ML20198R351

Forwards Open Items Associated W/Seismic Margins SER

ML20198R351
Person / Time
Site:	05200003
Issue date:	12/11/1997
From:	Joseph Sebrosky NRC (Affiliation Not Assigned)
To:	Liparulo N WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
References
NUDOCS 9801230288
Download: ML20198R351 (11)

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Dec:mber 11, 1997 Mr. Nicholas J. Liparulo, Manager Nuclear Safety and Regulatory Analysis Nuclear and Advanced Technology Division t

Westinghouse Electric Corporation P.O. Box 355 Pittsburgh, PA 15230

SUBJECT:

OPEN ITEMS ASSOCIATED WITH SEISMIC MARGINS SAFETY EVALUATION REPORT (SER)

Dear Mr. Liparulo:

The Civil Engineering and Geosciences Branch (ECGB) of the Division of Engineering has provided an SER for the seismic margins review specified in Chapter 55 of the Westinghouse AP600 Probabilistic Risk Assessment. However, the SER contained some open items. These open items have been extracted from the SER and can be found in the enclosure to this letter.

You have requested that portions of the information submitted in the June 1992, application for design certification be exempt from mandatory public disclosure. While the staff has not completed its review of your request in accordance with the requirements of 10 CFR 2.790, that portion of the submitted information is being withheld from public disclosure pending the staff's final determination. The staff concludes that these follow on questions do not contain those portions of the information for which exemption is sought. However, the staff will withhold this

letter from public disclosure for 30 calendar days from the date of this letter to allow Westing-house the opportunity to verify the staffs conclusions. If, after that time, you do not request that all or portions of the information in the enclosures be withheld from public disclosure in accor.

dance with 10 CFR 2.790, this letter will be placed in the Nuclear Regulatory Commission Public Document Room.

if you have any questions regarding this matter, you may contact me at (301) 415-1132.

Sincerely, original signed by:

Joseph M. Sebrosky, Project Manager Standardization Project Directorate Division of Reactor Program Management Office of Nuclear Reactor Regulation Docket No. 52 003 i

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Enclosure:

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cc w/ encl:: See next page DISTRIBUTION: See next page htMI DOCUMENT NAME: A:ECGB.,,SMA.RAI To receive a copy of this document, Indcate in the box: "C" Spop fwlthout attachment / enclosure "E" = Copy wjh attachment / enclosure

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Mr. B. A. McIntyre Ms. Cindy L Haag Advanced Plant Safety & IJcensing Advanced Plant Safety a Licensing Westinghouse Electric Corporation Westinghouse Electric Corporation Energy Systems Business Unit P.O. Box 355 Energy Systems Business Unit Box 355 Pittsburgh, PA 15230 Pittsburgh, PA 15230 Enclosure to be distributed to the following addresseus after the result of the proprietary evaluation received from Westinghouse:

Mr. Russ Bell Ms. Lynn Connor Senior Project Manager, Programs DOC-Search Associates Nuclear Energy Institute Post Office Box 34 1776 i Street, NW Cabin John, MD 20818 Oulte 300 Washington, DC 20006 3706 Mr. Robert H. Buchholz GE Nuclear Energy Dr. Craig D. Sawyer, Manager 175 Curtner Avenue, MC 781 Advanced Reactor Programs San Jose, CA 95125 GE Nuclear Energy 175 Curtner Avenue, MC-754 Mr. Sterling Franks San Jose, CA 95125 U.S. Department of Energy NE 50 Barton Z. Cowan, Esq.

19901 Germantown Road Eckert Seamans Cherin & Mellott Germantown, MD 20874 600 Grant Street 42nd Floor Pittsburgh, PA 1521g Mr. Charles Thompson, Nuclear Engineer AP600 Certification Mr. Frank A. Ross NE 50 UI Department of Energy, NE 42 19901 Germantown Road Office of LWR Safety and Technology Germantown, MD 20874 19901 Germantown Road Germantown, MD 20874 Mr. Ed Rodwell, Manager PWR Design Certification Electric Power Research Institute 3412 Hillview Avenue Palo Alto, CA 94303

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Open hems Associated with Seismic Margins Review Cpen item 73g.448F Mode Combination (from SER input Sootion 88.3.1.1.4) k Westinghouse stated that fragility parameters associated with modo combination are not included in the FA methodology because they do not effect the HCLPF values calculated using j

the probabillatic FA method. Westinghouse provided the following reasons in response to RAI 230.107.

