Ana Earthquake Spectra Seismic Damage Indicating Parameters at Nuclear Power Plants Affected by the 2011 Great East Japan...
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The Professional Journal of the Earthquake Engineering Research Institute PREPRINT This preprint is a PDF of a manuscript that has been accepted for publication in Earthquake Spectra. It is the final version that was uploaded and approved by the author(s). While the paper has been through the usual rigorous peer review process for the Journal, it has not been copyedited, nor have the figures and tables been modified for final publication. Please also note that the paper may refer to online Appendices that are not yet available. We have posted this preliminary version of the manuscript online in the interest of making the scientific findings available for distribution and citation as quickly as possible following acceptance. However, readers should be aware that the final, published version will look different from this version and may also have some differences in content. The DOI for this manuscript and the correct format for citing the paper are given at the top of the online (html) abstract. Once the final, published version of this paper is posted online, it will replace the preliminary version at the specified DOI. Seismic Damage Indicating Parameters at Nuclear Power Plants Affected by the 2011 Great East Japan Earthquake and Plant Shutdown Criteria Fred F. Grant,a) M.EERI, Yuchuan Tang,a) M.EERI, Greg S. Hardy,a) and Robert Kassawara b) M.EERI M.EERI, A rational and quantitative shutdown criterion is required for a nuclear power plant in response to seismic shaking to determine whether the plant must be shut down for inspection. The shutdown criterion is generally defined in terms of seismic damage indicating parameters. This paper presents seismic damage indicating parameters of the recorded free-field and in-structure motions at the Onagawa, Fukushima Daiichi, Fukushima Daini, and Tokai Daini nuclear power plants during the 2011 Great East Japan earthquake. The observed seismic damage indicating parameters largely exceed the current U.S. and Japan shutdown thresholds for nuclear power plants,; while minimal damage due to ground shaking was observed at the four Japanese plants. These observations indicate a potential for raising the current threshold without introducing any significant additiona l seismic risk to nuclear power plants. The insights presented in this paper can be used to guide regulation and industry methods for quickly evaluating the damage potential of future earthquakes that affect nuclear power plants. With some adjustment, a similar methodology and criterion could be applied to conventio na l structures and lifeline infrastructure. INTRODUCTION In the 1970s and 1980s, at least two U.S. nuclear power plants (NPPs), the Virgil C. Summer NPP in South Carolina and the Perry NPP in Ohio, were subjected to ground motions caused by nearby low-magnitude earthquakes (Whorton 1988; Monroe and Stevenson 1986). The recorded motions at the plant sites exceeded the operating basis earthquake (OBE) design response spectrum at high frequencies above 10 Hz (EPRI 1988). Although there was no a) b) Simpson Gumpertz & Heger Inc., Newport Beach, CA 92660 Electric Power Research Institute, 3420 Hillview Ave., Palo Alto, CA 94304 damage in either case, extensive analyses were required to verify that the safety-related equipment had not been degraded. These events prompted Electric Power Research Institute (EPRI) to initiate research to develop a rational criterion for determining when the OBE design limit has been exceeded at an NPP and, hence, when actions are required in accordance with U.S. Nuclear Regulatory Commission (USNRC) Reg. 10 CFR 100, Appendix S (USNRC 1973). The OBE exceedance is concerned only with critical safety-related structures and equipment, and not to other components whose plant damage state is not critical to safety. Since the OBE definition was generally based on free-field motion, the EPRI research focused on various methods of using the recorded motion obtained from site free-field recording instruments to ascertain if a potentially damaging earthquake had occurred. In 1988, EPRI reviewed various seismic damage indicating parameters (DIPs) derivable from an instrument recording (such as peak ground acceleration (PGA), Arias intensity, etc.). A new instrumental parameter, denoted as the cumulative absolute velocity (CAV) was developed in EPRI NP-5930 (EPRI 1988), which concluded CAV was the best instrume nta l parameter for determining the damage threshold of earthquake ground motions. Consequently, a two-level criterion for determining OBE exceedance, containing both