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:	Lauren Gibson Japan Lessons-Learned Division
To:	Bryan Hanson Exelon Nuclear
Gibson L, NRR/JLD, 415-1056
Shared Package
Ml17255A475	List: ML17255A524
References
CAC MF6598, CAC MF6599
Download: ML17255A524 (26)

Text

OFFICIAL USE ONLY SECURITY RELATED INFORMATION UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555--0001 September 25, 2017 Mr. Bryan C. Hanson 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. ML15230A235}, the U.S. Nuclear Regulatory Commission (NRG) informed you of the staff's plan to conduct a regulatory audit of Exelon Generation Company, LLC's (the licensee) Flood Hazard Reevaluation Report (FHRR) submittal related to the Near-Term 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 licensee's FHRR and the subsequent issuance of a staff assessment.

The 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 , Office Instruction LIC-111 , "Regulatory Audits," dated December 29, 2008 (ADAMS Accession No. ML082900195). Therefore, the purpose of this letter is to provide you with the final audit report, which summarizes and documents the NRC's regulatory audit of the licensee's FHRR submittal.

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

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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.

Sincerely, Lauren K. Gibson, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation Docket Nos. 50-277 and 50-278

Enclosures:

1. Audit Report (Non-Public)

2. Audit Report (Public) cc w/encl: Distribution via Listserv 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 1o 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 accident at the Fukushima Dai-ichi nuclear power plant, as documented in The Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident. 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.

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 10 CFR 50.54(f).

By letter dated August 12, 2015 (Agencywide Documents Access and Management System Accession No. ML15233A067), 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 is 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 similarities/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 participants can be found in Attachment 2.

DOCUMENTS AUDITED 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.

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OFFICIAL USE ONLY SECURITY RELATED INFORMATION EXIT MEETING/BRIEFING On June 17, 2016, the NRC staff closed out the discussion of the technical topics described above.

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OFFICIAL USE ONLY SECURITY RELATED INFORMATION Table 1: Peach Bottom, Units 2 and 3 Information Needs and Response Summary INFORMATION INFORMATION NEED REQUEST RESPONSE NEED NO.

1 All Flood-Causing Mechanisms - Comparison of In response to the information request (Exelon, 2016a), the Reevaluated Flood Hazard with Current Design licensee stated that the CLB and the CDB are synonymous, i.e.

Basis 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 information 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. Include 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 OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION INFORMATION NEED REQUEST RESPONSE NEED NO.

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 incorrectly compared with the current licensing basis (CLB).

Please confirm that this has been verified throughout the FHRR.

2 Local Intense Precieitatlon - Model bounda~ area The licensee stated (Exelon, 2016a) that the safety-related doors would not be subject to flow from the area outside the model Bac.kqround: The local intense precipitation (LIP) is domain because the doors have higher elevations than the ground equivalent to the 1-hour (hr), 1-square mile (mi 2) surface at the boundary location receiving flow from the outside probable maximum precipitation (PMP) as described in drainage area. Additionally, the licensee listed all critical door NUREG/CR-7046 (NRC, 2011; Exelon, 201 Sa). The locations and their elevations in Table 1 of the response (which modeling boundary area chosen by the licensee for the came from the FHRR) . The table's footnote states that the LIP analysis is only 0.13 mi2 , and doesn't capture all Turbine Building (Door 111) is at a non-safety-related structure flood-contributing rainfall within the site area. The and is not flood-protected. After reviewing, the staff requested a contour lines indicate a valley in the topographic map of reference indicating that the Turbine Building is not safety-related Figure 6.2 of calculation package PEAS-FLOOD-03 structure. For this request, the licensee provided an excerpt from Rev O (Exelon, 201 Sb) clearly show that runoff can the Updated Final Safety Analysis Report's (UFSAR) Appendix enter the Peach Bottom site from outside the current C.2.5.4, which describes that the Turbine Building is not one of model boundary through the northwest corner.

the structures included in a safe shutdown during flooding. The Request: Explain how the current model boundary area NRC staff confirmed the information from the UFSAR and no ca~tures the entire drainage 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 information need request.

needed.

