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  Transactions of the 17  th  International Conference on  Structural Mechanics in Reactor Technology (  SMiRT 17)  Prague, Czech Republic, August 17 –22, 2003    Paper # K03-2 1 Scenario Earthquakes for Korean Nuclear Power Plant Site Considering Active Faults In-Kil Choi 1) , Young-Sun Choun 1)  , Jeong-Moon Seo 1)   1) Korea Atomic Energy Research Institute, Daejeon, Korea   ABSTRACT Scenario earthquakes have been used for the design and safety assessment of nuclear power plant structures and equipments. Scenario earthquakes can be obtained from the deaggregation of Probabilistic Seismic Hazard Analysis (PSHA) results. In this study, the probability based scenario earthquakes were developed for the Korean nuclear  power plant site using the PSHA data. The magnitude and distance bins of the scenario earthquakes for the example Korean nuclear power plant site were M6.4, 9km and M6.2, 13km. The ground response spectra for the scenario earthquakes were developed using the attenuation equations adopted in PSHA (Probabilistic Seismic Hazard Analysis). To incorporate the near-fault earthquake effects, the ground response spectra were modified by the correction factor. KEY WORDS : scenario earthquakes, probabilistic seismic hazard analysis, hazard de-aggregation, control earthquakes, near-fault, ground response spectrum INTRODUCTION The seismic analysis and design have been performed based on a design basis earthquake. The design basis earthquake ground motion is generally specified as the design response spectrum. However the seismic safety of a nuclear power plant can not be secured by considering only the design basis earthquake, since the seismological situation of the nuclear power plant site is changed during the development of geosciences. A survey on some of the Quaternary fault segments near the Korean nuclear power plants is ongoing [1]. It is likely that these faults would be identified as active ones. If the faults are confirmed as active ones, it will be necessary to reevaluate the seismic safety of nuclear power plants located near the fault. This study is to develop the scenario earthquakes for the reevaluation of seismic safety of the nuclear power plant near the active fault. The probabilistic seismic hazard analysis for most of the Korean NPP sites have been completed. The hazard consistent earthquake scenario is developed as the probability based earthquake scenario using the existing results of probabilistic seismic hazard analysis. The scenario earthquake is specified in terms of the earthquake magnitude, M, and its distance, R, from the site under consideration. The probability based scenario earthquake is developed by the de-aggregation of the probabilistic seismic hazard analysis results according to the procedures of the US NRC R.G. 1.165 [2]. The spectral shape for the scenario earthquake is developed using the attenuation equations adopted in PSHA. The near-fault ground motion effect is incorporated into the response spectra, since the potential active fault is located near the nuclear power plant site. Near-fault ground motions have caused much damage in the vicinity of seismic sources during recent earthquakes. This is due to the pulse-type ground motion, which has a large amount of input energy. Finally, this study proposes the scenario earthquake for the nuclear power plant site and corresponding response spectra considering the potential active fault effect. SCENARIO EARTHQUAKE DEVELOPMENT METHODS Scenario Earthquakes   Tow typical method, probabilistic seismic hazard analysis and deterministic seismic hazard analysis, are generally used to define the scenario earthquakes. The purpose of the seismic hazard analysis is to evaluate the annual  probability of exceedance of various earthquake sizes at a given site, and to develop the spectral shapes of the motion from these earthquakes. This is called a probabilistic seismic hazard analysis to emphasize that its results are intrinsically probabilistic in nature. The seismic hazard of a nuclear power plant site is generally described by a series of seismic hazard curves, that is, a plot of the probability of exceedance vs. the ground motion intensity, such as peak ground acceleration, velocity or spectral acceleration. PSHA is very useful to define the scenario earthquakes, since it can determine the annual probability of exceedance for a ground motion intensity parameter [3]. The concept of the probability based scenario earthquake srcinates from McGuire studies [4]. The probability    2  based scenario