ML20149M680

Safety Evaluation Accepting Licensee 960807 Results of Analyses Re Operability Evaluation of Main Steam Sys W/Bent Rod Hangers at Plant

ML20149M680
Person / Time
Site:	Crystal River
Issue date:	01/17/1997
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20149M679	List: ML20149M676 ML20149M680
References
NUDOCS 9701240259
Download: ML20149M680 (3)

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1. , k" UNITED STATES NUCLEAR REGULATORY COMM12SION

. 5f WASHINGTON, D.C. 20666-0001

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION OPERABILITY OF THE MAIN STEAM SYSTEM WITH BENT HANGERS 1

l FLORIDA POWER CORPORATION l l

CRYSTAL RIVER NUCLEAR GENERATING PLANT. UNIT 3 DOCKET NO. 50-302 l

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1.0 BACKGROUND

On May 10, 1996, Florida Power Corporation (FPC) discovered during a routine walkdown, a severely bent rod hanger on each of the two main steam systems, CR-5 and CR-6, at Crystal River, Unit 3 (CR3). The root cause for the bent hangers was attributed to maintenance activities on adjacent isolation valves.

The operations staff at CR3 declared the rod hangers degraded, but were judged operable. At the request of the NRC Project Manager for CR3, FPC performed an evaluation of the systems, which was documented in an Operability Concern Resolution Evaluation Report, dated May 23, 1996. FPC concluded that the bent rod hangers were operable in their existing condition, but should still be replaced as soon as the work schedule permits. In Reference 1, FPC submitted for evaluation by the EMEB staff the supporting analyses justifying the operability conclusion. On June 12, 1996, the bent rod hangers were replaced with straight rod hangers, bringing the systems back into conformance with the licensing basis for CR3.

2.0 EVALVATION Due to an unknown loading condition, two rod hangers (MSH 27B on CR-5 and MSH 138 on CR-6) were found permanently bent (kinked) from the out-of-straight configuration. An FPC evaluation showed that the rods would experience Euler buckling (elastic) when subjected to compressive axial loads, indicating that the rod hangers could not have been bent inelastically by any uplift load unless significant vertical upward displacements of the pipe were experienced.

Since each hanger was bent roughly in two, FPC considered these hangers as formed two-member structures, and calculated the elastic stiffness based on the reported out-of-straight dimension (approximately 2 ir.tnes) using standard structural analysis. Each hanger stiffness was found to be smaller than the stiffness of a straight rod hanger by roughly an order of magnitude. In this condition the hangers can sustain equal compressive as well as tensile loading as long as the elastic limit of the rod material is not exceeded.

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. 2 FPC performed a linear elastic analysis of main steam system CR-5, in which the elastic stiffness of the bent hanger (MSH 27B) was specified. This system was chosen since this hanger would experience limited compression under j seismic loading. The purpose of this analysis was to demonstrate that the rod hanger remains operable, i.e., it would carry sufficient load such that the allowable stresses of the code of record, ANSI B31.1,1967, for the piping  ;

would not be exceeded, under the licensing basis sustained and occasional '

loading conditions. This demonstration is also considered applicable to the  ;

1 other main steam system.

l The staff reviewed the FPC analysis attached to Reference 1. The analysis

! showed that, with the reduced stiffness of the bent hangers, the stresses in

the 24" diameter main steam line remain below the allowable stresses.

However, the analysis did not account for secondary stresses induced in the

' piping and the adjacent hangers by the bending of the rods. The FPC analysis assumed that the pipe is stress free with the bent hangers and, therefore, did not account for these stresses. This, represents a deficiency in the FPC j approach to demonstrate operability. However, the FPC analysis also showed that there is ample margin to accommodate these secondary stresses, and

accordingly, the staff finds that in this case, the demonstration of the rod 3 hanger operability is acceptable. It should be noted that, in general, straight hangers which are subsequently bent inelastically while installed in

, the piping system will induce a significant increase in the loads of the

adjacent rod hangers and the stresses in the pipe. Operability of such rod i-hangers and the adequacy of the piping system can generally be assured on an j interim basis by a detailed evaluation, possibly based on non-linear analysis, l and would be acceptable on a case-by-case basis only. Such an evaluation is l s generally not recommended (Reference 2). Permanent plant operation in such a i

degraded condition is also not acceptable.

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3.0 CONCLUSION

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! ' Based on the analyses submitted by FPC, the staff finds that FPC has i

demonstrated the operability of two main steam systems at CR-3, each of which

! was found with an initially straight rod hanger in a bent condition. The i staff has reviewed the results of these analyses and finds them reasonable and i acceptable.

1 i The staff also finds that, in general, a detailed non-linear piping analysis

! may be necessary to demonstrate operability and adequacy of piping systems i with inelastically deformed rod hangers. Such an evaluation however, is l generally not recommended (Reference 2).

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Principal Contributor: M. Hartzaman, EMEB, NRR

! Date: January 17, 1997 4

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. REFERENCES

! 1. Letter of August 7,1996, from P. M. Beard, Jr., Florida Power Corporation, to the NRC Document Control Desk, with attachments.

2. Welding Research Council Bulletin No. 353, " Position Paper on Nuclear Pipe Supports." )

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