ML14322A739
| ML14322A739 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 11/13/2014 |
| From: | Mark D. Sartain Virginia Electric & Power Co (VEPCO) |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 14-273A | |
| Download: ML14322A739 (9) | |
Text
VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 November 13, 2014 U. S. Nuclear Regulatory Commission Serial No.: 14-273A Attention: Document Control Desk NLOS/ETS: RO Washington, DC 20555-0001 Docket Nos.: 50-338/339 License Nos.: NPF-4/7 VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS I AND 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION (RAI)
REGARDING FUKUSHIMA LESSONS LEARNED FLOODING HAZARD REEVALUATION REPORT By letters dated March 11, 2013 (Serial No.13-017) and March 31, 2014 (Serial No.14-133),
Virginia Electric and Power Company (Dominion) submitted a flooding hazard reevaluation report and seismic hazard and screening report, respectively. The seismic hazard and screening report indicates that the ground motion response spectrum (GMRS) exceeds the safe shutdown earthquake for North Anna. Based on this conclusion, in a May 14, 2014 letter the NRC requested additional information to complete the review of the flooding hazard reevaluation of the North Anna site.
As discussed in our June 16, 2014 letter (Serial No.14-273), a new, detailed seismic evaluation of the North Anna service water reservoir (SWR) was required to provide justification for the seismic capability of the service water impoundment under the updated seismic hazard (submitted in the March 31, 2014 letter) to respond to the NRC's RAI. Dominion committed to provide a response to the NRC RAI by November 15, 2014. The attachment to this letter provides Dominion's response to the NRC's request for additional information.
Should you have any questions or require additional information, please contact Mr. Thomas Shaub at (804) 273-2763.
Respectfully, Mark Sartain Vice President - Nuclear Engineering CRAIG D SLY Notary Public Commonwealth of Virginia COMMONWEALTH OF VIRGINIA )
) My Commission Expires December 31, 204,16 COUNTY OF HENRICO The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by Mr. Mark D. Sartain, who is Vice President - Nuclear Engineering, 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.
Acknowledged before me this I34'. day ofNDiw6-, 2014.
My Commission Expires: *.I l1t, Ndtary Public Aooo L(2 Serial No. 14-273A Docket Nos. 50-338/339 Page 2 of 2 Commitment contained in this letter: None
Attachment:
Response to Request for Additional Information Regarding Fukushima Lessons Learned - Flooding Hazard Reevaluation Report cc: U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Avenue, NE Suite 1200 Atlanta, GA 30303-1257 Dr. V. Sreenivas NRC Project Manager North Anna 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
Serial No. 14-273A Docket Nos. 50-338/339 Attachment RESPONSE TO REQUEST FOR ADDITONAL INFORMATION REGARDING FUKUSHIMA LESSONS LEARNED FLOODING HAZARD REEVALUATION REPORT Virginia Electric and Power Company North Anna Power Station Units I And 2
Serial No. 14-273A Docket Nos. 50-338/339 Flooding RAI - SWR Seismic Capability Attachment Page 1 of 6
Background
"Supplemental RAI No. 3.4-1 By letter dated November 26, 2013 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML133258122), the U.S. Nuclear Regulatory Commission (NRC) staff issued a request for additional information (RAI) to Virginia Electric and Power Company (Dominion) regarding the North Anna Power Station, Units 1 and 2, flood hazard reevaluation report submitted in response to NRC's March 12, 2012, 50.54(f) letter (ADAMS Accession No. ML12053A340). RAI 3.4-1 requested, in part, that the licensee provide evaluations of failure mechanisms applicable to the service water reservoir impoundment using present-day guidance, methods, and data, including an evaluation of the potential for seismically-induced failure of the service water reservoir impoundment dike under site-specific seismic hazards defined using present-day guidance and methods.
The licensee responded (ADAMS Accession No. ML13357A100) to RAI 3.4-1 by stating that a site-specific seismic hazard reevaluation will be completed in March 2014 and that the "adequacy of the seismic qualification for the service water reservoir impoundment dike may have to be revisited depending on the results of the seismic reevaluation and resulting station [probabilistic risk assessment] reevaluations." The licensee further indicates that "even if the impoundment dike was to fail as a result of the seismic event, the design of the emergency dike and diversion trench prevent water from the service water reservoir from reaching the plant." The RAI response does not provide information about the seismic capacity or performance of the emergency dike to support a conclusion that it would be available in the event that the service water reservoir impoundment dike were to fail under a seismic event.
