ML23214A194
| ML23214A194 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 09/05/2023 |
| From: | NRC/NRR/DORL/LPL2-2 |
| To: | Virginia Electric & Power Co (VEPCO) |
| References | |
| Download: ML23214A194 (1) | |
Text
North Anna Seismic Hazard Report
Overview This report provides the NRC staffs updated seis mic hazard curves and response spectra for the North Anna Nuclear Plant (North Anna) site that are based on the implementation of (1) a new seismic ground motion model for the central and eastern United States (CEUS) and (2) recent advances in site response analysis. The NRC staffs updated hazard curves and site amplification factors are included in an appendix to this report.
Background
In response to the March 11, 2011, Great East Japan Earthquake and tsunami, which triggered an accident at the Fukushima Dai-ichi nuclear power plant, the U.S. Nuclear Regulatory Commission (NRC) established the Near-Term Ta sk Force (NTTF) to conduct a systematic and methodical review of NRC processes and regulat ions and determine whether the agency should make additional improvements to its regulator y system. In SECY-11-0093, Near-Term Report and Recommendations for Agency Actions Following the Events in Japan, dated July 12, 2011 (NRC, 2011), 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 reactor licensees pursuant to Title 10 of the Code of Federal Regulations 50.54(f) (the 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). To assist licensees responding to the 50.54(f) request, the Nuclear Energy Institute developed Electric Power Research Institute (EPRI) Report 1025287, Seismic Evaluation Guidance: Screening, Prioritization, and Implementation Details (SPID) for the Resolution of Fukushima NTTF Recommendation 2.1 Seismic, dated November 27, 2012 (EPRI, 2012). Recipients of the 50.54(f) letter committed to following the SPID to develop seismic hazard and screening reports (SHSRs). By December 2017, the NRC staff had finished assessing the SHSRs for all operating U.S. nuclear power plants.
Under the process for the ongoing assessment of natural hazards information (POANHI),
described in SECY-16-0144, Proposed Resolution of Remaining Tier 2 and 3 Recommendations Resulting from the Fukushima Dai-ichi Accident, dated December 26, 2016 (NRC, 2016), the NRC staff continuously seeks out and integrates new natural hazards information for operating plants in the United States. The Office of Nuclear Reactor Regulations Office Instruction LIC-208, Process for the Ongoing Assessment of Natural Hazards Information, issued November 2019 (NRC, 2019), provides guidance to the staff on how to collect, integrate, and evaluate new information for consideration in its regulatory decision-making. This report presents the NRC st affs latest understanding of seismic hazards at the North Anna site following the POANHI framework.
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The North Anna site is located in Virginia adjacent to Lake Anna within the Piedmont physiographic province and is founded on competent metamorphic rock (gneiss and schist) of Paleozoic age.
Motivation After evaluating the SHSR submittals, the NRC staff captured in NUREG/KM-0017, Seismic Hazards Evaluations for U.S. Nuclear Power Plants: Near-Term Task Force Recommendation 2.1 Results, issued December 2021 (Munson et al., 2021), the information used to develop the GMRS at each of the U.S. nuclear power plants. This includes a compilation and synthesis of (1) information provided by licensees in their SHSRs, (2) information collected by the NRC staff during its reviews of the SHSRs, and (3) information subsequently collected by the NRC staff from the scientific and engineering literature pertaining to several of the nuclear power plant site s. In addition, NUREG/KM-0017 includes updated approaches and relationships, relative to those recommended by the SPID, that the NRC staff used to perform its analyses.
After the development of NUREG/KM-0017, a new Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 ground motion model (GMM) for Eastern North America called NGA-East was published by Goulet et al. (2018). In addition, the NRC staff also participated in a SSHAC Level 2 study, documented in Research Information Le tter (RIL) 2021--15, Documentation Report for SSHAC Level 2: Site Response, issued November 2021 (Rodriguez-Marek et al., 2021). This SSHAC Level 2 study implemented the SSHAC approach to performing site response analyses (SRAs). The SSHAC process, described most recently in NUREG-2213, Updated Implementation Guidelines for SSHAC Hazard Studies, issued October 2018 (Ake et al., 2018),
provides a structured and logical framework for the systematic evaluation of alternative data, models, and methods. This seismic hazard report for the North Anna site incorporates the NGA-East GMM in place of the EPRI (2013) GMM and lessons learned from the SSHAC Level 2 SRA study (RIL 2021-15) into a PSHA to develop updated seismic hazard curves and a GMRS for the site.
