ML102250213

Enclosure - Response to Technical Assistance Request - LaSalle Station and Byron Station Independent Spent Fuel Storage Installation (ISFSI) Pad Seismic Design REGION-III TAR ML101680320

ML102250213
Person / Time
Site:	Byron, LaSalle
Issue date:	08/12/2010
From:	Eric Benner NRC/NMSS/SFST
To:	Christine Lipa Division of Nuclear Materials Safety III
Tripathi B NMSS\SFST 492-3281
Shared Package
ML102250189	List: ML102250192 ML102250213
References
TAR ML101680320
Download: ML102250213 (11)

Text

Response to Region III Technical Assistance Request (TAR) [ML101680320]

Independent Spent Fuel Storage Installation (ISFSI) Pads At LaSalle [and Byron Nuclear Power] Stations DSFST Ticket Number: 201000010 Date: 8-12-2010 Background and Scope:

United States Nuclear Regulatory Commission (NRC) Region III requested assistance from the NMSS/Division of Spent Fuel Storage and Transportation, (NMSS/DSFST); by a Technical Assistance Request (TAR) dated October 16, 2009 [ML092890663] to perform a technical review of the LaSalle Station (LS) Independent Spent Fuel Storage Installation (ISFSI) Pad, to determine whether the licensees seismic analysis and design of the pad meet the regulatory requirements of 10 CFR Part 72. This technical review was limited to the licensees documentation, and the relevant calculations prepared by LS and/or their contractors and furnished together with the Technical Assistance Request (TAR). The results of the technical review were forwarded to the NRC Region III office on January 20, 2010 [ML1002005301].

On June 14, 2010 the staff received a TAR [Package Accession No. ML101680320]

from NRC Region III requesting further assistance with the review of several revised calculations associated with the ISFSI Pads at the LaSalle and Byron Nuclear Power Stations. These revisions were initiated as a result of staffs comments on the original sets of calculations for the requested ISFSI pad at LaSalle Station. The initial TAR response also resulted in similar changes that occurred for Byrons calculations. The current TAR requested to verify that the questions and concerns that arose from the initial calculation(s) review have been addressed in the recently revised calculations.

On July 30, 2010 staff received additional information from Region III, which the staff was asked to review. This new information included two letters from HOLTEC to Exelon (References 12 and 13) outlining HOLTEC's evaluation of the existing safety margins in the ISFSI pads for the LaSalle and Byron/Braidwood Stations, and a position paper on HOLTEC's SSI analysis methodology (Reference 14).

The licensee is required by 10 CFR 72.212(b)(2)(i)(B) to perform written evaluation to establish that the cask storage pads have been designed to adequately support the static and dynamic loads of the stored casks, considering potential amplification of earthquakes through soil-structure interaction, and soil liquefaction potential or other soil instability due to vibratory ground motion. At the LaSalle Station and at the Byron Station the licensees vendor, HOLTEC International, performed calculations as listed herein to demonstrate compliance with the above regulatory requirement. These calculations were performed to analyze the pad, taking into consideration the soil structure interaction in order to determine the loads on the pad under a design basis seismic event. These loads were used as input for the structural evaluation/design of the pad.

Action Requested by NRC Region III to DSFST NRC/HQ:

For assistance in resolution of the concerns identified above, Region III has requested DSFST staff to review the licensee revised calculations:

1) EXELON Calculation L-003346 Rev. 2, dated 5-13-10 (Ref. 1)

2) EXELON Calculation L-003347 Rev. 4, dated 5-12-10 (Ref. 2)

3) EXELON Calculation 2.4.3 BYR-08-027 Rev. 3, dated 6-4-10 (Ref. 4)

4) EXELON Calculation 2.4.3 BYR-10-095 Rev. 2, dated 6-11-10 (Ref. 5)

ISFSI Related Regulations:

Regulatory Requirements: ISFSIs are licensed under 10 CFR Part 72, Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor-Related Greater Than Class C Waste. 10 CFR 72.210 grants a general license for the storage of spent fuel in an ISFSI to all holders of a Part 50 reactor operating license. The conditions of this general license are given in

§72.212. Section 72.212(b)(2)(i) requires the general licensee to perform written evaluations, prior to use, that establish that:

(A) Conditions set forth in the Certificate of Compliance have been met; (B) Cask storage pads and areas have been designed to adequately support the static and dynamic loads of the stored casks, considering potential amplification of earthquakes through soil-structure interaction, and soil liquefaction potential or other soil instability due to vibratory ground motion; and (C) The requirements of § 72.104 have been met.

