ML13357A102

Response to Request for Additional Information for Flood Hazard Reevalution in Response to March 12, 2012 Information Request Regarding Flooding Aspects of Recommendation 2.1

ML13357A102
Person / Time
Site:	North Anna
Issue date:	12/13/2013
From:	Heacock D Virginia Electric & Power Co (VEPCO)
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
Download: ML13357A102 (18)

Text

VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 December 13, 2013 U.S. Nuclear Regulatory Commission Serial No.13-640 Attention: Document Control Desk NL&OS/ETS RO Washington, DC 20555 Docket Nos. 50-338/339 License Nos. NPF-4/7 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)

NORTH ANNA POWER STATION UNITS I AND 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION FOR FLOOD HAZARD REEVALUTION IN RESPONSE TO MARCH 12, 2012 INFORMATION REQUEST REGARDING FLOODING ASPECTS OF RECOMMENDATION 2.1 On March 12, 2012, the Nuclear Regulatory Commission (NRC) issued "Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident" to all power reactor licensees and holders of construction permits in active or deferred status. Enclosure 2 of the March 12, 2012 letter requested that each licensee perform a reevaluation of external flooding sources and report the results in accordance with the NRC's prioritization plan. In a letter dated March 11, 2013, Dominion submitted the flood reevaluation for North Anna Power Station to the NRC. In a letter dated November 26, 2013, the NRC transmitted a request for additional information (RAI) to complete the review of the North Anna flood hazard reevaluation. The attachment to this letter and enclosed CD-ROM provide the requested information.

If you have any questions regarding this information, please contact Mr. Thomas Shaub at (804) 273-2763.

Sincerely, David Heacock President and Chief Nuclear Officer Dominion Nuclear COMMONWEALTH OF VIRGINIA COUNTY OF HENRICO The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by David A.

Heacock, who President and Chief Nuclear Officer Dominion Nuclear of Virginia Electric and Power Company. He has affirmed before me that he is duly authorized to execute and file the foregoing document in behalf of that company, and that the statements in the document are true to the best of his knowledge and belief.

CRAIG D SLY i' A tlh4,6z" 2013. 1 Notary Public J*.-

Acknowledged before me this =I day of 4 Commonwealth of Virginia My Commission Expires: 1 , Reg. # 7518653

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My Commission Expires December 31, 20&61 I-ot&PbiA/

Serial No.13-640 Docket Nos. 50-338/339 Page 2 of 4 Commitments made in this letter: No new regulatory commitments

Attachment:

Response to Request for Additional Information for North Anna Power Station Units 1 & 2 Flooding Hazard Reevaluation Report (FHRR)

Enclosure:

Compact Disc (CD-ROM) - North Anna Power Station, Data in Response to NRC Request For Information, RAI Nos. 3.2-1 to 3.2-5 and 3.3-1 Enclosed with this letter is a compact disc (CD-ROM) which contains electronic data requested by the NRC. The enclosed CD-ROM is labeled:

Virginia Electric and Power Company (Dominion)

North Anna Power Station Data in Response to NRC Request For Information RAI Nos. 3.2-1 to 3.2-5 and 3.3-1 Serial No.13-640 Docket Nos. 50-338/339 License Nos. NPF-4/7 The enclosed CD-ROM contains the following files:

File Name File Size Sensitivity Figures Directory publicly available 01 FIGURE 2.1-1.pdf 738 KB publicly available 02 FIGURE 2.1-2.pdf 2,940 KB publicly available 03 FIGURE 2.1-3.pdf 488 KB publicly available 04 FIGURE 2.1-4.pdf 626 KB publicly available LIP HEC Files\HEC-HMS Files\NAPS Units 1 2 Directory publicly available 05 50 Run.log 4 KB publicly available 06 72 Hour PMP.log 2 KB publicly available 07 72 Hour PMP.met 2 KB publicly available 08 75 Run.log 4 KB publicly available 09 Control 1.control 2 KB publicly available 10 Control 2.control 2 KB publicly available 11 Local PMP.met 4 KB publicly available 12 NAPS Units 1 2.access 0 KB publicly available 13 NAPS Units 1 2.dsc 188 KB publicly available 14 NAPS Units 12.dss 44,300 KB publicly available 15 NAPS Units 12.gage 2 KB publicly available 16 NAPS Units 1 2.hms 2 KB publicly available 17 NAPS Units 1 2.1og 2 KB publicly available 18 NAPS Units 1 2.out 2 KB publicly available 19 NAPS Units 1_2.pdata 2 KB publicly available

