soil effects on the response of free-standing dry storage …papers.16ecee.org/files/soil effects on...

SOIL EFFECTS ON THE RESPONSE OF FREE-STANDING DRY STORAGE CASKS

Sharad DANGOL1, Luis IBARRA2, Steven BARTLETT3, Chris PANTELIDES4, David SANDERS5

ABSTRACT This study evaluates the seismic response of free-standing dry storage casks (DSCs) under long-term seismic events. Characterization of motion of three-dimensional free-standing structures is a highly nonlinear problem, especially when rocking and sliding motion occur simultaneously. Excessive sliding of such bodies may lead to impact with adjacent structures, whereas rocking may result in overturning or tip-over. Both scenarios can lead to unacceptable limit states to the system. The response of these free-standing bodies depends on several factors, such as aspect ratio (radius-to-centroidal height ratio, r/hcg), coefficient of friction, and ground motion characteristics, among others. This paper addresses soil effects on the predominant frequency of ground motion (surface rock motion), and ultimately on the DSC dynamic response. The ground motions initially developed for this project were for surface rock conditions. This was achieved by spectrally matching ground motions to the seismic hazard level or spectral accelerations developed for earthquake events of 10,000 and 30,000 year return periods. These rock spectral values cannot be used directly for soil site evaluation because soil effects were not considered. To account for these effects, the initial ground motions were subjected to deconvolution and convolution analyses. The resulting motions recovered at the surface of the soil-column show that soil softening shifts the ground motion’s predominant period to a longer period in addition to de-amplification of high frequency accelerations. This effect leads to larger rocking and translational displacements of DSCs, and may result in instability of free-standing structures. To investigate the soil-structure-interaction (SSI) effect on DSC response, a fully coupled, cask-pad-soil finite element (FE) model was developed. Numerical models of DSC with two different aspect ratios (r/hcg) were studied: 0.43 and 0.55. A ground motion recovered during the convolution process at a FE model’s soil column depth of 152.4 m (500ft) was used as the input acceleration. Results from FE simulations show that the change in frequency content of ground motions due to soil effects increases rocking and sliding displacements three to five times compared to those resulting from applying rock site motions with higher frequencies. Keywords Free-standing bodies; Dry Storage Cask; Soil Effects; Soil Structure Interaction; Rocking and Sliding. 1. INTRODUCTION Dry Storage Casks (DSCs) store spent nuclear fuel (SNF) at sites contiguous to nuclear power plants (NPPs), known as Interim Spent Fuel Storage Installations (ISFSIs). The DSCs can be either anchored or free-standing, and the seismic response of the latter is the focus of this research. Housner (1963) conducted one of the initial studies to determine free-standing body’s response; a work followed by several studies on rigid blocks ( Ishiyama, 1982a; Makris and Zhang, 1999; Hao and Zhou, 2012). Most investigations, however, simplified the problem by focusing only on pure rocking of planar rigid bodies (two dimensional blocks, 2D), with few exceptions (e.g., Ishiyama, 1982b). These studies show that free-standing rigid body response depends on several factors such as aspect ratio (radius-to-centroidal height ratio, r/hcg), coefficient of friction, and ground motion characteristics, among others. The main

1EIT, Dunn Associates, Inc., Salt Lake City, USA, [email protected] 2Associate Prof., Dept. of Civil and Env. Eng., University of Utah, Salt Lake City, USA, [email protected] 3Associate Prof., Dept. of Civil and Env. Eng., University of Utah, Salt Lake City, USA, [email protected] 4Professor, Dept. of Civil and Env. Eng., University of Utah, Salt Lake City, USA, [email protected] 5Professor, Dept. of Civil and Env. Eng., University of Nevada at Reno, Reno, USA, [email protected]

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goal of this paper is to study the effect of soil on the predominant frequency of ground motions, and ultimately on the DSC dynamic response. DSCs are considered a temporary SNF storage solution, and usually are licensed for 20 years, although they can be relicensed for operating periods of up to 60 years. Recent consideration of DSCs as a potential mid-term solution, in which the operating period may be extended for up to 300 years (US NRC, 2011) needs a reevaluation of DSC response under long return period seismic motions. For a 20-year compliance period, ISFSIs are designed for a Design Basis Earthquake (DBE) associated with a return period, T 2,000 years (US NRC, 1998), corresponding to a probability of exceedance

