ML20037B257

Safety Evaluation Supporting Amend 16 to License DPR-2

ML20037B257
Person / Time
Site:	Dresden
Issue date:	08/09/1976
From:	Office of Nuclear Reactor Regulation
To:
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ML20037B254	List: ML20037B253 ML20037B256 ML20037B257 ML20037B258
References
NUDOCS 8009100733
Download: ML20037B257 (4)

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO.16 TO LICENSE N0. DPR-2 COMMONWEALTH EDIS0N COMPANY DRESDEN NUCLEAR POWER STATION - UNIT N0. 1 DOCKET NO. 50-10 INTRODUCTION Sy letter dated March 16, 1976, Commonwealth Edison Company (CECO) requested changes to the technical specifications appended to Facility License No. DPR-2 for the Dresden Nuclear Power Station - Unit No.1 facility. The proposed changes involve establishing limiting conditions for oper ation and surveillance requirements for shock suppressors (snubbers) protecting safety related systems and components. The request is in response to our letter dated December 17, 1975, regarding i

safety-related snubbers.

l During our review of the proposed changes, we found that certain modifications to the proposal were necessary to meet Regulatory require-ments.

These changes were discussed with the licensee's staff. The l

licensee has accepted these changes and they will be incorporated into the specifications.

l EVALUATION Snubbers are designed to prevent unrestrained p'pe motion under dynamic loads as might occur during an earthquake or se;ere transient while allowing normal thermal movement during startup and shutdown. The consequence of an inoperable snubber is an increase in the probability

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of structural damage to piping resulting from a seismic or other postulated 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 their operability.

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_2 During the summer of 1973, inspections at two reactor facilities revealed a high incidence of inoperable hydraulic shock suppressors (snubbers) manufactured by Bergen Paterson Pipesupport Corpcration. As a result of those findings, the Office of Inspection and Enforcement required each operating reactor liceasee to inanediately inspect all Bergen Paterson snubbers utilized on safety systems and to reinspect them 45 to 90 days after the initial inspection.

Snubbers supplied by other manufacturers were to be inspected on a lower priority basis.

I Examination of defective snubbers at reactor facilities has shown that the high incidence of failures observed in the summer of 1973 was caused by severe degradation of seal materials and subsequent leakage of the hydraulic fluid. The basic seal materials used in Bergen Paterson snubbers were two types of polyurethane; a millable gum polyester type containing plasticizers and an unadulterated molded type. Material tests performed at several laboratories (Reference 1) established that the millable 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 unsuitable for application in the higher temperature environments.

Data are not currently available 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 installations.

An extensive seal replacenent program has been carried out at many reactor 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 been attributed to faulty snubber assembly and installation, loose fittings and connections and excessive pipe vibrations.

The failures have been observed in both PWRs 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 operability and surveillance requirements should be incorporated into the Technical Specifications. We have further concluded that these requirements should be applied to all safety related snubbers, regardless of manufacturer, in alllight water cooled reactor facilities.

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

Subject:

Hydraulic Shock Sway Arrestors.

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- The proposed technical specifications, as modified, and Bases provide additional assurance of satisfactory snubber performance and reliability.

The specifications require that snubbers be operable during reactor Because snubber protection is required operation and prior to startup.

only during 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. The licensee will be expected to commence repair or replace-nent of a failed snubber expeditiously. However, the allowance of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> p

is consistent with that provided for other safety-related equipment and provides for remedial action to be taken in accordance with 10 CFR 50.36(c)(2).

Failure of a pipe, piping system, or major ccmponent would not necessarily result from the failure of a single snubber to operate as designed, and even a snubber devoid of hydraulic fluid would provide The likelihood support for the pipe or component and reduce pipe moticn.

of a seismic event or other initiating event occurring during the time allowed for repair or replacement is very small.

Considering the large size and difficult access of some snubber units, repair or replacement in a shorter time period is not practical. Therefore, the 72-hour period provides a reasonable and realistic period for remedial action to be taken.

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 as nominally 18 months.

Experience at operating facilities has shown that the required surveillance program should provide an acceptable level of snubber perfonnance provided that the seal materials are compatible with the operating environment.

Snubbers containing seal materici which has not been demonstrated to be compatible with the operating environment are required to be inspected every 31 days until the compatibility is established or an appropriate seal change is completed.

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To further increase the level of snubber reliab'lity, the technical specifications require functional tests of snubbers at least once each refueling cycle.

The tests will verify proper piston movement, lock up and bleed.

We have concluded that the proposed additions to the technical specifi-cations, as modified, increase tne probability of successful snubber performance, increase reactor safety and therfore are acceptable.

. EtNIRONMENTAL C03FIDERATION We have determined that the amendment does not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact.

Having made this determination, we have further concluded that the amendment involves an action which is insignificant from the standyint of environmental impact and pursuant to 10 CFR 551.5(d)(4') that an environmental statement, negative declaration, or environmental impact :2ppraisal need not be prepared in connection with the issuance of this amendment.

I CONCLUSION We have concluded, based on the considerations discussed above, that:

(1) because the changes do not involve a significant increase in the probability or consequences of accidents previously considered and do not involve a significant decrease in a safety margin, the changes do not involve a significant hazards consideration, (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 Consnission'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:

August 9, 1976

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