Serial No.13-640 Docket Nos. 50-338/339 Page 3 of 4 20 NAPS Units 1 2.run 2 KB publicly available 21 NAPS Units 1 2 Ex 72 Hour.basin 8 KB publicly available 23 NAPS Units 1 2 Ex 50 .basin 12 KB publicly available 24 NAPS Units 1 2 Ex 75 .basin 12 KB publicly available LIP HEC Files\HEC-RAS Files Directory publicly available 25 Backup.pOl 4 KB publicly available 26 LocaIPMP.f02 6 KB publicly available 27 LocaIPMP.f03 4 KB publicly available 28 LocalPMP.gO1 22 KB publicly available 29 LocaIPMP.g03 4 KB publicly available 30 LocaIPMP.O01 188 KB publicly available 31 LocalPMP.002 188 KB publicly available 32 LocalPMP.003 114 KB publicly available 33 LocaIPMP.005 188 KB publicly available 34 LocaIPMP.pO1 4 KB publicly available 35 LocaIPMP.p03 4 KB publicly available 36 LocaIPMP.prj 2 KB publicly available 37 LocaIPMP.rO1 26 KB publicly available 38 LocaIPMP.r02 26 KB publicly available 39 LocaIPMP.r03 8 KB publicly available PMF HEC Files Directory publicly available 40 0.25 Rise PMF.Iog 2 KB publicly available 41 Basin 1 0.25 Rise .basin 4 KB publicly available 42 Control l.control 2 KB publicly available 43 Lake Anna Rise PMF.dsc 14 KB publicly available 44 Lake Anna RisePMF.log 0 KB publicly available 45 Lake Anna Rise PMF.out 2 KB publicly available 46 Met 1.met 2 KB publicly available 47 Rise.access 0 KB publicly available 48 North Anna Normall Pool Rise.dsc 6 KB publicly available 49 North Anna NormallPool Rise.dss 456 KB publicly available 50 North Anna Normall Pool Rise.gage 2 KB publicly available 51 North Anna Normall Pool Rise.hms 2 KB publicly available 52 North Anna Normall Pool Rise.log 2 KB publicly available 53 North Anna Normall Pool Rise.out 2 KB publicly available 54 North Anna Normall Pool Rise.pdata 2 KB publicly available 55 North Anna Normall Pool Rise.run 2 KB publicly available

Serial No.13-640 Docket Nos. 50-338/339 Page 4 of 4 cc: U.S. Nuclear Regulatory Commission, Region II Regional Administrator Marquis One Tower 245 Peachtree Center Ave. NE Suite 1200 Atlanta, Georgia 30303-1257 Dr. V. Sreenivas Project Manager U.S. Nuclear Regulatory Commission One White Flint North, Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 K. R. Cotton Gross Project Manager U.S. Nuclear Regulatory Commission One White Flint North, Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 NRC Senior Resident Inspector North Anna Power Station J. E. Reasor, Jr.

Old Dominion Electric Cooperative Innsbrook Corporate Center, Suite 300 4201 Dominion Blvd.

Glen Allen, Virginia 23060

Serial No.13-640 Docket Nos. 50-338/339 ATTACHMENT RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION FLOODING HAZARD REEVALUATION REPORT (FHRR)

VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)

NORTH ANNA POWER STATION UNITS I AND 2

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 1 of 13 BACKGROUND By letter dated March 12, 2012, the Nuclear Regulatory Commission (NRC) issued a request for information pursuant to Title 10 of the Code of Federal Regulations, Section 50.54(f) (hereafter referred to as the 50.54(f) letter). The request was issued as a part of implementing lessons-learned from the accident at the Fukushima Dai-ichi nuclear power plant. Enclosure 2 to the 50.54(f) letter requested licensees to perform a flood hazard reevaluation using present-day methodologies and guidance. By letter dated March 11, 2013 (Agencywide Documents Access and Management System (ADAMS)

Accession No. ML13074A925), Virginia Electric and Power Company (Dominion) submitted a response to Enclosure 2, Required Response 2 of the 50.54(f) letter. The NRC staff has determined that additional information is needed to complete its review.

In an email dated November 14, 2013, the NRC transmitted a draft request for additional information (RAI) and requested responses to the RAI within 30 days of receipt of the email. In a phone call on November 20, 2013 to discuss the RAI, Dominion agreed to provide the requested information within 30 days. Subsequently, in a letter dated November 26, 2013, the NRC transmitted the RAI.

NORTH ANNA FLOOD HAZARD REEVALUATION REPORT (FHRR) RAI RAI No. 3.2-1. LIP Flooding The licensee is requested to provide high resolution, digital versions of Figures 2.1-1 through 2.1-4 as these figures are important in understanding the direction and boundary of sub-basin flows.

