ML24136A162

OEDO-24-00083 (E-Mail) 10 CFR 2.206 - Diablo Canyon Power Plant, Unit Nos. 1 and 2 Seismic - Initial Assessment

ML24136A162
Person / Time
Site:	Diablo Canyon
Issue date:	05/15/2024
From:	Perry Buckberg Plant Licensing Branch IV
To:	Curran D, Leary C, Templeton H Environmental Working Group, Friends of the Earth, Harmon, Curran, Harmon, Curran, Spielberg & Eisenberg, LLP
Lee S, 301-415-3158
References
EPID L-2024-CRS-0000 10 CFR 2.206
Download: ML24136A162 (1)

Text

From: Perry Buckberg To: dcurran@harmoncurran.com; htempleton@foe.org; cleary@ewg.org Cc: Natreon Jordan; Daniel King; Anthony Shelton; James Kim

Subject:

Diablo Canyon Seismic Core Damage 2.206 petition - Initial Assessment Date: Wednesday, May 15, 2024 7:31:00 AM

Diane Curran, Hallie Templeton, Caroline Leary,

The Petition Review Board (PRB) has completed its initial assessment of your March 4, 2024, submittal related to Diablo Canyon, Units 1 and 2 (Diablo Canyon) seismic core damage that was referred to the 10 CFR 2.206 Petition process on March 12, 2024 (ADAMS ML24072A529). Your petition requested that the U.S. Nuclear Regulatory Commission (NRC) exercise their supervisory authority to order the immediate closure of Diablo Canyon due to the unacceptable risk of a seismically induced severe accident. Your petition is supported by the expert declaration of Dr. Peter Bird.

Specific concerns listed in your petition include:

1. Thrust faulting is neglected by Pacific Gas & Electric Companys (PG&Es) 2012 Seismic Source Characterization (SSC) model, because the model assumes that a majority of large earthquakes affecting Diablo Canyon are strike-slip and disregards the significant contribution of thrust faulting earthquake sources under the Diablo Canyon site and the adjacent Irish Hills. In addition, PG&E did not use a hanging-wall term for the modeling of potential ground motions from the Los Osos and San Luis Bay thrust faults.

2. The magnitude 7.5 (moment magnitude) January 2024 earthquake centered in the Noto Peninsula (Japan), with an average slip of 2 meters on the fault, is analogous to future potential thrust mechanism earthquakes beneath Diablo Canyon. Based on the slip rate of the Irish Hills adjacent to Diablo Canyon and the slip of the Noto earthquake, large thrust fault earthquakes will occur, on average, every 715 years near the Diablo Canyon site.

3. Uplift rates for the Irish Hills should be several times higher than the rates used by PG&E in its SSC model in 2012.

4. Seismic core damage frequency (SCDF), estimated by PG&E in 2018 to be 3x10-5, should be 1.4x10-3 per year (about once every 715 years) based on this higher recurrence rate for thrust earthquakes.

On March 28, 2024 (ML24088A238), I informed you that the NRC staff had determined immediate closure of Diablo Canyon was not necessary and that the concerns raised in your petition regarding seismic hazards at Diablo Canyon have screened into the 2.206 Petition Process warranting evaluation by a Petition Review Board (PRB).

The PRBs initial assessment is that the concerns represented in your petition do not meet the Management Directive (MD) 8.11 (ML18296A043) acceptance criteria in Section III.C.1(b)(ii). Specifically, The issues raised have previously been the subject of a facility-specific or generic NRC staff review and the petition does not provide significant new information that the staff did not consider in a prior review. Based on this, the PRBs initial assessment is to not accept your petition for further review.

The PRB is providing the following responses related to each of your concerns listed above:

1. The licensees seismic models (ML15071A045) developed in response to NRCs 10 CFR 50.54(f) request include the potential for thrust faulting, as both the Los Osos and San Luis Bay thrust faults were evaluated in great detail and considered by PG&E to be primary fault sources in the models used for the hazard calculations. For both thrust faults, the ground motion model developed by PG&E includes a hanging wall term to incorporate the potential for higher ground motions. The NRC staff assessment (ML16341C057) of PG&Es 2015 seismic hazard reevaluation includes confirmatory calculations of the hazard from both the Los Osos and San Luis Bay thrust faults and concludes that the licensee adequately characterizes the seismic hazard for Diablo Canyon, including the potential for thrust faulting near the site.