For primary components supprts:

i e SG supports and RPV supports The seismic response for these structures is from time history (TH) analyses and not from response spectra. Therefore, mode combination fragility parameters are not appropriate.

• Pressurizer supports The pressurtzer response is predominantly resulting from raodos around 23 Hz, and modes

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above 33 Hz which are in the rigid region. Therefore, tlw respor se is not multi-mode in character and thus modo combination fragility parametoa are not appropriate.

j Containment vessel:

The seismic response that contributes to the critical " failure mode" which is buckling is i

predominantly single mode and not multi-mode. Therefore, mode combination fragility _

parameters are not appropriate. The containment vessel is a cylindrical structure so that the i

earthquake response from each of the seismic components are uncoupled and any effect from variations due tc combination of earthquake components are negligible.

Containn.ont intamal structure and IRWST tank modules:

The seismic response is not made up of multiple modes as a multi-mass syskm, and therefore, variation in seismic response due to modo combination is not applicable.

However, Reference 2 (page 31g) states that the combination of response modes is random due to random phasing of the individual modal responses. This is true whether a response spectrum or a TH analysis is performed.- A TH analysis, conducted using a different earthquake record but with the same ground motion parameter value (e.g., pga), will result in different phasing between the Fourier components and hence a different peak response, it recommends that a 8, of 0.15 for structures with multiple important modes and 0.05 for simple structures, such as a containment building that responds primarily in a single mode. Westinghouse should revise the AP600 PRA Chapter 55 to include this mode combination randomness for the protodlistic FA method. This is an Open item.

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Open llem 720.444F Earthquake Component Combination (from SER input

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. For pdmary component supports (8G, RPV, and Pressuriser), containment vessel, and containment inlemal structure and IRW8T tank modules, Westinghouse stated that the critical I

component esismic load is t rt.W primarily on a single earthquake componard, and therefore, fragility parameters associated with the combination of earthquake components are

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not included. However, Reference 2 (page 3 27) recommends that a randoess (a,) for response be included in the FA since the actual response will be higher or lower and provides an effects of earthquake compon)ent combination Westinghouse rationale for not using this randomness for the probabilistic FA. Westinghouse responded by quoting "that an upper bound value of 8, equal to 0.18 can alwsp be used but may be excessively conservative for cases where the response is primarily from a single direction"In Refetonon 2 (page 3 27). Westinghoun -laimed that since the cruical seismic load is depende primarHy on a single earthquake component or the components are uncoupled, any effect from variations in response due to the combination of earthquake components are negligible (RAI 230,137, R1), However, this statement needs to be justified since for the example given, the containment will see ampression loads due to one horizontal earthquake along with shear i

loads due to the other horizontal earthquake, as well as compressive loads due to the vertical i

earthquake, The effects from all three components should be considered, For comparison, Reference 3 (page 15e) also shows the variability of a combination of earthquake components (S.) as 0.15 in addition to analysis and modeling error. Westinghouse should revise the AP600 PRA Chapter 55 to include this combination of earthquake components for the probabilistic FA l

method. This is an Open item.

t Open item 720.447F Strength (from SER input Section 58.2.1.2.1)

A. Variable Strength Factors Westinghouse used different margin factors for the different failure modes:

For buckling, the margin factor of 1,5 and composite LSD of 0,11 without material variability are used. The margin factor 1.5 was cased on the curve in ASME Code Case N 284, Revision 0 which was derived from lower bound tests. Using test data provided by l

Westinghouse, the staff performed a regression analysis based on methodology provided in NUREGICR 4604, and found that the median is higher than 1,5 times the lower bound curve.

Therefore, the median factor of 1,5 is acceptable The composite LSD of 0.11 is acceptable based on Reference 11 (page 847).

For shear stiengih of high-strength bolts, the mean shear strength of 0.6250 m,and the composite LSD of 0.05 are used.

