IR 05000260/1988039

Insp Rept 50-260/88-39 on 881219-22.Six Out of Eight Open Items Reviewed Closed.Major Areas Inspected:Techniques, Procedures & Criteria Used for Generating New Amplified Response Spectra (Ars) & Util Responses to Open Items

ML20246N218
Person / Time
Site:	Browns Ferry
Issue date:	06/17/1989
From:	Cheng T, Terao D Office of Nuclear Reactor Regulation
To:
Shared Package
ML20246N176	List: IR 05000260/1988039 ML20246N172
References
50-260-88-39, IEB-79-14, NUDOCS 8909080064
Download: ML20246N218 (12)

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*%--...+ f UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION TVA PROJECTS DIVISION Report No.: 50-260/88-59 Docket No.: 50-260 Licensee:  Tennessee Valley Authority 6N, 38A Lookout Place 1101 Market Street Chattanooga, TN 37402-2801 Facility N:ene: Frowns Ferry Nuclear Plant, Unit 2
 ' Inspection At: Browns Ferry Nuclear Plant Site Decatur, Alabama Inspection Conducted: December 19-22, 1988 Inspector:  g(b b$

Thbmas M. Cheng, Team Lea er

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Consultant: T. Tsai Approved by: 42 David Terao, Chief N'I bY Date Engineering Branch TVA Projects Division Office of Nuclear Reactor Regulation SCk$?,0hbjgo PDC

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I Special Inspection Relating to Design Calculations for the Seismic Design Program 1.0 INTRODUCTION AND BACKGROUND

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During the staff inspection of the Browns Ferry Nuclear Plant Unit 2 (BFN-2) Seismic Design Program conducted during the period from October 30 through November 10, 1988, a total of 32 items were identified as documented in NRC Inspection Report (VR) 50-260/88-38 dated April 19,1989(Reference 5.1). As summarized in Enclosure 3 of IR 50-260/88-38, 28 of the 32 items remained open at the end of the inspection. TVA is required to resolve these open issues before restart of BFN- In response to the open items in the design area related to the generation of amplified response spectra (ARS), TVA and its consultants Stone & Webster Engineering Corporation (SWEC) and Bechtel Power Corporation (BPC) performed additional calculations. The calculations included the generation of new ARS, the justification for using higher damping values for steel members, the com-parison of new ARS and original design ARS, the calculations of soil amplifi-cation factors and soil springs for soil-supported structures, and the consideration of coupling effects of the horizontal and vertical building response .0 PURPOSE AND SCOPE The purpose of this special inspection are (1) to review the techniques, procedures, and criteria used for generating the new ARS, and (2) to review TVA's responses to the open items identified in IR 50-260/88-38 relating to the ARS generation. These open items as identified in IR 50-260/88-38 are: CSG-2, C5G-3. CSG-4, CSG-5, CSG-6s CSG-9, CSG-16, and CSG-23. The staff will review the resolutions of the remaining open items identified in IR 50-260/88-38 in future inspection .0 DISCUSSION OF REVIEW FINDINGS During the period from December 19 through 21, 1988, the staff and its consultant conducted the second inspection of the BFN-2 seismic design program at TVA's office in Knoxville, Tennessee. The list of attendees of the entrance meeting is contained in Enclosure I of this Inspection Report. Following the entrance meeting, a three day inspection on the issues related to the ARS generation was conducted. The exit meeting was held on December 21,1988 and the attendees are listed in Enclosure In addressing staff's concerns in the areas of soil-structure interaction, percentage of mass participation in the structural responsest use of higher

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(structural damping ratios, and amplification of ground motion from the top-
-of bedrock = through soil stratum TVA used the artificial ground motion time history previously found acceptable by the staff as documented in a letter from G. Zech (NRC) to S. White (TVA) dated January 25, 1988 (Reference 5.2). Thi artificial ground motion time history was used with the 9ewly-developed structural models to generate ARS' at locations of interest for all safety-related structures.- The staff and its consultant reviewed TVA's analyses and found that the techniques and general procedures used for generating ARS of the reactor building and four soil-supported safety-related buildings are consistent with the BFN-2 FSAR requirements and are acceptable. The staff
. consultant also reviewed TVA's responses to the open items from IR 50-260/83-38'

