ML20205H399
| ML20205H399 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 08/13/1986 |
| From: | Kemper J PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC |
| To: | Muller D Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20205H406 | List: |
| References | |
| NUDOCS 8608200069 | |
| Download: ML20205H399 (5) | |
Text
f-PHILADELPHIA ELECTRIC COMPANY 23011 M ARKET STREET P.'
. BOX 8699 O
PHILADELPHI A. PA.19101 JOHN S. KEMPE R V IC E-PR ESI D E N T anormaanensAnonataAnca August 13, 1986 Mr. Daniel R. Muller, Director BWR Project Directorate #2 Division of Boiling Water Reactor Licensing U. S. Nuclear Regulatory Commission Washington, DC 20555
Subject:
Peach Bottom Atomic Power Station Units 2 & 3 Docket Nos. 50-277 and 50-278 Use of Energy Absorbers as Replacements for Snubbers
References:
1)
PECo Letter to NRC (H. R Denton),
dated July 25, 1985, Same Subject 2)
NRC Letter to PECo (J. S. Kemper),
dated November 13, 1985
Dear Mr. Muller:
In the reference (1) letter we described our intent to install energy absorbers in place of snubbers at the Peach Bottom Atomic Power Station (PBAPS) under the provisions of 10CFR50.59(a). Reference (2) provided comments from the NRC relative to the application of 10CFR50.59(a) provisions. This letter responds to your comments.
Summarizing the information provided in our reference (1) letter, the concept of using energy absorbers as dynamic supports is both old and simple:
If dynamic energy transmitted to the piping is dissipated the system response can be significantly reduced. This idea has not been widely applied in nuclear plants mainly because the design computation necessary for proper documentation has been very costly.
The technology recently developed by Bechtel Power Corporation (Bechtel) permits the application of the energy absorber concept to nuclear power plants in a cost effective manner. This technology has been adapted using current linear analytical methods.
Energy absorber technology has been benchmarked to test results that satisfy the intent of Regulatory Guide 1.61.
In applying the energy absorber technology, unique modal damping ratios are calculated and used in conjunction with the otherwise traditional linear response spectra type seismic analysis.
These calculated damping ratios are rigorous and are correlated to test results and physical laws.
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The document titled " Technical Basis for Energy Absorbers as Supports of Nuclear Power Plant Piping Systems" which has been submitted with the Reference (1) letter provides the complete basis of the energy absorber technology. On the basis of this document and the relevant aspects of the application of energy absorber supports, we believe that existing Regulatory Guides 1.61 and 1.92 permit the use of energy absorbers under the provisions of 10CFR$0.59(a).
The following is offered in response to your comments in Reference (2):
Comment 1:
Elimination of snubbers and replacement with energy absorbers would involve a change in the Technical Specifications in the opinion of the NRC staff.
Response
1:
Recently, the technical specifications for PBAPS, Units 2 & 3 were revised to delete specific snubber listings as allowed by the NRC generic letter #84-13. Since the energy absorbers are not snubbers, they will require no functional ISI testing. However, they will receive the same visual ISI examination for structural integrity that is employed for standard pipe support components like springs and rigid supports. No Technical Specification change is required.
Comment 2:
The number and location of the energy absorbers in a given piping system would be determined by the use of a non-linear computer program together with the structural hysteric properties of the absorbers. Neither of these features have been reviewed in detail by the staff and found to be acceptable.
Response 2:
For seismic analysis and analysis of other dynsmic load cases for which a response spectra is available, the linear response spectra methodology is used with modal damping ratios commensurate with the specific energy absorbers used. Non-linear time history analysis methods would only be used for those load cases for which the load is defined in terms of a time history, such as water hammer. The documentation submitted in Reference 1 describes the details of this methodology.
Comment 3:
The proposed replacement of snubbers with energy absorbers in a previously analyzed safety system could result in a potential decrease in the margin of safety. The reanalysis and verification of adequate safety margins concerning snubber replacement represent a potential unreviewed safety question in the opinion of the NRC staff.
Response 3:
Systems on which energy absorbers are applied are analyzed and all calculated responses are verified to comply with code requirements and other plant licensing requirements. Energy absorbers actually enhance the margins of safety in at least four ways:
(a) Energy absorbers have no moving parts, or activation mechanisms.
An energy absorber may be viewed as a ductile rigid support.
On the other hand, snubbers have failed on many occasions, mainly due to their complex mechanisms. Snubbers may lock during thermal expansion causing an unanalyzed condition to occur, or they may remain unlocked during a dynamic event.
Both cases cause unanticipated system response and may result in loads that exceed allowable stresses for both pipe and equipment.
It is for this reason that particular emphasis is placed on snubber ISI functional testing. Thus, energy absorbers are expected to enhance the system reliability and safety.
(b) Snubbers are effectively rigid restraints for dynamic events.
Should any unexpected dynamic event occur that is more severe than previously analyzed, system response and snubber load will increase proportionately. On the other hand, the higher response will cause the energy absorber to dissipate more energy and system response will increase in less than a direct proportion.
(c) Snubbers or other elastic type supports, will transmit higher loads from the supporting structure to the piping under higher than design seismic events. The limited and predictable yield force of energy absorbers will serve to limit the load transmitted to the piping system even for higher than design seismic events.
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(d) Other enhancements to safety margins with energy, absorbers, include a lower. dynamic amplification factor tha'n snubbers for rapid transient events, and the built-in limit stop for redundancy and protection from severe unanticipated dynamic events such as water banner.
To support our response to comment (3), a report on the" reanalysis of the PBAPS HPCI steam supply line with energy absorbers as,
replacements for snubbers is enclosed for your;information.f The report includes a description of the system and'its configuration and a summary of the piping stresses and restraint loads.
. t' USNRC Regulatory Guide 1.84 Rev. 24 issued in June 1986 has.
e approved the use of Code Case N-420 on " Linear Energy Absorbing Supports for Subsection NF,' Classes 1, 2 and 3 Construction, / ','
Section III, Division 1" subject ~to the,following:
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Each applicant should provide the following information.' '/
prior to implementing the Code Case:
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1.
Indication of systems in which energy-absorbing supports are to be used.
i 2.
Fatigue design.
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3.
Piping system analysis result considering inelastic behavior of supports.
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4.
Plans for 1nservice inspection of energy absorbers.
i Items 1 and 3 above are addressed in the enclosure to this letter.
Items 2 and 4 above were addressed in Sections 8.0 and 9.0 respectively in Bechtel's proprietary document Enclosure 2 to our reference (1),
letter.
Based on the above discussion,"we conclude that:
j a.
A technical specification revision is not required.
b.
Thereisnoreduction.inthemargin'$liafety.
c.
This modification does not involve an unreviewed safety question.
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We believe that the information provided in this submittal will enable your Staff to concur with our conclusions. We plan to implement the change on HPCI steam supply line during the upcoming refueling outage for Unit 2 in February 1987.
Please do not hesitate to contact us should you or the staff require additional information.
1 Sincerely, l
f W-,AM l
l EDP/pdO6048610
Enclosure:
(Copy of Summary Report)
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