For shour strength of weld, the mean shear strength of 0.840 m% and the composite LSD of 0.10 are used.

Based on response to RAI 230.133, Revision 1, the failure mode for SG upper support rin girder flange joint bolts is tension failure, This is not presented in the AP600 PRA. Based on

- References 5 and 8, the composite LSD is specified as 0.02,

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3 Reference 2 (page 315) specifies the composite LSDs as 0.13,0.10 and 0.19 fc de tension strength of bolt, the shear strength of bolt, and the shear strength of weld, respectively. These values are used for common materials. Therefore, Westinghouse should revise the AP600 PRA Chapter 55 to increase the composite LSDs for these common materials. This is an Open item.

Open item 720.448F Review of Large Supports (from SER input Section 55.2.1.3)

For the RPV and SGs, it seems that the variability of the, flow responses are not properly accounted for, The calculated B, values of 0.27 and 0.I u1 Table 551 of the AP600 PRA Chapter 55 ere considered to be too low in comparison w?, typical values of 0.5 to 0.6 in the past seismic PRA studies. The staff requested the rationale for the nonconservative evaluation of variabilities. If it is intended to use conservative FRS to compensate for this nonconservative assumption (i.e., low GJ, the staff requested Westinghouse explain quantitatively that the net results fer the HCLPF calculations are still on the conservative side (230.133, R1).

Westinghouse responded that a noncons9rvative evaluation of variabilities was not used.

Comparisons to typical values from past generic seismic PRA studies cars be misleading since they reflect a large population of different types of compments having different types of failure modes, and therefore, potentially have large variability. The values used in the AP600 SMA for total variabilities reflecting both randomness and uncertainty components are appropriate for the critical fraglNy modes of the primary components.

The goveming failure modes associated with the RPV and SG are related to bolts within the primary component supports:

SG upper support ring girder flange joint bolts tension failure RPV support box hold down bolts shear failure Inelastic energy absorption or ductility was not considered since the governing failure modes are local without large energy absorption capability. This reserve margin factor associated with ductility generally has a large variability which significantly contributes to the variability B,. A review of ABWR fragility data associated with the RPV primary component r.upports reported in their SSAR (Seismic Capacity Analysis, Amendment J1) was made. It was found that the variability used was not near the 0.5 to 0.6, but much nearer to the 0.27 and 0.29 values used in the AP600 SMA (cf., the variability B, of ABWR RPV pedestal, support, and shroud without ductility are 0.36,0.33, and 0.36, respectively). The staff believes that West:nghouse considered neither response variabilities from mode combination and earthquake component combination nor capacity variabilities from tension strength of bolt, shear strength of bolt, and shear strength of weld as discussed above. Therefore, the AP600 PRA Table 55-1 should provide the latest variabilitles used for RPV and SGs supports. This is an Open item.

Open item 720.449F inelastic Energy Absorption (from SER Section 55.2.2)

The increased capacity due to inelastic energy abseption is defined using recognized deterministic methods. It is only applied to the column structural elements which act as shear walls in the Shield Building roof. The formulation defining ductility margin follows the effective frequency / effective damping approach given in Reference 7 (OITS 5038).

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it is stated that the inelastic energy absorbing factor, FJ, is estimated for the column structural

-- elements in the Shield Building roof, for which the CDFM approach in Reference 6 is used. It is also stated that an additional margin factor is considered te scoount for a higher damping value

'due to inelastic responses. However, the formulation for the F, factor in the PRA should be used to modify the linear responses for which a liaear (lower) damping value (e.g., 7 pecent for concrete structures) is used. To account for both the F, factor and a higher damping value is considered to be a double counting of the nonlinear respr:nse effects and should be avoided.

Westinghouse agreed that double counting of nonlinear response effects should be avoijod and -

modified the HCLPF calculations (RAI 230.134, R1). According to the calculations in References 8 and g, the HCLPF value of the Shield Building is controlled by the failure of the tension ring, and its capacity is estimetod to bo 0.648g. In the calculation, an inelastic energy absorption factor of 1.1g is assumed for the columns only, and a factor of 1.0 is assumed for other i

componsats of the Shield Building. In evalesting the seismic margin of tension ring, the effects of biaxial bending and the torsional moment (along the ring axis) are not considered properly.