as previously' discussed. The review findings and conclusions are sumw rized below: 3.1 (3.5.1)* Comparison of New ARS and Original Design ARS The original desigr. ARS for BFN-2 were generated based on the El Centro earth-quake ground motion time history and a single-stick lumped-mass structural model. Recently TVA generated a set of new ARS based on the artificial ground motion time history and newly-developed structural models (i.e., a multi-stick structural model for generating ARS inside drywell,'a single-stick structural model for generating ARS outside drywell of the reactor building, and a two-dimensional model for soil-supported structures). The peaks of the ARS were broadened by t10% of the corresponding frequencies for the reactor building and by 115% for those of soil-supported structures. The staff reviewed the new ARS and the comparison of the new ARS with the original design ARS and found that the new ARS with the peak broadening exceed the original design ARS in some frequency range as anticipated. As discussed in Section of this report, TVA committed to assess the impact of new ARS on the evaluations (systems and components) which were completed based on the original design ARS. Therefore, the staff considers this item close (CSG-2) 3.2 (3.5.2) Dampino Values for Steel Structural Members Inside Drywell For the new model of structural components inside drywell TVA proposed to use (1) 5% damping for reinforced concrete members, biological shield wall and concrete pedestal, (2) 7% damping for the fuel elements. (3) 3.5% and 1% dam-ping for the control rod drive (CRD) housings in the horizontal and vertical directions respectively, (4) 2% damping for the star-truss, stabilizer, re-fueling bellows and the horizontal component of springs K and K, of the i reactor pressure vessel (RPV), and (5) 1% for the remaining steeT component 'As discussed in IR 50-250/88-38, the staff considered that all damping values proposed were consistent with those specified in the FSAR and are acceptabl However, the 2% damping value used for steel members such as the star-truss dnd the stabili2er is inconsistent with the 1% damping value described in the BFN FSAR Section 12.2.8.2 for steel members inside drywell. During the-inspection, TVA provided its justification for using 2% damping for these steel members. The staff also reviewed the applicable sections of FSARs and NRC safety evaluation reports (SER) of facilities of a similar vintage to that of BFH and found that a 2% damping value was used for similar types of steel

*The number in the parenthesis denotes the section number in IR 50-260/88-3 O
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y - l l members in the seismic analyses of these plants. Therefece, the staff finds the use of a 2% damping value for the star-truss, the stabilizer and the hori-lzontal component of spring constants K3 and K2 f r the RPV acceptable and this item is closed. (CSG-3) 3.3 (3.5.3) Soil Amplification Factors (SAF) for Soil Supported Structures The soil-supported safety-related structures at BFN include the diesel gener-(DGB), the standby gas treatment building (SGTB), the residual ator building heat removal (RHR) service water tunnel, the equipment access lock, and the off-gas treatment building (OGTB). These structures are either supported on the surface of the layered soil stratum or buried in the soil with the excep-tion of the OGTB which is supported on bedrock and buried in soil. Because the input ground motion is required to be specified at bedrock for consistency with the FSAR, the staff questioned the amplification factors used to calculate the input ground motion for the analysis of each of the soil-supported and soil-buried structures when the new ARS were generate As discussed in IR 50-260/88-38 TVA agreed to: (1) apply an amplification factor of 1.6 which is consistent with the FSAR to the horizontal free-field input motion defined at top of bedrock for generating the new ARS of the DGB, SGTB, RHR service water tunnel, and equipment access lock, (2) compute the vertical soil amplification factor for all soil-supported structures using the same theory and method for calculating the horizontal amplification factors, and (3) calculate the amplification factor for the OGTB roof by an inter-polation technique. The calculated soil amplification factors for different buildings are tabulated below: Building / Location Horizontal Vertical Diesel Generator Building .1 Standby Gas Treatnent Building .2 RHR Service Water Tunnel .0

- Base Slab Off-Gas Treatment Building .3 According to TVA, no new ARS needs to be generated for equipment access loc Therefore, the amplification factors were not calculated for this structur Also, because the RHR Service Water tunnel is s::pported on piles which were driven all the way to the bedrock, there is no soil amplification in the vertical direction, i.e., SAF= The staff and its consultant reviewed the calculations of the amplification factors for all soil-supported structures and found that the theory and method used for calculating the soil amplification factors are consistent with those to which TVA committed during the first seismic design program inspection (Reference 5.1) and the calculated amplification factors (both horizontal and vertical directions) for all structures are reasonable. This item is consi-dered close (CSG-4)

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7.4(3.5.4) Soil Spring Constants for Soil-Supported Structures As discussed in IR 50-260/88-38 TVA agreed to compute the horizontal soil

 . spring constants for the RHR service water tunnel and equipment access lock using the same theory and method specified in the FSAR for the diesel generator building (DGB) and standby gas treatment building (SGTB). TVA also agreed to use the same theory and method for calculating the vertical soil-spring constants of all soil supported structures. The staff's review of the soil-spring constant calculations for all of safety-related structures including the re-calculation of the DGB and SGTB soil spring constants found that the techniques and method used by TVA are consistent with those specified in the FSAR and the calculated soil-spring constants are reasonabl Therefore, the staff considered this item close (CSG-5)