T/1 5 × 10-4/year. Then, the probability of exceeding the DBE )]0([ xP in 20 years is 1%. To

obtain the same probability of exceedance of 1% in 300 years, a return period of T 29,850 years needs to be considered, resulting in 3.3 × 10-5/year (Dangol and Ibarra, 2013). For this reason, the ground motion records used in the study were spectrally matched to earthquake events with long return periods of 10,000 and 30,000 years (Abrahamson, 1993; McGuire et al., 2001). The target spectra were developed using NUREG 6728 (McGuire et al., 2001) guidelines for Western US (WUS) rock sites. For each return period, two target spectra were developed: (i) Near Field Ground Motions, NFGMs, with magnitude M = 6 and distance R = 2 km, and (ii) Far Field Ground Motions, FFGMs, with M = 8 and R = 20 km. Thereafter, two ground motion sets, with 15 ground motions each, were spectrally matched to the four target rock spectra, as shown in Figure 1 (Dangol, 2017). These spectral shapes for 10,000 and 30,000 year return periods, and ultimately the spectrally matched ground motions, are only appropriate for rock sites. Although amplification factors to convert rock spectra to soil spectra are available in current bridge codes, their implementation includes several simplifications. Bartlett (2004) recommends site specific response analyses for soft and stiff soils. The softening leads to higher shear strain and damping, resulting in (i) de-amplification of high frequency spectral accelerations, and (ii) longer predominant period of the spectral shape.

(a) (b)

Figure 1. Target response spectra (Western US rock): (a) 10,000 year event, (b) 30,000 year event This study investigates the effect of soil on the response of free-standing cylindrical dry storage casks. For this purpose, a fully coupled cask-pad-soil Finite Element (FE) model was developed and separate deconvolution and convolution analyses were performed to obtain input motions for the FE model. The studied DSCs have aspect ratios (r/hcg) of 0.43 and 0.55, although this paper only presents findings for the former cask. 2. ANALYSIS MODEL A fully coupled cask-pad-soil model was created in LS-DYNA (LSTC, 2012), consisting of 4 free-standing casks on a concrete pad, which in turn is on top of a 152.4 m soil column, divided in 28 layers (Figure 2). The soil column diameter was set to 15 times the diagonal of a pad with dimensions of 29.41 × 9.45 × 0.61 m. This large soil column is necessary to approximate the soil semi-space, and to minimize the effects of reflected waves within the soil. The soil column also satisfies the US Army Corps of Engineers (1993) recommendation of having a soil column with a length of at least seven times the

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largest pad dimension, in this case, diagonal of the concrete pad in this case. Nodes at the outer ring of each layer (Figure 2) were constrained together to have the same nodal displacements. The process was repeated multiple times for each layer except the bottom edge nodes. This technique allows the soil column to behave globally as a one-dimensional soil column, while allowing for local disturbances and movements (Luk et al., 2005; Ko et al., 2009). Figure 2 shows the case of a full-scale model with four casks placed in the middle section of the pad, while the rest of the pad was loaded with equivalent cask surface loads (q), see Figure 3. The cask characteristics are presented in Table 1.

Figure 2. FE Model for Cask-Pad-Soil full-scale model

Figure 3. Schematic of casks and pad for full-scale model (Cask r/hcg = 0.43)

Table 1. Dimensions and weight of full-scale casks and surface loads used in FE models

Description FS.43 (r/hcg = 0.43) FS.55 (r/hcg = 0.55) Radius (r), m 1.32 1.45 Height (h), m 6.06 5.56 Centroidal Height (hcg), m 3.05 2.63 Weight, kN 907.44 1051.50 Bottom Area (A), m2 5.46 6.56 Bottom Stress, kPa 166.29 160.30 Weight of 4 Casks, kN 3629.75 4206.10 Surface Load (q), kPa 35.80 44.6

