ML20043B267
| ML20043B267 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood  |
| Issue date: | 05/21/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20043B241 | List: |
| References | |
| NUDOCS 9005250171 | |
| Download: ML20043B267 (4) | |
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SAFETY EVALUATION REPORT BY DIVISION OF ENGINEERING TECHNOLOGY, MECHANICAL ENGINEERING BRANCH CO*PONWEALTH EDISON COMPANY IMPLD'ENTAT10N OF, SEISMIC STOPS AT BYRON UNIT NO. 2 1.
BACKGROUND On May 2, 1969, Commonwealth Edison Company (Ceco) proposed at a meeting with theMechanicalEngineeringBranch(EMED)todemonstratetheimplementationat Byron Unit No. 2 of a type of pipe support termed " seismic stops' (Reference 1).
These stops were developed by a CECO contractor, Robert L. Cloud Associates (RLCA) and are passive supports which are activated only during a seismic or other dynamic event, similar to snubbers or other energy absorbing mechanisms.
Thedemonstrationwillconsistofapilotstudyinwhichallsnubbers(13)on reactor coolant loop bypass line (2RC04/19) will be replaced by an optimited number of seismic stops (8).
The objectives of this study are to establish the feasibility and applicability of seismic stops as snubber replacements for balance-of-plant safety related piping, and to demonstrate t1e applicability of a RLCA proprietary computer program 'GApPipE" developed specifically for performing analyses of piping systems with these seismic stops.
!!. EVALVATION Figure 1 shows a diagram of a seismic stop. This design permits the free thermal expansion of the pipe under operating conditions without binding since the piston-and-rod are able to move freely in the cavity between the two cylinders. The gap size is determined by the largest expected thermal expansion of the attached pipe et the support point. Under seismic or other vibrating loads the piston moves within the cavity and impacts the cylinder wall end. This impacting dissipates energy, in effect introducing a damping mechanism in the pipe support structure.
The stops are designed and tested per ASME Section III, Subsection NF.
Since the structural load-displacement properties of a seismic stop are highly non-linear, the analysis of a systen with such stops cannot be performed on a linear-elastic basis. A number of computer programs are available commercially for performing non-linear analyses wit 1 gapped elements which could be used for analysis of piping system with seismic stops. These are ordinarily based on time-history numerical procedures.
However, RLCA has developed a proprietary i
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piping analysis program, GAPPipE, for performing analyses of piping systems with seismic stops. This program is based on equivalent-linearization/ iteration procedures and standard piping (modal analysis with capability for uniform (URS) j or independent support motion ISM) response spectrum analysis.
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The staff and its consultant the Brookhaven National Laboratory (BNL) have reviewedthetheoreticalbasIsfortheequivalentlinearization/ iteration procedures used in GAPSIPE, the design concept of the seismic stop, the test data to support this concept, and the proposed implementation in Byron Unit No. 2.
From February 25 to March 1 1990, BNL personnel conducted an audit of various aspects of the GAPPIPE calculation methods, an evaluation of the test data, and the GAPPIPE post-test analyses. RLCA has performed a significant amount of testing of seismic stops, es in reoorted References 1 through 4 The tests were conoutted in the United States and Germany. The results indicate that soismic dops perform on an equivalet basis with shubbers, and providt similar levels of safety assurance when subjected to seismic loading, SNL has also reviewed the GAPPIPE calculations as applied to the pliot study at Byron Unit No. 2.
In these calculations RLCA applied the modal damping values of ASME Section !!!, Code Case N-411, with URS analysis. Guidelines for the use of this code case are provided in Regulatory Guide 1.84, Revision 24 and later revisions. They concluded (Reference 5) that GAPPIPE provided results comparable to these obtained by other nor.-linear programs such as ANSYS. They have recommended t.he implementation of the seismic stops in the pilot study.
However, they have also recommended that an independent confirmatory analysis of the piping system in the pilot study be performed to verify the GAPPIPE results in an actual case. Such confirmatory analyses have also been performed in the evaluation of other proposed piping seismic restraints. We concur with these recommendations.
Since GAPPIPE has the capability of using the ISM response spectrum analysis method, we will require that application of this method conform to the guidelines of NUREG-1061 Volume 4 (Reference 6).
Revised conditions for the application of the ISM method have as yet, not been agreed upon and therefore, will require acceptance on a case-by-case basis.
The licensee is requested to inform the comission, in writng, of any significant changes in the estimated completion schedule identified in the staff's safety evaluation. The licensee is also requested to inform the Comission, in writing when the action has actually been implemented and the modificationisdetermInedtobeoperational.
111. CONCLUSION 1.
Based on our review of the design methodology and test data, we find the implementation of seismic stops in the proposed Byron Unit No. 2 pilot st,pdy acceptable.
2.
Since this is a new concept in restraining piping under seismic and other dynamic excitation, we will require that Ceco perform a visual inspection at the first outage after installation of all seismic stops installed in the study and the pipe to which the stops are attached. The licensee has agreed to this requirement.
3.
We find the program GAPPIPE acceptable for evaluating the dynamic response of piping systems containing seismic stops based on the URS method and
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1 Code Case N-411. The use of the ISM response spectrum analysis method is acceptable provided it conforms to the guidelines in NUREG-1061, Volume 4 I
Proposed alternate rules for application will require case by-case evaluation.
4 Since the GAPPIPE methodology is also recently developed, a confirmatory analysis will be >erforned by BNL of the piping system chosen for the i
pilot study. Suct analyses have also been performed for other types of proposed non-linear supports.
5.
Final acceptability of the seismic stops for BOP piping at Byron Unit 1
No. 2 will depend on achieving a satisfactory comparison of BNL and RLCA piping calculations, and satisf actory results of the in-service inspection at the first outing after installation.
IV.
PEFERENCES 1.
Internal NRC memorandum from from L. N. 01shan, " Meeting Summary -
Non Linear Piping Analysis Used to Justify Removal of Snubt,ers and Pipe Supports," dated May 24, 1989.
2.
EPRI Report NP-6442, "A Simplified Piping Support System with Seismic Limit Stops." Prepared by Robert L. Cloud Associates, Berkeley, CA.,
July 1989.
3.
Cloud, R. L, et. al., " Seismic Stops vs. Snubbers, A Reliable Alternative," Nuclear Engineering and Design, Vol. 107, 1988, pp. 205 213.
4.
Yang, M.
S., et al., " Shaking Table Testing of a Piping System Supported by Seismic Sto)s," Transactions of the 10th International Conference on Structural Mecianics in Reactor Technology, Vol. K2, pp. 769-774.
5.
Memorandum from N. Simos and P. Bezier, BNL, to M. Hartzman, NRC,
" Evaluation of the Seismic Stops Concept for Restraining Piping Povement, dated liarch 19, 1990."
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6.
NUREG-1061, " Report of the U.S. N.R.C. piping Review Committee," Vol. 4 December 1984 I
Principal Contributor:
Dated:
May 21, 1990
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