ML17096A766

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Submittal of Mitigating Strategies Assessment for Flooding Report
ML17096A766
Person / Time
Site: Diablo Canyon  Pacific Gas & Electric icon.png
Issue date: 04/06/2017
From: Franke J
Pacific Gas & Electric Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
DCL-17 -024, EA-12-049
Download: ML17096A766 (25)
v • d • e


Text

Pacific Gas .and Electric Compa*nytil Jon A. Franke P.O. Box 56 Vice President Avila Beach, GA 93424 Generation Technical Services 805.545.4888 E-Mail: J6FD@pge.com April6, 2017 PG&E Letter DCL-17 -024 U.S. Nuclear Regulatory Commission EA-12-049 ATIN: Document Control Desk 10 CFR 50.54(f)

Washington, D.C. 20555-0001 Docket No. 50-275, OL-DPR-80 Docket No. 50-323, OL-DPR-82 Diablo Canyon Units 1 and 2 Submittal of the Diablo Canyon Power Plant Mitigating Strategies Assessment for Flooding Report

References:

1. NRC Order Number EA-12-049, "Issuance of Order to Modify Licenses with Regard to Requirements for Mitigation Strategies for Beyond-Design-Basis External Events," dated March 12, 2012
2. NRC Letter, "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," dated March 12, 2012 (ADAMS Accession No. ML12056A046)
3. PG&E Letter to NRC, DCL-15-034, "Final Response to Request for Information Pursuant to 10 CFR 50.54(f) Regarding Recommendation 2.1 Flooding," March 11, 2015
4. PG&E Letter to NRC, DCL-16-016, "Updated Response to Request for Information Pursuant to 10 CFR 50.54(f) Regarding Recommendation 2.1 Flooding," February 8, 2016
5. NRC Letter, "Diablo Canyon Power Plant, Unit Nos. 1 and 2 - Interim Staff Response to Reevaluated Flood Hazards Submitted in Response to 10 CFR 50 .54(f) Information Request- Flood-Causing Mechanism Reevaluation (CAC Nos. MF6039 and MF6040)," March 30, 2016 (ADAMS Accession No. ML16083A551)

A member of the STARS Alliance Callaway

  • Diablo Canyon
  • Palo Verde
  • Wolf Creek

Document Control Desk PG&E Letter DCL-17-024 April 6, 2017 Page 2

6. PG&E Letter to NRC, DCL-16-077, "Pacific Gas and Electric Company's Notification of Full Compliance with Commission Order Modifying Licenses with Regard to Mitigation Strategies for Beyond-Design-Basis External Events (Order Number EA-12-049) for Diablo Canyon Power Plant Units 1 and 2,"

July 28, 2016

Dear Commissioners and Staff:

On March 12, 2012, the NRC issued Reference 1 to Pacific Gas and Electric Company (PG&E) regarding mitigation strategies required for Beyond-Design-Basis External Events. Also on March 12, 2012, the NRC issued Reference 2 to request information associated with (among others) Near-Term Task Force (NTTF) Recommendation 2.1 for Flooding. One of the requests for information in Reference 2 directed licensees to submit a Flood Hazard Reevaluation Report (FHRR). PG&E submitted the FHRR in Reference 3 and updated it in Reference 4. Based on the updated FHRR, the NRC issued the interim staff response (Reference 5), which concluded that the FHRR was suitable for the assessment of mitigating strategies in response to Reference 1.

Table 2 of Reference 5 provided the Diablo Canyon Power Plant (DCPP) flood elevations to be considered in performing a mitigating strategies assessment (MSA) with respect to the local intense precipitation (LIP) reevaluated flood hazard, which is the sole reevaluated flood-causing mechanism that was not explicitly bounded by the DCPP current licensing basis. Note 1 of Table 2 stated that PG&E was expected to develop flood event duration (FED) parameters and applicable flood associated effects (AEs) to conduct the MSA.

The enclosure to this letter contains the PG&E MSA for flooding, which includes the requested FED parameters and applicable flood AEs that were used in the MSA.

With two exceptions, the MSA concludes that the current FLEX strategies can be deployed as designed and submitted in the DCPP Final Integrated Plan (Enclosure 3 of Reference 6). The two exceptions are to provide instructions for placing water-activated flood barriers prior to blocking open one door if flood water is present, and to stage these barriers in the control room with other FLEX equipment.

This communication contains a regulatory commitment (as defined by NEI 99-04).

The commitment is contained in Attachment 1 to the enclosure.

If you have any questions or require additional information, please contact Mr. Scott Maze at 805-542-9591.

A member of the STARS Alliance Callaway

  • Diablo Canyon
  • Palo Verde
  • Wolf Creek

Document Control Desk PG&E Letter DCL-17-024 April 6, 2017 Page 3 I state under penalty of perjury that the foregoing is true and correct.

Executed on April6, 2017.

ice President, Generation Technical SeNices mem6/4539/50465780 Enclosure cc: Diablo Distribution cc/enc: William M. Dean, NRC/NRR Director Kriss M. Kennedy, NRC Region IV Administrator Christopher W. Newport, NRC Senior Resident Inspector Joseph M. Sebrosky, NRC Project Manager Balwant K. Singal, NRR Senior Project Manager A member of the STARS Alliance Callaway

  • Diablo Canyon
  • Palo Verde
  • Wolf Creek

Enclosure PG&E Letter DCL-17-024 Page 1 of 21 Diablo Canyon Power Plant Mitigating Strategies Assessment for Flooding NEI 12-06, Revision 2, Appendix G Submittal

