ML23068A086

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NRC-2022-000210 - Resp 1 - Final, Agency Records Subject to the Request Are Enclosed, (Part 4 of 4)
ML23068A086
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Issue date: 03/01/2023
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NRC-2022-000210
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Text

Scott L. Batson

(_~DUKE Vice President

~ ENERGY Oconee Nuclear Station Ouke Energy ON01VP I 7800 Rochester Hwy Seneca. SC 29672 o 864.873.3274

f. 864.873. 4208 ONS-2015-028 Scolt. Blltson@duke-energy.com March 6, 2015 ATTN: Document Control Desk 10 CFR 50.54(f)

U.S. Nuclear Regulatory Commission (NRG)

Washington, DC 20555 Duke Energy Carolinas, LLC (Duke Energy)

Oconee Nuclear Station, Units 1, 2 and 3 Docket Numbers 50-269, 50-270, 50-287 Renewed License Numbers DPR-38, DPR-47, and DPR-55

Subject:

Supplemental Information Regarding NRC 2008 and 2012 Requests for Information Pursuant to 10 CFR 50.54(f) Pertaining to External Flooding at Oconee Nuclear Station (ONS).

References:

1. NRC Letter, Information Request Pursuant to 10 CFR 50.54(() Related to External Flooding, Including Failure of the Jocassee Dam, at Oconee Nuclear Station, Units 1, 2, and 3, (TAC NOS. MD8224, MD8225, and MD8226), dated August 15, 2008 (ADAMS Accession No. ML081640244).
2. NRC Letter, Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50. 54(() Regarding Recommendations 2. 1, 2. 3, and 9. 3, of the Near- Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, dated March 12, 2012 (ADAMS Accession No. ML12053A340).
3. Duke Energy Letter, Revised Flood Hazard Reevaluation Report per NRC's Request for Additional Information, dated March 6, 2015.

Ladies and Gentlemen, This letter supplements Duke Energy's response to the NRC's 10 CFR 50.54(f) letters dated August 15, 2008 (Reference 1), and March 12, 2012 (Reference 2), for the purpose of communicating Duke Energy's approach to address the regulatory matters associated with flooding caused by a postulated Jocassee Dam failure.

The approach is discussed in more detail in the enclosure to this letter. The enclosure also refers to a letter submitted concurrently with this one, which transmits a revised Flood Hazard Reevaluation Report (FHRR) for Oconee Nuclear Station (Reference 3).

This letter does not create or revise any Regulatory Commitments.

ONS-2015-028 March 6, 2015 Page 2 Should you have any questions concerning this letter, or require additional information, please contact Brian McCabe at (919) 812-8324.

I declare under penalty of perjury that the foregoing is true and correct. Executed on March 6, 2015.

Sincerely, Scott L. Batson Vice President Oconee Nuclear Station

Enclosure:

Supplemental information: 10 CFR 50.54(f) letters dated August 15, 2008, and March 12, 2012.

ONS-2015-028 March 6, 2015 Page 3 cc:

Victor Mccree, Regional Administrator U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, Georgia 30303-1257 William Dean, Director, Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Mail Slop O-13H16 Rockville, MD 20852-2738 Michele Evans, Director, Division of Operating Reactor Licensing U.S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Mail Stop O-8E1 A Rockville, MD 20852-2738 Jack Davis, Director, Japan Lessons Learned Project Division U.S. Nuclear Regulatory Commission One While Flint North 11555 Rockville Pike Mail Stop O-13H16 Rockville, MD 20852-2738 Brian Holian, Deputy Director for Engineering and Corporate Support U.S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Mail Stop O-13H16 Rockville, MD 20852-2738 Jeffery Whited (by electronic mail only)

U.S Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop O-8B1A Rockville, MD 20852 James R. Hall, Project Manager (ONS)

(by electronic mail only)

U.S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop O-8G9A Rockville, MD 20852 Eddy Crowe NRC Senior Resident Inspector Oconee Nuclear Station

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 1 of 13 Enclosure Supplemental information:

10 CFR 50.54(f) letters dated August 151 2008, and March 121 2012.

INDEX

1. PURPOSE
2. EXTERNAL FLOODING LICENSING HISTORY
3. CONFIRMATORY ACTION LETTER (JUNE 22, 2010)
4. EXTERNAL FLOODING MODIFICATIONS
5. FLOOD HAZARD REEVALUATION REPORT
6. PLANT RESPONSE TO EXTERNAL FLOODING
7. REGULATORY PATH FORWARD
8.

SUMMARY

9. REFERENCES

Attachment:

June 22, 2010, Confirmatory Action Letter - Commitments to Address External Flooding Concerns

ONS-2015-028 Enclosure March 6, 2015 Encl'. Page 2 of 13

1. PURPOSE' This enclosure supplements information provided by Duke Energy in response to the NRC's 10 CFR 50.54(f) letters dated August 15, 2008 (Reference 1), and March 12, 2012 (Reforence 2).