Additionally, 10 CFR 72.212(b)(3) requires that a licensee review the Safety Analysis Report (SAR) referenced in the Certificate of Compliance and the related NRC Safety Evaluation Report (SER), prior to use of the general license, to determine whether or not the reactor site parameters, including analyses of earthquake intensity and tornado missiles, are enveloped by the cask design bases considered in these reports.

NRC/HQ - DSFST Staffs Evaluation of ISFSI Pad Analyses and Design:

LaSalle Station:

The ISFSI at LS includes a main reinforced concrete pad 2 feet thick by 90 feet wide by 246 feet long as an onsite storage pad for accommodating the HI-STORM 100 Cask Systems. The concrete pad is a stand-alone structure designed to accommodate up to 90 casks in a 6 x 15 array. The ISFSI at LS is licensed under the provisions for a 2

general license specified in Part 72, which is granted to holders of a Part 50 operating license. HOLTEC has been issued a Certificate of Compliance (CoC) No. 1014 Amendment # 3 (Ref. 10) for the HI-STORM 100 Cask System by the NRC.

As a condition of utilizing their general license to operate an ISFSI, 10 CFR 72.212 requires licensees to perform written evaluations that establish that, for their site-specific conditions, the conditions set forth in the Certificate of Compliance have been met and that cask storage pads and areas have been designed to adequately support the static and dynamic loads of the stored casks, considering potential amplification of earthquakes through soil-structure interaction (SSI), and soil liquefaction potential or other soil instability due to vibratory ground motion. A report by Exelon, Dynamic Analysis of HI-STORM 100 Cask on LaSalle ISFSI Pads Revision 4 Calc. L-003347 (Ref. 2) was prepared to fulfill the requirements of 10 CFR 72.212.

Since the Safe Shutdown Earthquake (SSE) response spectra at the control level were acknowledged in the LS Updated Safety Analysis Report (SAR) (Ref. 8) to be similar to Reg. Guide 1.60 (Ref. 9) spectra, a set of time histories, appropriate to Reg. Guide 1.60 spectrum with 5% damping were developed (documented in Appendix H of Ref. 2) and were used for the analyses of the ISFSI pad at LS.

Byron Station:

• The Byron Nuclear Power Station has the same ISFSI configuration as the Braidwood Station, but is sited on a stiffer subgrade (having a higher average wave speed than one at the Braidwood Station). The ISFSI pad at Byron/Braidwood (B/B) has a maximum of 96 freestanding casks on a reinforced concrete pad. The ISFSI pad proposed for Byron Station and Braidwood Station are identical. Further, both ISFSI pads are driven by response spectra set matching a Regulatory Guide 1.60 (Ref. 9) seismic event at the far-field free surface rock outcrop. Since the substrate underlying the Braidwood Station is softer than the substrate underlying Byron Station, and the peak ground acceleration (PGA) for Braidwood Station at the far field free surface rock outcrop is larger, an analysis using the substrate properties and the PGA associated with the Braidwood Station (0.26g) that bound the desired results for the Byron Station was performed. The ISFSI pad at Byron Station is 198 feet x 116 feet x 2 feet accommodating in an 8 x 12 array up to 96 loaded HI-STORM casks. The method of analysis for the B/B Station ISFSI pad (Ref. 5) appeared similar to the method developed for TVAs Sequoyah ISFSI as described in Ref.

3, and used for the ISFSI pad at LaSalle Station. The structural analysis of the ISFSI pad was performed in a separate report (Ref. 4). As the methodology used is identical, the summary and conclusions presented below applies to ISFSI pads at LaSalle Station and at Byron Station.

3

Comment L-1: Evaluation of Uncertainties in Soil-Structure Interaction (SSI) Analysis In the design basis dynamic analysis of the LaSalle ISFSI pad (Ref. 2) the methodology used to develop the SSI model and ensuing SSI analyses used best estimate soil properties. The staff found no evidence that separate SSI models were developed and analyses performed for lower bound and upper bound soil properties. The dynamic analysis provided no basis for not including these soil properties. The following three references illustrate the need to perform SSI analyses for lower bound and upper bound soil properties.