Reference 6 (page 6 5) suggested that in lieu of computing F,, for all but the most brittle failure

' modes, F, can be conservatively chosen as being equal to 1.25 which is as low as any of the results presented in Reference 7 for shear wall structures. Most components have both ductile and non-ductile failure modes. Non-ductile failure modes must be checked. Unless the capacity-i of the lowest non-ductile failure mode exceeds the yield capacity ofine lowest ductile failure mode by at least 125 percent, the component CDFM capacity should be defined by non-ductile failure modo capacity wih F, = 1.0. The 1.25 factor accounts for the variability in the yield and i

brittle capacities.

Due to a very low shear span ratio (about 0.2) the Shield Building columns are considered to fail in a very brittle diagonal failure under in-plane shear loading. The assumed F, of 1.ig is, therefore, considered inadequate for this type of brittle failure mode.

j The tension ring is expected to fall predominantly in tension. The failure mode (even if the torsional effects are considered) is not considemd to be a b'ittle rupture under cyclic earthquake loads, Therefore, it is considered adequate to use a F, of greater than 1.0 (but not greater than 1.25)in the HCLPF evaluation of the tension ring.' Although this assumption yielded a Lonservative HCLPF value, nevertheless the calculation package should be revised, and Table 55-1 of the PRA should be updated to reflect the new HCLPF values. This is an Open item.

i f3pc n item 720.450F Damping (from SER input Section 55.2.2.3)

A margin factor associated with damping is defined recognizing ths; damping of rainforced concrotc can increase from 7 percent te 10 percent when cracking is present.- This margin factor p

is equal to the ratio of the spectral accelerations at 7 percent and 10 percent damping for the dominant building structure frequency.

However, Reference 6 (page 2-48) recommended that the higher damping values (e.g.,

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10 percent for concreto structures) only be used when fixed base linear elastic analyses are performed since these higher values are likely to incorporate some radiation of energy back into 11 the foundation media (rock or soil) and some hysteretic or:srgy dissipation from nonlinear L

- behavior. Rsference 8 shows a margiri factor of 1.1 for damping was used. Westinghouse

should revise the AP600 P'tA Chapter 55 for the damping margin factor of 1.0 to be used. This 1

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Open Mem 720.451F Generic Fragility Data (from SER input Section 55.2.5)

Generic fragility data were used when insufficient information was available to define the HCLPF value using one of the methode described above. Those cases where this approach was used were:

Reactor intomals and core assembly that includes fuel Control rod drive mechanism (CRDM) and hydraulic drive units e

Reactor coolant pump e

e Accumulator tank Piping e

e Cable trays e Valves Battery rads e

e Main control room operation and switch stations

• Ceramic insulators Reference 10 was used for all of the components listed above except ceramic insulators, for which recognized industry low-fragility data were available.

The staff believes that the,uggested generic fragility values are intended for a preliminary analysis only. These generic values should not be used for critical components which are important to plant risks. In addition, for components with new design features, it should be confirmed that the new design features do not potentially contribute to lowering fragility values.

Such examples may include the fuel rods, for which some differences in design (e.g., different outside diameter and additional gas space below the fuel pellets) are observed compared with the typical four loop design.

Westinghouse responded that Reference 10 prcvides a summary of generic fragility data for preliminary analysis only; however, they are representative of the antic',7ated capacity.

Westinghouse has identified a COL item which requires verification of as-built conditions conforming to the seismic margin esaluation, it is stated in the AP600 PRA Chapter 59, Section 59.10.6, "The cornbined license applicant referencing the AP600 certified design will confirm that the as-built plants conforms to the design used as the bases for the seismic margin evaluation" (RAI 230.136). The verification of as-built conditions is discussed in Subsection 55.3 of this FSER.

The staff requested information on HCLPF magin for rigid components with non-ductile supports. The SSF design load and the RLE for the AP600 are 0.3g and 0.5g, respectively.

Therefore, a HCLFF margin of 1.67 is implied for all the safety related equipment and components. To achieve this HCLPF margin, a median margin factor of at least 4.2 is needed.