The purpose of this enclosure is to document Duke Energy's regulatory approach assciciated with flooding levels caused by an upstream Jocassee Dam failure including:

a) Oconee Nuclear Station (ONS) plans for maintaining ongoing compensatory measures documented in Confirmatory Action Letter (CAL) 2-10-003 (Reference 3),

b) An update on the status of upstream E.lam failure mitigation modifications, and c) The protection and mitigation strategies being implemented to address flood levels up to and including the bounding flood at the Oconee site due to dam failure.

The actions discussed in this enclosure reflect Oconee Nuclear Station's robust defense-in-depth approach to mitigate external floods and our commitment to enhancing safety margins associated with such floods.

Duke Energy has submitted a revised Flood Hazard Reevaluation Report (FHRR) (Reference 4) concurrent with this letter. Pending NRC staff review, the new FHRR Jocassee Dam break analysis will become a new beyond design basis event at Oconee to be documented in the ONS Updated Final Safely Analysis Report (hereinafter referred to as the new flooding licensing basis). This is discussed in more detail in Sections 5 and 7 of this Enclosure.

There has been previous correspondence between Duke Energy and the NRC (References 5 and 6) pertaining to using the NRC's January 28, 2011, Safet~ Evaluation (SE) (Reference 7) on the Ocone.e inundation study to satisfy the requirements of the March 12, 2012, 10 CFR 50.54(f)

"Recommendation 2.1 : Flooding;' as the dam failure hazard in the FHRR for the site. IJn Reference. 6, the NRC indicated, If your review of Recommendation 2.1 using the standards of the RF/ {Request for Information] indicate that the Jocassee Dam will not fail due. to overtopping or seismic failure, and the NRG staff review supports this determination, then the NRC will accept the January 28., 2011 SE as defining the bounding flood at the Oconee site due, to dam failure." Duke Energy has completed ,ts review and concluded that the Jocassee Dami will not fail due to overtopping or seismic failure, and has provided documentation which supports that conclusion to the NRC (Reference 4). This is discussed in more detail in Section 5 of 1this Enclosure. Pending NRC staff review, Duke Energy will use the January 28, 2011, SE as defining the bounding beyond design basis flood at the Oconee site due to dam failure .

Duke Energy is implementing a defense-1n-depth flood response strategy at Oconee that includes physical protection barriers and mitigation strategies. The physical protection barriers l~ lude assive_ desigc

'"""-~--H- --*-_

- -.-- which are

  • above site gra e

, s such as the StandbY. Shutdown Facility (SSF) flood wa!J.1.§___

above the flood lev.el l - h t rr1ne revrsedl=HRR, and msl). Diver~e and redunda~t flood~ng mitig~~,,i~Ppi9 ,~ ,,

I 1m ~.men _ed at Oconee through the bounding beyond design basis flood (t;t,c1 1 * * ~

11 1 1

' , , - 'i_. ( due to dam failure. These physical protection barriers ana m, 1ga 10n strategjes are discussed in more detail in Sections 4 and 6 of this Enclosure. The miti(~ation strategies will be completed in the 4th quarter of 2016.

Duke Energy has worked expeditiously and continues to make significant progress on upstream dam failure mitigation modifications. Those modifications will be completed by June 2016. The status of these modifications is discussed in more detail jn Section 4 of this Enclosure.

Duke Energy recently completed a review of the compensatory measures documented! in CAL 2-10-003. While these measures were intended to be interim in nature, Duke Ene~rgy has

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 3 of 13 concluded that it is prudent to maintain the ongoing compensatory measures. This is discussed in more detail in Section 3 of this Enclosure.

The comprehensive actions reflected in the referenced modifications, compensatory measures, physical protection barriers, and mitigating strategies at Oconee reflect our commitment to safety and our robust defense-in-depth strategy to mitigate external floods. Duke Energy looks forward to continued dialogue with the NRC on this matter, including further discussions on the regulatory path forward.

2. EXTERNAL FLOODING LICENSING HISTORY Oconee Nuclear Station was licensed in 1973 and 197 4 based on an evaluation that showed that a Jocassee Dam failure was not a credible event based on the design and construction of the dam to prevent hydrologic failure and seismic failure. In 1984, Oconee developed a PRA that was reviewed by the NRC in 1985 which showed that a random failure of the Jocassee Dam had a low event frequency (Reference 8). This was confirmed during the 1994 Generic Letter (GL) 89-13 Service Water Inspection (Reference 9) and the NRC Staff Assessment of the IPEEE in March of 2000 (Reference 10). NRC questions raised in 2008 concerning Duke Energy and industry methods for calculating dam failure frequency led to an NRC request to perform a deterministic failure analysis of upstream dams. On August 15, 2008, the NRC issued a 10 CFR 50.54(f) letter that led to a deterministic failure analysis of the Jocassee Dam (Reference 1). Oconee provided the deterministic analysis results on August 2, 2010 (Reference 11 ), which was then evaluated in a Safety Evaluation issued by the NRC on January 28, 2011 (Reference 7). Duke Energy committed to and implemented compensatory measures to mitigate flooding from a Jocassee Dam failure. These compensatory actions were documented in the June 22, 2010, CAL (Reference 3). On April 29, 2011, Duke Energy also provided to the NRC a list of proposed modifications and response strategies for mitigating the external flooding impacts for a beyond design basis flood (Reference 12).