The licensee's seismic hazard and screening report (ADAMS Accession No. ML14092A416) submitted by the licensee on March 31, 2014, concluded that North Anna screens in for a seismic probabilistic risk assessment as the results of the screening evaluation indicates that the ground motion response spectrum (GMRS) exceeds the safe shutdown earthquake (SSE).
Request for Supplemental Information The licensee is requested to provide justification for the seismic capability of the service water impoundment under the updated seismic hazard. In that case, the licensee is requested to: Evaluate the seismic performance of the service water reservoir impoundment dike considering the updated seismic hazard information for the site and considering all relevant seismic failure modes. Relevant seismic failure modes include, but are not limited to, slope stability, settlement, cracking and resulting potential for internal erosion, as well as liquefaction and lateral spreading.
Serial No. 14-273A Docket Nos. 50-338/339 Flooding RAI - SWR Seismic Capability Attachment Page 2 of 6 The licensee is requested to provide a technical basis for the ability of the emergency dike to withstand the updated seismic hazard such that it is able to convey sufficient outflow from a postulated seismically-induced failure of the service water impoundment dike system. Specifically, the licensee is requested to provide the following supplemental information related to the dike failure flood hazard analyses:
" Postulate plausible service water reservoir impoundment dike failure scenarios, and estimate breach parameters conservatively, including width, time, and breach outflows from the reservoir.
- Provide information related to the seismic stability and/or failure analyses of the emergency dike system to evaluate the seismic performance of the emergency dike under the updated seismic hazard considering all relevant seismic failure modes (e.g., slope stability, settlement, cracking and resulting potential for internal erosion, as well as liquefaction and lateral spreading).
If the seismic failure of the emergency dike system is deemed credible, the licensee is further requested to:
" Postulate failure scenarios of the emergency dike caused by a seismic event and/or piping event caused by upstream breach flow and to evaluate the corresponding flooding impacts on the plant site. If the consequences (i.e., floods) from the plausible seismic failure scenarios are judged not to affect the site, provide a detailed justification for this conclusion, including the effectiveness of the emergency dike and intercepting channel system.
" Provide electronic versions of the input files used for any hydrologic flood routing related to the dike failure flood analyses.
" Confirm whether or not the dike failure flooding scenario will be included in the Integrated Assessment, and provide relevant associated effects and flood event duration parameters (as applicable), including the warning time the site will have to prepare for the dike failure, the period of time the site is inundated, and the period of time necessary for water to recede off the site.
As an alternative to a postulated failure of the service water impoundment and subsequent analysis of the emergency dike and (if applicable) site flooding, the licensee may provide justification for the seismic capability of the service water impoundment under the updated seismic hazard. In that case, the licensee is requested to:
- Evaluate the seismic performance of the service water reservoir impoundment dike considering the updated seismic hazard information for the site and considering all relevant seismic failure modes. Relevant seismic failure modes include, but are not limited to, slope stability, settlement, cracking and resulting potential for internal erosion, as well as liquefaction and lateral spreading."
Serial No. 14-273A Docket Nos. 50-338/339 Flooding RAI - SWR Seismic Capability Attachment Page 3 of 6 Dominion Response Dominion has chosen to take the alternative approach and has evaluated the North Anna Power Station (NAPS) Service Water Reservoir (SWR) for the effects of an extreme seismic event consistent with the ground motion response spectrum (GMRS) submitted in the Seismic Hazard and Screening Report for the site in Dominion letter Serial No.14-133 dated March 31, 2014 [ADAMS Accession No. ML14092A416]. The evaluation provides justification for the seismic capability of the SWR under the loadings consistent with the updated seismic hazard. The evaluation considered seismic failure modes including slope stability, settlement, cracking and resulting potential for internal erosion, as well as liquefaction and lateral spreading.
The seismic demand used as input to the evaluation was developed based on the maximum ground acceleration from the GMRS-consistent seismic ground motion analysis for the SWR area. The earthquake moment magnitude used for slope stability and liquefaction analyses is 7.1, which is the largest magnitude low frequency earthquake for a mean annual frequency of exceedance (MAFE) of 10-4 hazard level. The peak ground acceleration is 0.28g, based on the mean acceleration profile corresponding to the SWR area.