Methods Reference Rock Hazard For the reference rock PSHA, the NRC staff used the distributed seismicity zones (DSZs) from the SSHAC Level 3 Central and Eastern United States Seismic Source Characterization for Nuclear Facilities (CEUS-SSC) model in NUREG-2115, Central and Eastern United States Seismic Source Characterization for Nuclear Facilities, issued January 2012 (NRC, 2012).
Specifically, the NRC staff selected the DSZs that are located within 500 kilometers of the site.
For this reevaluation, the NRC staff used the SSHAC Level 2 update to the CEUS-SSC seismicity catalog and recurrence parameters (Gatlin, 2015), which primarily impact the DSZs that encompass Monticello Reservoir and Lake Keowee in South Carolina as well as the 1886 Charleston earthquake sequence. In addition, the NRC staff selected the RLME sources that are within 1,000 kilometers of the site. To develop the reference rock seismic hazard curves for the site, the NRC staff used the NGA-East GMM (2018) to compute the median and logarithmic standard deviation of the spectral accelerations. Because the NGA-East GMM implements the 3
rupture distance parameter, the NRC staff devel oped virtual rupture planes for each of the distributed source zones surrounding the site. For each virtual rupture, the NRC staff used the CEUS-SSC hazard input document (NRC, 2012) to specify the size of the rupture plane and the orientation of the rupture plane in terms of the strike and dip angles, dip direction, and rupture type (e.g., reverse and strike slip). In contrast, to develop the hazard curves for NUREG/KM-0017, the NRC staff used point source approximations for the CEUS-SSC and EPRI GMM (EPRI, 2013) combination.
Figure 1 shows the distribution of the virtual ruptures for one of the four alternative CEUS-SSC seismotectonic DSZ configurations along with the resulting 10-Hertz (Hz) mean hazard curves developed using the NGA-East GMM. In particular, Figure 1 shows the distribution of the surface projection of the updip segments of the virtual rupture planes for each of the four seismotectonic DSZs within 500 kilometers of the site. As expected, the Extended Continental Crust Midcontinent Craton Atlantic Margin (ECC-AM) source zone, which surrounds the site, is the largest contributor to the 10 Hz reference rock mean hazard curves at the 10 -4 annual frequency of exceedance (AFE) level. Similarly, Figure 2 shows the distribution of the virtual ruptures for one of the three alternative CEUS-SSC maximum-magnitude DSZ configurations along with the resulting 10 Hz mean hazard curves developed using the NGA-East GMM. The Mesozoic-and-Younger ExtensionNarrow Configuration (MESE-N) source zone, which surrounds the site, is the largest contributor to the 10 Hz reference rock mean hazard curves at the 10-4 AFE level. Figure 3 shows the RLME sources within 1,000 kilometers of the site, and their contribution to the 1 Hz reference rock mean hazard, from using the NGA-East GMM. The New Madrid Fault System and Charleson RLME sources are the largest contributors to the 1 Hz reference rock mean hazard curves at the 10 -4 AFE level. Figure 4 shows the contribution from all of the DSZs relative to the RLMEs, as well as the total mean hazard for the 1 and 10 Hz mean reference rock hazard curves, from using the NGA-East GMM. For both the 1 and 10 Hz mean reference rock hazard curves, the DSZ sources provide the largest contribution at the 10 -4 AFE level. Finally, Figure 5 shows the mean 1,000-, 10,000-, and 100,000-year return period mean reference rock uniform hazard response spectra (UHRS) for the North Anna site from using the EPRI GMM (blue) and the NGA-East GMM (red). For this reevaluation, the NRC staff used the NGA-East single station standard deviation and for the comparison shown in Figure 5, the NRC staff used the EPRI GMM ergodic standard deviation. As shown in Figure 5, the spectral accelerations from using the NGA-East GMM are similar or slightly higher than those from using the EPRI GMM, across the entire frequency spectrum.