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2.1 Cask and Pad Submodel The four casks were generated using hexahedral solid elements. To preserve the aspect ratio and weight of the specimen, cask models were divided into halves and the density of each half defined accordingly to achieve desired weight and aspect ratio. To account for possible bending or flexibility of the pad, as its thickness is much smaller than the lateral dimension, “thick shell” elements were used to model the concrete pad. Table 2 presents the material properties used for the casks and the concrete pad. Two models were created for FS.43 and FS.55, but only the model and simulations for FS.43 are presented in this paper. Mass proportional damping, as well as a scale factor for vertical damping, were determined by trial and error to account for energy loss. The trial and error consisted of varying the scale factor for damping, that is, the coefficient of restitution during impact. The scale factor was varied until the model satisfactorily reproduced the experimental results.

Table 2. Material properties of dry storage cask and pad model

Specimen Material Properties

Young’s modulus, E (kN/m2)

Poisson’s ratio, ν

Density, ρ (kg/m3)

FS.43 2.78 × 107 0.2 2.76 × 103 (bottom half) 2.83 × 103 (top half)

Concrete Pad 2.78 × 107 0.2 2.29 × 103 2.2 Soil Model Solid elements were also used to model the soil column. To simulate the embedment of the concrete pad in a simplified model, the pad and soil were “tied” or bounded together. Material properties for the soil layers were defined as the strain compatible properties obtained from the deconvolution and convolution analysis performed in ProShake (EduPro Civil Services Inc., 2005) and DEEPSOIL (Hashash et al., 2016), respectively; using an equivalent linear approach. The density (ρ) and Poisson’s ratio ( ) for all soil layers were set as 2,002 kg/m3 and 0.4, respectively. The elastic modulus and damping for each soil layer were obtained from the strain compatible properties in the convolution analysis, and are presented in the following section. 2.3 Interface Contact between Cask and Pad Contact was defined between the cask and pad, and between the overpack and MPC, using soil column the “automatic_surface_to_surface” contact definition with a baseline concrete pad-steel overpack friction coefficient μs = μk = 0.55. The contact definition used in the model adopts a penalty contact algorithm. The response of surfaces interacting through frictional contact can be highly non-linear; therefore, explicit time integration schemes were used to analyze the model. An explicit code was implemented because of its capability to solve highly non-linear problems. Ground motion acceleration input was applied at the base of the soil column. 2.4 Representation of Soil Damping The LS DYNA’s material library has a non-linear soil model that could be implemented, however, the deconvolution and convolution analyses were performed using an equivalent linear method. Because a linear elastic material cannot account for energy dissipation, damping in the soil the column model was represented using Rayleigh damping.

KMC (1) where α and β represent the mass and stiffness proportional damping factors, respectively, and can be expressed as

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n ; n / (2)

)}/(4/{2 iin VsH (3) In Equation (2), ξ is the damping fraction for each layer obtained from the convolution process, and ωn is the natural frequency of the soil column determined by Equation (3), where Hi and Vsi are the thickness and strain-compatible shear wave velocity of each i-th soil layer obtained from DEEPSOIL analysis, respectively. 3. CONVOLUTION AND DECONVOLUTION ANALYSIS A site study for a typical WUS rock and soil profile was performed to account for the soil effect on ground motion characteristics. The motions obtained after the deconvolution and convolution processes were used as input in the FE simulations. The spectral shape of rock motion can be significantly modified by soil effects (Bartlett, 2004), and generally soil sites de-amplify the PGA in the high frequency region. To perform the site specific response analysis, the computer programs ProShake (EduPro Civil Services Inc., 2005) and DEEPSOIL (Hashash et al., 2016) were used for the deconvolution and convolution processes, respectively. The equivalent linear (EQL) method was used for both programs. Deconvolution analysis converts the surface rock motion into the motion at the required depth. These motions are then passed through the desired soil profile in the convolution process. The spectrally matched free-field WUS rock motion was deconvolved to a depth of 2,000 m of the generic WUS rock profile (Boore and Joyner, 1997; Bartlett, 2004). The deconvolved motion at a depth of 2,000 m was obtained as “outcrop” motion in ProShake. For the convolution process, the deconvolved motion was then applied at the base of the same profile, with the top 152.4 m of rock replaced with a standard soil profile for a typical NPP site (Figure 4a). For the convolution process, soil properties of the top 152.4 m were defined as per the Darendeli (2001) material model for granular soils, with over-consolidation ratio OCR = 1 and Ko = 0.4. However, replacing only the top 152.4 m resulted in a sharp discontinuity, leading to an artificial amplification of the spectral accelerations of the convolved soil spectra. To remove this discontinuity in the shear wave velocity profile, soil layers below 152.4 m were also adjusted by modifying the material properties according to Darendeli’s model (Figure 4b). The soil profile in Figure 4b was then used to convolve the ground motion to obtain the ground motion at the soil surface. Figure 4c shows the maximum frequency each layer of soil can support.