Dominion Response The requested high-resolution, digital versions of Figures 2.1-1 through 2.1-4 of the North Anna Flooding Hazard Reevaluation Report are provided in the folder named "Figures" on the enclosed Compact Disc (CD-ROM).

RAI No. 3.2-2, LIP Flooding The licensee is requested to provide electronic versions of the input files used for HEC-HMS and HEC-RAS modeling in the local intense precipitationanalyses.

Dominion Response Electronic versions of the input files used for HEC-HMS and HEC-RAS modeling in the local intense precipitation analyses are provided in the folder named "LIP HEC Files" on the enclosed CD-ROM.

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 2 of 13 RAI No. 3.2-3. LIP Flooding The licensee is requested to provide a description of the methods used to incorporate elevation measurements and the magnitude of the elevation uncertainty in the local intense precipitation analyses. Discuss the related uncertainty associated with the onsite flood level estimates.

Dominion Response Ground topography was obtained from an aerial light detection and ranging (LIDAR) survey. The ground elevations from this survey have an accuracy of +/-0.5 foot.

Additionally, a ground survey was performed along the CE, CW, and SW Flow Paths (flow paths designated in the FHRR) where safety-related equipment is located. The ground survey data has an accuracy of +/-0.10 foot on non-hard ground surfaces and an even higher accuracy of +/-0.03 foot on hard surfaces such as pavement and concrete.

The ground survey data were used to develop the HEC-RAS cross sections for the CE, CW, and SW Flow Paths. HEC-RAS cross sections in other areas were developed from the topography shown on Figures 2.1-1 through 2.1-4 of the North Anna Flood Hazard Reevaluation Report.

In addition to using the very accurate ground elevation data with an uncertainty of +/-0.03 foot in hard surface, safety-related areas to develop the local intense precipitation flood model, the HEC-HMS and HEC-RAS models incorporate highly conservative assumptions such as: use of conservative Manning's roughness coefficients, the entire site considered impervious and modeled with a curve number of 98, reduced time of concentration values, and the storm drains considered to be non-operational. The combined effect of these conservative assumptions more than compensates for the uncertainty associated with modeling the ground elevation such that estimated flood levels can be considered as the maximum possible flood levels due to the local intense precipitation (LIP) over the site.

RAI No. 3.2-4, LIP Flooding The licensee is requested to discuss roof drainage features (e.g., scuppers, gutter outlets, etc.) for plant buildings, and how runoff from these drainage features are incorporated into the HEC-HMS modeling in the local intense precipitation flood analyses.

Dominion Response The roof drainage features are assumed to be clogged or non-operational during the probable maximum precipitation (PMP) event. Runoff from the roofs is assumed to fall off the roofs and contribute to the surface runoff on the ground. Thus, roof runoff is accounted for in the HEC-HMS model by combining the surface area of the roof with the ground surface area in each sub-basin. Both the roof surface area and ground surface

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 3 of 13 areas are modeled as impervious surfaces. No credit is taken for detention storage of runoff on roof structures or for any runoff that would be discharged directly to Lake Anna through roof drains connected to the storm drain system.

RAI No. 3.2-5. LIP Flooding The licensee is requestedto discuss how the vehicle barriersystem was incorporatedin the HEC-HMS modeling in the local intense precipitationanalyses.

Dominion Response The local PMP analysis performed for the North Anna FHRR considers two types of vehicle barriers at the North Anna Units 1 and 2 site, jersey barriers and bollards. As indicated in Section 2.1.1 of the report, jersey barriers are essentially 32 inch high concrete walls. When the jersey barriers are located above grade, they are treated as blockages to flow in the HEC-HMS and HEC-RAS analysis. The bollards consist of concrete posts approximately 3.25 ft high and 0.9 ft in diameter with approximately 4.0 ft of open space between the bollards. Therefore, flow through the bollards is considered in the HEC-HMS and HEC-RAS modeling.

As indicated in Section 2.1.1, the vehicle barriers on the south and east sides of the protected area consist of above grade jersey barriers with the exception of two openings. One opening is located at the southeast sally port, where bollards are placed to allow flow out of the protected area. This is where Flow Path SE exits the protected area. The other opening in the jersey barriers is located at the eastern sally port entrance where Flow Path CE exits the protected area. For the other areas along the southern and eastern edges of the protected area, the vehicle barrier acts as a wall and blocks flow.

The vehicle barrier on the west side of the protected area is also an above grade jersey barrier. However, it is located at the bottom of the future Unit 3 excavation and is located far below the Units 1 and 2 site grade and therefore, does not block flow exiting the Units 1 and 2 site to the west.