a response spectrum check and a CAV check, was developed in EPRI NP-5930. The computational algorithm for CAV was further refined and standardized in EPRI TR-100082 (EPRI 1991) to make the CAV value representative of strong ground shaking rather than coda waves (small amplitudes that can continue for a long time after the strong shaking). Using the standardized CAV, denoted as CAVST D, EPRI TR-100082 proposed 0.16 g-sec as the CAVST D threshold of potential damage for NPPs. This CAVST D threshold was adopted in USNRC Regulatory Guide (RG) 1.166 (USNRC 1997), together with a response spectrum check, for determining OBE exceedance at an operating NPP based on free-field ground motion. The underlying basis for the CAVST D threshold criterion was a large volume of measured empirical data that verified that below the threshold, damage would not occur to structures and equipment of “good design and construction” (USNRC 1997). Campbell and Bozorgnia (2011) proposed a variant of CAVST D, denoted as CAVDP , to incorporate both the CAVST D and the response spectrum checks of the USNRC RG 1.166 shutdown criterion. They developed an empirical prediction equation for CAV DP . Furthermore, Campbell and Bozorgnia (2012) investigated the relationship between CAVDP and instrume nta l seismic intensity measures to correlate CAVDP with the qualitative descriptions of damage associated with macro-seismic intensity scales. Several NPPs in Japan have experienced earthquake motions with CAVST D values on the order of ten to twenty times the USNRC CAVST D threshold and have remained functio na l (EPRI 2009), which tends to suggest that the threshold is conservative. The conservatism was previously investigated by Campbell and Bozorgnia (2012) by correlating CAVST D with macroseismic intensity scales, from which building damage level can be inferred. The successful performance of the Japanese NPPs subjected to large-CAVST D events and the analysis by Campbell and Bozorgnia suggest that the threshold could be increased while still maintaining a reasonable and defendable level of conservatism relative to earthquakes that actually have the potential to damage NPP structures, systems, and components (SSCs). However, the very limited seismic experience data at NPPs may be insufficient to provide a robust base for raising the CAVST D threshold level. The seismic data collected at four Japanese NPPs in the 2011 Great East Japan (GEJ) earthquake make a valuable addition to the scarce seismic database of NPPs (EPRI 2009). This paper presents insights on the seismic DIPs observed at the Fukushima Daiichi, Fukushima Daini, Onagawa, and Tokai Daini NPPs during the 2011 GEJ earthquake. First, the earthquake and its impacts on the affected NPPs are introduced. Then seismic DIPs of the acceleration recordings at the plant sites are calculated and analyzed. In addition, this paper investigates the soil amplification and soil-structure interaction effects on CAVST D by comparing the free-field ground surface motions to the basemat recordings. Based on the seismic experience from the NPPs subjected to the 2011 GEJ earthquake, the conservatism of the current CAVST D threshold and the suitability of using free-field CAVST D as OBE exceedance threshold are discussed. This study may be used to guide regulation and industry methods for quickly evaluating the damage potential of an earthquake for NPPs. NUCLEAR POWER PLANTS AFFECTED BY THE 2011 GREAT EAST JAPAN EARTHQUAKE On 11 March 2011 at 2:46 p.m. local time, a Mw 9.0 earthquake occurred off the Pacific coast of Northeastern Japan, rupturing an area approximately 450 km (280 miles) long and 150 km (93 miles) wide (Government of Japan 2011). The earthquake caused a massive tsunami that inundated over 561 km2 (217 square miles) of land along the northeastern Japan coastline (Government of Japan 2011). The epicenter of this earthquake was off the coast of Miyagi Prefecture, as shown in Figure 1, with a focal depth of approximately 24 km (15 miles) (IAEA 2011). The earthquake directly affected four Japanese NPPs on the northeastern coast of Japan as shown in Figure 1: Fukushima Daiichi NPP (1F), Fukushima Daini NPP (2F), Onagawa NPP (O), and Tokai No.2 NPP (T2). Based on the surface projection of the fault model by Yokota et al. (2011), the four plants have similar Joyner-Boore distance of about 20 km (12 miles), which is the closet distance to the surface projection shown in Figure 1. The earthquake resulted in automatic shutdown of eleven power-generating units that were operating at the time of the earthquake (Table 1). The four NPPs were subjected to strong ground shaking in the 2011 GEJ earthquake. The seismic intensities at the plant sites are estimated at the level