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Figure 4. (a) Typical shear wave velocity (Vs) – Standard Profile for nuclear power plant sites, adapted from (Luk et al., 2005); ; (b) Soil profile for convolution analysis [with modified Vs below 500ft (152.4 m), smooth

transition]; (c) Maximum frequency supported by modified soil profile

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Figure 5 presents the average soil layer properties before and after convolution analysis performed in DEEPSOIL (Hashash et al., 2016). The reduction in shear velocity (Vs) for each layer is an indication of soil softening. As soil strain increases the shear modulus decreases, leading to a softer soil with lower Vs and higher damping values. This reduced Vs was used to determine the elastic modulus of each soil layer using Equation (4). Figure 6(a-h) compares spectra developed for WUS rock to the average spectra obtained for soil after the deconvolution and convolution processes. Each figure shows 15 individual spectra obtained for the top layer of soil after the convolution performed in DEEPSOIL, as well as the average (mean) of the fifteen spectra. As can be seen, after convolution, the spectral acceleration for the longer period region is amplified while this is not always true for vertical motions; the horizontal motions show a decrease in the short period region (T < 0.4 s), effectively elongating the predominant period of the motion (Figure 6a, 6c, 6e and 6g).

2)1(2 ii VsE (4)

Figure 5. Average strain compatible properties of soil obtained from convolution analyses [top 152.4m].

4. VALIDATION OF FE MODEL 4.1 Cask-pad only interaction model Experimental results from 1:2.5-scale model dynamic tests performed on the shaking table at the University of Nevada, Reno (UNR) (Maree et al., 2015; Nielsen, 2016) were used to validate the cask-pad interaction model. The experiments were simulated in LS-DYNA (Dangol, 2017). The experimental and FE responses were compared for the FS.43 scaled casks under 75% of the 10,000 year Chi-Chi motion (rock) to validate the FE model (Dangol, 2017). Global damping was applied to account for energy loss during impact (i.e., coefficient of restitution), along with a scale factor for vertical damping. Global damping is a mass proportional damping that is applied to the nodes globally. After approximating a damping factor, the scale factor for vertical motion was determined by trial and error until the models satisfactorily reproduced the experimental results. Thereafter, a FE model for a full-scale FS.43 was created. The validated 1:2.5 scale model and the full-scale model were then subjected to 100% of 10,000 year Chi-Chi motion (rock). Figure 7 compares the response of the full-scale and 1:2.5 scaled model, after scaling the response of the 1:2.5 model, according to the similitude law (Dangol et al., 2016). As observed, the displacement and rocking angle time history of both models are similar. Thus, the full-scale model and the modeling techniques were considered to be validated. Table 3 presents the summary of peak response of the 1:2.5 scale model scaled obtained from the similitude law. These values represent the response that an equivalent full-scale cask would experience.

