ML20127K414

Safety Evaluation Accepting Amend 12/change 20 to DPR-22 Changing Hydraulic Snubber TS

ML20127K414
Person / Time
Site:	Monticello
Issue date:	09/15/1975
From:	Office of Nuclear Reactor Regulation
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ML20127K386	List: ML20127K382 ML20127K407 ML20127K414 ML20127K426
References
NUDOCS 9211200398
Download: ML20127K414 (3)

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r -UNITED STATES NUCLEAR REGULATORY COMMISSION

W AS HIN GT O N, D. C. 2 0 5 5 5 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO. 12 TO LICENSE NO. DPR-22 (CIRNGE NO. 20 TO THE TECHNICAL SPECIFICATIONS)

NORTi!ERN STATES POWER COMPANY MONTICELLO NUCLEAR GENERATING PLANT DOCKET NO. 50-263 INTRODUCTION During the summer of 1973, inspections at two reactor facilities revealed a high incidence of inoperable hydraulic shock suppressors (snubbers) manufactured by Bergen Paterson Pipe Support Corporation. As a result of those findings, the Office of Inspection and Enforcement required each operating reactor licensee to immediately inspect all Bergen Paterson snubbers utilized on safety systems and .to reinspect then 45 to 90 days

. after the initial inspection. Snubbers supplied by other manufacturers were to be inspected on a lower priority basis.

Since a long term solution to climinate recurring failures was not immediately availabic, the Division of Reactor Licensing sent a letter dated October 2,1973, to operating facilities (including Monticello) utilizing Bergen Paterson snubbers specifying continuing surveillance requirements and requesting a submittal within one year of proposed Technical Specifications for a snubber surveillance program. On August 15,1975, .':orthern States Power Company proposed Technical Specifications for hydraulic snubbers at the Monticello reactor.

During our review of the proposed change, we found that certain modifications were necessary. These modifications were discussed with Northern States and have been incorporated into the proposed Technical Specifications.

EVALUATION ,

Snubbers are designed to prevent unrestrained pipe motion under dynamic loads as might occur during an earthquake.or severe transient while allowing normal thermal movement during startup and shutdown.

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The consequence of an inoperable snubber is an increase in the probability l of structural damage to piping resulting from a seismic or other pcstulated i event which initiates dynamic loads. It is, therefore, necessary that snubbers installed to protect safety system piping be operable during reactor operation and be inspected at appropriate intervals to assure j their operability.

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Examination of defective snubbers at reactor facilities has shown that the high incidence of failures observed in the surmer of 1973 was caused by severo degradation of seal materials and subsequent leakage of the

hydraulic fluid. The basic seal materials used in Bergen Paterson 4 snubbers were two types of polyurethane; a millabic gum polyester type i containing plastici

ers and an unadulterated molded type. Shterial tests performed at several laboratories (Reference 1) established that the millabic gum polyurethane deteriorated rapidly under the temperature and moisture conditions present in many snubber locations. Although the molded polyurethane exhibited greater resistance to these conditions, it also may be unsuitabic for application in the higher temperature

! environments. Data are not currently availabic to precisely define an upper temperature limit for the molded polyurethane. The investigation indicated that seal materials are available, primarily ethylene propylene compounds, which should give satisfactory performance under the most severe conditions expected in reactor in:ta11ations.

! An extensive seal replacement program has been carried out at many reactor i facilities. Experience with ethylene propylene seals has been very

good with no serious degradation reported thus far. Although the seal replacement program has significantly reduced the incidence of snubber failures, some failures continue to occur. These failures have generally i been attributed to faulty snubber assembly and installation, loose 3 fittings and connections and excessive pipe vibrations. The failures have been observed in both PNRs and BWRs and have not been limited to units manufactured by Bergen Paterson. Because of the continued incidence of snubber failures, we have concluded that snubber opera-j bility and surveillance requirements should be incorporated into the Technical' Specifications. We have further concluded that these require-ments should be applied to all safety related hydraulic snubbers, regardless of manufacturer, in all light water cooled reactor facilities.

4 We have developed the attached Technical Specifications and Bases to provide additional assurance _of satisfactory snubbers performance and 4 reliability. The specifications require that snubbers be operable during T

(1) Report H. R. Erickson, Bergen Paterson.to K. R. Goller, NRC, October 7, 1974,

Subject:

Hydraulic Shock Sway Arrestors 2

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4 reactor operation and prior to startup. Because snubber protection is required only during relatively low probability events, a period of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> is allowed for repair or replacement of defective units before the reactor must be shut down.

An inspection program is specified to provide additional assurance that the snubbers remain operable. The inspection frequency is based upon maintaining a constant level of snubber protection. Thus the required inspection interval varies inversely with the observed snubber failures.

The longest inspection interval allowed in the Technical Specifications after a record of no snubber failures has been established is nominally-18 months. Experience at operating facilities has shown that the required i surveillance program should provide an acceptable level of snubber performance-provided that the seal materials are compatible with the operating environment. Snubbers containing seal caterial'which has not

been demonstrated to be compatibic with the operating environment are required to be inspected every 31 days until compatibility is established or an appropriate seal change is completed.

To further increase the level of snubber reliability, the Technical Specifications require functional tests and internal inspections of snubbers at least once each refueling cycle. The tests will verify proper piston movement, lock up and bleed, and the internal inspections will monitor for wear, breakdown and deterioration that cannot be observed by the external inspections.

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CONCLUS:0NS We have concluded that the proposed additions to the Technical Specifi-cations, as modified, increase the probability of successful snubber

. performance, increase reactor safety and we therefore find them acceptabic.

We u.ve concluded, based on the considerations discussed above, that:

(1) because the change does not involve a significant increase in the probability or consequences of accidents previously considered and does not involve a significant decrease in a safety margin, the change does not involve a significant ha ards consi.deration, (2) there is reasonable assurance that the health and safety of the public will not be. endangered by operation in- the proposed manner, and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.

Date:

SEP 151975 4

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