Enclosure PG&E Letter DCL-17-024 Page 2 of 21 Table of Contents

1. Executive Summary ........................................................................................................ 3
2. Background ..................................................................................................................... 3
3. Site Description ............................................................................................................... 5
4. Characterization of MSFHI (NEI 12-06, Revision 2, Section G.2) ................................... 5
5. Basis for Mitigating Strategies Assessment (NEI 12-06, Revision 2, Section G.3) .......... 9
6. Assessment of Current FLEX Strategy (NEI 12-06, Revision 2, Section G.4.1) ............ 11
7. Assessment for Modifying FLEX Strategies (NEI 12-06, Revision 2, Section G.4.2) ..... 18
8. List of Abbreviations and Acronyms .............................................................................. 19
9. References ................................................................................................................... 20

Enclosure PG&E Letter DCL-17-024 Page 3 of 21

1. Executive Summary The Diablo Canyon Power Plant (DCPP) Flood Hazards Reevaluation Report (FHRR) submitted in response to the NRC's March 2012 10 CFR 50.54(f) letter (Reference 1) determined that, for some locations, one potential external flood-causing mechanism, a Local Intense Precipitation (LIP) event, could result in flood water levels at DCPP that are not bounded by the design basis flood elevation. As required by the NRC, this Mitigating Strategies Assessment (MSA) was performed to evaluate the impact of the postulated LIP event on the DCPP FLEX strategies.

In support of the MSA, a more detailed LIP analysis was performed, "DCPP Local Intense Precipitation (LIP) Analysis 2D Modeling" (Reference 11 ). To determine flow path and volume of water entering plant structures and potential impact on structures, systems, and components (SSCs), an additional flooding analysis internal to buildings was conducted, "Study Calculation: Diablo Canyon Local Intense Precipitation (LIP)

Effect on Building Internals" (Reference 10). These calculations determined that the water levels from a LIP event are bounded by the Current Licensing Basis values, and that no programmatic, procedural, or plant modifications are required. The MSA determined that the existing FLEX strategies can be implemented as designed, provided that the following actions are completed:

  • Change plant procedures to provide instructions for placing water-activated flood barriers prior to blocking open door-355 if flood water is present.
  • Stage water-activated flood barriers in the control room to support this activity.

These actions will be completed by the required compliance date for the forthcoming regulation 10 CFR 50.155, "Mitigation of Beyond-Design-Basis Events."

2. Background

On March 12, 2012, the NRC issued Reference 1 to request information associated with (among others) Near-Term Task Force (NTTF) Recommendation 2.1 for Flooding. On January 22, 2016, the NRC issued JLD-ISG-2012-01, Revision 1, "Compliance with Order EA-12-049, Order Modifying Licenses with Regard to Requirements for Mitigating Strategies for Beyond-Design-Basis External Events" (Reference 5). One of the Required Responses specified in Reference 1 directed licensees to submit an FHRR.

By letter dated March 11, 2015, PG&E provided the FHRR Revision 0, for the DCPP, Units 1 and 2 (Reference 2). By letter dated February 8, 2016, PG&E submitted a revised FHRR, Revision 1, which contained a new LIP and associated site drainage analysis (Reference 6). A description of interim actions implemented to address the potential effects of the unbounded flooding mechanism was required by the original 10 CFR 50.54(f) request, and these measures were described in both the original and revised FHRR submittals (References 2 and 6). These actions consisted of a warning time notification process and a procedure to deploy sandbags at all affected doors when heavy rainfall is forecast, and were inspected by the NRC as part of the Tl-190

Enclosure PG&E Letter DCL-17 -024 Page 4 of 21 inspections. Based on Reference 6, on March 30, 2016, the NRC issued an interim staff response letter (Reference 3) to PG&E, also known as the "MSFHI Letter."

Reference 3 transmitted a summary of the NRC staff's assessment of the reevaluated flood-causing mechanisms described in the DCPP FHRR, Revision 1 (Reference 6),

and documented the NRC staff's conclusion that the reevaluated flood hazards information for DCPP was suitable for an assessment of mitigating strategies developed in response to Order EA-12-049.

The mitigating strategies for DCPP, termed FLEX strategies, developed in response to Order EA-12-049, were documented in the Final Integrated Plan (FIP) submitted by Reference 9 on July 28, 2016. As documented in Reference 13, the NRC staff concluded that the DCPP FLEX strategies, if implemented appropriately, would adequately address the requirements of Order EA-12-049. The FLEX Design Basis Flood Hazard is defined as the controlling flood parameters used to develop the FLEX flood strategies.

By Reference 7, the NRC affirmed that the licensees need to address the reevaluated flood hazards within their mitigating strategies for Beyond-Design-Basis External Events (BDBEEs). Guidance for performing MSAs is contained in Appendix G, "Mitigating Strategies Assessment for New Flood Hazard Information," of Reference 4, endorsed by the NRC (with conditions) in Reference 5. Appendix G of Reference 4 defines the reevaluated flood hazard information from the FHRR, Revision 1, as the mitigating strategies flood hazard information (MSFHI), and describes the MSA for flooding as containing the following elements:

  • Section G.2- Characterization of the MSFHI
  • Section G.3 - Basis for Mitigating Strategy Assessment- Comparison of the MSFHI and FLEX Design Basis (DB) Flood to determine whether the MSFHI is bounded by the flood hazard that was used to develop the FLEX strategies
  • Section G.4.1 -Assessment of Current FLEX Strategies- If the MSFHI is not bounded in all aspects as described in Section G.3, this section provides guidance for evaluating the existing FLEX strategies against the impacts of the MSFHI to determine whether the FLEX strategies can still be implemented without change.
  • Section G.4.2 -Assessment for Modifying FLEX Strategies - If the FLEX strategies cannot be implemented without change, this section provides guidance to determine whether the FLEX strategies can be modified to address the identified impacts from the MSFHI.
  • Section G.4.3- Assessment of Alternate Mitigating Strategies (if necessary)
  • Section G.4.4- Assessment of Targeted Hazard Mitigating Strategies (if necessary)

The sections that are applicable to DCPP are G.2, G.3, G.4.1, and G.4.2. This MSA document provides the information requested by those sections. Elevation data in this document are based on the North American Vertical Datum of 1988 (NAVD88).

