ML24008A211
| 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: |
| 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
4 Repeating Large Magnitude Earthquakes Summer Charleston Seismotectonic Zones Mmax Zones Central and Eastern U.S. Seismic Source Model (NUREG 2115)
Summer Summer ECC_AM PEZ_N ECC_GC MESE_N NMESE_N New Madrid
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).
- Licensees sensitivity analysis demonstrated reduction in SCDF from credit for operator actions to reset relays.
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
- 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 Path Forward