The SWR is located approximately 500 ft south of the station site area. There are two structures located along the northern side of the reservoir, the Service Water Pump House (SWPH) and the Service Water Valve House (SWVH). The SWR was built in an area of sloping terrain where the natural surface varied from about El. 326 ft in the northwest to about El. 280 ft in the southeast. With the exception of the embankment crest near the SWPH and SWVH, the crest of the reservoir dike is at El. 320 ft. The bottom of the SWR is at El. 305 ft, and the maximum design water surface within the pond is El. 315 ft, at which level the pond capacity is approximately 88 acre-ft (roughly 29,000,000 gallons).
During normal operation, the reservoir water depth varies between 8 and 10 ft (water surface El. 313 ft to 315 ft). The reservoir has a perimeter of approximately 3,000 ft. In the west and northwest portions of the SWR, about 500 ft of the reservoir perimeter was excavated below existing ground surface. The remainder (2,500 ft) of the SWR perimeter is formed by an impounding earthen embankment (also referred to as a dike).
During the design, construction, and licensing process, the SWR and its component materials were subjected to extensive subsurface exploration, laboratory testing, analyses, and instrumentation monitoring. Numerous borings were performed and the soil analysis used standard penetration test (SPT) sampling and thin-walled tube samplers. The borings performed in the SWR area encountered fill, residual soil, and saprolite grading to sound rock. The soils underlying the SWR area are primarily residual soils and saprolites similar to those encountered at the main plant site. Typically, silts of low to moderate plasticity (ML and MH) are found in the upper portions of the soil profile. These finer materials transition to coarser-grained saprolite soils (SP, SM, and SP-SM) which are encountered in the lower portions of the profile. Sound bedrock is found at depths of about 65 ft to 100 ft below original ground surface.
Serial No. 14-273A Docket Nos. 50-338/339 Flooding RAI - SWR Seismic Capability Attachment Page 4 of 6 Slope Stability Evaluation The slope stability evaluation analyzed the most critical portion of the slope of the SWR embankment, where failure of that portion of the slope could result in flooding of the plant.
This slope is located on the northeastern SWR embankment, near the SWPH. The cross-section has an upstream height of 21 ft and a downstream height of 26 ft, measured from the crest of the embankment to the point where the downstream slope becomes significantly less steep. The embankment slope is 2.7:1 Horizontal to Vertical. The embankment is founded on the native saprolite and there is a compacted core material in the embankment, which extends to the upstream side of the embankment. Rock fill is placed in a relatively thin layer on the upstream side of the embankment with a much thicker layer of compacted rock fill on the downstream side. There is a 'transition filter' placed between the rock fill and the compacted core. Conservative physical properties and strength parameters for the rock fill, filter materials, compacted core, and native saprolite were selected for the analysis.
The factors of safety (FS) for the analysis are defined as:
- End of Construction (undrained), minimum FS = 1.3
- Long-term Static (drained, non-seismic), minimum FS = 1.5
- Seismic, minimum FS = 1.1 The slope stability calculation was performed by computer analysis (SLOPE/W Program) using the Morgenstern-Price method. The analysis was performed for both seismic and non-seismic (static) cases.
For the non-seismic case, analyses were run for drained and undrained conditions. The minimum computed factor of safety for static analysis was 1.8, which is above the minimum acceptable (1.3 for undrained analysis and 1.5 for drained analysis).
For the seismic case, the analyses used a pseudo-static approach for undrained conditions. In the pseudo-static approach, the horizontal and vertical seismic forces are assumed to act on the slope as a constant static force. This is a very conservative analytical approach, since the actual seismic event occurs for only a short period of time, and during that time, the forces alternate direction at a relatively high frequency. In addition, the pseudo-static analysis is run using peak seismic acceleration, while the mean acceleration during the seismic event is significantly less than the peak value.
Modifications to the pseudo-static approach, supported by industry research, were used in order to address the over-conservatism of the method. These modifications involved reasonable reductions of the ground acceleration value used as analysis input and, in some cases, reduction of the ground strength parameters. A total of twelve modified pseudo-static cases were run for the seismic stability analysis - six for the downstream and six for the upstream embankment slopes. These twelve cases were used to evaluate the seismic stability of the embankment.