3 Local Intense Precieitation - Missing site structures In response to the information request (Exelon, 2016a), the licensee stated that these structures were not included in the model because thev have been removed from the site.

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Background:

Some structures that could obstruct flow are not considered in the FL0-20 modeling (Exelon, The NRC staff reviewed the information provided by the licensee 2015b). and concluded it was sufficient to address the information need request.

Request: Provide justifications for why these structures were not represented in the model, or correct if needed.

4 Current Licensing Basis-Flooding in Streams and In response to this information need request (Exelon, 2016a), the Rivers licensee clarified that the differences in discharge were due to accounting for flood control projects (i.e. dams) versus

Background:

The staff noticed that the discharge of unregulated flow.

1,750,000 cubic feet per second (cfs) does not correspond to the discharge of 1,625,000 cfs utilized The NRC staff reviewed the information provided by the licensee previously by Three Mile Island (TMI) near Harrisburg and concluded it was sufficient to address the information need (Exelon, 2015a).

request.

Request: Please clarify or explain the difference in the numbers presented .

5 Rivers and Streams In response to this information need request, the licensee provided a table of values listing the storm orientation associated

Background:

The NRC staff noticed that the licensee with each storm centering (Exelon, 2016a).

did not provide the storm orientations for other storms in Table 3.2.4.2.1.1 of the FHRR (Exelon , 2015a) . The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need Request: Please clarify if the orientation of all storms request.

other than the TMI Watershed Centroid differed from the selected TMI Watershed Centroid storm.

6 Rivers and Streams In response to this information need request, the licensee clarified that the discharges were presented for informational purposes

Background:

The NRC staff noticed that the results for only, as both sets of values were derived from the same HEC-TMI such as discharge are presented on page 76 (and HMS model (Exelon, 2016a). Upon further request for others) in the FHRR (Exelon, 2015a).

clarification , the licensee presented a comparison of hydrographs from HEC-HMS and HEC-RAS to confirm that both models were OFFICIAL USE ONLY SECURITY RELATED INFORMATION

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Original Request: Please clarify why the results of providing roughly equivalent routing over the entire range of discharge are presented for TMI in this report instead of discharges considered (Exelon, 2016b).

Peach Bottom's resu lts.

The NRC staff reviewed the information provided by the licensee Supplemental Request: and concluded it was sufficient to address the information need request.

Background:

Hydrologic routing methods performed by the Hydrologic Engineering Center - Hydrologic Modeling System (HEC-HMS) tend to be much more simplified than the equations used by the unsteady Hydrologic Engineering Center - River Analysis System (HEC-RAS). Due to these computational differences, HEC-HMS routings may differ significantly from HEC-RAS over an equivalent reach of a river at certain discharges, particularly if the reach contains 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 mi). It is anticipated that there may be some differences in results between models, particularly as discharges exceed the calibration 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.

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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

Background:

In the Calibration and Validation Results individual storms, while the three individual storms had been used section of the FHRR (Exelon, 2015a), the staff noticed to derive three individually calibrated models, which in turn had that the text indicated the HEC-HMS model was been combined to produce the combined model (Exelon, 2016a) .

calibrated against Tropical Storm Lee, Hurricane Agnes, and Hurricane Ivan. No other storms are The NRC staff reviewed the information provided by the licensee indicated in this section, yet the heading indicates and concluded it was sufficient to address the information need model validation was performed. request.

Req~est: Please clarify what storm event(s) were considered for model validation.