(1) ASCE Standard 4-98 (Ref. 6) Section 3.3.1.7 states the following:

The uncertainties in the SSI analysis shall be considered. In lieu of a probabilistic evaluation of uncertainties, an acceptable method to account for uncertainties in SSI analysis is to vary the low strain soil shear modulus. Low strain soil shear modulus shall be varied between the best estimate value times (1 + Cv) and the best estimate value divided by (1 + Cv), where Cv is a factor that accounts for uncertainty in the SSI analysis and soil properties. If sufficient, adequate soil investigation data are available, the mean and standard deviation of the low strain shear modulus shall be established for every soil layer. The Cv shall be established so that it will cover the mean plus or minus one standard deviation for every layer. The minimum value of Cv shall be 0.5. When insufficient data are available to address uncertainties in soil properties, Cv shall be taken as no less than 1.0.

(2) Section 2.0 of HOLTECs report documenting the seismic analysis methodology for ISFSI pads (Ref. 3) states the following:

For the site specific earthquake as seismic input, solve the dynamic model for assumed sets of soil properties (representing lower bound, best estimate, and upper bound values) to obtain. results.

(3) In a letter from HOLTEC International to E. William Brach, Director, Spent Fuel Project Office, discussing HI-STORM manufacturing and deployment issues (Ref. 7),

HOLTEC makes the following statement:

Three discrete soil-structure interaction analyses are performed by HOLTEC using the lower bound estimate, best estimate and upper bound estimate of the soil shear modulus, respectively, to account for the uncertainties in the characterization of the soil properties.

Finding: 10 CFR 72.212 requires, in part, that, a written evaluation be performed to demonstrate that the IFSFI pad has been designed to support the static and dynamic loads. ASCE 4-98 (Ref. 6) industry guidance, HOLTEC's commitments (Ref. 7), and prior NRC requirements have established that SSI analyses must include an evaluation 4

of the impact of the lower and upper bound soil properties. Contrary to the above, Exelon's evaluation did not include an evaluation of the upper and lower bound soil properties. Therefore, the NRC determined that the requirements of 72.212 were not satisfied for the IFSFI pads at LaSalle Station.

Comment L-2: HOLTECs Soil-Structure Interaction (SSI) Analysis Methodology The SSI analysis methodology used by HOLTEC for the seismic analysis of the ISFSI pad/cask system can be separated into four distinct parts. The first part of the methodology develops a two degree-of-freedom (2 DOF) spring/mass soil model. In this model the soil springs are determined based on the guidelines of ASCE 4-98 and strength of materials principles. The soil masses are then determined using an iterative process such that when the control motion is applied to the base of the model, it produces a ground surface response spectrum that closely matches the ground surface response spectrum produced by the SHAKE2000 (Ref.11) program. In this first part three 2 DOF spring/mass soil models are developed, one for each translational DOF, using the approach described above. Rotational and torsional soil springs are developed from ASCE 4-98 and strength of materials principles, and moment of inertia properties are developed from the geometry of the translation soil mass beneath the pad.

The second part of the methodology develops the non-linear ISFSI pad/cask model.

The pad is modeled as a 6 DOF rigid plate and each cask is modeled as a 6 DOF rigid cylinder. Each cask is attached to the pad at several circumferential locations around the cask/pad interface by compression only contact elements that also incorporate friction and sliding. This model allows the cask to tip and slide on the pad.

The third part of the methodology combines the 2 DOF spring/mass soil model with the ISFSI pad/cask model. The approach HOLTEC takes is to attach the ISFSI pad/cask model to the first mass (i.e., the mass attached to the ground spring) of the soil model to create the final SSI dynamic analysis model.

The first three parts of the SSI methodology develop the combined soil/pad/cask model.

The fourth part of the methodology incorporates the control motion, i.e., the design basis acceleration time history, into the SSI model and executes the analysis.

Assessment of the Methodology: Within the time constraints for responding to the TAR, the staff was able to review the referenced calculations, but the staff was not able to perform independent calculations or consult with third party experts. Based on this limited review, the staff found the first and second parts of the methodology to be a reasonable approach for constructing an independent approximate 2 DOF spring/mass soil model and an independent 6 DOF non-linear ISFSI pad/cask model. However, the staff has concerns with the third and fourth parts of the methodology, i.e., the approach used to combine the soil model with the pad/cask model, and the application of the control motion to the non-linear SSI model.