3.5(3.5.5) Coupling of Horizontal and Vertical Responses of Soil-Supported Structures In response to IR 50-260/88-38 TVA considered the coupling effects of hort-zontal and vertical responses when calculating the ARS for soil-supported structures such as the diesel generator building and the standby gas treatment building. The review of TVA's calculations found that the square-root-sum-of-squares (SRSS) rule was used for combining the co-directional responses, e.g., the vertical response and the vertical component of rotational responses due to the horizontal input ground motion. This technique is consistent with the guidelines of Regulatory Guide 1.92, " Combining Modal Responses and Spatial Components in Seismic Response Analysis" Revision 1 dated February 197 Therefore, the staff considers this item close (CSG-6) 3.6 (2.5.6) Impact of New ARS The staff's review of the comparison between the original design ARS (without peak broadening) obtained from El Centro ground motion time history and the new ARS (with peak broadening)* based on the artificial ground motion time history identified differences between the two sets of ARS. The new ARS in many cases, significantly exceeded the original design ARS. The staff had concerns about the poter.tial impact of these new ARS on the TVA's re-evaluation of miscel-laneous steel, drywell platforms, conduit and conduit supports, and supports completed based on the original design ARS. TVA is continuing to assess the impact of these differences on the completed evaluations and committed to pro-vide the staff its assessnent results dan complete. This issue remains ope (CSG-9) i 3.7 (3.5.7) Percentace of Mass Participation in the Vertical Seismic Analysis l I In conjunction with IR 50-260/88-38, the staff found that TVA considered only three structural modes for frequencies less than 20 Hz and included only 70% of the total building macs in the vertical seismic analysis of the reactor building. The same concern also applied to the vertical seismic analysis of other safety-related structures. In response to the staff's concern, TVA

 *The new ARS (with peak broadening) were developed in accordance with the staff acceptance criteria discussed in Reference _ - _ _ _ _ _ _ _ - _

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addressed this open item in two separate approaches. First, TVA included additional structural modes (higher than 20 Hz) in the generation of vertical ARS with participating modal mass up to at least 90% of total mass. This approach demonstrated that the re-generated ARS exceeded the ARS which con-tained only 70% mass participation by an amount less than 10% at a frequency around 20 Hz. Second, TVA derived a mathematical justification to show that the impact of excluding the structural modes higher than 20 Hz on the calcu-lated ARS is insignificant. TVA also showed that the contribution of the higher structural modes (frequencies between 20 Hz and 33 Hz) to the vertical ARS is negligible for the safety-related structures other than the reactor i building because these structures are relatively rigio (all vertical frequen-cies are higher than 20Hz) in the vertical direction. The staff reviewed the calculations for the ARS generation, the comparison of the two sets of ARS (one set considered structural frequencies up to 20 Hz and the other included the higher structural modes with the mass participation up to 90% of the total building mass), and the deriveo mathematical justification and found that the justifications presented by TVA ara reasonable. This item is considered closed. (CSG-16) 3.8 (3.5.8) Definition of Zero-Period Acceleration of New ARS For the new ARS. TVA calculated the spectral accelerations only up to 20 Hz (the rigid frequency of buildings as defined in the BFN FSAR) and defined the peak of floor acceleration tfme history (or the spectral acceleration at 33 Hz or higher) as the zero-period acceleration (ZPA) for the new ARS. In comparing the spectral acceleration at 20 Hz and the peak of the floor acceleration time history, the difference, in some cases, was approximately 20%-30%. TIA was requested to assess the significance of this difference for all newly-generated ; ARS as documented in IR 50-260/88-38. In order to address staff's concern and to demonstrate that this difference is insignificant in the seismic evaluation of systems. TVA plans to address this issue in two ways:

 (1) TVA will provide a comparison of two sets of ARS. In the first set, the ARS will be calculated only up to 20 Hz and the ZPAs will be defined based on the peak floor accelerations. For the second set, the spectral accelerations will be calculated up to 50 H (2) Based on the comparison of the two sets of ARS. TVA will perfom an impact study using three selected piping sy C ems outside drywell and one inside drywell .

