ML24008A211

Evaluation of Seismic Risk for V. C. Summer Under the Process for Ongoing Assessment of Natural Hazard Information (Poanhi) - January 25, 2024 (Slides)

ML24008A211
Person / Time
Site:	Summer
Issue date:	01/25/2024
From:	Ed Miller, Clifford Munson, Shilp Vasavada NRC/NRR/DEX, Plant Licensing Branch II, NRC/NRR/DRA
To:	Dominion Energy South Carolina
Williams S, NRR/DORL/LPLLPL
Shared Package
ML24008A223	List: ML24008A211 ML24008A218
References
Download: ML24008A211 (1)

Text

Evaluation of Seismic Risk for VC Summer Under the Process for Ongoing Assessment of Natural Hazard Information (POANHI)

January 25, 2024 DIVISION OF RISK ASSESSMENT - SHILP VASAVADA DIVISION OF ENGINEERING AND EXTERNAL HAZARDS - CLIFF MUNSON DIVISION OF OPERATING REACTOR LICENSING - ED MILLER

Purpose:

• Present to Dominion staff the methodologies and assumptions used in the NRC staffs evaluation of preliminary seismic risk estimate for VC Summer based on the site-specific POANHI hazard

• Objective:

• Provide Dominion staff with the option to offer additional information for the NRC staff to refine its analysis and support its decision 2

Key Messages

• The staffs evaluation has not identified an immediate safety concern; VC Summer continues to operate safely

• Site-specific POANHI seismic evaluation indicates a need for further refinements to the NRC staff evaluation

• NRC staffs review may benefit from additional plant-specific modeling details and insights that Dominion could provide 3

Central and Eastern U.S. Seismic Source Model (NUREG 2115)

ECC_AM New Madrid PEZ_N NMESE_N Summer Summer Summer MESE_N Charleston ECC_GC Repeating Large Seismotectonic Zones Mmax Zones Magnitude Earthquakes 4

POANHI Seismic Hazard

• Figure shows ground motion response spectra (GMRS) from NRC staffs recent evaluation (red) compared to previous evaluations resulting from Near Term Task Force Recommendation 2.1 (blue)

• NRC staffs recent evaluation developed used latest models

• NGA-East ground motion model

• Central and Eastern U.S. seismic source model (NUREG 2115) including update to seismicity catalog to remove dependent events and reservoir induced earthquakes in South Carolina

• Because Summer is a hard-rock site NRC staffs evaluation did not implement a site response analysis 5

POANHI Decision-Making Flowchart 6

Seismic Risk Using POANHI Hazard -

Approach

• Used latest information available to NRC staff to obtain plant-level seismic risk evaluation.

• Approach was consistent with approach discussed in public meetings.

• Dominion may be able to provide more realistic modeling assumptions

• Used average of 1, 5, 10, and 100 Hz results consistent with approach in flowchart and discussions in public meetings.

• NRC staff consideration of reasonable variations in plant parameters yielded similar results 7

Insights from Licensees SPRA

• The dominant initiator for SCDF (potential core damage) was seismically induced loss of offsite power.

• Dominant risk contributors for SCDF include:

• Relay chatter

• Operator action to continue auxiliary feedwater after battery depletion (if station loses all AC power).

• Operator action to manually start (safety systems)ESFAS after failure of automatic ESFAS.

• Licensees sensitivity analysis demonstrated reduction in SCDF from credit for operator actions to reset relays.

• No quantitative credit for FLEX equipment or actions in the SPRA.

8

NRC Staff Evaluation of Insights from Licensees SPRA

• Seismically induced loss of offsite power expected to remain dominant initiator for SCDF.

• Changes in dominant risk contributors are likely due to POANHI hazard in 1 - 10 Hertz (Hz) and high frequency (>33 Hz) range.

o Increase in 1 - 10 Hz range for updated POANHI hazard can increase relative importance of contributors besides loss of offsite power.

o Decrease in >33 Hz range for updated POANHI hazard can decrease relative importance of relay chatter failures.

• Impact of operator actions to reset relays and FLEX credit hazard on risk reduction is unknown based on information available to the NRC staff.

9

Path Forward

• NRC staff analysis and decision will benefit from improved realism that could be provided by the licensee

• Licensee has no response obligations, but NRC staff will consider any additional information provided 10