This is based on assumptions that a relativelylow variability of 2, of 0.40 is used for a fragility estimate, and the seismic design is performed up to the limits of the code design allowables.

For relatively flexible / ductile comporents, such as piping, the design criteria in the PRA is considered to give a sufficient margin to achieve the above median factor of 4.2 However, for dynamically rigid components whose support structures are considered to have a non-ductile failure mode, such as elastic buckling and shear failure in fillet welds or anchor bolt joints, the s

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f design requirements in the SSAR may not be sufficient to provide this safety' margin. Acco to the staff's estimate, an additional m.Jian margin factor of 2.1 to 3.0 is necessary to achieve the aforementioned HCLPF margin of.1.67 for relatively rigid components with non-ductile -

support structures.

Westinghouse responded that the components in the AP600 design generally have margin _ _

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- factors in excess of the range of 2.1 to 3.0. The calculated margins of several specific examples x

were included in the response to RAI 230.13g, Revision 1.

For the hpEt?M case where the component has rigid response characteristics with non-ductile support structures, Westinghcuse stated that generic fragility data that is in the public domain, and is also used in Reference 10, does not reflect components having low HCLPF values. Therefore, this hypothetical case would be very plant specific. The AP600 component support designs do not deviate from those seen in other plants and reflected in the generic fragility data, and, therefore, have HCLPF values below seismic margin requirements if this hypothetical case did exist, it would be a plant specific case, and it would probably not provido the seismic margin commitments of AP600, unless the as-built plant is verified to conform to the

- seismic margin of 0.Str. It is stated in PRA Chapter 5g, Section 5g.10.6, "The combined license applicant referencing the AP600 certified design will confirm that the as-built plants confonti to the r9 sign used as she bases for the seismic margin evaluation." Therefore, thl4 case would be ider&d and addressed by the COL applicant (RAI 230.139, R1).

c However, it is not clear how an as-built verification program would identify such deficiencies in median margin factors for other supports which r..ay be designed up to the code allowsbie values. This is required to be verified by a COL Action item. This is an Open item.

g Open item 720.452F Verification of Equipment r ragility Data (from S'IR input Section 55.3)

Since no walkdowns can be performed at this time, the staff requested Westinghouse to show how the key assumptions for structures, systems, and components cont'dered in the SMA can be verified for the as-built and as operated plant conditions. Examples of this include proper anchorage of equipment and seismic fragility of electri::al/ electronic equipment which may be different in the futurec Westinghouse responded that a verification that the as-built plant confirms L

the basis of the seismic margin evaluatior Mil be performed by the COL applicant (RAI 230.115).

i Westinghouse stated in Section 55.2.2.5 of AP600 PRA that the seismic margin evaluat!on has focused on demonstrating that the design of nuclear island structures, safety-related equipment,

' and equipment supports can cany the loads induced by the RLE. This evaluation incorporates L

as-specdied equipment data. After the plant has been built, it will be necessary to perform a

- verification of the seismic margin assessment for the installed conditions. The AP600 PRA SectiJn 59.10.6, Revision 9 provides the COL information for the AP600 PRA, including the SMA. The COL action item for seismic margin, as stated in the PRA, is the COL applicant referencing the AP600 certified design will confirm that the as-built plant conforms to the design 7 used as the basis for the seismic margin evaluation. It is the responsibility of the COL applicant to define how this confirmation is performed (RAI 230,112),

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The staff agrees that there needs to be a verification program to confirm the data arid assumptions made in the SMA for allitems. The description of how this will be accomplished is

. lacking and should be described. The process of identifying w,at data and assumptions need to t

be verifisk w and where they will be documented, and how the verification process will be L

conducte.c <,.he COL applicant thould be included in the AP600 PRA. For example, where t

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generic fragility data from Reference 10 was used, will a new SMA be performed by the COL applicant to confirm the assumed HCLPF values? Based on the above discussion, this is an Open item.