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Table 3. Equivalent peak (absolute) response of full-scale casks

Ground Motion

Cask Rocking Angle

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Displacement (m) Cask Top

Displacement (m) Residual Displacement

Cask Bottom (m) X Y X Y X Y X Y

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FS.55 0.010 0.015 0.035 0.023 0.070 0.103 0.003 0.008 FS.43 0.059 0.085 0.163 0.190 0.433 0.540 0.050 0.083

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FS.55 0.021 0.014 0.080 0.108 0.193 0.175 0.058 0.080 FS.43 0.042 0.042 0.135 0.145 0.368 0.335 0.090 0.088

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FS.55 0.039 0.028 0.183 0.218 0.368 0.320 0.063 0.050 FS.43 0.082 0.044 0.140 0.245 0.575 0.435 0.013 0.073

4.2 Soil column model Ground motions were applied at the base of the soil column to verify that surface soil spectra could be obtained from deconvolution and convolution. The four casks of Figure 2 were removed, but their weight was included by applying a pressure load on top of the entire pad. Once the simulation was complete the spectra of the motion obtained at the “Soil Top Far Edge” (Figure 2) were compared to those obtained

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after the convolution process (Figure 6). The FE input motions from the convolution process are presented in Figures 8 and 9. During the convolution process, the ground motion at the base of Layer 28 (i.e., at the depth of 152.4 m) was also obtained as output in DEEPSOIL (Hashash et al., 2016) as a “within layer motion.” This motion was then applied to the base of the soil column. Figure 10 compares the original rock spectra (10,000-year), soil spectra obtained from DEEPSOIL convolution, and that obtained from the FE model at the edge of the soil column. The soil spectra obtained from DEEPSOIL and the FE model are in good agreement, indicating that the FE model can reproduce the surface soil motion. 5. RESPONSE OF FULLY COUPLED CASK-PAD-SOIL MODEL 5.1 Cask with Aspect Ratio 0.43 (FS.43) The validated model (Figure 2) was then subjected to ground motions applied at the model’s soil column base. These motions were obtained from the DEEPSOIL convolution at a depth of 152.4 m. This section presents the fully coupled cask-pad-soil model performance for FS.43 (r/hcg = 0.43). The results show that the change in the ground motion dominant frequency due to SSI plays a relevant role in the casks’ response. Figure 11 shows the casks in motion subjected to the convolved 30,000-year Chi-Chi record, representing one of the most extreme seismic excitations that a cask could experience. As observed, the casks undergo large rocking, tumbling, and nutation motions, leading to the top two casks colliding with each other. In spite of the large rocking and horizontal cask displacements, no overturning was observed for this extreme excitation. However, the maximum recorded rocking angle of 0.38 radians is close to the theoretical critical angle α = 0.41. Figures 12-14 show the rocking and displacement responses for the same case. The large difference in the bottom and top center cask horizontal displacements in Figures 12-13 is a result of the large rotations experienced by the cask (Figure 14). Also, each cask responds differently as time progresses, as observed in the rocking angle time history of Figure 14.

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As a result of the impact between Cask1 and Cask 2, large accelerations were observed in the casks during impact. Figure 15 shows the horizontal (X and Y direction) acceleration time histories at the top center of the two casks. As observed, at the time of impact, Cask1 experiences maximum (absolute) accelerations of 7.06 g and 7.18 g in the X and Y directions, respectively. Cask2 experiences absolute maximum accelerations of 10.44g (X direction) and 4.69g (Y direction). Four additional simulations for soil motions corresponding to the 10,000-year and 30,000-year Chi-Chi and Erzican were carried out. Table 4 summarizes peak rocking angles and displacements for each case. A comparison of the corresponding peak rocking angle values from Tables 3 and 4 shows that the SSI effect is critical in the response of DSCs. Rocking of DSCs under soil motion increases by an average factor of 2.8, suggesting that the elongation of the dominant period of the ground motion can cause excessive movement of free-standing DSCs.