Site Response Analysis SRAs, which are used to develop site adjustment (or amplification) factors, depend on several factors, including the site strata (material type, stiffness, and thickness) and their response to dynamic loading. Because this information is site specific, the ability to accurately model the site response depends on the quantity and quality of site-specific geologic and geotechnical data available, and on the interpretation and use of these data to develop input models for assessing amplification (or deamp lification) of ground motions. The resulting are assessed for a wide range of input ground motions as part of understanding the changes in the soil and rock response as input ground motions increase.
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The NRC staff followed the site response approach described in RIL 2021-15, which uses a logic tree for systematically identifying and propagating epistemic uncertainties in the SRA. As described in RIL 2021-15, to produce a truly probabilistic estimate of the seismic hazard at the control point elevation, it is necessary to estimate both the epistemic uncertainties and the aleatory variability of the soil and or rock dynamic response, and to propagate these through the SRA and the calculation of the site hazard curves.
Site Exploration. As described in the NTTF R2.1 SHSR submitted by the Virginia Electric and Power Company (VEPCO; Heacock, 2014) and su mmarized in section 2.3.9 of NUREG/KM-0017, the field investigations for North Anna consisted of a number of borings through the upper rock beneath the site. Geophysical investigations for the proposed Unit 3 (Dominion Energy, Inc., 2013) include borehole geophysical measurements (suspension logging) in five boreholes to estimate shear wave velocities (, with the deepest measurement at a depth of about 75 meters beneath the site.
Basecase Profiles. VEPCO stated in its NTTF R2.1 SHSR (Heacock, 2014) that the North Anna site consists of a thin veneer of saprolitic so ils (clay, clayey silt, and sand) overlying weathered rock grading into hard metamorphic igneous rock (gneiss and schist) from the Cambrian age Chopawamsic Belt. The North Anna reactor buildings are founded on sound rock, which VEPCO described as Zone III-IV moderately to slightly weathered rock. For its site response analysis, VEPCO used the top of the weathered rock at a depth of 1 meter, which corresponds to an elevation of 82 meters mean sea level, as the control point elevation for the North Anna site. For its SHSR, VEPCO developed a basecase profile that extends to a depth of 41 meters below the control point elevation. The entire profile consists of 13 meters of weathered rock, 17 meters of moderately weathered rock, and 11 meters of slightly weathered rock with of 1,295 meters/second (m/s), about 1,600 m/s, and 2,682 m/s, respectively. Based on the rock type (hard metamorphic igneous rock), the profile reaches the NGA-East reference rock of 3,000 m/s at a fairly shallow depth beneath the plant. VEPCO stated (Heacock, 2014) that the multiple datasets collected through its geophysics program for the proposed Unit 3 support the modeling of a single basecase profile rather than also developing lower and upper basecase profiles.
As VEPCO conducted multiple recent geophysical fiel d investigations to characterize the rock strata beneath the North Anna site, the NRC staff used VEPCOs layer thicknesses and for its best-estimate basecase profile.
Consistent with the NRC staffs effort to capture a wider range of uncertainty (RIL 2021-15), the NRC staff developed lower and upper basecase profiles by multiplying its best-estimate basecase profile by scale factors of 0.82 and 1.21, respectively, which corresponds to an epistemic logarithmic standard deviation of 0.15. The weights for the lower, best-estimate, and upper basecase profiles are 0.3, 0.4, and 0.3, respectively. Figure 6 shows the three lower, best-estimate, and upper basecase profiles used by the NRC staff. The lower epistemic value used by the NRC staff to determine the lower and upper basecase profiles is due to the staffs conclusion that the lithology of the strata beneath North Anna site likely has a low range in.
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Site Kappa. To estimate the site kappa ( ), which captures the overall attenuation (i.e., intrinsic and scattering attenuation) of the geologic profile, the NRC staff used the four - models from Campbell (2009), where is the effective quality factor of shear waves, which captures both the frequency-independent component of intrinsic attenuation and small-scale scattering.
For each of the four - models, the NRC staff estimated a for each layer in the basecase profiles, then used the estimated,, and layer thickness to determine a for each layer. Summing these values for each layer and adding the reference value of 6 milliseconds (msec) provides an estimate of the total. The NRC staff used a weight of 0.25 for each of the four - models. Assuming a lognormal distribution for with a logarithmic standard deviation of 0.2 from Xu et al. ( 2020), the NRC staff developed a nine-point discrete distribution. This results in 45 values and associated weights for each of the basecase profiles, which the NRC staff then resampled using the approach from Miller and Rice (1983) to reduce the distribution to three representative values and associated weights. These three values and weights, which are listed in Table 1, range from 6 msec to 7 msec for the three basecase profiles. The limited range in for the North Anna site is due to the shallow and high-velocity rock basecase profiles.
