Seismic safety at Diablo Canyon

Diablo Canyon Updated Seismic Assessment was submitted in response to Senate Bill 846 on March 6, 2024, in an updated evaluation of the seismic hazard associated with the DCPP. This assessment was mandated by SB 846 to ensure that the plant's operations beyond 2025, particularly in the context of California's energy reliability, are based on the most current and thorough seismic data available. The assessment ensures that Diablo Canyon's infrastructure maintains adequate safety margins, fulfills legislative and regulatory requirements, and supports ongoing seismic planning and public transparency, reinforcing confidence in the plant's safe and reliable operation.

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Seismic Probabilistic Risk Assessment (SPRA) for Diablo Canyon Power Plant was a key milestone fulfilling the NRC's 10 CFR 50.54(f) request under NTTF Recommendation 2.1. Seismic. Using modern probabilistic methods and updated fault data, it quantified the plant's seismic risk and confirmed adequate safety margins. Accepted by the NRC in 2019, the SPRA closed the post-Fukushima seismic review, establishing Diablo Canyon's current seismic risk baseline and demonstrating compliance with national safety standards through a fully modernized, science-based evaluation.

In 2012, the U.S. Nuclear Regulatory Commission (NRC) issued a request for information pursuant to 10CFR50.54(f), requiring that all operating nuclear plants in the U.S. perform a site-specific probabilistic seismic hazard assessment (PSHA) and develop a ground motion response spectrum (GMRS). Licensees were required to re-evaluate the seismic hazard using present-day NRC regulatory criteria and guidance. For plants in the Western United States, including Diablo Canyon, the directive required that the site-specific PSHA be performed using the SSHAC Level 3 process to develop the seismic source characterization and ground motion characterization models. The following reports were developed to fulfill the requirement for an updated PSHA and GMRS under the SSHAC Level 3 process:

• Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 Study
  • Seismic Source Characterization (SSC)
  • SWUS Ground Motion Characterization (GMC)
• Seismic Hazard Re-evaluation Report
• 3D Velocity Model Report
• Development of Amplification Factors Report

Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 Study represents one of the most rigorous approaches to estimating seismic hazard for nuclear facilities. This report incorporated a broad range of expert opinions and evaluated accounting for uncertainties in fault behavior, earthquake recurrence, and ground motions. Two simultaneous SSHAC Level 3 studies were carried out for DCPP, one focused on Seismic Source Characterization (SSC) and the other on Ground Motion Characterization (GMC). Together, these studies provided a scientifically robust and defensible estimate of seismic hazard for the site.

Seismic Hazard Re-Evaluation Report was a formal update of the plant's earthquake hazard assessment, building on previous studies such as the SSHAC-3 study and earlier fault investigations. Its purpose was to integrate all recent geological, seismologic, and probabilistic analyses to confirm that Diablo Canyon's seismic design remained adequate.

3D Velocity Model Report presents the updated three-dimensional representation of the subsurface geologic structure and seismic wave velocities beneath and around the plant site.

Development of Amplification Factors Report is a project report that quantifies how local site conditions amplify seismic ground motions. It evaluates how the soil, sediment layers, and rock beneath and around the plant can increase the intensity of shaking during an earthquake.

CCCSIP provided a comprehensive regional assessment of faults affecting DCPP. PG&E performed these studies following the recommendation of the California Energy Commission (CEC) in a report issued in response to state legislation (Assembly Bill 1632, or AB 1632). AB 1632 (Blakeslee, Chapter 722, Statutes of 2006) directed the CEC to assess the potential vulnerability of California's largest baseload power generation facilities (1,700 megawatts or greater) to a major disruption due to a seismic event or plant aging.

Reports developed under the CCCSIP refined estimates of earthquake magnitudes, fault activity, and potential ground motions at the plant site by analyzing onshore and offshore geologic data. CCCSIP studies incorporating advanced geophysical and seismic methods strengthened the scientific basis for DCPP's seismic design, supported regulatory compliance, and reinforced public and NRC confidence in the plant's long-term earthquake safety.

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Point Buchon Report provided an updated analysis of the Hosgri-Point Buchon fault system offshore of Diablo Canyon. It offered detailed mapping and evaluation of fault geometry, activity, and earthquake potential, which was used to refine seismic hazard assessments for the plant. The report ensured that Diablo Canyon's seismic design remained adequate, supported regulatory compliance, and contributed to the ongoing evaluation of the plant's long-term earthquake safety.

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Shoreline Fault Zone Report provided the first comprehensive study of the newly identified Shoreline Fault. PG&E evaluated the fault's characteristics and earthquake potential, concluding that its maximum credible earthquake would not exceed the plant's seismic design basis.

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The 2011 Nodal Zland Earthquake Database compiled detailed earthquake focal mechanism data for the central California coast near Diablo Canyon. By analyzing these "nodal planes," PG&E and seismologists gained a clearer understanding of regional fault orientations, slip styles, and stress patterns.

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2010 Shoreline Fault Zone Progress Report was PG&E's first assessment of the newly discovered Shoreline Fault located offshore from Diablo Canyon Power Plant. Submitted to the NRC, the report presented preliminary studies of the fault's size, orientation, and seismic potential, concluding that it did not pose a greater than the plant's existing seismic design basis. This progress report was important for maintaining regulatory transparency and laid the groundwork for more detailed seismic evaluations in subsequent years.

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In SER report, the NRC confirmed that PG&E's seismic studies and engineering evaluations were thorough and that Diablo Canyon's design provided sufficient safety margins against earthquakes. By endorsing the LTSP findings, the SER fulfilled a key licensing requirement, strengthened public and regulatory confidence in the plant's safety, and established the regulatory basis for all future seismic evaluations at plant.

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The DCPP Long-Term Seismic Program (LTSP) was PG&E's multi-year scientific and engineering effort to evaluate and address earthquake hazards at the Diablo Canyon Power Plant (DCPP) on the central California coast. The work was performed to reevaluate the seismic design bases for the Diablo Canyon Power Plant, as specified in the Unit 1 Full-Power Operating License, DPR-80, Condition 2.C.(7), issued by the Nuclear Regulatory Commission (NRC) on November 2, 1984. The Program improved understanding of:

• The neotectonic setting and earthquake potential of the south-central coast of California
• Site ground motion characteristics and plant responses
• Plant capabilities and seismic margins.

The results of the Program clearly demonstrated the adequacy of the seismic design and seismic margins for the plant's structures, systems, and components.

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