Enclosure PG&E Letter DCL-17-024 Page 5 of 21

3. Site Description The DCPP site consists of approximately 750 acres located in San Luis Obispo County, California, adjacent to the Pacific Ocean and roughly equidistant from San Francisco and Los Angeles. The DCPP site occupies a coastal terrace that ranges in elevation from 62.9 to 152.9 ft NAVD88 (60 to 150 ft mean sea level (MSL)) above sea level and is approximately 1,000 ft wide. The seaward edge of the terrace is a near-vertical cliff.

With the exception of the intake and discharge facilities, plant grade is at elevation 87.9 ft NAVD88 (85ft MSL) and entrance to major plant buildings is at or above this elevation. In addition, the plant site is generally sloped away from the major plant buildings and toward the ocean or Diablo Creek. Topography and plant site arrangement limit flood design considerations to local floods from Diablo Creek and sea wave action from the Pacific Ocean.

4. Characterization of the MSFHI (NEI 12-06, Revision 2, Section G.2)

The only unbo~nded external flood-causing mechanism from the FHRR (Reference 6) for the site is from LIP. All other flood-causing mechanisms were determined by the FHRR to be bounded by the existing licensing basis, and confirmed in the NRC interim staff response letter (Reference 3). Table 2 of the enclosure to the NRC interim staff response letter describes the reevaluated flood hazards that exceed the current design basis. This table lists the stillwater elevation/ reevaluated hazard elevation at 40 points of interest and notes that waves/run up is minimal at all doors. As described in the FHRR (Reference 6) the PMP for the DCPP area was calculated in accordance with NUREG/CR-7046 (November 2011). The probable maximum precipitation (PMP) was then used as input into a LIP and probable maximum flood (PMF) evaluation for the entire DCPP site. A LIP 20 evaluation (Reference 14) was conducted to determine the water surface elevation associated with the effects of the LIP inside the protected area where the power block and other safety-related structures and commodities are located.

The calculation also determined the duration of flooding and associated hydrodynamic loading near the potential water entry points to the auxiliary building, fuel handling building, and turbine building (including the safety-related diesel generator room air inlet louvers locations and areas where important safety-related components for the diesel generator fuel oil transfer system are located).

As part of DCPP's effort to support the MSA, additional detailed modeling based on the LIP event was conducted. As a result, two new calculations were issued.

  • Calculation titled, "DCPP Local Intense Precipitation (LIP) Analysis 20 Modeling" (Reference 11 ), included a more detailed LIP evaluation of water depth and potential entry points to safety-related structures on the rooftop of the Auxiliary Building, and determined new water depths above door thresholds and duration of inundation.
  • Calculation titled, "Study Calculation: Diablo Canyon Local Intense Precipitation (LIP) Effect on Building Internals" (Reference. 10), performed

Enclosure PG&E Letter OCL-17-024 Page 6 of 21 analysis of doors and areas that experienced inundation, to determine flow path and volume of water entering buildings assuming no LIP interim actions (i.e., sandbags) were deployed, and to evaluate the impact on FLEX from LIP.

As documented in the Reference 11 calculation, the flood heights from a LIP event vary at different OCPP locations due to the site-specific terrain and watershed pathways.

Based on the results of Reference 11, the water depth above the door thresholds in the power block area and surrounding structures varied between 0 ft and 1.01 ft. The duration of time-dependent water depths varied between 0.00 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> and 8.80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.

Table 1 below, from Reference 11, provides the MSFHI parameters for the LIP event that are not bounded by the current design basis hazard. Note that inundation does not begin at any door until 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> after the start of the Ll P event. The calculation in Reference 10 showed that without deployment of sandbags (i.e., the interim actions described in the FHRR Revision 1), water intrusion from flooding would not prevent any safety-related SSCs from performing their intended function, and that a total volume of 338,566 gallons of water would be conveyed to the Auxiliary Building sump and Pipe Tunnels. Since the OCPP Updated Final Safety Analysis Report (UFSAR), Section 9.2.3.3.7(2)- Internal Flooding Protection, states that, "[a] volume of 345,000 gallons in the auxiliary building pipe tunnel for sump overflow storage is available to receive water flooding," there is adequate volume to store water resulting from beyond-design-basis (BOB) flooding. Consequently, the interim actions implemented as described in the FHRR, Revision 1 (Reference 6), are not required to respond to LIP flooding since BOB flooding levels will not negatively impact safety-related SSCs within the plant.

The potential associated effects caused by a LIP flooding event are analyzed based on direction given in NEI 12-06, Revision 2. Guidance defines "flood height and associated effects" as the maximum stillwater surface elevation plus the following factors:

  • hydrodynamic loading, including debris;
  • wind waves and run-up effects;
  • effects caused by sediment deposition and erosion;
  • concurrent site conditions, including adverse weather conditions;
  • . groundwater ingress; and
  • other "pertinent factors" including flood event duration and warning time.

Not all the associated effects indicated above are applicable to LIP flooding events.

Only hydrostatic and hydrodynamic loading are considered in the LIP 20 calculation after screening out other associated effects (Reference 11 ). Hydrostatic and hydrodynamic loadings were calculated at each door, and the hydrodynamic loading was found to be insignificant. As stated in Reference 3, waves/runup is considered minimal, and no additional loads need be evaluated. As also stated and supported in Reference 11, the shallow flow depth (approximately 1 ft) and low velocity (approximately 1 ft/sec) due to LIP would not be expected to cause debris impact loading, sediment deposition or erosion at the OCPP site (Reference 11 ), and do not

Enclosure PG&E Letter DCL-17-024 Page 7 of 21 need additional consideration. As a result, the stillwater elevation and reevaluated hazard elevations are the same.