Serial No. 14-273A Docket Nos. 50-338/339 Flooding RAI - SWR Seismic Capability Attachment Page 5 of 6 The computed factors of safety for dynamic analysis for the downstream slope range from 1.2 to 1.6, with an average factor of safety of 1.5. The factor of safety from each of the cases exceeds the minimum value of 1.1, and is therefore acceptable.
The computed factors of safety for the upstream slope range from 1.0 to 1.5, with an average factor of safety of 1.3. The computed factor of safety from five of the six runs exceeds the minimum value of 1.1. However, one run, using the Duncan & Wright1 approach which conservatively reduces the ground strength parameters used as input, resulted in a factor of safety of 1.0. Through additional evaluation of this one case, it was further determined that this analytical result only indicated the potential to affect the rock fill layer and does not represent a failure or breach of the embankment.
Since there is no predicted embankment failure or breach, flooding of the NAPS site due to slope failure, or cracking and resulting erosion effects, is not credible.
Liquefaction Evaluation Soil liquefaction is a process by which loose, saturated, granular deposits lose a significant portion of their shear strength due to porewater pressure buildup resulting from cyclic loading, such as that caused by an earthquake.
Soil liquefaction can occur leading to foundation bearing failures and excessive settlements when:
- the ground acceleration is high
" the soil is saturated, i.e., below the water table, and
" the site soils are sands or silty sands in a loose or medium dense condition The liquefaction evaluation used the state-of-the-art method described by Youd et al. 2 The evaluation was performed based on the measured SPT N-values for the soil samples from the borings performed in the SWR area and on the shear wave velocity profile estimated for the SWR. The analysis was performed using an age factor of 2.0 and a minimum acceptable factor of safety of 1.1. Most of the SPT N-values from the borings were considered to represent the undisturbed in-situ condition of the subsurface material. The SWR shear wave velocity profile used for the analysis was taken as the profile developed for the SWPH and SWVH with the elevations shifted up by 5 ft to compensate for relative ground surface elevation differences.
I Duncan, J.M. and Wright, S.G., Soil Strength and Slope Stability, John Wiley & Sons, Inc., NJ, 2005.
2 Youd, T.L., Idriss, I.M., Andrus, R.D., Arango, I., Castro, G., Christian, J.T., Dobry, R., Finn, W.D.L., Harder Jr., L.F., Hynes, M.E.,
Ishihara, K., Koestor, J.P., Liao, S.S.C., Marcuson III, W.F., Martin, G.R., Mitchell, J.K., Moriwaki, Y., Power, M.S., Robertson, P.K.,
Seed, R.B., and Stokoe II, K.H. (2001). "Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils," J. Geotech. & Geoenv. Engrg., ASCE, 127(10), 817-833.
Serial No. 14-273A Docket Nos. 50-338/339 Flooding RAI - SWR Seismic Capability Attachment Page 6 of 6 A section of the perimeter of the SWR consists of an earth embankment. The embankment was constructed from structural fill using selected in-situ soils compacted to meet engineering specification requirements such that liquefaction of the dike material need not be considered.
The analyses based on SPT N-values and shear wave velocity values show the SWR area will not liquefy during the design seismic event. Therefore, lateral spreading is not a concern.
In addition, an assessment of seismic settlement of the SWR dike was performed based on the measured settlement due to the magnitude 5.8 Mineral VA earthquake that occurred in 2011. The estimated settlement from a magnitude 7.1 earthquake is approximately 0.5 inch and is not significant to the stability of the SWR dike.
The lack of the potential for liquefaction and minimal estimated settlement during the extreme seismic event precludes a concern for cracking of the dike and any associated erosion effects.
Since there is no predicted SWR or embankment failure due to liquefaction or settlement effects, flooding of the NAPS site is not credible.
Summary The evaluation provides justification for the seismic capability of the SWR under the loadings consistent with the updated seismic hazard. The evaluation considered seismic failure modes including slope stability, settlement, cracking and resulting potential for internal erosion, as well as liquefaction and lateral spreading. Therefore, SWR embankment failure is not considered credible and seismic failure of the service water impoundment need not be postulated.