5

In the third part of the methodology the ISFSI pad/cask model is attached to one of the two masses of the 2 DOF soil model which was constructed to approximate the dynamic response characteristics of the continuum soil column between the rock outcrop (or appropriate soil layer) and the ground surface. This is likened to attaching a virtual soil mass (or added mass) to the ISFSI pad. This may be a reasonable approach. However, HOLTEC offers no explanation or justification for why this approach creates an SSI analysis model that would be expected to produce results comparable to results from models that have been accepted by the NRC for the SSI analysis of nuclear power plant (NPP) facilities.

With respect to the fourth part of the methodology, the staff notes that in every SSI methodology that has been reviewed and approved by the NRC the control motion is applied at a rock outcrop or comparable soil layer. This is physically how the earthquake ground motion arrives at the site. The seismic waves arrive at the rock outcrop, are filtered and amplified by the soil layers between the rock outcrop and the ground surface and generate motion to the ISFSI pad.

In the HOLTEC SSI methodology the rock outcrop (which is also the base of the SSI model) is fixed, and therefore cannot move and transmit seismic waves. Instead, in the HOLTEC methodology the control motion (acceleration time history), rather than being applied to the rock outcrop, as it would be in a real physical model, is applied as an inertia force time history to each mass point (i.e., cask cg., pad cg., soil mass cg., etc.)

in the SSI model. This methodology is non-physical and counter intuitive. The staff recognizes that this non-physical methodology may be theoretically correct for a linear analysis; however, the staff has no evidence that this methodology is applicable to a non-linear problem wherein a cask is allowed to slide, tip or loose complete contact with the pad. Furthermore, what the staff finds most unusual is that this SSI analysis methodology has no referenced sources to the technical literature. The methodology appears to have been exclusively developed by HOLTEC. To the staffs knowledge this SSI analysis methodology has never undergone a rigorous third party expert review and benchmarking to demonstrate the validity of this approach.

Finding: HOLTEC did not provide adequate justification and documentation for use of a new SSI analysis methodology, which has never been peer-reviewed and benchmarked, appears inconsistent with industry practice and guidance, and has never been approved by the NRC.

Comment L-3: Use of a Single Set of Acceleration Time-Histories A single set of three dimensional (two horizontal and one vertical) acceleration time-histories were developed to envelop the 5% damped Reg. Guide 1.60 response spectra to perform the non-linear SSI analysis. The use of a single set of 3-D time-histories is not standard practice for performing a non-linear SSI analysis.

6

ASCE 4-98 (Reference 6) Section 3.2.2.3 "Nonlinear Analysis" states the following in paragraph (d):

"In general, more than one set of acceleration time histories, meeting the requirements of Section 2.3, should be used, and the results of the analyses shall be averaged."

NUREG/CR-6865 (Reference 16) also discusses this same issue and states the following in Section 4.1:

"...the seismic response of a dry cask using one time-history might not always lead to a predictable response. It is increasingly obvious that a suite of earthquake inputs should be examined in order to obtain statistically stable mean and standard variation in the response to form the basis for design decision.

This would require multiple runs using several earthquake records."

NUREG/CR-6865 uses a suite of 5 different sets of earthquake time histories to perform the non-linear SSI analyses for a single spectral shape. Figure 5.11 from the NUREG shows the displacement response at the top of the cask for each of the 5 sets of time-histories used in the NUREG. The difference in maximum response among the 5 sets varies by as much as a factor of 6 for the same spectral shape. This shows that the effect of the differences in frequency content and phasing within the 5 sets of time-histories has a significant influence on response.

Finding: Due to the potentially large differences in response that can result from using different earthquake time-histories as input to a non-linear SSI analysis, the staff finds that HOLTEC's use of only a single set of acceleration time-histories to perform a non-linear SSI analysis may have significantly underestimated the predicted seismic response.

Comment L-4: ISFSI Pad Important-to-Safety HOLTEC's letter of July 23, 2010 (Reference 12) states the following:

"HOLTEC notes that in the HI-STORM FSAR, the ISFSI pad is considered as a "Not-Important-to-Safety" (NITS) component for establishing structural safety factor. There is no provision in the HI-STORM FSAR to use ASCE 4-98 for on-grade ISFSI pads."

The HI-STORM FSAR provides for the design of the HI-STORM Cask. The NRC staff reviewed the HI-STORM FSAR and subsequently issued an SER and CoC for the HI-STORM Cask. The HI-STORM FSAR does not provide for the design of ISFSI pads.

ISFSI pads are designed separately to meet the requirements of 10 CFR Part 72.212, and to conform to the general licensee's site parameters and the HI-STORM CoC.

