concluded it was sufficient to address the informat i on need issues (Exelon, 2015b). The use of interpolated cross request. sections rather than actual topographica l data can impact the computed WSE, discharge, and hydrograph timing. Request: Please clarify what conveyance and energy loss issues were experienced.

Please clarify whether the Courant condition (or similar) was used to evaluate appropriate cross section spacing. Exelon, 2015a , "Peach Bottom Atomic Station , Flood Hazard Reeva l uation Report , Revision O," Enc l osure to Letter from James Barstow to the NRC Document Control Desk,

Subject:

" Exelon Generation Company, LLC Response to March 12 , 2012 , Request for Inform at i on Enc l osure 2 Recommendation 2.1, Flooding, Required Response 2, Flood Hazard Reevaluation Report," August 12 , 2015 , ADAMS Accession No. ML 15233A066 (Non-Public).

Exelon, 2015b, "Pertinent Site Data," CD-A , Enclosure to Letter from James Barstow to the NRC Docume nt Control Desk ,

Subject:

" Exelon Generation Company, LLC Response to March 12, 2012 , Request for Informat i on Enc l osure 2 Recomme n dation 2.1, Flood i ng, Requi r ed Response 2, Flood Hazard Reevaluation Report ," August 12 , 2015 , ADAMS Access i on No. ML 15233A066 (Non-Pub li c). Exelon, 2016a, No title , Exelon Responses to NRC Informat i on Need Requests for Peach Bottom and TMI , February 10 , 20 1 6. Exelon , 2016b , No title, Exe l on Submittal Responses to NRC Information Need Requests for Peach Bottom and TMI , Apr il 4, 2016 .. NRC , 2011, "Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the Un it ed State o f Amer ic a ," NUREG/CR-7046 , November 2011, ADAMS Accession No. ML 1 1321A195 (Public). NRC, 2012 , Letter from Eric J. Leeds, Director , Office of Nuclear Reactor Reg ul at ion and Michael R. Johnson, Directo r, O ffic e of New Reac tor s , to All Power Reactor Licensees and Holders of Construct i on Perm it s i n Act i ve or Deferred Status,

Subject:

" Request for Information Pursuant to Ti tl e OFFICIAL USE ONLY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION 10 of the Code of Federal Regulations 50.54 (f) Regard i ng Reco mm endat i ons 2.1 , 2.3 , and 9.3 , of t he Ne ar-Term Tas k F orce Rev i ew o f In s i g hts from the Fukushima Da i-lch i Accident ," Marc h 12, 2012 , ADAMS Accession No. ML 1 2056A046 (Pub l ic) OFFICIAL USE ONLY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION ATIACHMENT1 Peach Bottom Atomic Power Station, Units 2 and 3 Audit Document List 1. Exelon , 2015 , " Peach Bottom Atomic Station , Flood Hazard Reevaluation Report , Revision 0 ," Enclosure to Letter from James Barstow to the NRC Document Control Desk ,

Subject:

"Exelon Generation Company , LLC Response to March 12 , 2012 , Request for Information Enclosure 2 Recommendation 2.1, Flooding , Required Response 2 , Flood Hazard Reevaluation Report ," August 12 , 2015 , ADAMS Accession No. ML 15233A066 (Non-Public). 2. Exelon , 2015b, Beyond Design Basis External Event -Flood Re-Evaluation

-HEC-RAS Model of Susquehanna River Development and Calibration , Analysis No. PEAS-FLOOD-06 , October 2014. Provided on CD-Ras an Enclosure to Letter from James Barstow to the NRC Document Control Desk,

Subject:

"Exelon Generation Company , LLC Response to March 12 , 2012 , Request for Information Enclosure 2 Recommendation 2.1 , Flooding, Required Response 2 , Flood Hazard Reevaluation Report ," August 12, 2015, ADAMS Accession No. ML 15233A068 (Non-Public).