While pursuing the above flooding actions, on March 12, 2012, Oconee received a second 10 CFR 50.54(f) industry letter in response to the 2011 Fukushima accident, requesting information regarding Recommendations 2.1, 2.3 and 9.3 of the Near Term Task Force (Reference 2). Both the 2008 and 2012 10 CFR 50.54(f) letter responses included an analysis of the flooding effects from a postulated failure of the Jocassee Dam, and a proposed response strategy to mitigate the new Jocassee flooding hazard. ln response to a question from Oconee on how to address the two overlapping flooding information requests, the NRC issued a September 20, 2012, letter where the NRC provided the direction that, 'The NRC intends to maintain the CAL dated June 22, 2010 active until it can be superseded by regulatory action related to the Fukushima responses." (Reference 6). The information request associated with Recommendation 2.1 also states that the flooding hazard information gathered in the responses thereto will be applicable to the resolution of Generic lssue-204, "Flooding of Nuclear Power Plant Sites Following Upstream Dam Failures," which the NRC had initiated in 2010 based on its determination that, " ... the initial regulatory approach to this specific issue has evolved considerably and in a manner that addresses weaknesses in the previous approach.ft

( Reference 13).

The UFSAR still reflects that the Jocassee Dam was constructed with sufficient seismic margin and overtopping margin such that failure is not considered credible. No site dam flooding prevention features were included as part of the original design and construction of ONS beyond the robust design and construction of the Jocassee Dam. An examination of the subsequent licensing history of ONS concerning external flooding shows that while there has been significant interaction with the NRC, the licensing basis has not changed from what is currently reflected in the UFSAR.

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 4 of 13 Going forward, Duke Energy will incorporate the beyond design basis flooding results from the updated FHRR analysis (Reference 4) into the ONS licensing basis as a new, beyond design basis event analyzed in the UFSAR. These actions are also consistent with the direction established in our October 17, 2011, RAI response letter (Reference 14) issued after the Fukushima event and prior to the March 12, 2012, Fukushima 10 GFR 50.54(f) request to reevaluate external flooding hazards using updated hazard information and present-day regulatory guidance and methodologies. Modifications to support mitigation of this new flooding licensing basis will be completed in 2016, as directed by the September 20, 2012, NRG letter (Reference 6).

Duke Energy will implement mitigation strategies to address a bounding Jocassee Dam flooding event as described in the January 28, 2011, Safety Evaluation (Reference 7). This beyond design basis bounding flood will be addressed with mitigating strategies based on Fukushima NRG Order EA-12-049. The mitigating strategies will be documented in the UFSAR.

3. CONFIRMATORY ACTION LETTER (JUNE 22, 2010)

The safety of the site from a postulated failure of Jocassee Dam continues to be ensured today by CAL (Reference 3) compensatory actions that were put in place for Jocassee flooding mitigation in 2010, and continue to be maintained. The CAL was issued on June 22, 2010, to confirm commitments made by Duke Energy in a June 3, 2010, letter. The letter included a commitment to implement compensatory measures at the Oconee Site and Jocassee Dam to mitigate external flooding hazards resulting from a potential failure of the Jocassee Dam. The compensatory measures listed in the enclosure were put into place until final resolution of the inundation of the Oconee site from the failure of the Jocassee Dam has been determined. The compensatory measures that provide Jocassee flooding mitigation (early warning, shutdown, and core cooling) were inspected by the NRC in June 2010, and no findings were documented in the associated July 7, 2010, inspection report (ML101880769). The CAL compensatory measures were inspected again and documented in the April 2013, NRC Tl-187 Inspection for 2.3 Flooding Walkdowns (ML13115A063), and the November 2013, NRG Tl-190 Inspection of Fukushima Interim Actions (ML 1331 BA936). No findings were identified in the NRC inspections which included reviews of the notification and response procedures, reasonable simulations to verify response time, equipment staging and equipment material condition.

Duke Energy recently completed a review of the compensatory measures documented in CAL 2-10-003. While these measures were intended to be interim in nature, Duke Energy has concluded that it is prudent to maintain the ongoing compensatory measures that support mitigation strategies. Table 1 below lists the CAL compensatory measures by type. The table also describes whether the action is a onetime action or on-going, and whether the action will be maintained.

Table 1 June 22, 2010, CAL Compensatory Measures Complete I On-going To be 1

(see Attachment 1) (onetime action) action maintained

-Perform External Flood Analysis (1) X Initial Jocassee Dam Failure Table Top Exercise (11) X I

Jocassee Inspections {4) X X Jocassee Monitoring (5, f S:-12, & 13) X X Jocassee Response Equipment (9, 10, & 14) X X Jocassee Warning Time & I X X I Oconee Response_Process (2, 3, 6, & 15) I I

ONS-2015-028 Enclosure March 6, 2015

4. EXTERNAL FLOODING MODIFICATIONS Site modifications are required to protect the SSCs needed to support flooding mitigation for a range of potential breach events. The Jocassee flood licensing basis for the site will be the eference 4 ~ reach event where peak water levels at site grade will reach

' ~ ;;:_ I :.:1i:- fi 1:'

  • 11T' 6 1

. The flooding modifications that are being constructed at t e s1 e w1 a ress a range o pos u ated breach events, including the new flooding licensing basis, and ,are grouped in three flooding levels:

1. Flooding levels with peak levels below site grade at 796' msl.

1

2. Flooding levels above site grade but below the top of the SS F flood walls at ~
3. Flooding levels above the SSF flood walls from l'\J.,:! 1 -_ - .. ; s_~,,-- J *1 - I ~----~

_J _

1

, 1-The modifications to respond to the above range of flooding levels are described in Table 2 below and will be completed by June 2016. Additional mitigation strategy modifications to comply with NRC Order EA-12-049 will be included in a separate response associated with that Order.