of 5-Upper to 6-Upper in Japan Meteorological Agency (JMA) intensity scale (Government of Japan 2011). In terms of the Modified Mercalli Intensity (MMI) scale, the seismic intensity was estimated by Internatio na l Atomic Energy Agency (IAEA) as VII−VIII at the four NPPs according to a USGS ShakeCast Report (USGS 2011). The Reactor Building basemat peak accelerations exceeded the origina l design basis for 1F Units 1–5 and 2F Units 3–4 (IAEA 2011), but not for T2 NPP (JAPC 2011). (No information has been made available to the authors regarding the original design basis of Onagawa NPP.) Figure 1. Epicenter of the 2011 GEJ earthquake and affected NPPs. (Figure reprinted from GRS 2011 with surface projection of the fault model by Yokota et al. 2011 overlaid schematically.) Table 1. Status of nuclear power generating units affected by the 2011 GEJ earthquake (IAEA 2011) Unit Before Earthquake After Earthquake After Tsunami 1F1 1F2 1F3 1F4 1F5 1F6 2F1 2F2 2F3 2F4 O1 O2 O3 T2 Operating Operating Operating Outage Outage Outage Operating Operating Operating Operating Operating Reactor Start Operating Operating Automatic scram1 Automatic scram Automatic scram Cold shutdown3 Cold shutdown Cold shutdown Automatic scram Automatic scram Automatic scram Automatic scram Automatic scram Automatic scram Automatic scram Automatic scram Loss of cooling2 Loss of cooling Loss of cooling Loss of SFP cooling4 Cold shutdown Cold shutdown Cold shutdown Cold shutdown Cold shutdown Cold shutdown Cold shutdown Cold shutdown Cold shutdown Cold shutdown 1 ”Scram” is used to designate the shutdown of the nuclear reactor fission process by insertion of control rods 2 Loss of cooling function for reactor fuels 3 A reactor coolant system at atmospheric pressure and at a temperature below 200 degrees Fahrenheit following a reactor cooldown 4 Loss of cooling function to external heat exchangers for the Spent Fuel Pool (SFP) Despite the strong ground shaking, the safety-related SSCs at the four NPPs remained functional until the arrival of the tsunami wave 30 to 45 minutes after the main shock. The structural elements of the Onagawa NPP were remarkably undamaged, according to a review by an IAEA expert team (IAEA 2012). Following the earthquake, all the safety-related systems at Fukushima Daiichi NPP operated as designed until the tsunami arrived (ANS 2012, Government of Japan 2011). The earthquake shaking, to the extent that has been confir med, was not responsible for the damage to safety-related SSCs according to the Tokyo Electric Power Company (TEPCO) (National Diet of Japan 2012). At Fukushima Daini and Tokai No. 2 plants, the observed ground motions did not exceed the design basis, and no shaking damage to safety-related SSCs is reported in the 2011 IAEA report (IAEA 2011). STRONG MOTIONS RECORDED AT PLANT SITES AND SEISMIC DAMAGE INDICATING PARAMETERS Seismometers were present and operational at the Fukushima Daiichi and Daini, Onagawa, and Tokai No. 2 plant sites at the time of the 2011 GEJ earthquake. For example, Figure 2 shows some of the seismometers at the Fukushima Daiichi site. Acceleration recordings of the main shock of the 2011 GEJ earthquake were retrieved by plant operators for most of the seismometers. The digitized recordings have been released by Japan Association for Earthquake Engineers (JAEE 2011a; JAEE 2011b; JAEE 2011c). Figure 2. Seismometers Installed at Fukushima Daiichi NPP, 2011 GEJ Earthquake: Dense Surface Array (Left) and Unit 6 Reactor Building (right). (Figure reprinted from JAEE 2011c.) During the main shock of the 2011 GEJ earthquake, 141 seismometers at the four plant sites recorded acceleration time histories. While many seismometers recorded accelerations in all the three orthogonal directions (i.e., E-W, N-S, and U-D), some did not. At each plant site, the seismometers are distributed at various locations (e.g., free-field ground surface, boreholes away from buildings, different elevations in Reactor Buildings (RBs) and Turbine Build ings (TBs)). The seismometers and their recordings are categorized herein based on location for each NPP to investigate the spatial variation of the motions. Table 2 lists the number of recorded horizontal components by location category and by plant. Although vertical recordings are also available, this study focuses on horizontal components for two reasons: (1) horizontal components are found to generally have greater CAVST D than vertical component at the same location, though exceptions exist; (2) to avoid mixing the DIPs of horizontal and vertical components in statistical analysis. Table 2. Number of Horizontal Motion Components Retrieved Location Category Free-field at grade Borehole (RB basemat level) Other depths in borehole RB