Nonlinear Dynamic Properties. For the equivalent linear (EQL) SRA, nonlinearity is incorporated using strain-compatible site properties (i.e., shear modulus and damping ratio) for each layer. The strain-compatible properties model both the shear modulus reduction and the increased damping that are expected as the intensity of shaking increases. To model the nonlinear response within the uppermost 8-meter-thick lower velocity layer in the profile, the NRC staff used the EPRI rock modulus reduction and damping (MRD) curves (EPRI, 1993). The NRC staff used a weight of 0.5 for the EPRI rock MRD curves and a weight of 0.50 to capture the possibility that the weathered rock behaves linearly under seismic loading.
Table 2 provides the layer depths, lithologies,, unit weights, and dynamic properties for the NRC staffs three basecase profiles. It is importa nt to note that the NRC staff has adjusted the critical damping ratio values in each of the layers of the profiles, which are treated as having a linear response, so that each profile as a whole has the appropriate value. Figure 7, which shows tornado plots for the reference rock peak ground acceleration (PGA) value of 0.76 g, shows the site response logic tree nodes that contribute to the variance of the. Each tornado plot in Figure 7 is associated with one of the four oscillator frequencies of 1, 5, 10, and 100 Hz. For each of the four frequencies, the epistemic uncertainty in the basecase contributes the most to the variance in the.
Input Motions. Input motions used for the SRA were generated as outcrop motions at the reference rock horizon, located at the bottom of the basecase profiles. The NRC staff used random vibration theory to generate the input motions after first developing an input Fourier amplitude spectrum (FAS) using seismological source theory (i.e., single-corner frequency Brune source spectrum). To develop the FAS, the NRC staff used the source and regional attenuation parameters recommended in the SPID for Eastern North American rock sites and then used random vibration theory to develop corresponding 5 percent damped acceleration 6
response spectra. The NRC staff developed 12 input FAS assuming a magnitude ( ) of 6.5 and 12 different source-to-site distances, as recommended in the SPID.
Analysis Methodology. To develop for the North Anna site, the NRC staff used traditional EQL analysis and the recently developed kappa-corrected EQL analysis, which adjusts the high-frequency control point (i.e., top of profile) FAS from the EQL SRA to be consistent with the target value. In particular, the NRC staff used the kappa-corrected EQL analysis methodology (Xu and Rathje, 2021) with the modification in which the EQL control point FAS remains unmodified below a specified transition frequency, and then a slope equal to the target value is imposed at frequencies above the transition frequency (RIL 2021-15). To capture the uncertainty in this transition frequency value, the NRC staff selected three frequencies for which the FAS amplitude equals 5 percent, 11 percent, and 17 percent of its peak value, with weights of 0.2, 0.6, and 0.2, respectively.
To capture the spatial variability in site properties across the site, the NRC staff generated randomized profiles around each of the basecase profiles using the Toro (1995) model, which quantifies the aleatory variability through a depth-dependent standard deviation of the natural log of the velocities. The logarithmic st andard deviation values used by the NRC staff for the North Anna site were based on site-specific data and are shown in Table 2. In addition to randomizing the profiles, the NRC staff also randomized the MRD curves following the logit function approach used in the SPID and described in RIL 2021-15.
For each terminal branch of the site response logic tree, the NRC staff developed 60 randomized profiles and then determined the by dividing the computed control point response spectrum by the outcrop response spectrum for the reference condition. Next, the NRC staff computed a median and logarit hmic standard deviation for the, using the 60 from the randomized profiles, for each terminal branch of the logic tree. To facilitate implementing the medians and logarithmic standard deviations into the PSHA seismic hazard integral, the NRC staff reduced the median s from the over 200 logic tree terminal branches to seven discrete fractiles and weights using the resampling procedure outlined by Miller and Rice (1983). As recommended by Rodriguez-Marek et al. (2021), to ensure that estimates of the SRA capture enough epistemic uncertainty in the median, the NRC staff implemented a minimum logarithmic standard deviation value of 0.15, which causes the seven median fractiles to spread apart if necessary.