3. Exelon , 2015c , Beyond Design Basis External Event -Flood Re-Evaluation

-HEC-HMS Rock Run Creek Hydrologic Calculation , Analysis No. PEAS-FLOOD-07 , July 2015. 4. Exelon, 2014a, Beyond Design Basis External Event -Flood Re-Evaluation

-HEC-RAS Rock Run Creek Probable Maximum Flood (PMF) Hydraulic Calculation, Analysis No. PEAS-FLOOD-08 , July 2014. 5. Exelon , 2014b, Beyond Design Basis External Event -Flood Re-Evaluation

-Ice Effects , Analysis No. PEAS-FLOOD-09 , April 2014. 6. Exelon , 2015d, Beyond Design Basis External Event -Flood Re-Evaluation

-Site-Spec i fic Probable Maximum Precipitation (PMP) and Climatology Calculation , Analysis No. PEASFLOOD-11 , July 2015. 7. Exelon , 2015e, Beyond Design Basis External Event -Flood Re-Evaluation

-HEC-RAS Probable Maximum Flood (PMF) Water Level, Analysis No. PEAS-FLOOD-16, April 2015. 8. Exelon, 2015f, Beyond Design Basis External Event -Flood Re-Evaluation

-Combination Flooding.

Analys i s No. PEAS-FLOOD-20 , April 2015. 9. Exelon , 2015g , Beyond Design Basis External Event -Flood Re-Evaluation

-Error and Uncertainty Calculation , Analysis No. PEAS-FLOOD

-21 , July 2015. OFFICIAb USE ONbY SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION ATTACHMENT 2 List of Exelon Audit Participants Name Organization 1 . David Distel Exelon 2. Joe Bellini Aterra Solutions

3. Jesse Lucas Exelon 4. Shaun Kline Exelon 5. Vined Aggarwal Exelon 6. Chuck Behrend Exelon 7. Tom O'Reilly Exelon 8. Kyle Kaminski Exelon Consultant

9. Tim Dean Exelon Consultant

10. James Barbis Exelon Consultant 11 . W i lliam Mcsorley Exelon OFFICIAb USE ONbV SECURITY RELATED INFORMATION OFFICIAL USE ONLY SECURITY RELATED INFORMATION B. Hanson

SUBJECT:

NUCLEAR REGULATORY COMMISSION REPORT FOR THE AUDIT OF EXELON GENERATION COMPANY LLC'S FLOOD HAZARD REEVALUATION REPORT SUBMITTAL RELATING TO THE NEAR-TERM TASK FORCE RECOMMENDATION 2.1-FLOODING FOR PEACH BOTTOM ATOMIC POWER STATION , UNITS 2 AND 3 DATED SEPTEMBER 25, 2017 DISTRIBUTION

PUBLIC LGibson , NRR RidsNrrDorllpl1 Resource RidsRgn1 MailCenter Resource RidsOpaMail Resource RRivera-Lugo , NAO MWillingham , NRO NSanfilippo, NRA JLD A/F LQuinn-Willingham , NAO RidsNrrDorl Resource RidsNrrLASLent RidsAcrsAcnw

_MailCtr Resource ACampbell, NRO KSee , NRO SDevlin-Gill , NRO RidsNRAJLD Resource RidsNroDsea Resource RidsNrrPMPeachBottom Resource RidsOgcMailCenter Resource CCook , NAO MShams , NRR BHarvey , NAO A A S D M Accession Nos.: Pka: ML17255A475; Non Public: ML17255A501; Public: ML17255A524 OFFICE NAR/JLD/JHMB/PM NRR/JLD/JHMB/LA NAR/JLD/JHMB/BC NAO/DSENAHM 1 !TA NAME LGibson Slent NSanflipo KSee (SWvman forl IDATE 109/15/2017 109/13/2017 I 0911 s12017 I 0911512017 OFFICE NRO/DSENAHM1/BC NRR/JLD/JHMB/PM NAME CCook LGibson ISDev l in-Gill for) DATE 09/15 /2017 09/25/2017 OFFICAL RECORD COPY OFFICIAL USE ONLY SECURITY RELATED INFORMATION I