basemat 1F NPP 42 4 18 16 2F NPP 6 5 7 12 O NPP 2 2 4 10 T2 NPP 2 2 4 10 RB mid-height RB operating deck Other locations in RB Turbine Building Total 2 4 2 4 92 10 4 4 24 72 6 10 6 0 40 2 2 2 0 24 The acceleration recordings of the main shock of the 2011 GEJ earthquake exhibit three notable features. First, the acceleration amplitude is very high (e.g., PGA 1.1g in the E-W direction at the surface array seismometer C02 at Fukushima Daiichi NPP). Second, the shaking lasted quite long, as measured by effective duration (e.g., 70.8 sec for the E-W recording by the seismometer G1 in a borehole at Fukushima Daini NPP). The effective duration is defined as the time interval in which the cumulative energy of an acceleration record increases from 5% to 75% of the final value. Third, there is strong spectral content at high frequencies (above 10 Hz), compared to the recordings at NPPs under other historical Japan earthquakes (e.g., the Kashiwazaki–Kariwa NPP under the 2007 Niigata Chuetsu- Oki earthquake). For each acceleration recording, this study computed 5% damped response spectrum, Fourier amplitude spectrum, power spectral density, the Husid curve of cumulative energy, as well as various DIPs. The DIPs considered include CAVST D, Root Mean Square (RMS) acceleration over the effective duration, zero period acceleration (ZPA), Arias intensity, JMA intensity, and effective duration. Assuming a lognormal distribution for some DIPs, Figure 3 shows the estimated median and ±1 natural logarithmic standard deviation (±σ) by location category (refer to Table 2) and plant based on the recorded horizontal components. In this figure, each colored marker represents the median value, while the black bar shows the ±σ interval. The estimation may not be credible for the categories with only two or four recordings. Figure 3 provides a straightforward comparison of the DIPs across various motion categories as well as across plant sites. It is apparent from Figure 3 that the RB basemat motion was amplified to the operating deck. The DIPs of the operating deck recordings have large variation due to complexities in the structural responses. In general, the DIPs were amplified from depth in borehole to ground surface, which reflects site amplification since a soil layer exists between the RB basemat and ground surface with a thickness of ~15 m at Tokai and less than 10 m at the other NPPs (JAEE 2011a,b,c). The RBs at Onagawa NPP are founded on rock with shear wave velocity Vs of 1000–1500 m/s (JAEE 2011a), and the RBs at the other three plants are founded on soft rock with Vs of ~500 m/s (JAEE 2011b,c). Therefore, soil–structure interaction is not significa nt enough to cause big difference between the DIPs of the RB basemat recordings and those of the borehole recordings at the RB basemat level. Figure 3. DIPs of Horizontal Components by Location Category and by Plant (Median and ±σ Range). EXCEEDANCE OF JAPANESE NPP AUTOMATIC SHUTDOWN SETTINGS Table 1 shows that the 2011 GEJ earthquake triggered automatic shutdown of eleven operating power generating units. According to JAPC (2011), TEPCO (2011) and TohokuEPCO (2011), the automatic shutdown settings for the power generating units were horizonta l and vertical acceleration limits at various floors in the RBs. The Japanese shutdown settings are given in the unit Gal (i.e., cm/s2 ). The recorded floor accelerations during the 2011 earthquake exceeded all the shutdown settings at the four NPPs, except that the recorded horizontal peak floor acceleration (225 Gal) at the Tokai Unit 2 RB basemat is less than the corresponding shutdown setting of 250 Gal. For each unit at each NPP, Figure 4 shows the recorded peak floor acceleration (shaded bar) and the corresponding shutdown setting (hollow bar) for the largest exceedance among several shutdown limits in one RB. For instance, the largest exceedance occurred in the vertical acceleration of the second basement floor for Fukushima Daini Units 1, 2, and 4 RBs while in the horizontal acceleration of the second floor for Unit 3 RB. Figure 4. Recorded Peak Floor Accelerations (Shaded Bars) Exceed Japanese Automatic Shutdown Settings (Hollow Bars) with Values in Unit Gal or cm/s2 . EXCEEDANCE OF CURRENT USNRC REGULATORY SHUTDOWN THRESHOLD The USNRC criterion for determining exceedance of the OBE includes a CAVST D threshold and a response spectrum check (USNRC 1997) based on recorded free-field ground motions. As introduced before, the CAVST D threshold is 0.16 g-sec, which can be triggered by any one of the three directional components recorded at a seismometer. In the response spectrum check, the OBE response spectrum is exceeded if any one of the three components (two horizonta l and one vertical) of the 