Open hem 720.453F Spatial Interaction (from SER input Section 55.4)

The staff requested Westinghouse that spatial interactions (e.g., seismic impact between adjacent componants and Seismic II/l interactions) be included in the SMA. Westinghouse responded that the interaction between the Turbine Building and the north end of the Auxiliary Building is explicitly discussed in PRA Section 55.5.8 (RAI 230.114).

As part of SMA, the selsmic interaction between the Turbine Building and the Nuclear Island was evaluated. It was determined that:

• The adjacent Auxiliary Building structuralintegrity will not be lost with the failure of the Turbine Building.

it is not likely that the size and energy of deris from the Turbine Building will be large enough e

to result in penetration through the Auxiliary Building roof structure.

Even though it is not likely that the Turbine Building debris could be large enough or have sufficient energy for penetration through the Auxiliary Building roof structure, thi* event was evaluated. The consequences of damage to the safety-related equiptrW in the Auxiliary Building was investigated. it was determined from this investigation that, should an event occur inat causes the failure of equipment in the upper elevations of the Auxiliary Building, the results of the SMA analysis, the plant HCLPF value, and the insights derived from the SMA would not be affected. Moreover, according to the AP600 focused PRA results, steam line break events that would result from damage to equipment in upper elevations are not dominant contributors to the core damage frequency. Further, any loss of equipment in the upper elevations would not affect the passive safety systems used to put the plant in a safe shutdown condition should an event occur.

The information presented in Sections 55.5.8 and 55.2.2.6 of the AP600 PRA does address the concem of seismic interaction between the Turbine Building and the Auxiliary Building. In the AP600 PRA Section 55.3.3, Annex Building, Diesel Generator Building, and Radwaste Building are assumed to have fal'ed for the SMA. No credit is taken for systems in those buildings. The interaction between the other building and the Nuclear Island is assumed to have no detrimental effect on the Nuclear Island structures. However, the AP600 PRA does not address how the failure of the Annex Building and/or the Radwaste Building affects the safety related structures and components of the Nuclear Island. The concem ofinteraction effects also includes potential impac1 from deflection of adjacent componen's or collapse of non-seismic Category I structures and components. This is required to be verified by a COL Action item. This is an Open item.

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References:

1.

Westinghouse AP600 SSAR Appendix H, Seismic Margin Assessment, Revision 1, dated July 22,1994.

2.

J. R. Benjamin and R. P. Kennedy, " Methodology for Developing Seismic Fragilities,"

EPRI TR-103959, June 1994.

3.

Uncertainty and Conservatism in the Seismic Analysis and Desion of Nuclear Facilities.

Working Group on Quantification of Uncertainties, American Society of Civil Engineers, ISBN 0-87262-547 8,1986.

4.

Greimann, L., et al., "Probabilistic Solsmic Resistance of Steel Containment,"

NUREG/CR-3127, January 1984.

5.

Westinghouse letter from B. A. McIntyre to Brookhaven National Laboratory, "Information for Review of AP600 Large Support Seismic HCLPF Evaluations," dated August 13, 1997.

6.

J. W. Reed, et al., "A Methodology for Assessment of Nuclear Power Plant Seismic Margin," Revision 1, EPRI NP-6041 SL, August 1991.

7, R. P. Kennedy, et, al., Enoineerino Characterization of Ground Motion - Task 1. Effects of Characteristics of Free-Field Motion on Structural Response. NUREG/CR 3805, May 1984.

8.

Westinghouse letter from B. A. McIntyre to Brookhaven National Laboratory, "Information for Review of AP600 Shielding Building Seismic Margin HCLPF Evaluation," dated October 17,1997.

9.

Westinghouse letter from B. A. McIntyre to Brookhaven National Laboratory, "Loarting information from ANSALDO Calculation No.1277-S3C-006, Revision 2, For the AP600 Shielding Building," dated October 30,1997.

10.

Advanced Light Water Reactor Utility Requirements Document, Volume lit, ALWR Passive Plant, Chapter 1, Appendix A, PRA Key Assumptions and Groundrules, Revisions 5 & 6, Issued 12/93, 11.

Greimann, Lowell and Fanous, Fouad, " Reliability of Containments under Overcmesure,"

Pressure Vessel and Piping Technology, A Decade of Progress,1985, pp. 835-856.

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