Therefore, statements made in the HI-STORM FSAR regarding the safety classification 7

of the ISFSI pad are not relevant since the HI-STORM FSAR is not the design document for the ISFSI pad.

It is important to note that the ISFSI pad is one of the few dry storage system components that is specifically called out in the regulations. 10 CFR Part 72.212(b)(2)(i)(B) states:

"The general licensee shall: Perform written evaluations, prior to use that establish that; Cask storage pads and areas have been designed to adequately support the static and dynamic loads of the stored casks, considering potential amplification of earthquakes through soil-structure interaction...."

Furthermore, NUREG/CR-6407 "Classification of Transportation Packaging and Dry Spent Fuel Storage System Components According to Important-to-Safety" (Reference

17) classifies the ISFSI concrete pad as "Important-to-Safety" Category C.

Finding: The ISFSI pad is Important-to-Safety.

Comment L-5: Minimum Factor-of-Safety for the ISFSI Pad HOLTEC's letter of July 23, 2010 (Reference 12) states the following:

"...the minimum computed factor-of-safety for bending for the structurally most limiting alignment of casks on the LaSalle pad is 1.32. This governing factor-of-safety corresponds to the load combination that includes dead load, live load, and long-term settlement. Specifically, it does not correspond to the load combination that includes the seismic load. (For the seismic load combination case, the minimum safety factor, f, is 1.52 - much larger.)"

The LaSalle ISFSI design calculation (Reference 1) specifies the structural design code for the ISFSI pad to be ACI-349-01 (Reference 15). Section 9.2.2 of ACI-349 states the following:

"Where the structural effects of differential settlement, creep, or shrinkage may be significant, they shall be included with dead load D in Load Combinations 4 through 11."

Load Combination 4 includes dead load, live load, and seismic (SSE). By including differential settlement with dead load in the load combination that includes seismic, the Code is combining loads in a realistic manner in the sequence in which they would be expected to occur. In HOLTEC's load combinations differential settlement is not included with dead load so that differential settlement and seismic loads are never combined. Thus HOLTEC has not complied with Section 9.2.2 of ACI-349, and as a result the minimum factor-of-safety of 1.32 is expected to be significantly lower when differential settlement and seismic loads are properly combined.

8

Finding: HOLTEC has not included the effects of differential settlement and seismic loads in the same load combination as required by ACI-349. As a result the minimum factor-of-safety of 1.32 is expected to be significantly lower when differential settlement and seismic loads are properly combined.

Comment L-6: Choice of Upper Bound and Lower Bound Soil Properties Comment L-1 above addressed the evaluation of uncertainties in SSI analysis, the requirements of ASCE 4-98, and HOLTEC's commitment to perform SSI analyses for upper bound and lower bound soil properties.

HOLTEC's letter of July 23, 2010 (Reference 12) states the following:

"Specifically, this letter addresses the guidance in ASCE 4-98, in which the Upper Bound soil properties are defined by a 50% increase in the Shear Modulus of each substrate layer (based on testing), and Lower Bound properties are defined by a 33.3% decrease in the layer Shear Modulus."

The staff notes that the use of a 50% increase and 33.3% decrease in best estimate soil properties depends on whether sufficient, adequate soil investigation data are available to establish the mean and standard deviation of the low strain shear modulus for each soil layer. In the absence of adequate soil data ASCE 4-98 requires the best estimate data to be increased by 100% and decreased by 50% to establish upper bound and lower bound properties. These larger percentages would be expected to further increase the seismic results.

Finding: HOLTEC has not shown that sufficient, adequate soil data exists such that a 50% increase and 33.3% decrease in best estimate soil properties can be used to establish the upper and lower bound properties for SSI analysis. In the absence of adequate soil data ASCE 4-98 requires the best estimate data to be increased by 100%

and decreased by 50% to establish upper bound and lower bound soil properties.

These larger percentages would be expected to further increase the seismic results.

SUMMARY

AND CONCLUSIONS:

NRC/HQ -DSFST staff reviewed the dynamic analysis, the structural qualification and other pertinent documents presented to the NRC/HQ staff for the LaSalle Station and Byron Station ISFSI pads. The purpose of the dynamic analyses performed by the licensee was to evaluate the seismic response of the ISFSI pad under the SSE for the site, and to qualify the structural design of the ISFSI pad. Given the timeframe allowed for the review the NRC/HQ staffs review consisted of an inspection focusing on safety significant items of the four revised licensee calculations and the additional documents received on July 30, 2010.