8 Rivers and Streams In response to this information need request, the licensee clarified that Tropical Storm Lee was selected for calibration because it

Background:

In the Calibration and Validation Results was the most recent of the large events and it best matches the section of the FHRR (Exelon, 201 Sa), the staff noticed

~eometry used in the HEC-RAS model (Exelon, 2016a). The that the text indicated that the HEC-RAS model was licensee further explained that the validated storms all matched calibrated against Tropical Storm Lee and validated ob~erved high water marks at the plant site within 0.1 feet (ft) ,

against the March 1936 Storm, Hurricane Ivan and while not concurrently over- or under-predicting Hurricane Agnes events.

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.

9 Rivers and Streams In response to this information need request, the licensee provided values of mean absolute error, root mean squared error

Background:

The staff noticed in Table 3.2.6.2.1 (and and cor:elation coefficient (R) to demonstrate the timing match by elsewhere in text) of the FHRR (Exelon, 201 Sa) that comparing values of computed and observed discharge from the model calibration was indicated to be acceptable in same time step over the duration of the hydrograph (Exelon, relation to stage only.

2016a). The licensee also provided graphical comparison Request: Please clarify if timing of the hydrograph was between computed and observed hydrographs that allowed better of any concern in calibratina the HEC-RAS model to be OFFICIAL USE ONLY SECURITY RELATED INFORMATION

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used in unsteady flow, and if so, what was acceptable comparison of the timing of the rising , falling and peak portions of hydrograph timing differences for calibration. the hydrographs.

The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need request.

10 Ice Jams In response to this information need request the licensee indicated that the cumulative conservatisms included in other

Background:

The staff noticed that there was no parameters would encompass the uncertainties pertaining to ice discussion of certain details regarding the modeling of volume and roughness (Exelon, 2016a). The licensee also the ice jam release in the FHRR (Exelon, 2015a).

demonstrated that water surface profiles in the vicinity of the Request: Please clarify if the hydraulic modeling of the Peach Bottom site were relatively insensitive to the ice released ice jam took into account the volume of ice parameters selected due to the backwater influences posed by released from the jam and if increased roughness was considered to account for the interaction of ice flows The NRC staff reviewed the information provided by the licensee passing downstream.

and concluded it was sufficient to address the information need request.

11 Error/ Uncertainty In response to this information need request, the licensee clarified that the results presented were in response to sensitivity in

Background:

The staff noticed that the uncertainty Conowingo gate operations, not as a statistical uncertainty range listed in Table 3.9.3.3.1 of the FHRR (Exelon, analysis, as was done for topographic uncertainty (Exelon ,

2015a) was not related to any statistic such as the 2016a) . The licensee further clarified that topographic uncertainty standard deviation was not considered simultaneously with gate sensitivity.

Request: Please clarify if the uncertainty range The NRC staff reviewed the information provided by the licensee presented represents one standard deviation or two and concluded it was sufficient to address the information need standard deviations in elevation .

request.

12 Rivers and Streams In response to this information need, the licensee clarified that the

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Background:

The staff noticed text pertaining to - - and noted that the typographical error does not that states that the crest elevation is impact results presented in the FHRR (Exelon, 2016a).

while the HEC-RAS model uses an invert elevation of - The NRC staff reviewed the information provided by the licensee

• I (assume this is also in NAVD88 per description in and concluded it was sufficient to address the information need section 3.2.2.12) (Exelon, 2015a). The invert elevation request.

of the crest impacts the routing and the discharge through the dam.

Request: Please clarify which crest elevation is correct.

13 Rivers and Streams In response to this request for information, the licensee provided discussion on the applicability of the lower roughness values for Original Request higher discharges (Exelon, 2016a). The licensee also provided

Background:

The staff noticed in Figure 3.2.2.15.1 of aerial imagery overlaid with the location of abrupt changes in the FHRR (Exelon , 2015a) that some low roughness roughness values. The NRC staff reviewed the information values (n=0.0143) are used in the downstream reach of provided by the licensee and concluded it was sufficient to the HEC-RAS model. Also, abrupt changes in address the information need request for the first portion of the roughness from 0.0143 to 0.03 to 0.0375 are included request (low roughness values).