The vehicle barrier on the northern edge of the protected area consists of jersey barriers. However, these jersey barriers do not block flow. The grade elevation for the Units 1 and 2 intake area located north of the vehicle barrier and protected area is several feet lower than the protected area grade elevation. The bottom of the jersey barriers along the northern edge are placed on the intake area site grade and act as a retaining wall. The tops of the jersey barriers are located at or below the protected area site grade. Therefore, runoff flows from the protected area to the north and passes over the vehicle barrier (which acts as a weir), down into the intake area, and eventually to Lake Anna.

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 4 of 13 RAI No. 3.3-1. Stream and River Flooding The licensee is requested to provide electronic versions of the input files used for HEC-HMS modeling in the flooding of rivers and streams analyses.

Dominion Response Electronic versions of the HEC-HMS input files used in the streams and rivers flooding analysis (Lake Anna probable maximum flooding (PMF)) are provided in the folder named "PMF HEC Files" on the enclosed CD-ROM.

RAI No. 3.4-1, Dam Failure Flooding Standard Review Plan Section 2.4.4 states: "If any onsite water control or storage structures exist above the site grade, failures of these from any cause should be determined and the potential for flooding of SSC important to safety and doors and openings identified." Consistent with this statement, staff requests the following information related to potential site flooding caused by a failure of the service water reservoir impoundment discussed in Section 2.3.4 of the licensee's flood hazard revaluationreport (FHRR).

A. In Section 2.3.4 of the Flooding Hazard Reevaluation Report (FHRR), the licensee states that the service water reservoir dike is a "Seismic Class I structure, and has been evaluated to preclude overtopping, piping, slide and other sources of failure." The licensee is requested to describe the evaluation(s) performed for plausible failure causes (i.e., hydrologic, seismic, and sunny-day failure modes) for the service water reservoir impoundment and, if applicable, provide an evaluation of subsequent site flooding using a hierarchicalframework.

Specifically, the licensee is requested to provide evaluations of failure mechanisms consistent with using present-day guidance, methods, and data, including:

1. evaluation of the potential for seismically-induced failure under site-specific seismic hazards defined using present-day guidance and methods,

2. evaluation of the potentialfor hydrologically-inducedfailure (e.g., overtopping) under the local intense precipitationevents applicable to the site, and

3. evaluation of the potential for sunny-day failure.

If crediting existing evaluations, the licensee is requested to provide justification that existing evaluations are consistent with present-dayguidance, methods, and data or bounding of flood hazards that would result from an evaluation based on present day guidance, methods, and data. The licensee is requested to: (1) describe the models and assumptions that were used in the evaluations and (2) provide an explanation of the consistency of these evaluations with present-day guidance and methods as well as current data. If the consequences (i.e., floods)

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 5 of 13 from any of these failure modes are judged not to affect the site, the license is requested to provide a justification for this conclusion, including a description of pertinentevaluations, models, and assumptions.

B. Section 2.3.4 of the FHRR describes an emergency dike and intercepting trench capable of diverting water from a failure of the service water reservoir impoundment. The licensee is requested to provide a detailed description of the site's topographic or flow division features, including maps showing their location.

In addition, the licensee is requested to provide an evaluation of the effectiveness of dike/trench systems (including the potential for erosion and sedimentation) following failure of the service water reservoir impoundment under the plausible failures modes. The licensee is requested to describe the models and assumptions that were used in the evaluations.

Dominion response As discussed with the NRC staff reviewers via telephone on November 20, 2013 and, as documented in Section 2.3.4 of the North Anna FHRR, Dominion's assessment is that a potential failure of the service water reservoir impoundment dike would not affect the plant site by flooding. Therefore, consistent with the NRC's 50.54(f) letter, Dominion determined that flooding from a potential failure of the North Anna service water impoundment dike could be screened out as mechanisms that are not applicable at the plant site with respect to the Flooding Hazard Reevaluation.

However, the NRC staff reviewer explained that the combination of the North Anna FHRR and the North Anna Power Station Updated Final Safety Analysis Report (UFSAR) provided them insufficient information to conclude that the North Anna service water reservoir impoundment dike was not a potential flood hazard for the plant site.

The staff reviewer then requested that, in lieu of a detailed evaluation, Dominion could provide some additional information on the design of the service water reservoir, the reservoir impoundment dike, and the emergency dike and diversion channel beyond the discussion in the North Anna UFSAR.

Description of North Anna Service Water Reservoir:

The service water reservoir sits in a knoll slightly above the nominal plant site grade elevation. The top of the reservoir dike berm is approximately Elevation 320'. The bottom of the reservoir is approximately Elevation 305'. The elevation of the plant site grade is nominally 271'. The reservoir location is approximately 500 feet south of the site protected area and approximately 800 feet west of the plant discharge canal.