Table 2

- Flooding No Modification Description Levels Relocate the back-up power transmission line towers I 100kV (FANT) above the floodplain to supply .e mergency power through

. Back-up Power I 1 CT-5 to the site. This modification includes the acquisition 1 Line Tower of new right-of-way and relocation of 22 transmission Relocation towers.

Protect the Condenser Circulating Water {CCW) imbedded piping (cooling water supply) by armoring the eastern East Slope slopes supporting the site grade. Th[s modification 2

2 Scour includes approximatel'y 150,000 ft of scour protection (rip 1, 2, & 3 Protection rap, concrete block, shotcrete, energy dissipaters, gabion mattresses) on the east slopes of the ONS power block, to I prevent flood waters from eroding the embankment.

1 Protect the intake dike and the imbedded Condenser I

Circulating Water piping (cooling water supply) by I armoring the intake dike in high water velocity locations and providing grass cover support as approved by FERC.

Intake Dike I 3 Scour Protection The modification includes scour protection (s'h otcrete) where the intake dike meets the east bank, and an 1 , 2, & 3 auxiliary drain system at the bottom of the intake dike slope to improve drainage for the existing good grass cover that is credited for preventing flood waters from eroding the embankment.

Protect the Condenser Circulating Water imbedded piping (cooling water supply) and reduce initial flooding levels by I

directing water away from the site grade. The modification Discharge 4

Diversion Wall includes a 206' long flood diversion wall, with a top 1, 2, & 3 elevation of 828' msl, located near the Condenser Circulating Water discharge structure and designed to deflect flood waters ~w~ from the ONS Power Block.

I The modification includes one manually operated weir Turbine gate located at the entrance of a 72" turbine building drain 5 Building Drain 1 line to prevent flood waters from back-flowing into the tsolation building basement.

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 6 of 13

5. FLOOD HAZARD REEVALUATION REPORT Revised FHRR - 2015 Eva,uation Results On September 15, 2014, the NRC issued a Request for Additional Information requesting that Duke Energy resubmit the 2.1 ONS FHRR with a revised dam failure analysis, applyin1;i alternate breach parameter estimations (Reference 15). By following the guidance provided in JLD-ISG-2013-01 it was determined that the Von Thun and Gillette breach parameter regression equations would provide a conservative yet plausible estimate for Jocassee: Dam breach parame,ters. By applying the Von Thun and Gillette equations to the input data for the Jocassee Dam, as well as exercising engineering judgment, the resulting breach parameters utilized in the revised FHRR are as shown in the Table 3 below.

Table 3 Revised FHRR Jocassee Dam Breach Parameters (t II ll -~ I Bottom Width IL/ I Side Slopes -- --

Final Bottom Elevation

-Breach Development _,

Time ta Peak Outflow Time to Empty Reservoir (99.5% ~eservoir volume emptied)

HEC-RAS Computed Peak Outflow at Jocassee Dam To simulate a potential sunny-day failure of the Jocassee Dam, an unsteady flow modE~I of the Jocassee/Keowee/Hartwell reservoir system was developed usfng the USACE HEC-RAS (Version 4.1) program. HEC-RAS is a 1-D dynamic flow model with unsteady flow mo(jel components used in estimating inundation due, to hypothetical dam failures. Additionally, a TUFLOW FV 2-D hydraulic model was developed to simulate the complex flow paths moving between the Keowee and Little River arms of Lake Keowee including the ONS Intake Canal and Dike that abuts the ONS site at the south forming an island during dam breach simulations. The 2-D analysis was performed to add detail to the HEC-RAS analysis and mode) the potential inundation in critical areas identified around ONS. The revised FHRR evaluation shows flooding at the ONS site is primarily caused by high J<eowee Pam taltwater elevations. Table 4 below shows the results from the 1-D and 2-D models at various locations on site.

