ML25253A439
| ML25253A439 | |
| Person / Time | |
|---|---|
| Issue date: | 09/10/2025 |
| From: | NRC/NRR/DANU |
| To: | Nuclear Energy Institute |
| References | |
| Download: ML25253A439 (1) | |
Text
Submittal Date: July 3, 2024 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: SELECTION OF A SEISMIC SCENARIO FOR AN EPZ BOUNDARY DETERMINATION Letter: ML24187A095, WP: ML24187A096 Purpose of the White Paper: The Nuclear Energy Institute (NEI) stated that the purpose of this white paper is to develop a technology-inclusive methodology for selecting a seismic scenario to be used in an analysis for determining the boundary of a plume exposure pathway emergency planning zone (EPZ) for advanced reactors.
FEEDBACK AND OBSERVATIONS The feedback and observations on this white paper are preliminary and subject to change. The feedback and observations are not regulatory findings on any specific licensing matter and are not official agency positions.
This feedback is to support discussion during a public meeting with NEI to discuss their white paper Selection of a Seismic Scenario for an EPZ Boundary Determination, scheduled for September 17, 2:00 - 3:00 pm ET.
Feedback and Observations on Specific Questions
- 1. Selection of the Beyond Design Basis Earthquake (BDBE) for scenario development:
The proposed analysis is a deterministic screening analysis, based on selecting an initiating seismic event (2 x ground motion response spectrum (GMRS)) and then assessing whether the structures, systems and components (SSCs) fail or succeed. The GMRS peak ground acceleration (PGA) is based on Regulatory Guide (RG) 1.208, revision 0, A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion, and is equivalent to the design basis ground motion for seismic design category (SDC) 5 of standard ASCE 43, Seismic Design Criteria for Structures, Systems, and Components in Nuclear Facilities. NEI should consider adding clarification to clearly state that the GMRS is developed using the current version of RG 1.208.
The annual exceedance frequencies for 2 x GMRS ground motion levels for the sites studied in the white paper are on the order of 1.0E-5/year. This level of ground motion is considered an appropriate level for pass-fail determination coupled with an appropriate capacity criterion. For many of the safety significant SSCs in new and advanced reactors, this ground motion is considered appropriate because the 2 x GMRS will be larger (perhaps significantly larger) than the design basis ground motions.
Additionally, the white paper proposed the use of a 1.0g PGA cut-off for establishing the initiating seismic event. NEI should consider the following to determine if it is appropriate to use a 1.0g PGA cut-off:
- 1. Because the fragility determination accounts for a spectral shape and the natural frequencies of an SSC, the arbitrary cut-off of PGA at 1.0g PGA may lead to unconservative screening of the SSCs, meaning screening out of SSCs that should be retained in the analysis.
- 2. Although most of the contiguous U.S. has modest to low seismicity, there are regions in both the central and eastern U.S. and western U.S. where seismic hazards are high and
where the 1.0g PGA cut-off would be unconservative. Past arguments have proposed the 1.0g PGA cut-off because of an assumption that an earthquake with a PGA of greater than 1.0g would also destroy the needed infrastructure for emergency response.
However, at higher seismicity sites, the reviewers suggest that the community infrastructure needed to support emergency response will be commensurate with these higher hazard levels.
- 3. Using such a cut-off may fail to provide site-specific insights into the consequences of seismic events intended to inform the site-specific emergency plan.
- 4. NEI should consider providing a statement for applicants to justify the 1.0g PGA on a site-specific basis.
- 2. Failure or Success of SSCs:
The staff note that the use of C10% capacity implies that there is a 90% probability of survival of a given SSC at the reference earthquake level, that is 2 x GMRS in this case. Given the high level of the screening criterion, staff consider the use of C10% capacity acceptable for developing initial scenarios to inform the EPZ determination. However, the scenario needs to be carefully examined for the cliff-edge effects as discussed later in this enclosure to ensure that there is no significant increase in consequences arising from a small step increase in the earthquake severity.
NEI should further consider the pass-fail criterion for random non-seismic failures and operator actions based on the discussion below:
For the advanced reactor designs at issue here, especially the more passive designs and the designs that rely much less on human actions, this may or may not be a generalizable property of the design. However, if this proposed pass-fail criterion were used for todays large light water reactors, the analysis would assume that most of those failures would be successes, and the corresponding sequences would be dropped from the analysis.
NEI could consider in this methodology retaining these human errors and non-seismic failures in the first-stage analysis, study which sequences contain those types of events, and use case-by-case judgment as to whether the sequence should be retained or should be screened out, based on the role of these failures and the importance of the sequence.
- 3. Seismic scenario for EPZ and dose/consequence calculation:
With regard to the seismic scenario and consequence calculation, Section 11, Consequence Calculation, of the white paper states, in part, that:
The dose consequence calculation consists of two parts, a source term and an atmospheric dispersion and transport model. The source term would be a plant design-specific calculation assuming the plant damage state defined by the seismic scenario.
Additionally, Section 13, Summary and Conclusions, states that:
The framework applies a multiplier of 2 to the site seismic design basis (the GMRS) so as to enable a straightforward and early establishment of the EPZ sizing earthquake; applicants will not have to wait for completion of a site-specific SPRA.
NEI should consider providing specific guidance on how a plant damage state is determined for a scenario that may lead to radiological consequences given that an SPRA may not be available. NEI should consider providing an example to clearly demonstrate how a plant damage state is determined from the available information, how the seismic successes and failures are incorporated in the sequences, and guidance on demonstrating that the resulting scenario follows regulatory guidance, meets regulations, and is appropriate for EPZ sizing.
Lastly, NEI should consider describing actions to be taken to update or confirm the seismic scenario for an EPZ once a site-specific SPRA becomes available, for example actions to be taken by an applicant for an operating license with a previous construction permit.
- 4. Sensitivity Check:
Section 10, Sensitivity Analysis to Check for Cliff Edge Effect, of the white paper proposes that a sensitivity study should be performed, in which the analysis adds additional failures of SSCs whose C10% capacity is within 10% higher than the EPZ earthquake (i.e., 2 x GMRS), to guard against what NEI describes as potential cliff-edge effects. Staff notes that the cliff-edge effects, in the sense that a slight increase in the hazard severity would result in a large increase in consequences, depends on the nature of a scenario or sequence. The use of a mean fragility curve to determine the cliff-edge effects may mask the potential cliff-edge effects because of a large u.
Staff note that NEI should consider expanding the white papers discussion on the cliff-edge effects including the need for sensitivity analyses, and that an example for calculating cliff-edge effects could prove useful. For instance, a possible approach would be to perform a close examination for potential cliff-edge effects in addition to the use of the proposed criteria. Such an approach could involve examination of the cutsets of various scenarios or sequences for singletons and dominant contributors that may lead to cliff-edge effects. The governing failure modes could also be examined, such as a brittle anchorage failure mode or a failure mode associated with a threshold criterion (e.g., buckling). Finally, sensitivity studies could also be performed by failing an SSC when there is a possibility of cliff-edge effects.