As described in Sections 3.8.4.1 through 3.8.4.4, of the North Anna UFSAR, the service water reservoir impoundment dike is a safety-related, Seismic Class 1 structure. The UFSAR describes the physical properties and strength parameters of the design and construction of the impoundment dike. The design of the service water reservoir and dike system were approached conservatively, using design procedures, methods of analysis such as limit equilibrium slope stability studies for the steady state and design

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 6 of 13 basis seismic event conditions, and considerations typical of those used for major earth dams. As represented on Station Drawing 11715-FC-1D, the interior slope of the impoundment dike is a 3:1 slope. The exterior of the impoundment dike is a 2:1 slope.

Station Drawing 11715-FC-1D also indicates that both sides of the emergency dike have a 2:1 slope. These physical properties and strength parameters describe a robust design for the impoundment dike. Station Drawing 11715-FC-1D is provided in the Electronic Reading Room for use by the NRC reviewers.

As described in the FHRR, Section 1.2, Current Design Basis Flood Elevations, "NAPS was evaluated and determined to be in accordance with RG 1.59 and RG 1.102 to prevent the loss of safety-related functions resulting from the most severe flood conditions that can reasonably be predicted to occur at the site as a result of severe hydrometeorological conditions, seismic activity, or both."

As such, the service water impoundment dam, the emergency dike, and the diversion trench were designed and constructed to preclude flooding of the plant site. With these design features in place, water from the service water reservoir is prevented from reaching the plant site. Thus, the service water reservoir does not represent a flood hazard to the plant site.

Consideration for Potential for Reservoir Overfill/Instantaneous Breach:

The reservoir sits in a knoll slightly above the nominal plant site grade elevation. The reservoir impoundment is well above the peak flood level of the Lake Anna PMF. In addition, this knoll location prevents the possibility of Local PMP watershed flood water from contributing to the volume of water in the reservoir. Therefore, the reservoir cannot be overfilled by upstream watershed flood water.

As described in Section 2.1.2 of the NAPS FHRR, from the NOAA publication HMR 51, a 72-hour 10 square mile PMP depth of 43 inches was determined to be the maximum Local Intense Precipitation (LIP) event for the site. As described in the NAPS FHRR, this LIP event depth of 43 inches refers to the cumulative Local PMP for the 72-hour ten square mile Local PMP event (30.65 inches) plus the 40% antecedent 72-hour ten square mile Local PMP event (12.26 inches) or approximately 43 inches of cumulative rainfall.

The direct watershed area of the pond is slightly larger than the area at the top of the pond (i.e., includes a portion of the roadway on top of reservoir dike). In addition, the interior surface of the reservoir dike is sloped at 3:1. The service water reservoir is designed with approximately 5 feet of freeboard capacity above the maximum reservoir operating water level. Even though the total watershed area is slightly larger than the size of the reservoir, the volume of the water entering the reservoir by a 43-inch cumulative rainfall will not exceed the reservoir storage capacity with the 5-feet freeboard. Therefore, Dominion considers that the 43-inches cumulative rainfall from the LIP event will not cause flood water to overtop the reservoir dike.

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 7 of 13 This assessment is very conservative since the LIP assessment in the NAPS FHRR assumes three dry days between the 40% antecedent PMP prior to the additional full PMP. It is very conservative to assume the service water reservoir level is not returned to nominal operating level between these events. Thus, water accumulated in the pond associated with the LIP event will not overtop the service water reservoir impoundment dike.

The current license basis did not evaluate the potential for combined effects of Local PMP and wave action. Even so, the cumulative rainfall discussed above does not fill the reservoir and any wind generated waves within an area as small as the service water reservoir would not be expected to be of sufficient size, height, and force to create more than a small amount of intermittent wave overtopping.

Consideration for a Postulated Overflow Event:

As described in the UFSAR, even though the volume of the PMP runoff into the service water reservoir is not expected to exceed the capacity of the reservoir, the analysis conservatively assesses the possibility of overtopping the service water dike.

As described in the FHRR, Section 1.2.5, Service Water Reservoir Overflow Event, "the service water reservoir was designed and constructed with an emergency overflow dike and intercepting channel as discussed in UFSAR Section 3.8.4.7.5. Regardless of the initiating event, i.e., seismic, wind, or precipitation, the emergency dike controls and diverts potential service water reservoir overflow or breach flow to the WHTF, thereby preventing water from entering plant site structures. This event has no calculated flood level."