ONS-2015-028 Enclosure March 6, 2015 Encl. Pa9e 7 of 13 Tabte 4 Comparison of 1-D and 2-D Model Results:

Time to breach and maximum water surface elevations (Elevation in ft msl and Time in1 hours)

Breaching Keowee Dam Intake Dike 1-D 2-D 1-D 2-D Elevation Time Elevation I Time Elevation Time Elevation Time lb)1] lliUSC §S2-t0-l(d" /b)I *11F*

n/a n/a n/a n/a Maximum Water Surfaces Keowee Dam Intake Dike 1-D 2-D 1-D 2-D Elevation Time Elevation I Time Elevation Time Elevation Time tb113; le t*s C § S!-lo-11d* (l 1\ . )(Fl Maximum Water Surfaces Swale Tailwater 1-D 2-D 1-D 2-D Elevation Time Elevation I Time Elevation Time Elevation Time bl13i.16 TJ.3 C_ ~ ~.:'-lo<1tl1. lbl\71.F n/a n/a Maximum Water Surfaces SSF SSF 1-D 2-D 1-D .. 2-D *-

Elevation Time Elevation I Time Depth Time Depth Time b)1] 1 \o, USC § Sl-!-0-l(d** 1b!C):J61J.l'>.C 5 E~o..1.(dl:

n/a n/a t,11"'11F n/a n/a 1buf ,F:

11,,.:JU6U~.C S 82Jo-! d; Th e Keowee dam is nominally at an elevation o 1h r ffl -he peak water surface elevation a th e dam reachesr 1(:!U6u.;c.~ ~No-]1<lt 1bi('l1F. ). 1:. t31:,6 ,.r.s..:..

\ ;;n,-l1d b C-.(F/

The ONS Intake Canal Dike crest is nominally at an elevation of The peak water surface elevation in the ONS Intake Canal Dike reaches .- Cb- ~'"""

3)..,..

!6.,..,t""'

r_:,-~_c"""'_-§- - - t

~=-~o ; ,d: lt,117)(F:

The nominal elevation of the Yard is 796' msl. The Yard wou e exposed to flooding via downstream (bil'.!i lH*.~.c '~~0 0

  • 11tl 1 ib)r,)1.F, . The Yard would be inundated 1:o elevation Ic,, L II') J61J .s c s above grade) during the modeled dam failure ( (Cr>ICcu6 fl s c ~ ~~~ 0 -1rdl
  • ~ 'r.A\- ,*""-.u"'7,/C-, )\--;"')(f*

,., c :16 .JSL. 5s.:...""1, d1, .

1 ~-------~

l,11 7 !!Fl For further discussion and results, pleas.e refer to the revised FHRR.

Methodology and Assumptions Used to Develop Analysis The JLD-ISG-2013-01 guidance document discusses three types of dam failures : hydn:'.>logic failure, seismic failure , and sunny-day failure. The revised FHRR concludes that the first two of these failures are not credible failure modes. As a result of reviewing the dam structures at Jocassee and Keowee for criteria outlined in ANS 2.8 for design bas*1s floods and comparing them to the design basis requirements required by the regulating agency, FERC, it was determined that the Jocassee Development and Keowee Development have adequate margin to meet the criteria for overtopping and stability as required by NTTF Recommendation 2.1 .

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 8 of 13 As requested by the March 20, 2014, NRC Request for Additional Information {Reference 16) to update the previous analysis initially submitted in the 2013 FHRR, Duke Energy performed new seismic analyses for the Jocassee Dam. Duke Energy performed a new combination deterministic/probabilistic analysis using the guidance provided in JLD-ISG-2013-01 to determine the dam factor of safety and displacements at the ground motion response spectrum (GMRS) level. The calculated post-earthquake slope stability factors of safety are well above 1.0 and the estimated earthquake induced displacements are relatively small; therefore, the Jocassee Dam is expected to perform satisfactorily following the design earthquake ground

  • motions considered.

The NRC staff position on sunny-day failure states that no widely accepted current engineering practice exists for estimating failure rates on the order of 1x10*5 per year, therefore sunny-day failure should be assumed to occur and the consequences estimated. Due to this staff position Duke Energy has assumed a deterministic sunny-day failure of the Jocassee Dam.

Of the various modeling techniques presented in the JLD-ISG-2013-01, Duke Energy used the Hydrologic Model Method, utilizing the Hierarchical Hazard Assessment (HHA) process which is a progressively refined stepwise estimation of the flooding hazard. The advantage to this approach is a more realistic representation of the effects upstream dam failures and attenuation to the site. In order to develop the sunny-day failure analysis, Section 7.2 of the JLD-ISG-2013-01, Breach Modeling of Embankment Dams, was used to determine the appropriate methodologies and assumptions. The breach parameter analysis used multiple common regression equations for the breach parameters sensitivity study and to predict parameters.

The peak breach outflow, as calculated by the HEC-RAS model, was compared to multiple common regression equations for peak outflow. As recommended by the JLD-ISG-2013-01 guidance, Duke Energy used multiple modeling methods including regression, physics based, and historical failures to ensure the final results reflected a comparative analysis that is both conservative and plausible.

Comparison of 2010 and 2015 Evaluation Methodologies The 2010 Jocassee breach evaluation developed in response to the 2008 10 CFR 50.54(f) letter became the basis for the January 28, 2011, NRC Safety Evaluation. The new revised FHRR evaluation completed in 2015 differs from the 2010 evaluation with the primary difference being the approach used in the evaluations. In 2010, the goal was to maximize a flooding hazard by using conservative simplifying assumptions and methods. Some of these simplifying assumptions and methods included: not using regression equations to develop the breach parameters, using the default sinusoidal breach progression curve provided by HEC-RAS, and assuming the peak outflow occurs coincidentally with the breach fully progressed. These assumptions and methods were used to maximize flooding with no refined analysis performed beyond the initial screening level analysis. This approach is consistent with the NUREG/CR-7046 screening analysis approach.