earthquakes can be obtained from the de-aggregation of the PSHA results. It is intended to obtain  particular sets of earthquake source magnitude and distance for the specified probability level. Two typical methods for defining the probability based scenario earthquakes are generally used. The first one is the method developed by U.S. NRC. PSHA can provide important information to define the safe shutdown earthquakes (SSE) in the recent revision of U.S. NRC Regulatory Guide 1.165 [2]. The scenario earthquakes are called controlling earthquakes in the Regulatory Guide. The second one was developed by Japan Atomic Energy Research Institute [5,6] base on the Ishikawa and Kameda study [7]. The two procedures mentioned above were adopted to define the scenario earthquakes for the example site by Takada et al. [8]. This study shows that both methods generate the similar results. But the authors point out the methodological difference between them. The U.S. NRC Regulatory Guide 1.165 procedure can not identify the earthquake source location of the scenario earthquakes. And this procedure uses coarser bins for the earthquake magnitude and distances, and all results including earthquake source regions and faults are mixed up in the bins. The Ishikawa’s procedure [7] has the advantage that it produces the source contribution factors that is an effective indicator for identifying which earthquake sources and/or which fault(s) are most influential, and one or more scenario earthquakes can be determined. Finally, it is concluded that the U.S. NRC Regulatory Guide 1.165 procedure gives a global view of the scenario earthquakes, while the Ishikawa’s procedure provides a more precise view of the scenario earthquakes. US NRC Regulatory Guide 1.165 Procedure This procedure is srcinally an approach for determining the controlling earthquakes. In this study, the scenario earthquakes for a Korean nuclear power plant site were developed using this procedure. This procedure is based on a de-aggregation of the probabilistic seismic hazard in terms of earthquake magnitudes and distances. This section gives a simple description of the procedure [2]. Step 1 : Perform a site-specific PSHA. The hazard assessment (mean, median, 85 th  percentile, and 15 th  percentile) should be performed at 1, 2.5, 5, 10, and 25Hz, and the peak ground acceleration. A lower bound magnitude of 5.0 is recommended. Step 2 : Using the reference probability, determine the ground motion levels for the spectral accelerations at 1, 2.5, 5, and 10Hz from the total median hazard obtained in Step 1. Calculate the average of the ground motion level for the 1 and 2.5 Hz and the 5 and 10 Hz spectral acceleration pairs. Step 3 : Perform a complete probabilistic seismic hazard analysis for each of the magnitude-distance bins. Step 4 : From the de-aggregated results of Step 3, the median annual probability of exceeding the ground motion levels of Step 2 are determined for each magnitude-distance bin. Using the median annual probability, the fractional contribution of each magnitude and distance bin to the total hazard for the average of 1 and 2.5Hz, and 5 and 10Hz, respectively. Step 5 : Review the magnitude-distance distribution for the average of 1 and 2.5 Hz to determine whether the contribution to the hazard for distances of 100 km or greater is substantial. If the contribution to the hazard for distances of 100 km or greater exceeds 5%, additional calculations are needed to determine the controlling earthquakes using the magnitude-distance distribution for distances greater than 100 km. Step 6 : Calculate the mean magnitude and distance of the controlling earthquake associated with the ground motions determined in Step 2 for the average of 5 and 10 Hz. Step 7 : If the contribution to the hazard calculated in Step 5for distances of 100 km or greater exceeds 5% for the average of 1 and 2.5 Hz, calculate the mean magnitude and distance of the controlling earthquakes associated with the ground motions determined in Step 2 for the average of 1 and 2.5 Hz. Step 8 : Determine the SSE response spectrum. SCENARIO EARTHQUAKES FOR KOREAN NPP SITE In this study, the probability based scenario earthquakes for a Korean NPP site were developed using the method  proposed by the U.S. NRC Regulatory Guide 1.165[2]. The example NPP site is located in the southeastern part of Korean peninsular. Probabilistic Seismic Hazard Analysis   The probabilistic seismic hazard analysis was performed for the NPP site. The team approach developed by EPRI (Electric Power Research Institute) was adopted for the hazard analysis. Three seismicity expert teams and one attenuation team were composed to obtain the PSHA input parameters. At least one non-seismologist was included in each seismicity team. However, in the attenuation team, only one expert recommended several different attenuation equations with weight [9]. A questionnaire was made and distributed to the team. The contents of the questionnaire are as follows.    