ML20135J062

Submits Proposal to Use ASME Code Cases N-411 & N-397 on Dumping Values & Spectral Shifting,Respectively.Criteria Technically Justified & Provides More Realistic Piping Design Re Thermal & Seismic Requirements

ML20135J062
Person / Time
Site:	Three Mile Island
Issue date:	04/23/1985
From:	Hukill H GENERAL PUBLIC UTILITIES CORP.
To:	Murley T NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
5211-85-2082, NUDOCS 8509250180
Download: ML20135J062 (5)

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l GPU Nuclear Corporation i Nuclear o

=e me:r8o s Middletown, Pennsylvania 17057-0191 717 944 7621 TELEX 84-2386 Writer's Direct Dial Number:

April 23, 1985 5211-85-2082 Dr. Thomas E. Murley Region I, Regional Administrator U.S. Nuclear Regulatory Commission 631 Park Avenue King of Prussia, PA 19406

Dear Dr. Murley:

Three Mile Island Nuclear Station Unit 1 (THI-1)

Operating License No. DPR-50 Docket No. 50-289 Higher Variable Damping Value and an Alternate Peak Broadening Technique for j Piping System Analysis ,

GPU Nuclear Corporation is proposing to utilize new developments in piping criteria, namely the recent ASME Code Case on damping values (N-411) and spectral shifting (N-397), at Three Mile Island Unit 1, as described in the attachment to this letter.

These criteria are technically justified and provide a more realistic piping design considering thermal and seismic requirements. Consequently, GPUN will be able to eliminate unnecessary seismic restraints, thereby reducing personnel radiation exposure associated with their surveillance and maintenance.

Section 5.2.1.2.11 of the Updated FSAR currently indicates that for vital piping systems, the critical damping value is 0.5 percent. The ASME Code Case N-411 recommends a damping criterion of 5 percent for frequencies up to 10 Hz, linearly decreasing to 2 percent at 20 Hz, and thereafter held constant at 2 percent. In addition the code case N-397 allows the broadening of spectral peaks based on a probabilistic approach. Hence the FSAR damping value of 0.5 percent for both OBE and SSE appears to be needlessly conservative.

8509250180 050423 PDR ADOCK 05000289 P PDR GPU Nuclear Corporation is a subsidiary of the General Public Utilities Corporation ik 1 l

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, GPUN has performed a generic 10 CFR 50.59 evaluation for the impact of these code cases on the piping system and has determined that there are no

unreviewed safety questions. -In addition, if as a result of using ASME Code i cases N-411 and N-397, piping supports are moved, modified or eliminated,

! the expected increased piping displacements due to greater piping flexibility will be checked to assure that they can be accommodated and that there will be no adverse interaction with adjacent structures, components and equipment. Further, GPUN will perform a 10 CFR 50.59 evaluation of the effect of ASME Code Cases N-411 and N-397, for the individual support, to ,

i determine that there are no unreviewed safety questions prior to its modification. GPUN plans to update the FSAR to reflect these changes.

j This submittal is provided for information only.

! S cerely',

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H. D. Hukill -

] Director, TMI-1

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D ATTACHMENT-REVISED CRITERIA FOR SEISMIC ANALYSIS OF PIPING SYSTEMS--THREE MILE ISLAND UNIT 1 In recent years, a good deal of attention has'been focused on ways to increase the capacity factors of nuclear power plants. One area of research has been piping system design. GPUN believes that reduction in the use of seismic restraints would have a positive impact on plant availability, specifically:

--Fewer unplanned outages due to increased system flexibility

--Reduced snubber inservice inspection activities

--Reduced _need for plant access for maintenance

--Reduced personnel exposure to radiation Within the industry, research efforts using considerable test data have included examination of more refined methods of analysis to demonstrate seismic qualification of flexible piping systems. The following new criteria have been accepted by the ASME in Code Case N-397 and N-411. They will be applied to both ANSI B31.1 and ASME Section III, Division 1, Class 1, 2, and 3 piping.

A) Spectral Shifting--ASME Code Case N-397

. This technique seeks to remove some of the conservatism introduced by the peak broadening criteria of Reg. Guide 1.122 by using a probabilistic approach. In the particular case where there is more than one piping frequency located within the frequency range of a widened spectruin. peak, the floor spectrum curve may be more realistically applied in accordance with the following criterion.

Based on the fact that the actual natural frequency of the structure can assume only one single value within the frequency range defined by fj + 0.15 fj, not a range of values, only one of these piping modes can resFond with the magnitude of the peak spectral value. Therefore, the seismic analysis of a piping system using the broadened floor design response spectra may be accomplished by the following alternative:

1. Determine the natural frequencies (fe)n of the piping system to be qualified.

2. Consider all piping natural frequencies in the interval fj - 0.15 fj i (fe)n i jf + 0.15 f; ,

where fj is the frequency of maximum acceleration in the unbroadened spectra, and n = 1 to K (K is the number of natural frequencies within the interval).

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3. The piping system shall then_be evaluated by sequentially performing K + 3 analyses using the unbroadened floor design response spectrum and also the unbroadened spectrum modified by shifting the frequencies associated with each of the spectral values by a factor of +0.15, -0.15, and [(fe )n - fj]/fje where n = 1 to K.

4. The resultants of these separate seismic analyses shall then be enveloped to obtain the final resultant desired (pipe stress, support loads, accelerations, etc.).

If no piping system natural frequencies exist in the interval associated with the maximum acceleration peak, then the interval associated with the next highest peak shall be used in the above procedure.

B) Frequency Dependent Damping--ASME Code Case N-411 This Code Case is based on a large body of test data which shows current damping criteria for seismic design of nuclear power plant piping (Regulatory Guide 1.61) is unduly conservative.

The proposed new damping criterion is 5 percent for frequencies up to 10 Hz, lienarly decreasing to 2 percent at 20 Hz, and thereafter held constant at 2 percent (See attached Figure 1). This damping criterion applies for both operating basis earthquake and safe shutdown earthquake, and are independent of pipe diameter.

Conclusions We believe that the changes proposed by these code cases for higher damping values and for an alternative to peak broadening are more realistic but still result in a conservative design. These recomendations have been accepted by the American Society of Mechanical Engineers (ASME) in Code Cases N-397 and N-411 which sets the industry standards.

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