Home Nuclear Reactors Operating Reactors Reactor Safety Information Topics Process for the Ongoing Assessment of Natural Hazard Information (POANHI)

Seismic Activities Related to the Process for the Ongoing Assessment of Natural Hazard Information (POANHI)

Seismic Activities Before POANHI

In 2005, the NRC began looking broadly at potential changes in earthquake hazards in the Central and Eastern United States (CEUS). Through this effort, the NRC, the Department of Energy and the Electric Power Research Institute developed an updated model for CEUS earthquake sources. Following the events in March 2011 at Japan's Fukushima Dai-ichi plant, the NRC considered how best to incorporate lessons from Fukushima into consideration of U.S. seismic issues. The NRC established the Near‑Term Task Force (NTTF) to conduct a systematic and methodical review of NRC processes and regulations and determine whether the agency should make additional improvements to its regulatory system. The NRC staff recommended a set of actions to clarify and strengthen the regulatory framework for protection against natural hazards. In particular, NTTF Recommendation 2.1 (NTTF R2.1) instructed the NRC staff to issue requests for information to all power plant licensees pursuant to Title 10 of the Code of Federal Regulations 50.54(f) (“50.54(f) letter”). Enclosure 1 to the 50.54(f) letter requested that addressees reevaluate the seismic hazards at their sites, using present day NRC requirements and guidance to perform a probabilistic seismic hazard analysis (PSHA) and develop a site-specific ground motion response spectrum (GMRS).

The plants compared the new hazard curve to their original safe shutdown earthquake; if the new hazard was greater, additional actions were required. These actions could include examining if any immediate actions were necessary, determining seismic effects on spent fuel pools, and/or a detailed seismic risk evaluation. The seismic probabilistic risk assessments (SPRA), where appropriate, were completed, and the NRC staff reviewed the evaluations and any identified corrective actions. More information on the NRC's post-Fukushima seismic re-evaluation work is available on the NRC website.

NUREG/KM-0017, "Seismic Hazard Evaluations for U.S. Nuclear Power Plants: Near-Term Task Force Recommendation 2.1 Results" represents the current best knowledge and practices for characterizing the site-specific seismic hazards for each nuclear power plant (NPP) in the United States. This NUREG/KM builds on the hazard assessments performed by the US nuclear power plant licensees in response to the letter issued by the NRC under Title 10 of the Code of Federal Regulations (10 CFR) 50.54(f) [50.54(f) letter] and associated information requests (NRC, 2012) following the March 11, 2011, Great East Japan Earthquake and tsunami and resulting accident at the Fukushima Dai-ichi NPP. The U.S. Nuclear Regulatory Commission (NRC) staff are using the hazard characterizations provided in this NUREG/KM as a benchmark to evaluate new data, models, and methods consistent with the POANHI framework.

The Next Generation Attenuation for Central and Eastern North America (NGA-East) ground motion model (GMM) was a multi-disciplinary research project to develop a new ground motion characterization model for central and eastern North America. This ground motion characterization (GMC) model includes a set of new ground motion prediction equations and their associated weights in the logic-trees for use in a probabilistic seismic hazard analysis (PSHA). Consistent with the CEUS-SSC model development, NGA-East was conducted using the SSHAC Level 3 guidelines in NUREG-2117 and NUREG/CR-6372. The staff evaluated NGA-East and documented the results in RIL-2020-011.

RIL 2021-15, "Documentation Report for SSHAC Level 2: Site Response" documents work sponsored by the NRC that applied the SSHAC process for systematically identifying and propagating epistemic uncertainties in the site response analysis as has been previously applied to the seismic source characterization (SSC) and GMC analyses. The SSHAC process has been consistently applied to the SSC and GMC components of PSHAs performed for critical facilities for more than 15 years; RIL 2021-15 documents the application of the SSHAC process as applied to site response analysis.

Current Seismic Work

In December 2019, the NRC staff shared with stakeholders its plans for seismic hazard characterization updates using NGA-East Ground Motion Models (GMM) and CEUS seismic source models, with the latest geologic site profiles and the revised site response analysis approaches, consistent with the POANHI framework. This work includes the development of site-specific hazards reports by the NRC staff and a series of public meetings with stakeholders to facilitate sharing of information and a common understanding of the significance of new information.

Steps for Current Seismic Work

NGA-East GMM Seismic Screening Under POANHI

The NRC’s screening process (Step 2) is shown in the flow chart below. The NRC compares the GMRS associated with the NGA-East GMM to the plant’s safe shutdown earthquake (SSE) between the important frequency range of 1 to 10 Hz. If the new GMRS exceeds the plant SSE at 1, 2.5, 5, or 10 Hz, then the new GMRS is compared to the NTTF R2.1 GMRS for the same 4 frequency values between 1 and 10 Hz. If the new GMRS is more than 10 percent higher than the NTTF R2.1 GMRS, then the NRC staff perform a risk screening. To perform the risk screen, the NRC staff computes the new seismic core damage frequency (SCDF) at 1, 5, 10 and 100 Hz using an estimate of the plant capacity combined with the new seismic control point hazard curves. Previous estimates of the seismic core damage frequency using the NTTF R2.1 seismic hazard curves are then used to determine the change in seismic core damage frequency for these four frequency values. If the average change or delta seismic core damage frequency value for the four frequencies is less than 1E-05, then the screening process will be considered complete, and the NRC staff will contact the licensee and issue the hazard report and cover letter. If the average seismic core damage frequency for the four frequencies is greater than 1E-05, then the NRC staff will perform a further risk analysis.

Flow Chart for Seismic Screening Under POANHI

Stakeholder Interactions

In addition to the public meetings documented in the table below, members of the public and external stakeholders can submit any comments or feedback to the staff by emailing POANHI.Seismic@nrc.gov. As of March 2023, the NRC received two comments. The first comment was dispositioned during the public meeting on November 29, 2022. Comments will also be summarized in the POANHI annual report.

Task	Milestone	Status
Public Meeting to introduce the POANHI process	05/17/2022	Complete
Public Meeting to discuss NGA-East screening approach	11/29/2022	Complete
Public Meeting to provide an update on seismic work related to NGA-East	09/15/2023	Complete

Site-Specific Staff Results

The staff is producing site-specific evaluation reports (Step 3). In Step 5, these reports are sent directly to the licensee and made publicly available in the Agencywide Documents Access and Management System (ADAMS). The estimated calendar year (CY) issuance schedule and links to ADAMS for the completed site-specific evaluations are provided below.

Site	Issuance Date	Site	Issuance Date
Vogtle	May 15, 2023	Robinson	November 1, 2023
Sequoyah	July 31, 2023	Browns Ferry	July 31, 2023
Watts Bar	July 31, 2023	North Anna	September 5, 2023
Summer	3rd Qtr 2024 (estimated)	Peach Bottom	January 17, 2024
DC Cook	2nd Qtr 2024 (estimated)	Dresden	3rd Qtr 2024 (estimated)
Oconee	2nd Qtr 2024 (estimated)	Beaver Valley	3rd Qtr 2024 (estimated)
Callaway	4th Qtr 2024 (estimated)

Note: The following sites are not located in the CEUS and are therefore excluded from this analysis: Columbia, Diablo Canyon, and Palo Verde.