As described in Section 3.8.4 of the UFSAR, and in the FHRR, Section 2.3.4, Service Water Reservoir, "the Service Water Reservoir is a safety-related facility located approximately 500 ft south of the plant site area and partially enclosed by an earthen impounding dike. The dike is a Seismic Class I structure, and has been evaluated to preclude overtopping, piping, slide and other sources of failures. For added conservatism, the slopes and crest of the dike are protected by an outer rock shell that offers erosion protection in the unlikely event of overtopping during a major storm.

Further conservatism for flood protection of the station is provided by an emergency dike and intercepting channel on the south side of the station. A hypothetical dike failure event at the Service Water Reservoir would have to occur in the vicinity of the pump house to have any potential impact on the safety facilities of Units 1 & 2. During this highly unlikely event, the emergency dike and intercepting trench to the north would divert the flood wave from the breach and would conduct the flow east toward the Waste Heat Treatment Facility (WHTF). This dike is about 10 ft above the bottom of the trench. The flood flow path would cross an access road near the end of the emergency dike and continue on towards the discharge canal and the WHTF, which would not have any adverse impact on the safety facilities of the station. Therefore, a failure of the Service Water Reservoir, though unlikely, will not impact the safe functioning of Units 1

& 2."

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 8 of 13 Consideration for the Seismic Reevaluation:

Service water reservoir impoundment dam is a Seismic Class 1, safety-related structure. The North Anna Units 1 and 2 site seismic reevaluation required by the March 12, 2012, 50.54(f) letter will be completed about March of 2014. The adequacy of the seismic qualification for the service water impoundment dike may have to be re-visited depending on the results of the seismic reevaluation and resulting station PRA reevaluations.

However, as described above, even if the impoundment dike were to fail as result of a seismic event, the design of the emergency dike and the diversion trench prevent water from the service water reservoir from reaching the plant site. Thus, the service water reservoir does not create a flood hazard to the site.

Consideration of Failure by Piping, etc.:

The service water reservoir dike is a safety-related, Seismic Class 1 structure and, as such, is required to be instrumented, inspected, and maintained consistent with the design requirements. The dike is periodically inspected by site engineering and by external agencies responsible for inspection of such structures. These inspections identify early indications of the presence of trees, or rodent intrusion, or potential erosion areas to prevent weakening of the dike structure.

The dike design considerations and dike inspection and maintenance measures preclude issues associated with dike failure by other means such as piping failure.

Consideration for Size of Channel:

The elevation of the diversion channel bottom at the upper end of the channel nearest the reservoir impoundment dike is approximately the same as the elevation of the reservoir bottom. The diversion channel slopes downhill from the reservoir area to the roadway next to the station discharge canal. Beyond the roadway, the drainage area drops steeply into the station discharge canal. As indicated on Station Drawing 11715-FC-1D, the bottom of the diversion channel is approximately 35 feet wide and approximately 800 feet in length.

The emergency dike runs parallel to the diversion channel and remains approximately 10 feet above the bottom of the diversion channel for its entire length. The emergency dike runs from the reservoir to the roadway at the bottom of the diversion channel separating the diversion channel from the plant site.

During site planning and design, evaluations were performed to confirm sizing of the diversion channel and emergency dike. These evaluations assumed a postulated instantaneous 90-degree v-notch break from the top of the impoundment dike to the bottom of the service water reservoir. The initial flow rate of water exiting the postulated instantaneous break was calculated based on the maximum operating water level in the reservoir. This initial flow rate of water was used to determine the flow rate of water required to be managed by the diversion channel and emergency dike.

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 9 of 13 Since upstream watershed will not enter the reservoir, the volume of water estimated to exit a postulated break in the reservoir dike is limited to the finite volume of water in the service water reservoir. As the surface elevation of water in the reservoir decreases, the flow rate of water exiting the break will decrease significantly. Based on the estimated initial elevation of water and water volume at that elevation, the flow rate of water exiting the reservoir and thus the flow rate of water conveyed by the diversion channel will decrease significantly in a short period of time. Thus, the service water reservoir does not create a flood hazard to the site.

Other Items Discussed in the UFSAR:

The UFSAR describes other potential issues that could affect the integrity of the service water reservoir. These potential issues include such items as wear of the service water liner by water flowing across the liner, collapse of a service water header, and excessive liner leakage. These issues were addressed as part of the original design of the service water reservoir and are documented in the UFSAR.

Monitoring described in the UFSAR includes differential settlement of structures and buildings and monitoring of ground water and potential liner leakage. During the November 20, 2013 telephone conversation with the NRC staff, the reviewer questioned whether monitoring indicates a potential problem. Dominion's position is that monitoring itself is a protection and prevention tool for early detection of a concern for any such safety-related structure and any impounding structure, regardless of age or hazard rating. Monitoring and periodic inspection are not an indication of a defect or a concern, but are a requirement for regulated structures. The extent to which this occurs depends upon the critical nature of the structure.

