ML18052A622

Discusses Mod of Seismic Design Criteria to Incorporate ASME Code Case N-411 & How to Employ Use of Damping Values Other than 3,5 & 7 Not Given on Spectra Plots in NUREG/CR-1833

ML18052A622
Person / Time
Site:	Palisades
Issue date:	07/28/1986
From:	Kuemin J CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:	Office of Nuclear Reactor Regulation
References
RTR-NUREG-CR-1833 NUDOCS 8608040138
Download: ML18052A622 (6)

Text

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consumers Power POWERiNii NllCHlliAN'S PROliRESS General Offices: 1945 West Parnell Road, Jackson, Ml 49201 * (517) 788-0550 July 28, 1986 Director, Nuclear Reactor Regulation US Nuclear Regulatory Commission Washington, DC 20555 DOCKET 50-255 - LICENSE DPR PALISADES PLANT -

MODIFICATION OF THE SEISMIC DESIGN CRITERIA TO INCORPORATE ASME CODE CASE N-411 Prior to issuance of the June 1986 revision to NRC Regulatory Guide 1.84, ASME Code Case N-411, "Alternative Damping Values for Seismic Analysis of Classes 1, 2 and 3 Piping Systems,Section III, Division 1, was conditionally acceptable requiring approval of the NRC for use. The Regulatory Guide now stipulates the acceptable conditions for use and authorization for use is provided in 10CFR50.55a footnote 6.

Circumstances of Palisades prompted Consumers Power Company to seek NRC staff advice in the use of the subject code use. Although not required, the Staff asked we state how we intend to meet the conditions of the Regulatory Guide for code Case N-411 and further indicate how we intend to employ the use of damping values other than 3, 5 and 7 which are not given on the spectra plots in NUREG/CR-1833. I t is the purpose of this lett~r to respond to the Staff's request.

BACKGROUND The seismic design for the Palisades Plant was conducted in the 1966 to 1969 time period. The data and design methodologies employed were typical for plants of that era. The seismic design criteria employed today for re-evaluation and modification of existing systems differ very little from original guidelines. Seismic criteria for piping systems and major building structurals are essentially unchanged from the original design. Seismic criteria for equipment in the original design were not as well defined as for piping and buildings. Therefore, seismic equipment criteria presently employed at the plant tend to reflect more recent industry standards.

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Director, Nuclear Reactor Regulation 2 Palisades Plant Seismic Design Criteria July 28, 1986 The Systematic Evaluation Program (SEP) assessed older plant designs in terms of recent criteria to ensure that older plants reflect current design margins with respect to many design topics. Seismic design, SEP Topic III-6, was one of the design issues thoroughly investigated in the 1978 to 1982 time period at Palisades. Certain seismic topics are still being reviewed under the auspices of the "Integrated Plant Safety Assessment Report" (IPSAR).

The principal record documenting the SEP III-6 seismic review of the Palisades Plant was NUREG/CR-1833 (Reference 1). That evaluation was prepared for the NRC by Lawrence Livermore Laboratory. NUREG/CR-1833 evaluated overall structurals, selected equipment and four piping systems with regard to more recent seismic criteria. NUREG/CR-1833 employed the response spectral shape of US NRC Regulatory Guide 1.60 for free field seismic input motion to the structures. In addition to providing selective analysis results and a list of open items for further review, the NUREG provided amplified response spectra at certain elevations of the reactor and auxiliary buildings for evaluation of plant structural subsystems. It was the intent of the NRC that those amplified response spectra be employed in the evaluation of SEP III-6 open i, items until a suitable "site specific" response spectra could be developed.

Immediate follow on work to NUREG/CR-1833 was conducted employing those \_

spectra.

Site specific ground response spectra were provided to a number of plants by the NRC in Reference (2). Had those spectra been available two years earlier, they may have been used in NUREG/CR-1833 for Palisades. The site specific spectra curves for the various sites were all for 5 percent damping. However, a computational algorithm was provided for conversion to other damping values. .:

In Reference (3), the NRC provided a comparison of three ground response spectra considered for the evaluation of plant equipment at Palisades. The graphical comparison showed that for a given zero-period-acceleration, the Regulatory Guide 1.60 free field response spectra completely enveloped the site specific response spectra which in turn enveloped the Housner response spectra which the NRC incorrectly believed to be the basis for the Palisades amplified floor spectra. The NRC concluded that earlier Consumers Power Company submittals to them were incomplete due to the fact that the Housner spectra was simply too low for equipment evaluation. The Staff strongly encouraged use of the site specific spectra for SEP III-6 open item evaluation. Clearly, the Regulatory Guide 1.60 spectra would also have been acceptable to the Staff in that it enveloped the site specific spectra.