Table 1 - Reevaluated Flood Hazards for LIP External Flood Mechanism

  • (Source: Reference 11)

Stillwater Elevation MaxWD and above grid Maximum Water Depth Door/Unit Flood Duration Reevaluated surface (ft) Above Door Threshold No (ft) 1 (hours)

Hazard Elevation (ft NAVD88)3 A1.1 87.48 0.21 -0.01 0 A1.2 86.77 0.21 -0.03 0 A2.1 86.64 0.16 0.14 1.90 8U101 86.58 0.10 0.08 0.45 8U102 86.66 0.19 0.16 0.95 8U103 86.63 0.18 0.13 0.45 A3.1 86.69 0.24 . 0.19 1.35 A3.2 86.70 0.23 0.20 1.55 A3.3 86.62 0.17 0.12 0.25 8U104 86.66 0.19 0.16 0.95 101-1 86.98 0.20 0.18 0.80 102-1 86.97 0.24 0.17 0.55 119-1 87.01 0.23 0.21 0.85 122-1 87.22 0.45 0.42 4.90 C1.1 86.91 0.14 0.11 0.95 129 86.90 0.12 0.10 0.95 130 86.89 0.12 0.09 0.60 C1.2 86.84 0.07 0.04 0.20 BU101-2 86.68 0.21 -0.02 0.00 BU102-2 86.65 0.07 0.00 0.00 8U103-2 86.43 0.07 -0.07 0.00 8U104-2 86.43 0.07 -0.07 0.00 8U108-2 86.62 0.15 0.12 0.95 8U105-2 86.60 0.12 0.10 0.50 8U106-2 86.90 0.26 0.20 2.85 82.1 86.93 0.16 0.13 0.95 81.1 86.87 0.11 0.07 0.25 81.2 86.91 0.12 0.11 0.65 101-2 87.08 0.31 0.28 0.85 102-2 87.08 0.30 0.28 1.35

Enclosure PG&E Letter DCL-17 -024 Page 8 of 21 Stillwater Elevation MaxWD and above grid Maximum Water Depth Door/Unit Flood Duration Reevaluated surface (ft) Above Door Threshold No (ft) 1 (hours)

Hazard Elevation (ft NAVD88)3 119-2 87.10 0.32 0.30 1.35

' 122-2 87.10 0.32 0.30 1.35 192-1 87.21 0.43 0.41 6.20 191-1 87.04 0.25 0.24 2.70 194-1 87.02 0.25 0.22 0.75 192-2 87.19 0.40 0.39 8.80 191-2 87.11 0.33 0.31 2.40 194-2 87.10 0.37 0.30 0.80 363-1 116.87 0.09 0.07 0.45 361-1 117.14 0.36 0.34 1.90 360-1 117.14 0.35 -0.34 4.10 355-1 117.19 0.43 0.39 5.45 354-1 117.19 0.40 0.39 6.30 360-2 117.20 0.45 0.40 8.55 361-2 117.20 0.44 0.40 8.55 363-2 116.83 0.05 0.03 0.20 520 140.51 0.50 -0.07 0.00 521 140.51 0.47 0.13 0.60 525 140.51 0.46 0.46 2.10 528 140.51 0.50 -0.07 0.00 530 140.21 0.21 -0.29 0.00 540 140.20 0.20 0.03 0.05 541 140.21 0.21 0.04 0.05 565 140.20 0.20 0.20 4.25 575 141.01 1.01 1.01 7.20 587 140.52 0.52 0.52 4.40 588 - 141.01 1.01 0.84 2.00 589 141.01 1.01 1.01 6.35 609 163.46 0.13 -0.54 0.00 610 163.46 0.13 -0.54 0.00 523-2 140.96 0.96 0.79 1.95 524-2 140.51 0.50 -0.07 0.00 529-2 140.51 0.49 -0.07 0.00 530-2 140.21 0.21 -0.29 0.00 540-2 140.19 0.19 0.02 0.05 541-2 140.18 0.18 0.01 [0.00] 2 0.00 565-2 140.18 0.18 0.18 4.20 575-2 140.97 0.97 0.97 6.25 584-2 140.62 0.62 0.62 4.25

Enclosure PG&E Letter DCL-17-024 Page 9 of 21 Stillwater Elevation MaxWD and above grid Maximum Water Depth Door/Unit Flood Duration Reevaluated surface (ft) Above Door Threshold No (ft) 1 (hours)

Hazard Elevation (ft NAVD88)3 585-2 140.96 0.96 0.79 1.95 586-2 140.96 0.96 0.96 6.35 608-2 163.46 0.13 -0.54 0.00 611-2 163.47 0.14 -0.53 0.00 612-2 163.47 0.14 -0.53 0.00 LCRN 140.51 0.51 0.01 [0.00] 2 0.00 LCRS 140.52 0.52 0.02 [0.00] 2 0.00 Notes:

1. Maximum water depth above door threshold is calculated based on the difference between maximum stillwater elevation and door threshold elevation. A negative value indicates maximum stillwater elevation is below door threshold.
2. Inconsequential exceedance depth and duration less than the surface detention parameter result in no flooding time.
3. The values in this column are reported in the Reference 11 calculation in a local PG&E elevation datum. In order to convert these to NAVD88 elevation, 0.687 feet was added to the PG&E daturn values (reference PG&E Drawing 522316, Sheet 1, Revision 1 (Reference 16)).
5. Basis for Mitigating Strategy Assessment (NEI 12-06, Revision 2, Section G.3)

The current licensing basis (CLB) for flood hazards found no pending from local PMP events (UFSAR, Section 2.4.11 ). This is the flood hazard for which FLEX strategies were designed (i.e., the FLEX DB), except for the FLEX storage facilities, which were designed considering the reevaluated flooding hazard.