3 - Seismicity 1) Matrix of physical characteristics 2) Assessment of tectonic features according to the matrix of physical characteristics 3) Seismic source (source zone) and their inter-dependency 4) Maximum magnitude of each zone 5) Seismic parameters of each zone 6) Backup data (or interpretation) on the given figures - Attenuation (Strong ground motion) 1) Equations and their weights 2) Background Fig. 1 shows one of the seismic source maps which was used for the evaluation of seismicity by the expert team. Table 1 shows the attenuation equations recommended by the expert. As shown in this table, the attenuation expert recommended three attenuation equations for the peak ground acceleration and three attenuation equations for the spectral acceleration with individual weight. Using these PSHA input data proposed by the expert teams, the PSHA was performed for the site. Fig. 2 shows the seismic hazard curves for the example NPP site. Scenario Earthquakes The seismic hazard was de-aggregated to determine dominant magnitudes and distances at the prescribed exceedance level. In this study, the seismic hazard was de-aggregated at 1Hz, 5Hz, and 10Hz at the 10 -5  exceedance level according to the U.S. NRC Regulatory Guide 1.165 [2]. According to the guide, the seismic hazard should also  be de-aggregated at 2.5Hz. But the ground motion attenuation equations proposed by the expert did not include the equation for 2.5Hz. The fractional contribution of magnitudes and distance bin to the total hazard for 1Hz was used for the development of a low frequency scenario earthquake. Because the contribution of the distance bins greater than 100km contained less than 5% of the total hazard for the 1Hz, additional calculations to consider the effects of distant and larger events were not needed. Fig. 3 and 4 show the contribution of magnitude and distance bins for 1Hz and the average of the 5Hz and 10Hz, respectively. The scenario earthquakes for the example site were determined based on the contribution. Table 2 shows the magnitude and distance of scenario earthquakes for the example Korean NPP site. As shown in this table, the magnitudes and distances of the scenario earthquakes are very similar. It may be due to the small contribution of distant earthquakes of the 1Hz scenario earthquake. Near-fault Ground Motion Effects  Near-fault ground motions are ground motions that occur near an earthquake fault. In general, the near-fault ground motion records exhibit a distinctive long period pulse like time history with very high peak velocities. These features are induced by the slip of the earthquake fault. Near-fault ground motions, which have caused much of the damage in recent major earthquakes (Northridge 1994, Kobe 1995, Chi-Chi 1999), can be characterized by a pulse-like motion that exposes the structure to high input energy at the beginning of the motion. The recorded acceleration response spectra of recent major earthquakes are well enveloped by the design response spectra of the codes in the medium to high frequency range, but not in the low to medium frequency range [10]. The near-fault effects such as pulse-like motions can dramatically influence spectral content in large earthquakes. Some of these effects are most pronounced within about 10 km. The fault normal component is about 30% larger than the fault parallel component in the frequency range 0.2 to 0.5 sec due primarily to rupture directivity. The rupture directivity effects are strongest for strike slip motion on vertical faults but can also be significant for cases of directivity for sites located near dipping faults. Other factors, perhaps strongest at close distances, include hanging wall/foot wall site location as well as thrust verses strike slip or normal slip mechanisms. These additional factors can have significant impacts on spectral composition [11]. RESPONSE SPECTRA FOR SCENARIO EARTHQUAKES The spectral shape for the scenario earthquakes were developed using the attenuation equations proposed in the PSHA study. The spectral shapes for the scenario earthquakes normalized to 0.2g ZPA (Zero Period Acceleration) are shown in Fig. 5. The spectral shapes for WUS (Western US) and CEUS (Central and Eastern US) are also shown in Fig. 5 for comparison. The seismic response spectral shapes for design and analysis for WUS and CEUS sites were  proposed by McGuire et al. [11,12]. The response spectral shapes for the WUS site were developed from empirical attenuation equations in the WUS. For the CEUS, the WUS spectral shapes were modified with a transfer function  based on the random vibration model of strong ground motion that accounts for differences in source parameters,    4 crustal damping, and near-surface damping. The spectral shapes for the WUS and CEUS sites can be obtained from the following equations, respectively.   += 63 )exp()cosh(]/)(ln[ 5421 C C   f   f  C C  f  C C  PGA f  SA  (1) 2/1875421 963 )exp()exp( )cosh(]/)(ln[ ++= C C C   f   f  C C  f   f  C C  f  C C  PGA f  SA  (2) where, )(  f  SA  and  f    are the spectral accelerations and frequencies, respectively. i C   is a statistical coefficient defined as a function of magnitude and/or distance by creating a data set of response spectral shapes. The spectral shapes for scenario earthquakes using the WUS and CEUS spectral shapes are also shown in Fig. 5. As shown in this figure, the spectral shapes from the proposed attenuation equations are similar to the shapes from the CEUS 1 corner frequency model. Mean ground response spectra obtained from 270 earthquake records with magnitudes 3 to 5 which occurred in Korea are shown in Fig. 6 [9]. The fluctuation in the spectrum for magnitude 5 earthquakes is due to the very limited number of data. It is noted from Fig. 5 and Fig. 6 that the spectral shapes from the proposed attenuation equations are very similar to the mean response spectrum developed from the real earthquake data. The uniform hazard spectrum for the Korean nuclear power plant site was also very similar to the mean response spectrum developed from the real earthquake data [13]. This result shows that the ground motion attenuation equations used in the seismic probabilistic hazard analysis reflected relatively well the ground motion attenuation characteristics and the site soil condition. These features of near-fault ground motion are generally not considered in the seismic design of nuclear power  plant structures and components. Many researches have been performed to identify the characteristics of near-fault ground motion [14,15,16]. Ohno et al. [17] showed the range that the near-fault rupture directivity effect is dominant and proposed a method to correct the predefined response spectrum considering this effect. Fig. 7 shows the FN (Fault  Normal) to FP (Fault Parallel) response spectral ratio from 37 records of the 11 strong earthquakes. These strong motion records were strongly affected by the near-fault directivity effect. Based on this study, Nishimura et al. [18]  propose a correction factor to modify the response spectrum. The correction factor, )( i T  λ  , can be obtained from the following equations.   1)(  = i T  λ   for  Di  T T   ≤  (3) )/log(/)/log()5.2log( 10)(  D H  Di  T T T T  i T   ⋅ = λ   for i D  T T   <  (4) where, i T   denotes the period.  D T   (= 0.33sec) and  H  T   (= 5sec) are the control points of the design ground response spectrum. These equations express that only the spectral acceleration in long period range greater than 0.33 sec is increased due to the near-fault rupture directivity effect. And, it is assumed that the spectral acceleration amplification due to the near-fault rupture directivity effect does not appeare in short period range shorter than 0.33 sec. Fig. 8 shows the correction factor to incorporate the near-fault directivity effect on the response spectrum shape. As shown in this figure, the correction factors from the equation can express the near-fault rupture directivity effect which appeared in the strong earthquake records. Using the proposed equation, the spectral shapes for the scenario earthquakes were modified to incorporate the near-fault rupture directivity effect. Fig. 9 shows the modified response spectral shape for the two scenario earthquakes. CONCLUSIONS In this study, the probability based scenario earthquakes for the Korean NPP site were developed using the PSHA results. The magnitude and distance bins of the scenario earthquakes for the example Korean nuclear power plant site were M6.4, 9km and M6.2, 13km. The magnitude and distance of the two scenario earthquakes for 1Hz and the average of the 5Hz and 10Hz are very similar, since the contribution of distant earthquakes to the total hazard is very small.
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