After the natural disaster at Fukushima occurred and the resulting NRC 10 CFR 50.54(f)

Request for Information was issued, the goal of the 2015 evaluation was to develop a state-of-the-practice, accurate breach analysis that is both conservative and plausible. By following the guidance provided in JLD-ISG-2013-01 Section 7.2 "Breach Modeling of Embankment Dams",

the analysis complies with the NRC staff positions that the model results not rely on a single methodology and include a comparison of results for several models judged appropriate for the size and design of Jocassee. Key assumptions and engineering judgments are consistent with the physics of a dam breach events and were confirmed by histor"1cal data. These assumptions and engineering judgments include; breach growth in one direction limiting the breach size due to the breach initiating at one of the rock abutments, peak outflow occurs prior to full breach based on decreasing head differential and resulting water velocities, and the breach time recognizes time required to fully empty a reservoir as large as Jocassee. This approach is

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 9 of 13 consistent with the HHA approach of refined analysis described in NUREG/CR-7046.

Comparing the revised FHRR evaluation with the 2010 evaluation shows that by using JLD-ISG-2013-01, the updated breach analysis is founded in more technical rigor than the simplified screening approach used in 2010. Below in Table 5 is a comparison of the revised FHRR evaluation breach parameters wi(t, the parameters developed in the simplified e:valu_at1on completed in 2010.

Table 5

~o~parison of Jocassee Dam Breach Parameters

~-- Breach Parameter

('b : .tsr_*_;c_ 0°.::~c J ::._

i 2015 Revised FHRR -2010 Evaluation

- T Bottom Width

,Side

_ _ Slopes Final Bottom Elevation Breach Development Time 19_ !:eak Outflow

_H_EC-RAS Computed Peak Outflow at Jocassee Dam

6. PLANT RESPONSE TO EXTERNAL FLOODING The dam failure flooding response and associated modifications being constructed at the :site will address a range of p0stulated breach events including the new flooding licensing basis and are grouped in three flooding levels:
1. Flooding levels with peak levels below site grade at 796' msl.
2. Flooding levels above site grade but below the to , of th_e ~~ F _ood w3 lls a [: ,~ 1e-u;
3. Flooding levels above the SSF flood walls from " 1(1 1 * '- , ~ - ,- i1 ~ "'

The common entry point in Abnormal Procedures for all three categories of dam flooding events will be the declaration of a Condition "A" (Failure of one or more dam/dike is imminent or has occurred), or Condition "8 (Potentially hazardous flooding situation is developing). The Condition "A" response will require that all Oconee reactors be tripped and mitigating strategies be deployed in anticipation 0f flooding . Condition "B" will require the Shift Manager to ,evaluate the Condition "B" conditions and proceed with a shutdown if the conditions are degrading toward meeting Condition "A" criteria. The Abnormal Procedure response including mitigation strategies will involve the following parallel actions by procedure:

  • Portable pump alignments
  • Energize backup power supply through CT-5
  • Staff the SSF and ensure flood doors are secured
  • Activate the TSC/OSG & conduct site assembly The deployment of Mitigating Strategies *includes the staging and connecting of the equipment so that the equipment can be activated if required as the flooding scenario progresses based on fiooding levels and the loss of SSCs required to keep the reactor cores cool. The subsequent flooding respons.e based on the severity of the event is described below:

Flooding tevels with peak levels below site grade at 7.9 6' msl.

  • SSF and Mitigation Strategies provide back Up to plant SSCs

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 10 of 13 Flooding levels above site grade but below the top of the SSF flood walls atG, ~ ~- '* ~ - ,;,_ -, J

  • SSF is staffed and available
  • SSF provides feedwater and reactor coolant makeup if normal SSCs lost Mitigation Strategies provide backup to the SSF Flooding levels above the SSF flood walls, from jr': :;
  • _f __,_:_".: ' : .:.,".1 c 1- - :::-,

Plant SSCs provide feedwater and reactor coolant makeup until plant SSCs are lost SSF is staffed and available

  • SSF provides feedwater and re.actor coolant makeup when normal SS-Cs lost
  • When the SSF is lost. Mitigation Strategies are put into service providing:

Feedwater and reactor coolant makeup (diesel powered)

Instrumentation Spent fuel pool makeup Mitigation Strategies provide diverse and flexible capabilities to maintain the key safety functions: core cooling, containment, and spent fuel pool cooling if/when plant SSCs and the SSF are lost Backup equipment is available consistent with the N+1 guidance to support mitigation strategies.

Oconee's mitigation strategies for flooding provide steam generator feed, reactor coolant makeup, and multiple repower strategies; including guidance on where to measure key instrument readings usi.ng portable instruments for the beyond design basis bounding iflood event. New mechanical and electrical *connections are being* installed in the plant to a!low diverse capability_

The SG feed strategy uses one pump to feed all three units_ The RC makeup strategy will use one high pressure pump per unit to move inventory from the Borated Water Storage Tanks (BWST) into the RC system via currently installed vents and drains.