5% damped response spectra is larger than: (1) The corresponding design response spectral acceleration (OBE spectrum if used in the design, otherwise 1/3 of the safe shutdown earthquake ground motion (SSE) spectrum) or 0.2g, whichever is greater, for frequencies between 2 and 10 Hz, or (2) The corresponding design response spectral velocity (OBE spectrum if used in the design, otherwise 1/3 of the SSE spectrum) or a spectral velocity of 6 in/sec (15.24 cm/sec), whichever is greater, for frequencies between 1 and 2 Hz. If the response spectrum check and the CAVST D threshold are exceeded, then the OBE exceedance is triggered and plant shutdown is required. Campbell and Bozorgnia (2011) proposed a variant of CAVST D, denoted as CAVDP , to incorporate both the CAVST D and the response spectrum check of the USNRC RG 1.166 shutdown criterion. Following their definition, CAVDP is equal to the largest CAVST D among the three directional components if the USNRC OBE exceedance criterion is met; otherwise, CAVDP is assigned zero. Because their study is intended to be generic, they used the conservative minimum threshold values of 0.20g (spectral acceleration) and 15.34 cm/sec (spectral velocity) in the OBE exceedance criterion in the definition of CAV DP . To check against the USNRC shutdown criterion, only the free-field recordings at grade in Table 2 are considered below. The response spectrum check is performed with the generic threshold of 0.2g for spectral acceleration due to the unavailability of plant-specific OBE spectra. All free-field surface ground motions recorded at the four NPPs exceeded the 0.2g threshold between 2 and 10 Hz, by a factor of at least 5. The maximum CAVST D is selected among the three directional components from each seismometer. It is found that the maximum CAVST D exceeds the 0.16 g-sec threshold for every set of the free-field surface recordings. Following Campbell and Bozorgnia (2011), a number of raw data of CAVDP are calculated for each of the four Japanese NPPs. The exact count and the value range of the raw data are tabulated by plant in the “Raw Data” panel of Table 3. Based on the raw data, a median and natural logarithmic standard deviation (σ) of CAVDP are estimated and presented in the “Lognormal Distribution” panel of Table 3. Table 3 indicates that the CAVDP values of the free-field recordings at the four plant sites significantly exceed the USNRC CAVST D threshold of 0.16 g-sec. Furthermore, the observed free-field CAVDP values go beyond 0.77g-sec, at which minor shaking damage to the El Centro Steam Power Plant was observed after the 1979 Imperial Valley earthquake (EPRI 1991). The 0.77g-sec is the lowest CAVST D level at which damage was observed among the commercial and industrial sites investigated in the EPRI CAV studies (EPRI 1991). A conservatism factor is calculated in the right most panel of Table 3 by dividing the estimated median CAVDP by either the 0.16 g-sec threshold or 0.77 g-sec. The median CAVDP of the free-field surface recordings are approximately 35 to 50 times the regulatory threshold of 0.16g-sec. This is almost ten times the conservatism margin recognized in EPRI TR-100082 (EPRI 1991). The current regulatory CAVST D threshold is conservatively based on ground motions measured at commercial and industrial facilities and characterized by an MMI VII or greater (EPRI 1988). MMI VII was regarded as the threshold intensity of potentially damaging ground motion for “buildings of good design and construction.” This terminology was taken from the U.S. Geological Survey (USGS) version of the MMI scale and defined as “buildings which have reasonable earthquake protection, but have not necessarily been designed by an engineer ” (EPRI 1988). Yet the MMI scale describes effects on the general built environment, which is known to be less seismically rugged than NPPs; therefore the use of MMI VII as a surrogate for damage to NPPs is clearly conservative. Table 3. Observed CAVDP of Free-field Recordings at Grade (unit: g-sec) Plant 1F 2F O T2 Raw Data Count 21 3 1 1 Min. 5.09 4.20 8.33 5.58 Lognormal Distribution Max. 12.15 7.09 8.33 5.58 Median 7.91 5.36 8.33 5.58 σ 1.11 1.12 ─ ─ Conservatism Factor Median/0.16 49 34 52 35 Median/0.77 10 7 11 7 RELATIONSHIP BETWEEN CAV AND JMA INTENSITY Campbell and Bozorgnia (2012) developed a relationship between CAVDP and the JMA instrumental seismic intensities, IJMA, in order to correlate CAVDP with the qualitative descriptions of damage in the corresponding JMA intensity scales. The median relations hip between CAVDP and IJMA is shown in Figure 5 as the solid line along with the ±σ range (dashed lines), which were obtained by Campbell and Bozorgnia through regression analysis of the “PEER-NGA-PSV” database of earthquake records. The present study calculates the CAVDP and IJMA of the free-field surface motions recorded at the four NPPs. The calculated data are plotted in Figure 5 as the scattered markers. The observed CAVDP from the 2011 GEJ earthquake have a significant positive deviation (3σ to 5σ) from the median predicted by Campbell and Bozorgnia (2012). Although Campbell and Bozorgnia (2012) found their relationship between CAVDP and IJMA may underestimate CAVDP at large earthquake magnitudes by about 25%, the underestimation observed here is on the order of 80%. Campbell and Bozorgnia (2012) explained that these biases are caused by differences in the way that CAVDP and IJMA scale with the physical parameters (e.g., magnitude) of the earthquakes. They also discussed why the biased trends in the prediction residuals were not corrected, but how the biases could be removed if desired. Figure 5. Relationship between CAVDP and IJMA : Observations vs. Prediction Equation by Campbell and Bozorgnia (2012). CONCLUSIONS Based on 263 historical earthquake recordings, EPRI developed the CAVST D threshold of 0.16g-sec to establish the onset of damage to conventional buildings of good design and construction. The USNRC adopted this conservative CAVST D threshold, along with an OBE response spectrum check, to establish a NPP shutdown criterion that does not require shutdown for earthquake events non-damaging to NPP SSCs. This paper presents insights on the seismic DIPs (e.g., CAVST D) observed at the Fukushima Daiichi, Fukushima Daini, Onagawa, and Tokai Daini NPPs during the 2011 GEJ earthquake. Free-field surface horizontal motions at Onagawa NPP have CAVST D values of 7.9 g-sec to 8.3 g-sec with no damage to safety-related nuclear SSCs. At Fukushima Daiichi NPP, the recorded free-field surface horizontal motions have CAVST D values ranging from 5.0 g-sec to 12.2 gsec. The free-field horizontal CAVST D range is 3.9−7.1 g-sec and 5.4−5.6 g-sec for Fukushima Daini and Tokai No. 2, respectively. The CAVST D of the RB basemat motions in the 2011 GEJ earthquake is as large as 4.7 g-sec (horizontal) and 4.1 g-sec (vertical), with no earthquakeinduced damage to safety-related nuclear SSCs. The data presented in Table 3 suggest that the current regulatory CAVST D threshold may be conservative by more than an order of magnitude (factor of ten), which indicates the potential for raising the threshold without introducing any significant additional seismic risk to the U.S. nuclear fleet. Minimizing the conservatism in the shutdown criterion can prevent costly and unnecessary plant shutdowns due to non-damaging ground motions from nearby small-magnitude earthquakes or distant large-magnitude earthquakes. After an appropriate evaluation, the USNRC CAVST D threshold could also be used for NPPs outside of the United States. However, it is noted that the current regulatory threshold for NPPs response to earthquake s considers only ground shaking. The devastating consequence at Fukushima Daichi NPP of the tsunami caused by the 2011 GEJ earthquake indicates that future research on NPP shutdown criterion should also take into account earthquake-induced secondary hazards, if applicable. Although this paper focuses on damage indicating parameters for nuclear power plants, a similar methodology and criterion could also be used to rapidly assess whether conventio na l structures and lifeline infrastructure have been damaged after an earthquake to aid in emergency response and loss assessment activities. For instance, the damage potential indicated by CAV was investigated by Cabañas et al. (1997) for conventional structures and by Fahjan et al. (2011) for urban areas. ACKNOWLEDGEMENT The authors are grateful to Japan Association for Earthquake Engineers for providing the seismic recording data of the 2011 Great East Japan Earthquake at the Onagawa, Fukushima Daiichi, Fukushima Daini, and Tokai Daini nuclear power plants. We would also like to thank Dr. Kenneth W. Campbell and Dr. Julian J. Bommer for their constructive comments. REFERENCES American Nuclear Society (ANS), 2012. Fukushima Daiichi: ANS Committee Report, LaGrange Park, IL, available at http://fukushima.ans.org/report/Fukushima_report.pdf (last accessed 26 April 2016). Cabanas, L., B, B., M, H., 1997. An approach to the measurement of the potential structural damage of earthquake ground motions, Earthquake Engineering and Structural Dynamics, 26:79–92. Campbell, K.W., Bozorgnia, Y., 2011. Predictive equations for the standardized version of cumulative absolute velocity as adapted for use in the shutdown of U.S. nuclear power plants, Nuclear Engineering and Design, 241: 2558–2569. 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