In Reference (4), Consumers Power Company responded to Reference (3) by expressing reluctance to develop a new site seismic criteria in view of the results of analysis work performed in the evaluation of SEP III-6 open items.

While the Staff contention that the site specific spectra enveloped the Housner is correct, it is also true that the Housner spectra is not that employed in the horizontal seismic amplified response evaluation of Palisades Plant subsystems. The horizontal response spectra developed for Palisades are based upon the modified-Taft 1952 time history. The Palisades spectra neither OC0786-0123-NL,Q4

Director, Nuclear Reactor Regulation 3 Palisades Plant Seismic Design Criteria July 28, 1986 envelop nor are bounded by the site specific or by the Regulatory Guide 1.60 spectra. The site specific and Regulatory Guide 1.60 spectra are relatively low, broad-band spectra as compared with the highly peaked spectra used at Palisades.

• At the time of Reference (4) there was little motivation to incorporate new response spectra into the Palisades Plant design basis. This lack of motivation was due to the following:

0 Analyses of mechanical equipment and cabinetry indicated that items which failed NUREG/CR-1833 criteria failed FSAR criteria as well and items which passed either criteria passed the other. Thus, equipment analysis results seemed somewhat spectra independent.

0 Work funded by the NRC on portions of the residual heat removal, component cooling, auxiliary feedwater and regenerative heat exchanger letdown piping systems all met NUREG/CR-1833 criteria as they had earlier been shown to meet FSAR criteria as a result of IE Bulletin 79-14 work.

Again, the implication was that for large bore pipe the evaluation of the systems was somewhat spectra independent.

0 A significant amount of work would be involved in developing amplified floor response spectra from the free field site specific response spectra.

In the last year or so, there has been significant impetus for revising the seismic input spectra for Palisades in order to define new amplified (floor) response spectra. The primary thrust for revising spectra is to develop an overall seismic design basis which can accommodate ASME B&PV Code Section III, Code Case N-411. Code Case N-411 permits a significant increase in: response spectra damping values for piping analysis given that other segments of the plant seismic design criteria are consistent with current practice and guidelines. Code Case N-411 would, therefore, predict a lower subsystem response for a given set of spectral curves. Since the input (free field ground) spectra are integral parts of the seismic design basis, they too need to be made consistent with current practice. It is understood that broad-band input spectra such as those of Regulatory Guide 1.60 or the site specific spectra would be required to warrant use of Code Case N-411.

It is expected code Case N-411 usage at Palisades will be useful in changing out large bore pipe snubbers to passive restraints. It is highly likely that most requests from other nuclear plants for implementation of Code Case .N-411 are based upon such snubber reduction considerations. Recent work at Palisades indicates that Code Case N-411 can also be useful in evaluating the impact of modifications to small bore pipe (2~ inches and smaller in diameter). With present design response spectra, calculated acceleration responses of small bore pipe are very frequency sensitive in the 10 to 12 HZ range. Even in lower elevations of the reactor and auxiliary buildings, spectral ordinates can vary by a factor of 6 to 8 in this narrow frequency OC0786-0123-NL04

Director, Nuclear Reactor Regulation 4 Palisades Plant Seismic Design Criteria July 28, 1986 range. Such highly peaked spectra appear to represent an unrealistic design requirement and tend to result in detailed analyses of small bore systems which were never rigorously analyzed at or since construction and which do not lend themselves to precise evaluation.

BASIS FOR CODE CASE N-411 IMPLEMENTATION Consumers Power Company has reviewed Regulatory Guide 1.84 (Reference 5) and other Staff correspondence to licensees concerning the condi~ional use of Code Case N-411. It is believed that the intended use of Code Case N-411 by Consumers Power Company is relatively straightforward. For Palisades Plant applications, Consumers Power Company has not and does not intend to:

0 Employ ASME Code Case N-420 for inclusion of energy absorbers in systems.

0 Employ ASME Code Case N-397 involving spect~al shifting.

0 Employ time history methods in seismic dynamic analysis.

0 Employ Code Case N-411 where stress corrosion might occur.

0 Employ multiple input spectra support motion.

Consumers Power Company does intend to use Code Case N-411 for new analyses and reconciliation work. For work on snubber changeout, the large bore systems will not be likely to become more flexible as a result of Code Case N-411 use. For use in small bore pipe evaluation, Code Case N-411,will undoubtedly result in softer systems. In either case, potential increases in system displacements and equipment loads would be evaluated as a matter of normal practice.