In response to a Jocassee dam failure., Oconee Will use site procedures to shut down the units and perform a rapid cooldown, while deploying the SG feed portable mitigation strategy in parallel. The portable pump will be located above the inundation level and hoses will be secured throughout the yard to ensure capability is maintained through the inundation timefrarne. Based on thermal hydr.,.ulic analysis, RC Makeup will not be needed until after the flood waters recede. Debris clearing equipment is available if needed in the FLEX Storage Building, which is above the floodplain, to assi.s t in deployment.

An External Damage Mitigation Program will manage the mitigation strategies forflooding long-term. This program will include preventative maintenance activities to ensure health 01' the equipment. Oconee will house an additional set of equipment needed as well as ensuire the strategies have been designed to utilize Regional Response Center (RRC) equipment for backup capabiltty to allow indefinite coping.

The mitigation strategies are In progress and will be completed during the upcoming refueling outages (Unit 2 EOC27 fall 2015, Unit 3 EOC 28 spring 2016, and Unit 1 EOC29 fall 2016) with all required actions in place in 2016 to meet NRC Order EA-12-049.

7. REGULATORY PATH FORWARD Pending staff review of the new FHRR submitted concurrent with this supplement, the FHRR flood height will become the new licensing basis for Oconee. Specifically, Duke Energy will establish*the FHRR (Reference 4) Jocassee beyond des1gn basis flood where peak water levels

ONS-2015-028 Enclosure March 6, 2015 Encl. Pag1e 11 of 13 at site grade reach !i-< 1::.- --- ' .::. :-: "* lthe SSF flood walls) as the licensing basis for the site to be incorporated into Chapter 2 of the UFSAR.

Duke Energy performed a comprehensive review of Oconee's current licensing basis associated with external flooding. Based on that review, Duke Energy's current direction is to upd:ate the Oconee licensing basis and incorporate the results of the new FHRR Jocassee Dam bireak analysis in the UFSAR under 10 CFR 50.59 without a License Amendment. Changes to the UFSAR would be submitted to the NRC in accordance with 10 CFR 50. 71 (e). Other reigulatory processes are also available to codify the results of the Oconee external flooding reviews. Duke Energy looks forward to continued dialogue with the NRC on this matter.

As part of the site response to address a range of beyond design basis breach events, the analysis _submitted to t_he NRC _on August ~* 201 O (Case ~. 1OOW) (~E}~C~_I!f B) J\ wjll. ~ used 1

to establish the bounding flooding event with a peak flooding level of{ : :_c, " - - -- ' * "' above site grade) to be addressed w.ith mitigation strategies. This fiouding level is also reflected in the NRC's January 28, 2011 , SE (Reference 7). The mitigation strategies for NRG Order EA-12-049 including the new bound'ing flooding event response will be implemented for the site in the fall of 2016. The mitigating strategies response as required by NRC Order EA-12-049 will be included in the Chapter 3 of the site UFSAR.

As noted previously, the modifications discussed in Section 4 will be completed by June 2016.

Also , Duke Energy intends to maintain the compensatory measures outlined in CAL 2-10-003 as discussed in Section 3.

8.

SUMMARY

In summary, pending NRC staff review, the new FHRR Jocassee Dam break analysis will become the new licensing basis for external flooding at Oconee to be documented \n tt,e ONS Updated Final Safety Analysis Report, Chapter 2. This will continue to be tre.ated as a beyond design basis event within the context of our licensing basis and plant design basis. A!~;o, pending NRC staff rev.iew, Duke Energy will use the January 28, 2011 ,. SE to define th13 bounding flood at the Oconee site due to darn failure and document mitigation strategies in UFSAR Chapter 3.

As noted previousiy, Duke Energy is implementing a defense-in-depth flood response strategy at Oconee that includes physical protection barriers and mitigation strategies. The ph~rsical protection barriers include passive design features such as the SSF wall which is above the flood level in the FHRR (Reference 4). Diverse and redundant mitigation strategies am being implemented at Oconee through the bounding flood due to dam failure. Duke Energy continues to make significant progress on upstream dam failure mitigation modifications, and these modifications will be completed by June 2016. The mitigation strategies are in progress and will be completed in the 4th quarter of 2016. Finally, Duke Energy will maintaih the ongoin9 CAL 2-10-003 compensatory measures. These comprehensive actions reflect our commitment to safety and our robust defense-in-depth strategy to mitigate external floods at Oconee.

Duke Energy looks forward to continued dialogue with the NRC on this matter, including further discussions on the regulatory path forward .

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 12 of 13

9. REFERENCES
1) NRC letter, Information Request Pursuant to 10 CFR 50. 54(() Related to External Flooding, Including Failure of the Jocassee Dam, at Oconee Nuclear Station, Units 1, 2, and 3 (TAC Nos. MD8224, MD8225, and MD8226), dated August 15, 2008.
2) NRC letter, Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50. 54(() Regarding Recommendations 2. 1, 2. 3, and 9. 3, of the Near- Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, dated March 12, 2012.
3) NRC Confirmatory Action Letter 2-10-003, Confirmatory Action Letter- Oconee Nuclear Station, Units 1, 2, and 3 Commitments to Address External Flooding Concerns (TAC Nos.

ME3065, ME3066, and ME3067), dated June 22, 2010.

