ML19317G846
| ML19317G846 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 01/06/1977 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19317G838 | List: |
| References | |
| NUDOCS 8004010674 | |
| Download: ML19317G846 (4) | |
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- 1 NUCLEAR REGULATORY COMMISSION
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g wAsHIPeG70N,0. * 'D555 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO. 9 TO LICENSE NO. DPR-54 SACRAMENTO MUNICIPAL UTILITY DISTRICT RANCHO SECO NUCLEAR GENERATING STATIOM DOCKET NO. 50-312 Introduction By letters dated Auaust 7, 1975, September 27, 1976, and October 6, 1976, Sacramento Municipal Utility District (the licensee) requested amendment to Facility Operating License No. DPR-Sa. The purpose of the request is to incorporate provisions in the Rancho Seco Technical Specifications related to limitina conditions for operation and surveillance of shock suppressors (snubbers). We made changes in the licensee's October 6,1976, submittal after discussions with the licensee, by phone, December 10, 1076.
Background
Durina the summer of 1973, inspections at two reactor facilities revealed a high incidence of inoperable hydraulic shock suppressors (snubbers) manufactured by Bergen Paterson Pipesupport Cor70 ration.
As a result of those findings, the Office of Inspection and Enforce-ment required each operating reactor licensee to immediately inspect all Beraen 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.
Since a long term solution to eliminate recurring failures was not immediately available, the Division of Reactor Licensina sent a letter dated October 2, 1973, to operating facilities utilizing Bergen Paterson snubbers specifying continuina surveillance require-ments. By 'etter dated July 1,1975, we requested that the licensee submit propo:ed Technical Specification requirements for bydraulic snubbers. On August 7,1975, the licensee proposed Technical Speci-fications for hydraulic snubbers at the reactor and revised the proposal by letters dated September 27, 1976 and October 6, 1976.
During our review of the proposed chance, we found that certain modifications were necessary. These modifications were discussed with the licensee and have been incorporated into the proposed Technical Specifications.
8004010[ff
2 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.
The consequences of an inoperable snubber is an increase in the probability of structural damage to piping resulting from a sefsmic 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 tc assure their operability.
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 grea:ce resistance to these conditions, it also may be unsu'. table 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 avail-able, primarily ethylene propylene compounds, which should give satisfactory performance under the most severe conditions expected in reactor installations.
An extensive seal replacement 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 replace-ment program has signi.ficantly reduced the incidence of snubber failures, soce 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 PWR's and BWR's and have not been limited to u11ts manufactured by Bergen Paterson. Because of the continued incidence of snubber failures we have concluded that snubber operability and surveillance l
(1) Report H. k. Erickson, Bergen paterson to X. R. Goller, NRC, October 7, 1974,
Subject:
Hydraulic Shock Sway Arrestors
s requirements should be incorporated into the Tecnnical hecifications.
We have further concluded that these requirements shoult. be applied to all safety related snubbers, regardless of manufacturer, in all light water cooled reactor facilities.
The proposed Technical Specifications and Bases provide additional assurance of satisfactory snubber performance and reliability. The specifications require that snubbers be operable during reactor operation and prior to startup. - Because snubber protection is required only dur-ing 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 /> 11 allowed for repair
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or replacement of defective units before the reactor must be shut down.
Additional time before coing into cold shutdown condition is provided in order to reduce thermal cycling effects in the materials of the reactor coolant system. The licensee will be expected to coninenca repair or replacement 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 /> before shutdown is consistent with that provided for other safety-related equipment and provides for remedial action to be taken in accordance with 10 CFR 150.36(c)(2).
Failure of a pipe, piping system or major component would not neces#
sarily result from the failure of a single snubber to operate as designed, and even a snubber devoid of hydraulic fluid would provide support for the pipe or component and reduce pipe motion. The likelihood 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. There-fore, the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> periJd 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 main-taining a constant level of snubber protection. Thus the required inspection interval varies inversely with the observed snubber failures. The longest inspection interval allcwed in the Tecnnical Specifications after a record of no snubber failures has been established is nominally 18 month. Experience at operating facilities has shown that the required surveillance program should provide an acceptable level of snubber performance provided that the seal materials are compatible with the operating environment. Snubbers containing seal material which has not been demonstrated to be compatible with the operating environment are required to be inspected once each month until the compatibility is established or an appropriate seal change is completed.
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. To further increase the level of snubber reliability, the p.oposed Technical Specifications require functional tests once each refueling cycle. The tests will verify proper piston movement, lock up and bleed.
We have concluded that the proposed Technical Specifications, as modified, increase the probability of successful snubber performance, increase reactor safety and we therefore find them acceptable.
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 standpoint of environmental impact and pursuant to 10 CFP 851.5(d)(4) that an environmental impact statement, or negative declaration, and environ-mental impact appraisal need not be prepared in connection with the issuance of this amendment.
Conclusion We have concluded, based on the considerations discussed above, that:
(1) because the amendment 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 amendment does 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 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.
Dated:
January 6, 1977