For the areas identified in the FHRR, Revision 1, that experienced inundation due to Ll P, this MSA evaluates the impact on implementation of FLEX strategies at DCPP.

Since the analysis of external flooding events on the inside of power block structures (Reference 10) determined that the interim actions of deploying sandbags for Ll P is not required for the current design basis, FLEX strategies were assessed in this MSA report assuming these interim actions were not implemented.

Table 2 provides a comparison of MSFHI data from the BOB LIP event to the site's design basis/FLEX DB flood. Since the FLEX DB does not bound the Table 1 results (i.e., the MSFHI) a G.4.1 analysis is required.

Enclosure PG&E Letter DCL-17-024 Page 10 of 21 Table 2- Comparison of MSFHI LIP Flood Causing Mechanism to Plant and FLEX Design Basis Flood Hazard MSFHI FLEX Bounded Plant Design MSFHI (B) or Not

  • Flood Scenario Parameter DB Basis (NAVD88) Bounded Flood Flood (NB) by Hazard FLEX DB
1. Max Stillwater Elevation Dry Varies by Dry Site NB C/) (ft NAVD88) Site location, Note 1 t5

~ 2. Max Wave Run-up Dry Dry Site Note 2 B w Elevation (ft MSL) Site

"'C (J)

+""' 3. Max ro Dry

'(3 Hydrodynamic/Debris Dry Site Note 3 B 0 Site C/) Loading (psf)

C/)

<( 4. Effects of Sediment Dry

"'C Dry Site Note4 B c Deposition/Erosion Site ro 5. Other Associated Effects Dry (J) Dry Site Note 4 B

> (identify each effect) Site (J)

...J

6. Concurrent Site Dry

"'C Dry Site None B 0

0

, Conditions Site

u. Dry None
7. Effects on Groundwater Dry Site B Site Note 5 Dry N/A
8. Warning Time (hours) Dry Site B

+""'

Site Note 6 c

(J) c 9. Period of Site Preparation Dry N/A

> 0 Dry Site B W+=i (hours) Site Note 6

"'C ~ 10. Period of Inundation Dry 9 hrs 0  ::l oo Dry Site NB (hours) Site (Reference 11) u.

11 . Period of Recession Dry Dry Site Note 7 B (hours) Site Dry All modes

12. Plant Mode of Operations Dry Site B Other Site Note 8 Dry
13. Other Factors Dry Site None B Site Notes:
1. Table 6.3 in Reference 11 provides maximum stillwater elevation at the 76 doors that are impacted by LIP flooding. Conversion from the Local PG&E Datum to NAVD88 vertical datum requires raising the elevations by 0.687 feet.

Enclosure PG&E Letter DCL-17-024 Page 11 of 21

2. Based on the maximum water depths of less than or equal to one foot, waves/runup are minimal (Reference 3).
3. Debris flow is not a concern for water flow depth less than 1 foot and velocity less than 1 ft/sec. Maximum velocity values are less than 1 ft/sec for all commodities. Forces due to LIP flood event effects will not adversely impact the doors or power block and surrounding structures (Reference 2).
4. The shallow flow depth and low velocity due to LIP would not be expected to cause debris impact loading, sediment deposition and erosion, or other associated effects at the DCPP Site (Reference 11).
5. Groundwater at the site is limited to one Deep Well 0-2. No other significant groundwater has been encountered (UFSAR 2.4.14). The LIP event results in surface runoff and inundation that recedes to negligible levels within 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br />. Due to short duration of surface runoff, groundwater recharge will be extremely limited and is not considered.
6. No actions are required to respond to a LIP event.
7. Due to the site being a free-flowing site with a small upstream watershed, no additional time is associated with recession of the flood waters beyond the period of inundation.
8. The plant can be in any mode of operation during the LIP flooding event.
6. Assessment of Current FLEX Strategies (NEI 12-06, Revision 2, Section G.4.1)

The overall FLEX planned response to an extended loss of AC power (ELAP) and loss of ultimate heat sink (LUHS) will be initiated through normal plant command and control procedures and practices. Site emergency operating procedures (EOPs) or abnormal operating procedures (AOPs) govern the operational response. The FLEX strategies will be deployed in support of the EOPs/AOPs using FLEX Support Guidelines (FSGs), which will provide direction for using FLEX equipment in maintaining or restoring key safety functions.

Current FLEX Strategies assume a dry site based on the site design basis as required by NEI 12-06, Revision 2 (Reference 4). As described in the FHRR, site flooding occurs during the LIP event. The following discussion assesses the impacts of the MSFHI in place of the design basis on implementing FLEX strategies as required in NEI 12-06, Revision 2, and concludes that with the modification of placing water-activated flood barriers at one door, implementation of FLEX strategies is not affected by the impacts of the MSFHI.

Assessment of FLEX Strategies

  • The boundary conditions and assumptions of the initial FLEX design are maintained.

Enclosure PG&E Letter DCL-17-024 Page 12 of 21 The boundary conditions and assumptions for the initial FLEX design do not include any external flooding events at the site with the exception of the FLEX storage facilities.

The current FLEX strategy assumes a dry site and all events are time zero events without warning time. The FHRR, Revision 1, LIP flooding scenario results in flood waters above site grade which causes pending at several doors in the power block area (Reference 11).

In this evaluation, it is assumed that the ELAP/LUHS occurs approximately 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> after the beginning of the LIP rainfall event, corresponding to the peak LIP level. This is a conservative assumption because the end temporal distribution is the critical distribution for the DCPP site. For this distribution, rainfall amounts are approximately zero at the start of the event, and increase slowly until close to the time when peak rainfall occurs (Reference 11 ). Furthermore, Reference 10 shows that a LIP event will not cause an ELAP/LUHS, and this evaluation assumes that peak flooding from the LIP event results in an ELAP/LUHS.