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Figure 10. Spectra comparison for DEEPSOIL and FE model

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Cask1 Cask2 Cask3 Cask4

Time (s)

Y Rocking Angle Time History

Figure 14. Rocking angle time histories, cask-pad-soil model (full scale), r/hcg = 0.43, μs = 0.55, convolved

30,000-year Chi-Chi

Figure 15. Horizontal accelerations experienced by Cask1 and Cask2 (r/hcg = 0.43) top center (30,000-year Chi-

Chi, SSI)

0 10 20 30 40 50 60 70 80-12-8-4048

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0 10 20 30 40 50 60 70 80-12-8-4048

12

X A

ccel

erat

ion

(g) Cask1

Cask2

Time of Impact

Time of Impact

Y A

ccel

erat

ion

(g)

Time (s)

Cask1 Cask2

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Table 4. Peak (absolute maximum) responses of FS.43 (r/hcg = 0.43, full-scale cask-pad-soil model) under convolved soil motions

Ground Motion

Cask Rocking Angle

(rad) Cask Bottom

Displacement (m) Cask Top

Displacement (m)

Residual Displacement Cask

Bottom (m) X Y X Y X Y X Y

10,000-year Chi-Chi (Soil)

1 0.123 0.187 0.508 0.282 0.742 1.074 0.041 0.195 2 0.165 0.179 0.414 0.485 1.287 1.503 * * 3 0.184 0.203 0.624 0.498 1.390 1.691 * * 4 0.174 0.172 0.667 0.376 1.457 1.363 0.527 0.277

10,000-year Erzican (Soil)

1 0.114 0.126 0.229 0.151 0.594 0.862 0.098 0.076 2 0.113 0.126 0.245 0.215 0.931 0.865 * * 3 0.116 0.131 0.228 0.153 0.607 0.888 * * 4 0.121 0.116 0.219 0.153 0.657 0.808 0.062 0.059

30,000-year Chi-Chi (Soil)

1 0.375 0.326 1.277 0.697 2.136 2.281 * * 2 0.236 0.262 0.713 0.907 1.459 1.497 * * 3 0.349 0.230 0.707 1.548 2.047 2.515 0.381 1.427 4 0.336 0.194 0.686 1.227 1.855 1.981 0.227 0.355

30,000-year Erzican (Soil)

1 0.107 0.163 0.515 0.458 0.903 1.311 0.067 0.059 2 0.105 0.158 0.522 0.473 0.918 1.311 0.003 0.208 3 0.145 0.161 0.581 0.493 1.182 1.332 * * 4 0.121 0.159 0.471 0.516 0.898 1.366 * *

Note: * - Tumbling or nutation motion continues (not back to complete rest) 5. CONCLUSIONS Full-scale FE models of fully coupled cask-pad-soil systems were developed to assess SSI effect on the response of free-standing DSCs. The strain compatible soil properties used in the models were obtained from deconvolution and convolution analyses, which resulted in a change in spectral ground motion characteristics caused by the soil effect. Previous studies use deconvolution and convolution to obtain the same starting target motion, which does not account for the actual soil effect on the ground motion. This study used surface rock motion and performed a site specific soil effect study that resulted in ground motions with different spectral characteristics due the soil effect on the ground motion. The main conclusions are:

i. Deconvolution of rock motions and convolution back through the soil resulted in changes in spectral characteristics of the original rock motion. The dominant ground motion period elongates (T > 0.5s), and the high frequency content, including the PGA of ground motions, is de-amplified (muted) by soil effects.

ii. This change in frequency content has the largest impact on DSC response. The change in frequency increases rocking displacement from three to five times, and produces a similar increase in displacements compared to those resulting from application of rock motions.

iii. Simulations for the slender cask (r/hcg = 0.43) under the 30,000-year Chi-Chi motion showed excessive movement of casks, leading to impact between them, and casks experiencing impact accelerations exceeding 10 g.

iv. This study showed that the response of casks, within the same model, follows a similar trend during the early part of ground motion excitation. However, as time progresses, differences in the response occur; this indicates the sensitive nature of the seismic response of free-standing bodies.

6. ACKNOWLEDGMENTS This material is based upon work supported under a Department of Energy Nuclear Energy University Programs. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Department of Energy Office of Nuclear Energy.

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7. REFERENCES Abrahamson, N.A., 1993. Nonstationary Spectral Matching Program RSPMATCH. PG&E Internal Report.

Bartlett, S.F., 2004. Ground Response Analysis and Design Spectra for UDOT Bridges on Soft Soil Sites. UDOT research report, University of Utah, Salt Lake City.