9

Based on the findings presented herein regarding the seismic analysis and design of the LaSalle Station and Byron/Braidwood Station ISFSI pads, the staff concludes that the licensee did not demonstrate that the design of the ISFSI pads meet the requirements of 10 CFR Part 72.212(b)(2)(i)(B).

Path Forward: The resolution of the staffs findings including NRC acceptance of the HOLTEC methodology as a reliable methodology for performing non-linear SSI analysis is expected to require some time and effort. Given the immediate need for the licensee to load casks and move them onto the pad at these plants, and given the staffs findings, the staff must restrict the total load applied to the ISFSI pad by allowing the licensee to load a maximum of ten (10) casks and then only at every other cask location in each direction on the pad, so that for any cask on the pad an open (unused) location would be adjacent to it in both the length and width directions of the pad. Because this restriction on the number of casks and loading pattern significantly reduces the total load distribution on the pad, the staff concludes that for this reduced loading the concrete pad can adequately support the static and dynamic loads. Therefore, the staff has no safety concerns for the structural integrity of the ISFSI pad with this loading restriction in place, and therefore believes that the licensee can readily develop a timely and adequate written evaluation addressing these issues.

REFERENCES:

1) EXELON Analysis No. L- 003346, Structural Qualification of the ISFSI Pad at LaSalle under Static and Seismic Loading Revision 2, dated 05/13/2010.

2) EXELON Analysis No. L - 003347, Dynamic analysis of HI-Storm 100 Cask on LaSalle ISFSI Pads Revision 4, dated 05/12/2010.

3) HOLTEC Report No. HI-2012689 Revision 2, HOLTECs seismic Analysis Methodology for ISFSI Pad/Cask Assemblage Including Soil-Structure Interaction for Sequoyah Nuclear Plant, dated 4-10-2002.

4) EXELON Analysis No. 2.4.3 BYR-08-027, Structural Analysis of the ISFSI Pads at Byron and Braidwood Revision 3, dated 06/04/2010.

5) EXELON Analysis No. 2.4.3 BYR-10-095, Seismic Stability of HI-STORM 100 Casks on Byron/Braidwood ISFSI Pad using Classical Non-Linear Dynamics (CND) Model with Soil-Structure Interaction (SSI) Revision 1, dated 06/11/2010.

6) ASCE Publication 4-98, Seismic Analysis of Safety-Related Nuclear structures and Commentary, American Society of Civil Engineers.

7) Letter from HOLTEC International (Dr. K. P. Singh) to E. William Brach, Director, Spent Fuel Project Office, US NRC, HI-STORM Deployment Under CoC 72-1014 Manufacturing and Deployment Issue No. 1, Dated March 20, 2001.

10

8) LaSalle County Power Station Updated Final Safety Analysis Report, Rev. 17.

9) Nuclear Regulatory Commission, Office of Nuclear Regulatory Research, Regulatory Guide, 1.60, Design Response Spectra for Seismic Design of Nuclear Power Plants, Revision 1, December 1973.

10) Certificate of Compliance (CoC) No. 1014, Amendment 3.

11) SHAKE2000, Version 2.0, G. A. Ordonez, University of California at Berkeley, March 2003.

12) HOLTEC Letter dated July 23, 2010 to Exelon LaSalle ISFSI Pad Qualification, HOLTEC Project 1678 Document ID: 1678057.

13) HOLTEC letter dated July 26, 2010 to Exelon Byron/Braidwood ISFSI Pad Qualification, HOLTEC Project 1676 Document ID: 1678056.

14) HOLTEC Dry Storage Position Paper DS-348, "Seismic Excitation by Base Acceleration Time-History or by Applied Inertia Forces at the Mass Center - A Demonstration that Either Method is Acceptable," by Alan I. Soler, July 22, 2010.

15) ACI 349-01, "Code Requirements for Nuclear Safety Related Concrete Structures," American Concrete Institute, 2001.

16) NUREG/CR-6865, "Parametric Evaluation of Seismic Behavior of Freestanding Spent Fuel Dry Cask Storage Systems," February 2005.

17) NUREG/CR-6407, Classification of Transportation Packaging and Dry Spent Fuel Storage System Components According to Important-to-Safety, February 1996.

Prepared By: Gordon S. Bjorkman, NMSS/DSFST (202) 669-0164 Bhasker (Bob) P. Tripathi, NMSS/DSFST (301) 492-3281 Approved By: Christopher Regan, NMSS/DSFST (301) 492-3227 11