As evaluated in Reference 10, without initiation of FLEX, the progression of inundation inside the affected buildings would not adversely impact installed plant equipment.

As described below, the only impact on FLEX strategies occurs when blocking open Door 354 or Door 355 to control heat buildup in vital areas during an ELAP/LUHS event concurrent with a LIP event.

The reevaluated LIP hazard was considered in the design of the Primary and Secondary FLEX Equipment Storage Facilities and there are no adverse LIP flooding impacts to these facilities.

Deployment and staging of the first Phase 2 FLEX equipment is not expected to start until 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> after the ELAP/LUHS event, as documented in the DCPP BOB Validation of the FLEX Time Sensitive Actions report (Reference 12). Per Table 1, Reevaluated Flood Hazards for LIP External Flood Mechanism, the maximum water depth above grid surface in the vicinity of Phase 2 FLEX equipment deployment locations is 0.45 feet, with maximum inundation duration of 8.8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. Consequently, water will have receded prior to deployment and staging of Phase 2 FLEX equipment.

Predetermined, preferred staging routes and deployment paths have been identified and documented in the FSGs for the deployment of onsite FLEX equipment in Phase 2.

These deployment paths have been evaluated for potential soil liquefaction, which determined that the FLEX staging routes and deployment paths are not subject to liquefaction hazards (Reference 15). Additionally the deployment paths minimize travel through areas with trees, power lines, narrow passages and other potential debris to the extent practical. Due to the DCPP site arrangement, the LIP event results in surface runoff that rapidly dissipates following the LIP event. Furthermore, debris removal equipment stored on-site is protected from BDBEE hazards such that the equipment remains functional and deployable to clear obstructions from the designated deployment paths between the FLEX storage facilities and associated staging locations. The stored

Enclosure PG&E Letter DCL-17-024 Page 13 of 21 FLEX debris removal equipment includes front-end loaders equipped with buckets and lifting forks to move or remove debris from deployment paths.

Phase 3 of the FLEX strategies involves receipt of equipment from offsite sources including the National SAFER Response Center (NSRC) and various commodities such as fuel and supplies. Delivery of this equipment can be through airlift or via ground transportation. Debris removal for the pathway between the site and the NSRC receiving "Staging Areas" and from the various plant access routes may be required; however, in this scenario, plans have been created to airlift equipment from the various pre-identified staging areas to the site (Reference 9).

  • The sequence of events for the FLEX strategies is not affected by the impacts of the MSFHI (including impacts due to the environmental conditions created by the MSFHI) in such a way that the FLEX strategies cannot be implemented as currently developed.

The FIP (Reference 9) presents a Sequence of Events (SOE) Timeline for an ELAP/LUHS event at DCPP. Validation of each of the FLEX time constraint actions has been completed in accordance with the FLEX validation process outlined in NEI 12-06, Revision 2, and documented in Reference 12. Table 3 below is a revised version of the FIP table that has been modified to describe the potential impacts of the LIP event on the sequence of events actions for which Time Constraints have been established. As documented in this table, the only FLEX Time Sensitive Action .(TSA) that is impacted by the LIP event in such a way that FLEX strategies cannot be implemented as currently developed is TSA 14, "Doors to control room, cable spreading room, and battery charger/inverter rooms are blocked open."

Enclosure PG&E Letter DCL-17-024 Page 14 of 21 Table 3 - Sequence of Events Timeline Time Estimated Action Constraint Action Start Duration LIP Impact Item Finish Time (Hours)

Event Starts 0 N/A N/A Plant at 100°/o Power These actions are normal operator control room actions that are not affected by 1-10 LIP.

No impact - Local action occurring in the 11 Declare ELAP 34 mins 1 min 1 Control Room, an area that is not impacted by a LIP event No impact- Installed emergency battery powered and personal lighting is available in Control room 12 N/A 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> N/A the control room for 8 portable lighting hours, after which portable battery powered lighting will be used.

No impact - Phone call Assistance from Control Room, an 13 requested from 53 mins 1 min 1.25 area that is not NSRC impacted by a LIP event Minimal impact - Local actions would be required inside the Doors to control Turbine Building and room, cable Auxiliary Building, areas spreading room, that are not impacted by 14 and battery 53 mins 18 mins 1.5 a Ll P event. Water-charger/inverter activated flood barriers rooms are would be deployed as blocked open needed prior to blocking open doors to mitigate impact from LIP

Enclosure PG&E Letter DC L-17-024 Page 15 of 21 Time Estimated Action Constraint Action Start Duration LIP Impact Item Finish Time (Hours)

No impact - Local actions would be Vital DC battery required inside Auxiliary 15 load stripping is 55 mins 14 mins 1.5 Building and Turbine completed Building areas that are not impacted by a LIP event No impact - Ll P event Plant access 0.5 results in surface runoff 16 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> 6 assessment hours that rapidly dissipates following the Ll P event.

No impact - Local actions to deploy hoses would be required in the Deploy hoses to Spent Fuel Pool area SFP and open 1.8 17 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> 13 which is not impacted doors to FHB to hours by a Ll P event.

ventilate SFP Inundation levels would be minimal when doors are blocked open.

Transfer No impact- LIP TDAFWpump 18 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> 20 mins 17 inundation levels at this suction to 0-1 time would be minimal FWST Align second No impact- LIP vital battery and 19 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br /> 26 mins 19 inundation levels at this secure initial time would be minimal battery

Enclosure PG&E Letter DCL-17-024 Page 16 of 21 Time Estimated Action Constraint Action Start Duration LIP Impact Item Finish Time (Hours)

No impact - Local actions required inside buildings are in areas that are not impacted by a Ll P event. Exterior actions are limited to Perform plant 2.3 manual operation of 20 cooldown and 28 mins 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> steam dump valves.

depressurization Maximum inundation levels in this area would be less than 1. 01 feet and would not impact accessibility or operation of the valves.