Boore, D.M., Joyner, W.B., 1997. Site Amplification for Generic Rock Sites. Bull. Seismol. Soc. Am. 87.

Dangol, S., 2017. Dynamic Response of Free-Standing Dry Storage Casks and Its Variation under Long Return Period Sseismic Events. University of Utah.

Dangol, S., Ibarra, L., 2013. Effect of Vertical Accelerations on Dry Storage Cask Seismic Performance, in: Smirt-22. San Francisco.

Dangol, S., Ibarra, L., Maree, A., Nielsen, T., Sanders, D., Pantelides, C., 2016. Experimental Seismic Response of Scaled Storage Containments under Identical Loading Conditions, in: WM2016 Conference. Phoenix, Arizona.

Darendeli, M.B., 2001. Development of a New Family of Normalized Modulus Reduction and Material Damping Curves. University of Texas at Austin.

EduPro Civil Services Inc., 2005. ProShake v1.12.

Hao, H., Zhou, Y., 2012. Dynamic Response of Rigid Blocks to Simultaneous Horizontal and Vertical Ground Shock. Adv. Struct. Eng. 15, 1069–1082. doi:10.1260/1369-4332.15.7.1069

Hashash, Y.M.A., Musgrove, M.I., Harmon, J.A., Groholski, D.R., Phillips, C.A., Park, D., 2016. DEEPSOIL 6.1, Users Manual.

Housner, G.W., 1963. The Behavior of Inverted Pendulum Structures during Earthquakes. Bull. Seismol. Soc. Am. 53, 403–417.

Ishiyama, Y., 1982a. Motions of Rigid Bodies and Criteria for Overturning By Earthquake Excitations. Earthq. Eng. Struct. Dyn. doi:10.1002/eqe.4290100502

Ishiyama, Y., 1982b. Motions of Rigid Bodies and Criteria for Overturning By Earthquake Excitations. Earthq. Eng. Struct. Dyn. 10, 635–650. doi:10.1002/eqe.4290100502

Ko, Y.-Y., Hsu, S.-Y., Chen, C.-H., 2009. Analysis for Seismic Response of Dry Storage Facility for Spent Fuel. Nucl. Eng. Des. 239, 158–168. doi:10.1016/j.nucengdes.2008.09.006

LSTC, 2012. LS-DYNA.

Luk, V.K., Spencer, B.W., Lam, I.P., Dameron, R.A., 2005. Parametric Evaluation of Seismic Behavior of Freestanding Spent Fuel Dry Cask Storage Systems, NUREG/CR-6865. prepared for the Nuclear Regulatory Commission, NUREG/CR-6865, Washington DC.

Makris, N., Zhang, J., 1999. Rocking Response and Overturning of Anchored Equipment under Seismic Excitations, Final Report to PG&E. PEER Report 1999/06, University of California, Berkeley.

Maree, A.F., Nielsen, T., Dangol, S., Parks, J., Sanders, D., Ibarra, L.F., Pantelides, C., 2015. Shaking Table Experiments of Dry Storage Caks, in: 2015 Joint Conference AESE/ANCRiSST. Urbana-Champaign.

McGuire, R.K., Silva, W.J., Costantino, C.J., 2001. Technical Basis for Revision of Regulatory Guidance on Design Ground Motions: Hazard- and Risk-consistent Ground Motion Spectra Guidelines. prepared for the Nuclear Regulatory Commission, NUREG/CR-6728, Washington DC.

Nielsen, T.M., 2016. Experimental Investigation into the Long-Term Seismic Performance of Dry Storage Casks. University of Nevada, Reno.

US Army Corps of Engineers, 1993. Engineer Technical Letter No. 1110-2-339.

US NRC, 2011. Plan for the Long-Term Update to the Waste Confidence Rule and Integration with the Extended Storage and Transportation Initiative (SECY-11-0029).

US NRC, 1998. Exemption to 10 CFR 72.102(f)(1) Seismic Design Requirement for Three Mile Island Unit 2 Independent Spent Fuel Storage installation(SECY-98-071).

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