4) Duke Energy Letter, Revised Flood Hazard Reevaluation Report per NRC's Request for Additional Information, dated March 6, 2015.
5) Duke Energy letter, Response to Requests for Additional Information Regarding Modifications to Address External Flooding Concerns, dated June 14, 2012.
6) NRC letter, Oconee Nuclear Station, Units 1, 2, and 3 - Modifications to Address External Flooding Hazards (TAC Nos. ME7970, ME7971, and ME7972), dated September 20, 2012.
7) NRC letter, Staff Assessment of Duke's Response to Confirmatory Action Letter Regarding Duke's Commitments to Address External Flooding Concerns at the Oconee Nuclear Station, Units 1, 2, and 3 (ONS) (TAC Nos. ME3065, ME3066, and ME3067),

dated January 28, 2011.

8) NRC letter, Probabilistic Risk Assessment Study, Oconee Nuclear Station, Units 3 (Docket No. 50-287), dated August 14, 1985.
9) NRC letter, Notice of Violation, (NRG Inspection Report NOS. 50-269/94-31, 50-270/94-31, 50-287/94-31), dated December 19, 1994.
10) NRC letter, Review of Individual Plant Examination of External Events, (TAC NOS.

MA83649, M83650, and M83651 ), dated March 15, 2000.

11) Duke Energy letter, Oconee Response to Confirmatory Action Letter (CAL) 2-10-003, dated August 2, 201 0.
12) Duke Energy letter, Oconee Response to Confirmatory Action Letter (CAL) 2-10-003, dated April 29, 2011.
13) NRC Generic Issue (GI) 204, Screening Analysis Report for the Proposed Generic Issue on Flooding of Nuclear Power Plant Sites Following Upstream Dam Failures, dated July 2011.
14) Duke Energy letter, Response to Requests for Additional Information Regarding Necessary Modifications to Enhance the Capability of the ONS Site to Withstand the Postulated Failure of the Jocassee Dam, dated October 17, 2011.
15) NRC Letter, Request for Additional Information - Oconee Flooding Hazard Reevaluation Report (TAC Nos. MF1012, MF1013, and MF1014), dated September 15, 2014.

i 6) NRC Letter, Request for Additional Information Regarding Fukushima Lessons Learned Flood Hazard Reevaluation Report (TAC Nos. MF1012, MF1013, and MF10i4), dated March 20, 2014.

ONS-2015-028 Enclosure March 6, 2015 Encl. Page 13 of 13 Attachment June 22, 2010, Confirmatory Action Letter -

Commitments to Address External Flooding Concerns

1. Perform flooding studies using the Hydrologic Engineering Center-River Complete Analysis System (HEC-RAS) model for comparison with previous DAMBRK models to more accurately represent anticipated flood heights in the west yard following postulated failure of the Jocassee Dam.
2. Maintain plans, procedures (Jocassee and Oconee) and guidance documents implemented (Oconee) to address mitigation of postulated flood events which could render the Standby Shutdown Facility inoperable and are consistent with current perspectives gained following the HEC-RAS sensitivity studies and the subsequent 2D inundation studies. To the extent practical, the mitigation strategy is similar to existing extensive plant damage scenario (B.5.b) equipment, methods and criteria.
3. Duke Energy Hydro Generation will create a guidance document to consolidate river management and storm management processes. (Includes the Jocassee Development and the Keowee Development.).
4. Maintain a dam safety inspection program that includes: (1) weekly dam safety inspections of the Jocassee Dam by Duke Energy personnel, (2) dam safety inspections following any 2-inch or greater rainfall or felt seismic event, (3) annual dam safety inspections by Duke Energy, (4) annual dam safety inspections by FERC representatives, (5) five year safety inspections by FERG approved consultants, and (6) five year underwater inspections.
5. Maintain a monitoring program that includes: (1) continuous remote monitoring from the Hydro Central Operating Center in Charlotte, NC, (2) monthly monitoring of observation wells, (3) weekly monitoring of seepage monitoring points, and (4) annual surveys of displacement monuments.
6. Assign an Oconee engineer as Jocassee Dam contact to heighten awareness of Jocassee status.
7. Install ammeters and voltmeters on Keowee spillway gates for equipment condition monitoring.
8. Ensure forebay and tailrace level alarms are provided for Jocassee to support timely detection of a developing dam failure.
9. Add a storage building adjacent to the Jocassee spillway to house the backup spillway gate operating equipment (e.g., compressor and air wrench).
10. Obtain and stage a portable generator and electric drive motor near the Jocassee spillway gates to serve as a second set of backup spillway gate operating equipment.
11. Conduct Jocassee Dam failure Table Top Exercise with Oconee participation to exercise and improve response procedures.
12. Instrument and alarm selected seepage monitoring locations for timely detection of degrading conditions.
13. Provide additional video monitoring of Jocassee Dam (e.g., dam toe, abutments, and groin areas) for timely assessment of degrading conditions.
14. Obtain and stage a second set of equipment (including a 8.5.b-type pump) for implementation of the external flood mitigation guidance.
15. Conduct Jocassee Dam/Oconee Emergency Response Organization Drill to exercise and improve response procedures.
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