The UFSAR describes monitoring and inspection programs implemented to ensure these issues do not develop into structural concerns. These programs are initiated with procedures called Engineering Periodic Tests (PT) that include groundwater monitoring observations in piezometers installed in discrete critical areas of the service water reservoir embankment, inspection of the downstream slope, crest, downstream toe, and appurtenant structures of the service water reservoir, and survey monitoring of settlement monuments on the service water reservoir dike, service water pump house, service water valve house, and other safety-related category 1 structures. The periodic tests are performed on a semi-annual basis and tracked to establish trends with embankment and structure behavior. In addition, the NRC conducts a Dam Safety Audit with the assistance of regional Federal Energy Regulatory Commission (FERC) staff and qualified Dominion professional engineers on a biannual basis during which the items covered in the aforementioned periodic tests are reviewed and inspected to verify that dam safety is maintained. Finally, every 5 years, the service water reservoir in-service inspection is performed, during which the exterior areas of the dike (upstream and downstream slope, crest, downstream toe), the emergency dike, and diversion trench are inspected and photo-documented. In addition, exterior areas where the service water valve house and service water pump house meet the dike are reviewed for readily apparent signs of distress. This in-service inspection is subsequently

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 10 of 13 documented in a technical report which did not cite a concern related to slope instability, seepage, or other signs of distress that would indicate a potential failure or breach at the North Anna service water reservoir.

==

Conclusion:==

As described in the North Anna FHRR, Section 1.2, Current Design Basis Flood Elevations, "NAPS was evaluated and determined to be in accordance with RG 1.59 and RG 1.102 to prevent the loss of safety-related functions resulting from the most severe flood conditions that can reasonably be predicted to occur at the site as a result of severe hydrometeorological conditions, seismic activity, or both." Also, as discussed in UFSAR Section 3.8.4.7.5, "the service water reservoir was designed and constructed with an emergency overflow dike and intercepting channel." Regardless of the initiating event, i.e., seismic, wind, or precipitation, the emergency dike will control and divert potential service water reservoir overflow or breach flow to the Waste Heat Treatment Facility (WHTF), thereby preventing water from impacting the plant site. Therefore, with respect to the Flooding Hazard Reevaluation for North Anna, a dike failure event has no calculated flood level on the plant site and does not create a flood hazard to the site.

There are no changes or modifications to the facility that require evaluation beyond that prepared for the original site licensing basis. Neither LIP nor PMF affect the water entering the service water reservoir and, thus, as described in the FHRR, there are no changes to the analysis of potential flooding from the service water reservoir. That is, no water reaches the plant site and, thus, potential flooding from the service water reservoir is screened-out as a flooding hazard.

Basis for Screened-out Determination:

As described in Section 1.b of Requested Information, of Enclosure 2, Recommendation 2,1: Flooding, of the March 12, 2012, 50.54(f) letter, mechanisms that are not applicable at the site may be screened-out. Further, NEI FAQ-003, Hazard Screening, discusses how specific flooding mechanisms may be screened-out during the process of scoping flooding reevaluations where the flooding mechanisms are not plausible or applicable to the site.

Regardless of the initiating event, i.e., seismic, wind, or precipitation, water is prevented from entering the plant site from a breach or overflow of the service water reservoir because the emergency dike with the diversion channel controls and diverts potential service water reservoir overflow or breach flow to the WHTF. In addition, the service water reservoir dike is a Seismic Class 1 structure, and has been evaluated to preclude overtopping, piping, slide and other sources of failure. Therefore, this event does not create a flood hazard to the site and is screened-out as a mechanism not applicable to the site.

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 11 of 13 RAI No. 4.0-1, Integrated Assessment The March 12, 2012, 50.54(0 letter, Enclosure 2, requests the licensee to perform an integrated assessment of the plant's response to the reevaluated hazard if the reevaluated floods hazard is not bounded by the current design basis. The licensee is requested to provide the applicable flood event duration parameters (see definition and Figure 6 of the Guidance for Performing an IntegratedAssessment, JLD-ISG-2012-05) associatedwith mechanisms that trigger an Integrated Assessment. This includes (as applicable) the warning time the site will have to prepare for the event, the period of time the site is inundated, and the period of time necessary for water to recede off the site for the mechanisms that are not bounded by the current design basis. The licensee is also requested to provide a basis for the flood event duration parameters. The basis for warning time may include information from relevant forecasting methods (e.g.,

products from local, regional,or nationalweather forecasting centers).