It is understood that the existing seismic 'design response spectra for Palisades were not developed using current criteria such as those now employed in soil structure interaction, soil modulus definition, soil stiffness variation and spectral broadening (Regulatory Guide 1.122). The free field spectra does not conform to a broad band input as would be expected from multiple time history input (Regulatory Guide 1.60). It has been concluded that the most straightforward approach to be adopted by Consumers Power Company is to employ the Regulatory Guide 1.60 derived response spectra of NUREG/CR-1833.

NUREG/CR-1833 provides 3 horizontal spectra at 3 locations for the reactor building internals, 1 horizontal spectra on the reactor building shell, 6 horizontal spectra at three locations in the auxiliary building and single vertical spectra in both the auxiliary and reactor building. The damping values on the individual spectra plots are 3, 5 and 7 percent. A 2 percent damping curve would typically be generated by time history analysis or extrapolated for implementation of Code Case N-411 beyond 10 HZ. The OC0786-0123-NL04

Director, Nuclear Reactor Regulation 5 Palisades Plant Seismic Design Criteria July 28, 1986 feasibility of generating.the 2 percent curve directly is being evaluated.

Until the time that additional* 2 percent curves are generated, Consumers Power Company intends to employ the NUREG/CR-1833 damping curves for 3 percent beyond 10 HZ.

Beyond 10 HZ, Code Case N-411 proposes using damping curves based upon linearly interpolated damping values from 5 percent at 10 HZ and 2 percent at 20 HZ and then 2 percent for the damping value beyond 20 HZ. Clearly, using a 3 percent curve is conservative between 10 and 16.7 HZ. Between 16.7 and 20.0 HZ, the 3 percent damping curve would yield lower spectral accelerations than would be anticipated for the interpolated damping values. However, in this very narrow band region, the Regulatory Guide 1.60 damping curve for 3 percent is not expected to provide lower accelerations than a 2 percent damping curve from an amplified response spectra associated with the site specific spectra which the Staff had earlier endorsed for Consumers Power Company use. Beyond 20 HZ, spectra curves for all damping levels are essentially convergent. The difference in damping levels (2 to 3 percent) between 16 to 20 HZ yields very litt'le difference in spectra acceleration. In addition, spectral accelerations at these frequencies are not expected to make a significant contribution to the overall system response for P~lisades piping. Interpolation on spectral curves between damping values will be done logarithmically.

With the existence of NUREG/CR-1833 spectra, Consumers Power Company intends to:

0 Implement Regulatory Guide 1. 92 with regard to directional combinations.

and closely spaced modes.

0 Review the impact of higher mode (greater than 33 HZ) as contributing to overall response and interpret any potential response in terms of the modal effective mass fraction.

0 Employ equipment damping values consistent with Regulatory Guide 1.61 and use composite damping in the-system where appropriate.

It is noted that no major re-evaluation effort is planned for Palisades. Code Case N-411 is intended to be used to replace snubbers with struts and to render small bore piping designs more flexible and to make them more amenable to design without the detailed computer analysis required to evaluate natural frequencies precisely.

CONCLUSIONS Based upon a review of Palisades Plant seismic design data, recent literature and associated Consumers Power Company and NRC correspondence on seismic design issues, it is concluded that the Palisades seismic criteria for piping OC0786-0123-NL04

Director, Nuclear Reactor Regulation 6 Palisades Plant Seismic Design Criteria July 28, 1986 should be changed. It is judged that any such change should be based upon the incorporation of ASME Code Case N-411 into the design. It is therefore Consumers Power Company's intentions to fulfill the conditions defined in Regulatory Guide 1.84 for code Case N-411 *

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tKuemin Staff Licensing Engineer CC Administrator, Region III, USNRC NRC Resident Inspector - Pal*isades REFERENCES 1 NUREG/CR-1833, Seismic Review of the Palisades Nuclear Power Plant Unit 1 as part of the Systematic Evaluation Program, dated January 1981 2 Letter from D M Crutchfield, NRC to All SEP Owners (Except San Onofre),

dated June 17, 1981.

3 Letter from D M Crutchfield, NRC to D J VandeWalle, Consumers Power Company, dated September 6, 1983.

4 Letter from B D Johnson, Consumers Power Company, to D M Crutchfield, NRC, dated April 3, 1984.

5 US NRC Regulatory Guide 1.84, Design and Fabrication Code Case Acceptability ASME Section III Division 1, dated June, 1986.

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