Align ERGS No impact - Ll P 4.5 21 make-up pump 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> 20 inundation levels at this hours from BASTs time would be minimal No impact- Local actions would be required in areas around the Turbine FLEX Building and Auxiliary deployment 1.5 Building. Maximum 22 damage 1.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> inundation levels in assessment these areas throughout complete the LIP event would be less than 1.01 feet and would not impact accessibility.

480VAC No impact- LIP generator 4.3 23 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br /> 27 inundation levels at this repowers hours time would be minimal battery chargers Transfer No impact- LIP TDAFWpump 24 28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br /> 20 mins 29.5 inundation levels at this suction from 0-1 time would be minimal FWSTto CST EAFWand No impact- LIP 25 RWR equipment 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> 6 hours 39.9 inundation levels at this in service time would be minimal

Enclosure PG&E Letter DCL-17-024 Page 17 of 21 Time Estimated Action Constraint Action Start Duration LIP Impact Item Finish Time (Hours)

Align ERGS No impact - LIP 3.8 26 pump suction to > 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> 43.7 inundation levels at this hours RWST time would be minimal Align flexible No impact- LIP hoses to RWR 27 > 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> 6 hours 67 inundation levels at this supply header time would be minimal for SFP cooling Establish No impact- LIP 28 alternate fuel > 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> 15 mins <72 inundation levels at this supply<b) time would be minimal No impact- LIP Align large 28.3 29 > 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> 121 inundation levels at this generators<b) hours time would be minimal No impact- LIP Align EASW 16.3 30 > 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> 121 inundation levels at this pump hours time would be minimal The first TSA that is executed in an exterior yard area where flooding may occur is Action Item 20, "Perform plant cooldown and depressurization." According to the DCPP BOB Validation of FLEX TSAs (Reference 12), the estimated start time is 28 minutes after the ELAP/LUHS. The time constraint for this action is completion by 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />.

Required actions exterior to the building are limited to access through the 140 ft elevation "sundeck" area to manually operate steam dump valves. Access to this area likely would utilize Door 521. Ponding at this door would be below the door threshold when this TSA is started. Opening this door to pass through will not result in water entering the building. Pending above the thresholds at other doors in this area when this TSA is started would be:

Door 575 0.5 feet Door588 0.3 feet Door589 0.4 feet Door 523-2 0.3 feet Door 575-2 0.5 feet Door 585-2 0.4 feet Door 586-2 0.5 feet This level of flooding would not impact accessibility or operation of the steam dump valves.

Enclosure PG&E Letter DCL-17-024 Page 18 of 21 Action Item 14, the next TSA executed in an exterior yard area where flooding may occur, involves blocking open doors to control heat buildup in vital areas. The time constraint for this action is completion by 1.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. It is estimated to start at 53 minutes with an 18-minute duration determined for the current design basis. The DCPP LIP Effect on Building Internals calculation (Reference 10), which assumed that all doors remained closed, showed that water intrusion from external flooding would result in negligible water depths around safety-related SSCs, and that there is adequate available volume inside the Auxiliary Building to store water resulting from BOB flooding.

The doors listed in Table 4 experience inundation from the MSFHI for LIP event and are directed tq be open by plant procedures during an .ELAP/LUHS event. At the time these doors would be required to be open (i.e., when this TSA is initiated), only doors 354 and 355 would still be inundated, and the water level at these doors would have decreased to approximately half of the maximum (see Table 4).

  • Table 4 - Doors That Experience Flooding due to MSFHI and are Blocked Open to Control Heat Build Up Door Maximum Water Depth Water Depth Above Number Above Door Threshold (ft) Door Threshold at 6.75 hours8.680556e-4 days <br />0.0208 hours <br />1.240079e-4 weeks <br />2.85375e-5 months <br /> (ft) 530 0.00 0.0 530-2 0.00 0.0 354 or 355 1 0.39 0.2 129 0.10 0.0 521 0.13 0.0 612-2 0.00 0.0 Note 1: Operations has the option to open either door based on conditions.
  • The validation performed for the deployment of the FLEX strategies is not affected by the impacts of the MSFHI.

The impacts of the MSFHI result in the need to require placement of water-activated flood barriers at one door, prior to blocking it open, as needed for flooding. The affected TSA will be required to be revised, and will be re-performed in accordance with NEI 12-06, Revision 2, requirements. *

7. Assessment for Modifying FLEX Strategies (NEI 12-06, Revision 2, Section G.4.2)
  • If deployment locations of FLEX equipment are changed as a result of the evaluation per Section 6, the design considerations for the strategy should be reevaluated per Section 11.2.1.

Enclosure PG&E Letter DCL-17 -024 Page 19 of 21 Water-activated flood barriers will be required to be stored in the control room and deployed to one door when blocking it open. The control room is a protected location so the water-activated flood barriers will not be impacted by inundation there.

  • New or modified actions required for the strategy or existing actions that are impacted by the environmental conditions created by the MSFHI should be validated in accordance with Appendix E.
  • The procedure will be modified for implementing Action Item 14, blocking open doors to control room, cable spreading room, and battery charger/inverter rooms. In the revised procedure, if flood waters are present, direction will be provided for sole use of Door 355 and to require placement of water-activated flood barriers prior to blocking open the door. This new action will be validated in accordance with NEI 12-06, Revision 2, Appendix E requirements.
  • The flood protection features that support the modified FLEX strategies should meet the performance criteria provided in Section G.5.