in the model (Exelon, 2015b). Low roughness values The staff performed sensitivity analyses on the roughness values decrease the computed water surface elevation (WSE).

to determine the impact to computed water surfaces. Two general Justification for the use of low roughness values for the sensitivity analyses were conducted; the first included adjusting calibration effort is provided in the Exelon Transmittal of Manning's roughness values for the entire reach while the second Design Information (TODI) and Attachment 5 of the adjusted Manning's roughness values for an upstream reach.The TODI.

results indicated a relatively high sensitivity of the model to the Request: Please clarify or add justification to support roughness coefficients. The high sensitivity of the model to the assumption that the low roughness values would be roughness value was considered justification for an additional valid for higher discharges (i.e. probable maximum request for information.

flood [PMF]). Also, please clarify if abrupt changes in The licensee's second response (Exelon, 2016b) provided further roughness from 0.0143 to 0.03 to 0.0375 are supported information on a sensitivity analysis that the licensee conducted by physical changes in the river/floodplain or if the on roughness values in the upstream portion of the reach. The changes are a result of calibration efforts.

conclusion of their sensitivit anal sis was that hi her rou hness OFFICIAL USE ONLY SECURITY RELATED INFORMATION

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values in the upstream reach do not affect peak stages at the site.

Supplemental Request A thorough investigation of what would normally be considered a very low channel roughness value was documented in the Exelon

Background:

The response to the first portion of the TOOi and was deemed acceptable.

request (low roughness values) is acceptable. The second portion of the request pertaining to abrupt The NRC staff reviewed the information provided by the licensee changes in roughness is not fully addressed in the and concluded it was sufficient to address the information need response. Based on Figures 13.3 and 13.4 of the request.

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.

14 Rivers and Streams In response to this information need the licensee provided further clarification on the open rate of the gates and described the use of

Background:

The staff noticed that for -

the 10 gate clusters (Exelon, 2016a). While the written response

- an average open rate of did not directly address the request, the staff and licensee is assumed (Exelon, 2015a)). The rate at which gates discussed the re uest durin a conference call on Februa 12, OFFICIAL USE ONLY SECURITY RELATED INFORMATION

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are opened impacts the flood routing and computed 2016. During the call, the staff gained clarification on the order of discharges/stages. gate openings modeled within HEC-RAS. The discussion noted that the group ot l ml gates does open in -

Request: Please clarify whether the average open rate However, an example of when the - group would be of takes into account moving opened is when the model is going from to - gates.

the cranes. Also, please clarify whether using gate Prior to operation there would be two groups, a

• groups consisting of up to 10 gates influences - *and two groups open. The operation open/close rate (i.e. does HEC-RAS open . .I gates would include closing the and - groups and in opening another group.

The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need request.

15 Rivers and Streams The response from the licensee provided a narrative describing the conclusion that the inclusion of Unit 1 structures would not

Background:

The staff noticed that on page 69, significantly decrease the conveyance and therefore not increase Sections 3.2.3.8 and 3.2.3.9 of the FHRR (Exelon, water surfaces enough to adversely impact Unit 2 and Unit 3 2105a), that cross sections extend through Unit 1 and safety-related Structures, Systems, and Components (SSCs) other buildings in the vicinity. The buildings would likely (Exelon, 2016a) .

act as obstructions and would decrease conveyance through the cross sections. The NRG staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need Request: Please clarify whether Unit 1 and other request.

buildings in the vicinity were accounted for in either the cross section development (i.e. blocked obstructions) or in roughness values.

16 Dam Failure In response to this information need the licensee directed the reviewers to a separate calculation package (PEAS-FLOOD-20)

Background:

The staff noticed that for the where additional information pertaining to the selected breach parameters was contained (Exelon , 2016a). PEAS-FLOOD-20 was modeled (Exelon, 2015b). The breach size directly Revision 0 provided details regarding Gee's Method for estimating impacts downstream discharges and stages.

the breach width .