TVA agreed This item to complete remains ope its study)and provide the results for the staff revie (CSG-23 4.0 CONCLUSION As discussed in Section 3.0 of this report, six out of eight open items re-viewed during this inspection are closed. The two remaining open items are: CSG-9 (Impact of New ARS) and CSG-23 (Definition of Zero Period Acceleration of Hew ARS). In Enclosure 3 of this inspection report, the updated resolution status of all items documented in IR 50-260/88-38 are sunmarized. TVA is

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, continuf t.g to address these open items and plans to provide its responses to'these remaining open items to the staff for review when complete .0 REFERENCES 5.1 NRC Inspection Report No. 50-260/88-38, dated April 19, 198 .2 ~ Letter from G. Zech (NRC) to S. White (TVA), Subject: " Alternate Input for Seismic System Analysis - Artificial Time History," dated January 25, 198 .3 Summary of Meetings held on September 8 and 9,1988 Concerning Resolution of ]EB 79-14 Restart Issues, dated September 19, 198 '
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ENCLOSURE 1

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   , ENTRANCE MEETING DECEMBER 19, 1988 Tennessee Valley Authority .

W. A. Massie; < R. 1 McMahon W R. Cutsinger R. J.. Hunt

 .C. Tockstein-S.- Stone-D. R. Denton J. McCall-R. O. Hernandez Bechte1~ Power Corporation
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W. Tsengi - NRC - Office of Sper.ial Projects D. Terao T. Cheng T. N. C. Tsai. Consultant l l t.

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ENCLOSURE 2 EXIT MEETING L DECEMBER 21, 1988 _ . . ATTENDEES T. Cher.g: NRC/OSP R. D. Cutsinger CEB/BFEP Robert E. Day CEB Don R. Denton CEB R. Joe Hunt CEB W. A. Massie BFN $1'te Licensing R. J. McMahon Engineering Licensing F. L. Moreadith Engineering Manager, NE Karl S. Seidle Civil Engineering W. S. Tseng Bechtel - SF Tom Tsai NCT Engineering

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(- ENCLOSURE 3 i [[ ! i l STATUS OF SEISMIC DESIGN PROGRAM ITEMS Inspection Report Inspection Report 50-260/88-38 50-260/88-39 CSG-1 (Closed) ' Review of IEB 79-14 Program CSG-2 (Closed) Comparison of New ARS and 3. 5.1 ' Original Design ARS .! CSG-3 (Closed) Damping Values for Steel 3. .2-Structural Members Inside Drywell CSG-4- (Closed) Soil Amplification Factors for 3. .3 l Soil-Supported Structures j CSG-5- (Closed) Soil Spring Constants for 3. .4 ! Soil-Supported Structures l l CSG-6 (Closed) Coupling of Horizontal and 3. .5 l Vertical Responses of  ; Soil-Supported Structures l CSG-7 (0 pen) Design Criteria and Percent of 3. l I h'ork Completed CSG-8 (0 pen) Clarification of Design 3. Criteri,a Used-i CSG-9 (0 pen) Impact of New ARS 3. .6 i CSG-10 (0 pen) Assumption of Rigid Lower 3. Platforms in the Horizontal i Direction  ! CSG-11 (0 pen) Equivalent Static Analysis of 3. Drywell Platforms j CSG-12 (0 pen) Damping Values for Platform 3. Evaluation { i CSG-14 (0 pen) Thermal Effects on Drywell 3. l Platforms  ! l CSG-15 (Dpen) Load Interface 3.1.3 _ _ - - m

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    -2-Inspection Report inspectinn Report 50-260/88-38 50-260/88-39 CSG-16  (Closed) Percentage of Mass Participation 3. .7 in the Vertical Seismic Analysis CSG-17  (Closed) End Moments on Platform 3. Radial Beams CSG-18  (0 pen) Evaluation of Embedment Plate 3. Anchors of Radial Beams CSG-19  (0 pen) Platform Clip Angle Criteria 3. CSG-20  (0 pen) List of Beam Modifications 3. CSG-21  (Closed) Superseded Pages of Flatform 3. Design Calculation CSG-22  (0 pen) Design Criteria BFN-50-C-7100 through 7300 CSG-23  (0 pen) Definition of Zero Period 3. .8 Acceleration (ZPA)ofNewARS CSG-24  (Open) Buckling of HVAC Ductwork 3. CSG-25  (0 pen) Buckling of Aluminum Conduit 3. CSG-26  (0 pen) Allowable Stress for Aluminum 3. Conduit CSG-27  (0 pen) Flexible Conduit Program CSG-28  (0 pen) Use of Factor *I.33" to Increase 3.2.10 Stress Allowable CSG-29  (0 pen) Modeling of HVAC Supports 3. CSG-30  (Closed) Welding Allowables for HVAC 3. Supports CSG-31  (0 pen) Buckling of Conduit Rod Supports 3. CSG-32  (Oxn) Evaluation of Support Rod Hangers 3. CSG-33  (0 pen) Evaluation of Conduit Supports 3.4.5

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