All performance criteria provided in Section G.5 will be met by the deployment of water-activated flood barriers that are designed to handle the maximum water depths for the duration of inundation. The water-activated flood barriers required to support the modified procedure will be stored in the control room with other staged FLEX equipment. The barriers will be added to the FLEX Preventative Maintenance (PM)

Program. PG&E will complete the necessary changes to plant procedures, conduct required validation, stage water-activated flood barriers, and update the FLEX PM program by the required compliance date of the forthcoming regulation 10 CFR 50.155, "Mitigation of Beyond-Design-Basis Events."

8. List of Abbreviations and Acronyms
  • AMS -Alternative Hazard Mitigating Strategies
  • AOP -Abnormal Operating Procedures
  • BOB- Beyond-Design-Basis
  • BDBEE - Beyond Design Basis External Event
  • CLB- Current Licensing Bases
  • CST - Condensate Storage Tank
  • DB - Design Basis
  • EASW - Emergency Auxiliary Salt Water
  • ELAP - Extended Loss of AC Power
  • EOP - Emergency Operating Procedure
  • FHRR- Flood Hazard Reevaluation Report

Enclosure PG&E Letter DCL-17-024 Page 20 of 21

  • FIP- Final Integrated Plan
  • FLEX DB- FLEX Design Basis (flood hazard)
  • FWST- Fire Water Storage Tank
  • LIP~ Local Intense Precipitation
  • MSA- Mitigating Strategies Assessment
  • MSFHI - Mitigating Strategies Flood Hazard Information
  • MSL - Mean Sea Level
  • NAVD88 - North American Vertical Datum of 1988
  • NTTF- Near-Term Task Force
  • PM - Preventative Maintenance
  • PMF- Probable Maximum Flood
  • PMP - Probable Maximum Precipitation
  • RWR - Raw Water Reservoir
  • SFP- Spent Fuel Pool
  • SOE - Sequence of Events
  • SSCs - Structures, systems and components
  • THMS -Targeted Hazard Mitigating Strategies
  • TSA- Time Sensitive Action
  • WD - Water Depth
9. References
1. NRC Letter, "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," dated March 12, 2012.
2. Pacific Gas and Electric Company (PG&E) Letter to NRC, "Final Response to Request for Information Pursuant to 10 CFR 50.54(f) Regarding Recommendation 2.1 Flooding,"

dated March 11, 2015.

3. NRC Letter, "Diablo Canyon Power Plant, Unit Nos. 1 and 2 - Interim Staff Response to Reevaluated Flood Hazards Submitted in Response to 10 CFR 50.54(f) Information Request- Flood-Causing Mechanism Reevaluation (CAC Nos. MF6039 and MF6040),"

dated March 30, 2016.

4. Nuclear Energy Institute (NEI), Report NEI 12-06 [Rev 2], "Diverse and Flexible Coping Strategies (FLEX) Implementation Guide," dated December 2015. *
5. U.S. Nuclear Regulatory Commission, JLD-ISG-2012-01, Revision 1, "Compliance with Order EA-12-049, Order Modifying Licenses with Regard to Requirements for Mitigation Strategies for Beyond-Design-Basis External Events, dated January 22, 2016.
6. Pacific Gas and Electric Company (PG&E) Letter to NRC, "Updated Response to Request for Information Pursuant to 10 CFR 50.54(f) Regarding Recommendation 2.1, Flooding," dated February 8, 2016.
7. NRC Memorandum, "Staff Requirements- COMSECY-14-0037 -Integration of Mitigating Strategies for Beyond-Design-Basis External Events and the Reevaluation of Flooding Hazards," dated March 30, 2015.
8. NRC Letter, "Supplemental Information Related to Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Flooding Hazard Reevaluations for Recommendation 2.1 of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident," dated March 1, 2013.
9. PG&E Letter to NRC, "Pacific Gas and Electric Company's Notification of Full Compliance with Commission Order Modifying Licenses with Regard to Mitigation Strategies for Beyond-Design-Basis External Events (Order Number EA-12-049) for Diablo Canyon Power Plant Units 1 and 2," dated July 28, 2016.
10. PG&E Calculation No.: 9000042232-000-00 (Revision 0), "Study Calculation: Diablo Canyon Local Intense Precipitation (LIP) Effect on Building Internals," dated March 22, 2017.
11. PG&E Calculation No.: 9000042281-000-00 (Revision 0), "DCPP Local Intense Precipitation (LIP) Analysis 2D Modeling," dated March 22, 2017.
12. PG&E Report, "Diablo Canyon Power Plant Beyond-Design-Basis Validation of the FLEX Time Sensitive Actions, Units 1 & 2," Revision 4, dated July 28, 2016.
13. NRC Letter, "Diablo Canyon Power Plant, Unit Nos. 1 and 2- Safety Evaluation Regarding Implementation of Mitigating Strategies and Reliable Spent Fuel Instrumentation Related to Orders EA-12-049 and EA-12-051," dated December 28, 2016.
14. PG&E Calculation No.: 9000041561-001-00 (Revision 1), "Study Calculation: Diablo Canyon Power Plant Local Intense Precipitation Analysis 2D Modeling," dated January 26, 2016.
15. PG&E Report, "Staging and Deployment Walkdown Report, Revision 2," dated October 30, 2015.
16. PG&E Drawing 522316, "Diablo Canyon Power Plant Hi-Resolution Topography Survey Power Block Area," Sheet 1, Revision 1, February 19, 2014.

Enclosure Attachment 1 PG&E Letter DCL-17-024 Regulatory Commitments Commitment # 1 PG&E will complete the necessary changes to plant procedures, conduct required validation, stage water-activated flood barriers, and update the FLEX Preventative Maintenance Program by the required compliance date of the forthcoming Regulation 10 CFR 50.155, "Mitigation of Beyond-Design-Basis Events."

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