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Request: Please clarify how the width The NRG staff reviewed the information provided by the licensee for the breach was determined. and concluded it was sufficient to address the information need request.

17 Dam Failure In response to this information need the licensee provided additional information pertaining to the hydrographs and

Background:

The staff noticed that in Tables 3.3.6.1 associated _volume of water for the two scenarios (Exelon, 20 16a).

and 3.3.6.2 of the FHRR (Exelon, 2015a), that the Further review of the hydraulic modeling showing the -

maximum WSE for the seismically induced dam failure is than the maximum WSE for the indicates the highly variable stage-precipitation driven dam failure but maximum discharge rating curve is appropriately modeled.

discharges are not significantly different.

The NRG staff reviewed the information provided by the licensee Request: Please clarify why the discharges are and concluded it was sufficient to address the information need relatively similar even though the stages are much request.

different (i.e. is it due to backwater effects?)

18 Error/Uncertainty In response to this information need, the licensee clarified that because calibration and validation criteria were satisfied, further

Background:

The staff noticed in Section 3.9 of the examination of uncertainty was deemed unnecessary (Exelon, FHRR (Exelon, 201 Sa) that the text describes a 2016a). While additional examination of uncertainty would sensitivity analysis conducted to determine the impacts increase knowledge about the system , the response from the to stage due to gate failure. No licensee was sufficient to address the information need request.

other uncertainty is described in the text. The ranges in WSE due to both knowledge and model uncertainty for hydrology and hydraulics may be relatively significant.

Request: Please clarify why uncertainty due to hydrologic and hydraulic analyses (other than failure of

((Gonowingo Gates))) is not described.

19 Rivers and Streams In response to this information need, the licensee discussed the ne~d to use interpolated cross sections to maintain the gradually vaned unstead flow re ime which is the fundamental basis for OFFICIAL USE ONLY SECURITY RELATED INFORMATION

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Background:

The staff noticed that the HEC-RAS the HEC-RAS program equations (Exelon, 2016a). While 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 available. The

- the remainder are interpolated (Exelon, 2105b). The information provided by the licensee 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 impact on WSEs, peak discharges, and hydrograph timing within the model.

20 Flood Parameters and Comearison with COB In response to this information need request, the licensee provided additional background on the 1936 event and noted that

Background:

The staff noticed that on page 151, the bathymetry in the current HEC-RAS model is likely not Hydraulic Model CalibrationNalidation Events row that representative of the 1936 bathymetry (Exelon, 2016a). The the March 1936 event is listed here as a validation licensee also noted that the event was listed as a validation event event; however, Table 3.2.6.2.1 shows that the HEC-but it did not contain enough substantial evidence to incorporate '

RAS model WSE is 1.91 ft lower than the reported changes into the HEC-RAS model. Based on the discussion historical WSE at Peach Bottom (Exelon, 2015a).

pertaining to the event's lack of available data, reliability of the Model validation reflects how well a model performs for available data, and potential changes in river bathymetry since simulations other than the event for which it was 1936, the staff acknowledges that it may be more appropriate to specifically calibrated.

remove the 1936 event from consideration as a validation 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.

21 Rivers and Streams In response to this information need request, the licensee discussed how contraction/expansion coefficients are generally

Background:

The staff noticed that in calculation not used with an unsteady flow model (Exelon, 2016a).

package "Cale PEAS-FLOOD-06, Section 2.1 .2.3, Additionally, a sensitivity analysis was conducted to show that sheets 13-14 (Exelon, 2015b) that water surfaces increased by a maximum of just over 1 ft at the site contraction/expansion coefficients appear to have been with an overly conservative assumption of using set only on the non-interpolated cross sections.

contraction/expansion coefficients at each cross section.