2. The petition did not provide sufficient factual information to conclude that the 2024 Noto Peninsula earthquake can be used as an analogous thrust earthquake beneath Diablo Canyon with an associated slip of 2 meters for a magnitude 7.5 earthquake.

However, PG&E, based on the estimated length (70 kilometers [km]) and width (13 km) of the Los Osos fault and using the magnitude-area relation of Hanks and Bakun (2014), estimated a maximum moment magnitude of 7.0 for the Los Osos fault.

Similarly, PG&E modeled a maximum moment magnitude of 6.3 for the San Luis Bay fault based on its estimated length (15 km) and width (11 km). In addition to considering earthquakes on these two faults individually, PG&E also modeled several larger earthquake ruptures occurring on these two faults linked together with adjacent faults such as the Shoreline and Hosgri faults. The NRC staff assessment of PG&Es 2015 seismic hazard reevaluation concludes that the maximum magnitudes for the Los Osos and San Luis Bay faults are appropriate due to their estimated lengths and widths and that PG&Es hazard reevaluation adequately considered the potential for larger linked earthquake ruptures occurring on multiple adjacent faults.

3. The petitions postulated magnitude recurrence rate of 1.4x10-3/yr for large thrust fault earthquakes near Diablo Canyon, is based on the slip (2 m) from a single earthquake in Japan (2024 Noto earthquake) and an uplift rate for the Irish Hills (2.88 millimeters per year [mm/yr]) that is several times higher rather than the rates inferred from geologic field observations in the region surrounding Diablo Canyon. Based on geologic studies in the region, PG&E assumed an uplift rate for the Irish Hills that ranges from about 0.15 to 0.35 mm/yr and apportioned this rate to several scenario thrust earthquakes in the region. The PRB concludes that a long-term slip rate of 2.88 mm/yr for the Irish Hills is inconsistent with the slip rates inferred from geologic studies in the region. The NRC staff assessment of PG&Es 2015 seismic hazard reevaluation concludes that PG&E adequately characterized the potential for thrust fault earthquakes in the vicinity of the Diablo Canyon site.

4. The calculation of SCDF involves consideration of the seismic hazard curve and equipment fragility. Seismic hazard curves are developed based on the characterization of all potential seismic sources in the region, including their estimated fault slip rates. The PRB finds that it is inappropriate to estimate a new SCDF using modeled slip rates that are several times higher than those inferred from geologic field observations in the region surrounding Diablo Canyon. The NRCs assessment (ML18254A040) of PG&Es 2018 seismic probabilistic risk assessment concludes that PG&E adequately characterized the risk to the Diablo Canyon.

In conclusion, the NRC staff determined that the assessment (ML16341C057) of PG&Es 2015 seismic hazard reevaluation and the assessment (ML18254A040) of PG&Es 2018 seismic probabilistic risk assessment adequately characterized the seismic hazard and seismic risk for the Diablo Canyon.

I offer you the opportunity to clarify or supplement your petition in a virtual public meeting with the PRB. If you decide to take advantage of this opportunity, the meeting with the PRB would be conducted consistent with the format described in MD 8.11 Section III.F. The PRB will consider your statements and information presented at the meeting, along with the original petition, in making its final determination on whether to accept your petition for review. Please indicate whether you wish to have this public meeting by May 29, 2024.

If you have any questions regarding this e-mail, please feel free to contact me at Perry.Buckberg@nrc.gov.

Thanks, Perry Buckberg Senior Project Manager / Agency 2.206 Petition Coordinator U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation office: (301)415-1383 perry.buckberg@nrc.gov Mail StopO-8B1a, Washington, DC, 20555-0001