Finally, because the logarithmic standard deviation for each spectral frequency does not vary significantly across the terminal branches of the logic tree, the NRC staff used a single mean value for each frequency. In addition, to avoid double-counting the aleatory variability already captured by the GMM, the NRC staff adjusted the logarithmic standard deviation to include only the portion of the standard deviation associated with the nonlinear site response.
Figure 8 shows the seven median values (top) and the average logarithmic standard deviation (bottom) as a function of input reference rock spectral acceleration for the 1 and 10 Hz spectral frequencies. As shown in Figure 8, the median range from about 0.8 to 2.0 and remain constant with higher input spectral accelerations. The lower half of Figure 8 shows both 7
the total and the nonlinear values of the logarithmic standard deviation, the latter of which are implemented into the PSHA hazard integral. Figure 9 shows the seven median values versus frequency at the 10 -4 AFE spectral acceleration value for each of the 23 NGA-East GMM spectral frequencies as well for PGA, which is plotted at 200 Hz. Overall, the North Anna site produces a flat across the entire frequency range with a small amplification around 10 Hz.
Control Point Hazard and Ground Motion Response Spectra The NRC staff calculated the mean control poi nt hazard for the North Anna site using Convolution Approach 3 from NUREG/CR-6728, Technical Basis for Revision of Regulatory Guidance on Design Ground Motions: Hazard-and Risk-Consistent Ground Motion Spectra Guidelines, issued October 2001 (McGuire et al., 2001), which convolves the predetermined mean reference condition hazard with the. For each NGA-East GMM spectral frequency, the NRC staff convolved the mean reference condition hazard curve with the seven to determine the final mean control point hazard. Using the mean control point hazard curves, the NRC staff then determined the 10 -4 and 10-5 UHRS in order to calculate the final GMRS for the site, which are provided in Table 3. Figure 10 shows this final GMRS (red curve) compared to the GMRS (black curve) developed for NUREG/KM-0017, and the GMRS (blue curve) in VEPCOs SHSR (Heacock, 2014) and VEPCOs se ismic probabilistic risk assessment (SPRA; Stoddard, 2018). The years in the legend for Figure 10 show when the GMRS were developed either by VEPCO or the NRC staff. As shown in Figure 10, the final GMRS from this study is similar to the previous GMRS for the frequencies out to about 25 Hz and then is moderately lower than the previous GMRS above 25 Hz. The previous GMRS, which were developed using the EPRI GMM, only have high-frequency spectral acceleration values at 25 and 100 Hz. As such the spectral accelerations between 25 and 100 Hz are based on generic estimates of spectral shapes for the CEUS. In contrast, the NGA-East GMM, provides several high-frequency values between 25 and 100 Hz (25.0, 33.3, 40.0, 50.0, 66.7, and 100.0 Hz) such that the GMRS for this study does not rely on generic estimates of high-frequency CEUS spectral shapes.
Data Tables Appendix A provides the data tables for the North Anna site. Tables A-1, A-2, and A-3 give the reference rock mean hazard curves for 23 spectral frequencies ranging from 0.1 to 100 Hz and for PGA. Tables A-4 through A-27 give the medians and logarithmic standard deviations for the 23 spectral frequencies and for PGA. Tables A-28, A-29, and A-30 give the control point hazard mean hazard curves for the 23 spectral frequencies and for PGA.
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Table 1 Site Kappa ( ) Values for Each Basecase Profile
Profile Kappa Distribution Lower Case Base Case Upper Case (sec) Weight (sec) Weight (sec) Weight 0.0066 0.2476 0.0065 0.2476 0.0064 0.2476 0.0067 0.5505 0.0066 0.5505 0.0065 0.5505 0.0068 0.2476 0.0067 0.2476 0.0066 0.2476
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Figure 5 1,000-, 10,000-, and 100,000-year return period mean reference rock UHRS for CEUS-SSC and EPRI GMM (blue curves) and CEUS-SSC and NGA-East GMM (red curves)
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Figure 6 Shear wave velocity (VS) basecase profiles for North Anna; thick horizontal black line indicates the reference rock horizon; best estimate basecase profile shown as solid blue line; lower and upper range basecase profiles shown as dotted red and purple lines, respectively
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Figure 10 GMRS for the North Anna site 23
Appendix AData Tables