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Contraction/expansion coefficients may have an impact on computed WSEs. The NRC staff reviewed the information provided by the licensee and concluded it was sufficient to address the information need Reg~e~t: Please clarify whether contraction/expansion request.

coeff1c1ents were set on the non-interpolated cross sections only (supplied geometry appears to not have any contr~ction/expansion coefficients on interpolated cross sections) and what impact this may have on computed WSEs.

22 Rivers and Streams In response to this information need request the licensee noted that because of the minor vertical datum change expected and the

Background:

The staff noticed that in calculation distance between the bridge and the Peach Bottom site as well as package "Cale PEAS-FLOOD-06, on Sheet 19, Table the existence of - dams between the bridge and the Peach 2.2 that footnote 1 for the SR-472 bridge notes that the Bottom site, any impact from changing the bridge elevation would datum is unknown so mean sea level was assumed for be negligible (Exelon, 2016a).

~he datum. A higher or lower bridge may have an impact on computed WSEs at the site. The NRC staff reviewed the information provided by the licensee Request: Please clarify whether any sensitivity and concluded it was sufficient to address the information need analyses were completed to evaluate effects of a higher request.

or lower bridge and if so, what was the impact.

23 Rivers and Streams In response to this information need request, the licensee noted that the effects of submergence are better accounted for using

Background:

The staff noticed that in calculation HEC-RAS computed curves than user-specified rating curves package "Cale PEAS-FLOOD-06", that the text in (Exelon, 2016a) . The staff agrees with this assertion but notes section 2.4 notes that the rating curves for _

that at where the differences between the were not HEC-RAS computed and "known " curves are greatest, di.re~tly used in the HEC-RAS mooel (generic equations submergence does not appear to be a concern (the maximum w1thrn HEC-RAS were used instead) (Exelon, 2015b).

tailwater during the PMF appears to be roughly 40 ft below the The rating curves directly impact computed discharges outlets) . A cursory sensitivity analysis was conducted by the staff and stages.

using the user-defined rating curves that were included in the

~-----~--------------------~o~r~i~i~ na~l~m~oo~e~l~fo~r whicha ear~h~ebeen OFFICIAL USE ONLY SECURITY RELATED INFORMATION

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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 sensitivity to the selected computation method.

describe the potential impact on discharges and stages downstream, especially considering the differences in The NRC staff reviewed the information provided by the licensee computed vs. known rating curves reported in Section and concluded it was sufficient to address the information need 7.5 of calculation package "Cale PEAS-FLOOD-06". request.

24 Rivers and Streams In response to this information need request, the licensee noted the distance of the bridges from the site would result in negligible

Background:

The staff noticed that in calculation effects on the computed WSE at Peach Bottom (Exelon, 2016a).

package "Cale PEAS-FLOOD-06, Section 7.1.3, sheet 51 that the energy method was used to evaluate high The NRC staff reviewed the information provided by the licensee flows through the US-30 bridge and the SR-462 bridge and concluded it was sufficient to address 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 evaluate 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.

25 Rivers and Streams In response to this information need request, the licensee provided further clarification on the acceptance criteria,

Background:

The staff noticed that in calculation specifically that the +/- 0.1 ft criterion only applies at one location package "Cale PEAS-FLOOD-06", Sections. sheet 95, (Exelon, 2016a). While typical calibration methods would use a the first (#1) acceptance criteria notes that the global value for calibration acceptance rather than at one point to calibration event is within +/- 0.1 ft of observed value; ensure the entire model is reasonably replicating an event rather however, Table 7.4 on sheet 72 shows only one point than at one location only.

out of four at which the modeled WSE is within 0.1 ft of 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 and concluded it was sufficient to address the information need shown in Table 7.4. The calibrated model was used to request.

simulate the PMF and has a direct impact on the computed WSEs.

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Request: Please clarify the apparent contradiction between Sections 7.4.1 .2 and 8.0.