Dominion Response As discussed in the North Anna Power Stations Unit 1 and 2 FHRR, the only flooding mechanisms that affect North Anna Power Station are the LIP event and the PMF event. However, only the LIP event results in accumulation of water on the site that could impact the site. The parameters of the PMF event are essentially the same as the existing current licensing bases (CLB) PMF event. Other events such as storm surge, dam failures, seiche, tsunami, ice induced flooding, and channel migration have no impact on the North Anna Power Station.

As illustrated in Figure 6 of JLD-ISG-2012-05, Guidance for Performing the Integrated Assessment for External Flooding, the flood event duration has three phases - warning period for site preparation for flood event, period of inundation, and period of recession of water from site. The North Anna FHRR follows these prescribed phases.

Warning period for site preparation for flood event:

As defined in the NRC's JLD-ISG-2012-05, the "warning period for site preparation for flood event" is defined as the point when conditions are met for entry into flood procedures until arrival of flood elevation water on site.

During this period, North Anna Power Station implements actions to mitigate the effects of the LIP for when the storm arrives at the site at time (T) equals 0 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. Prior to T = 0 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />, i.e., T << 0 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />, the station begins to implement mitigation actions in accordance with Procedure 0-AP-41, Severe Weather Conditions. The initiating conditions implemented for Procedure 0-AP-41 are (at T << 0 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />) when either hurricane force winds are projected within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />, or a High Wind Warning projects winds greater than 35 mph, or a Severe Thunderstorm Warning issued for area, or a Severe Thunderstorm Watch issued for area, or a predicted rainfall of greater than 12 inches is projected for site. Therefore, the warning period can begin up to 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> or greater before arrival of the storm at T = 0 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> per 0-AP-41.

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 12 of 13 The North Anna FHRR estimates the LIP discharge rate and flood elevations at the site based on assumed central period peak precipitation rates. The LIP event for North Anna was based on a 6-hour, one square mile Local PMP event. The postulated LIP event depicted in Table 2.1-10 and Figures 2.1-7 through 2.1-10 of the North Anna FHRR describes an event where accumulated water on site reaches initial flood level at approximately T = 2:32 hours (i.e., approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 32 minutes from the beginning of the 6-hour storm).

Thus, as defined in the NRC's JLD-ISG-2012-05, the "site preparation for flood event" period is the combination warning period (i.e., variable up to 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> or greater) plus the period from the beginning of the storm until accumulated water on site reaches initial flood level (i.e., approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 32 minutes). Therefore, the duration for "site preparation for flood event" as depicted in the NAPS FHRR is variable from 2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and up to 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> or greater.

Period of inundation:

As defined in the NRC's JLD-ISG-2012-05, the "period of inundation" is defined as the point from arrival of flood elevation water on site until flood water begins to recede from the site.

As discussed above, the accumulated water on site is depicted to reach initial flood level at approximately T = 2:32 hours (i.e., 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 32 minutes from the beginning of the 6-hour storm). As stated in Section 2.1 of the FHRR, the peak elevations on the site occurs at approximately T = 3:04 hours (i.e., 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 4 minutes from the beginning of the 6-hour storm). Therefore, the duration for the period of inundation (as defined by the NRC's JLD-ISG-2012-05) is approximately 32 minutes.

Period of recession of water from site:

As defined in the NRC's JLD-ISG-2012-05, the "period of recession of water from site" is defined as the point from when flood water begins to recede from the site until the flood water has receded from the site and the plant is in a safe and stable state that can be maintained indefinitely.

As stated in Section 2.1 of the FHRR, the peak water elevation on the site occurs at approximately T = 3:04 hours (i.e., 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 4 minutes from the beginning of the 6-hour storm). As depicted in Table 2.1-10 and Figures 2.1-7 through 2.1-10 of the North Anna FHRR, the level of water on site is expected to recede below flood level at approximately T = 3:36 hours (i.e., 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 36 minutes from the beginning of the 6-hour storm). Therefore, the duration for the period of recession (as defined by the NRC's JLD-ISG-2012-05) is approximately 32 minutes.

The level of water on site is expected to continue to recede from the site after T = 3:36.

As depicted in Table 2.1-10 and Figures 2.1-7 through 2.1-10 of the North Anna FHRR, the water is expected to remain below flood level through the end of the 6-hour event and beyond.

Serial No.13-640 Docket Nos. 50-338/339 Attachment, Page 13 of 13 As described in Section 3.0 of the FHRR, for the LIP event, the flood elevation above site grade ranges from approximately 0.3 to 2.9 feet depending on the location on the site.