26 Rivers and Streams In response to this information need request the licensee noted (and provided graphical supporting documentation) that the

Background:

The staff noticed that in calculation oscillations downstream of Conowingo Dam do not influence the package "Cale PEAS-FLOOD-06", HEC-RAS model conditions upstream (Exelon, 2016a).

results that footnotes in Tables 7.5 and 7.7 indicate 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 and concluded it was sufficient to address the information need other model inconsistencies.

request.

Request: _Please clarify whether large oscillations (>5ft

?ve~_15 minutes) downstream of Conowingo Dam are 1ust1f1ed and whether or not they impact water surfaces upstream.

27 Rivers and Streams In response to this information need request, the licensee acknowledged that the large initial adjustments are a model issue,

Background:

The staff noticed in calculation package but noted that the initial adjustments in the water surface are "Cale PEAS-FLOOD-06", in the HEC-RAS model initial frequently encountered in models during their "spin-up" period conditions/results section, the results of the HEC-RAS (Exelon, 2016a). The WSE stabilizes after 14 hours0.583 days <br />0.0833 weeks <br />0.0192 months <br /> and is not model sho_w large initial adjustments in stage at certain expected to influence the peak PMF discharge or WSE.

cross sections (e.g. for Lee simulation, stage drops 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 information provided by the licensee (Exelon, 201 Sb). Large adjustments at the start of the and concluded it was sufficient to address the information need simulation may reflect inconsistent initial conditions request.

within the HEC-RAS model.

Request: Please clarify if this is based on observed data or is a model issue.

28 Rivers and Steams In response to this information need request, the licensee discussed model stabilitv and the need for interoolated cross OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION INFORMATION INFORMATION NEED REQUEST RESPONSE NEED NO.

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 sections were limited to every 1000 ft., then interpolated The NRC staff reviewed the information provided by the licensee every 100 ft. to avoid conveyance and energy loss and concluded it was sufficient to address the information need issues (Exelon, 2015b). The use of interpolated cross request.

sections rather than actual topographical 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.

Sources:

Exelon, 2015a, "Peach Bottom Atomic Station, Flood Hazard Reevaluation Report, Revision O," 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. ML15233A066 (Non-Public).

Exelon, 2015b, "Pertinent Site Data," CD-A, 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. ML15233A066 (Non-Public).

Exelon, 2016a, No title, Exelon Responses to NRC Information Need Requests for Peach Bottom and TMI , February 10, 2016.

Exelon, 2016b, No title, Exelon Submittal Responses to NRC Information Need Requests for Peach Bottom and TMI , April 4, 2016 ..

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

NRC, 2012, 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 OFFICIAL USE ONLY SECURITY RELATED INFORMATION

OFFICIAL USE ONLY SECURITY RELATED INFORMATION 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 (Public)

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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. ML15233A066 (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. ML15233A068 (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-Specific Probable Maximum Precipitation (PMP) and Climatology Calculation, Analysis No. PEAS-FLOOD-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. Analysis 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.

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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 illiam 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 JLD A/F RidsNRAJLD Resource LGibson , NRR LQuinn-Willingham, NAO RidsNroDsea Resource RidsNrrDorllpl1 Resource RidsNrrDorl Resource RidsNrrPMPeachBottom Resource RidsRgn1 MailCenter Resource RidsNrrLASLent RidsOgcMailCenter Resource RidsOpaMail Resource RidsAcrsAcnw_MailCtr Resource CCook, NAO RRivera-Lugo, NAO ACampbell, NRO MShams, NRR MWillingham, NRO KSee , NRO BHarvey, NAO NSanfilippo, NRA SDevlin-Gill, NRO ADAM S 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 NSanflipo LGibson Slent KSee (SWvman forl IDATE 109/15/2017 109/13/2017 I0911 s12017 I0911512017 I OFFICE NRO/DSENAHM1/BC NRR/JLD/JHMB/PM CCook LGibson NAME ISDevlin-Gill for)

DATE 09/15 /2017 09/25/2017 OFFICAL RECORD COPY OFFICIAL USE ONLY SECURITY RELATED INFORMATION