ML20082B669
| ML20082B669 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 07/01/1991 |
| From: | Shelton D CENTERIOR ENERGY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 1944, GL-83-28, TAC-75237, NUDOCS 9107160274 | |
| Download: ML20082B669 (3) | |
Text
CENTERDOR ENERGY Donald C. Shelton 300 Madison Avenue Vce President Nuclear Toledo. OH 43652-0001 Davis Besse (419)249 2300 1
Docket Number 50-346 License Number NPF-3 Serial Number 1944 July 1, 1991 United States Nuclear Regulatory Commission Document Control Desk Vashington, D.C.
20555 Subj ec t : Reactor Trip Breaker Preventive Haintenance Frequency Gentlement The purpose of this letter is to inform the Nuclear Regulatory Commission (NRC) staff of a change to the preventive maintenance frequency for Reactor Trip Breakers (RTB) at the Davis-Besse Nuclear Power Station (DBNPS), Unit 1.
NRC Generic Letter Number 83-28,
" Required Actions Based on Generic Implications of Salem ATVS Events,"
dated July 8, 1983 (Toledo Edison (TE) letter Log Number 1322),
required licensees to establish a preventive maintenance and surveillance program for reactor trip breakers (Items 4.2.1).
Toledo Edison's commitments relative to the preventive maintenance and surveillance program requested by GL 83-28 are documented in Toledo Edison's letters to the NRC dated November 7, 1983 (Serial Number 1000) and March 29, 1985 (Serial Number 1130). The NRC request for additional information and evaluation of TE's reactor trip breaker preventive maintenance program are documented in NRC etters to Toledo Edison dated January 30, 1985 (TE letter Log Number 1685) and November 12, 1985 (TE letter Log Number 1864).
Toledo Edison committed to perform preventive maintenance described in letter Serial Number 1130 at 12 month intervals.
The plant operates on a nominal 18 month fuel cycle. Although preventive maintenance can be performed at power, it requires removing the breaker from its cubicle and the associated Reactor Protection System (RPS) channel is tripped at least six times during performance of maintenance and post maintenance testing. With a RTB removed or RPS channel tripped, the plant is in a half trip condition and is, therefore, vulnerable to spurious reactor trips.
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s Docket Number 50-346-i License Number NPF-3 Serial Number 1944 Page 2
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In the NRC Safety Evaluation Report (TE letter Log Number 1864) related to TE's RTB preventive maintenance program, the NRC staff identified l
several RTB parameters which are indicative of RTB reliability.
The NRC staff considers that undervoltage device dropout' voltage, trip I
shaft torque, breaker response time and breaker insulation resistance-
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are acceptable trending parameters to forecast breaker degradation or failure. Trending of those_ parameters.has indicated-that there has.
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been no degradation of RTB performance over the'past eight years of RTB maintenance history.
In view of the increased-vulnerability to spurious reactor trips during the on-line performance of annual RTB-j preventive maintenance, and that no adverse trend is indicated by the i
maintenance history, Toledo Edison has concluded that RTB preventive maintenance should be performed only when the plant is shutdown for refueling (18 month interval with a maximum 25% extension to permit scheduling coincident with planned refueling outages). The increase in preventive maintenance interval is not expected to result in a reduction in RTB reliability as measured by continued trending of these.
parameters.
If any of these parameters fall outside the acceptance criteria following the increase of the preventive maintenance interval, then the interval vill be retutned to twelve months, unless the root cause of the failure is determined to be unrelated to the' preventive maintenance interval.
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Preventive maintenance vill continue to be performed in-house in accordance with General Electric (GE), the RTB manufacturer, service advisories with the exception of the recommended 12 month preventive--
maintenance intervals.
GE had previouslyfaudited TE's in house RTB i
maintenance program and found it to be acceptable. The reactor trip breaker maintenance history demonstratesethe effectiveness:of TE's program and that the RTBs have been maintained in exceptionally good
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condition.
The change in preventive maintenance interval does not affect the degree of monthly functional testing required by Technical Specifications. Technical Specification 43/4.3.1, Reactor-Protection System (RPS) Instrumentation, requires-monthly functional testing of-the reactor trip breakers.
Performance of the monthly functional' test involves tripping each breaker twice, once using the undervoltage device and once by means of the shunt-trip attachment.
The monthly functional tests provide continued assurance of reactor trip breaker reliability.
In addition to reactor trip breakers, the DBNPS Reactor Protection System design has always incorporated a diverse means-of removing power
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from the Control Rod Drive Mechanisms (CRDM), thereby effecting a reactor trip, independent of action of the reactor: trip breakers.
The diverse means consists of interrupting power to the CRDM programmer lamps and interrupting the gate drive circuits for the CRDM motor -
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return supply silicon controlled rectifiers (SCR).
The existence of this diverse means of de-energizing CRDMs reduces reliance on the RTBs-l and would tend to compensate for any unanticipated adverse effects of l
increasing the RTB preventive-maintenance. interval. ' Technical.
Specification 3/4.3.1, Reactor Protection System Instrumentation, includes operability and surveillance requirements for this diverse reactor trip mechanism (Table 3.3-1, Functional Unit-15, SCR Relays).
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Docket Number 50-346 License Number-NPF-3 Serial Number 1944 Page 3 In summary, Toledo Edison is increasing the RTB annual preventive maintenance interval to a refueling interval of nominally eighteen months. This change is being made to eliminate RTB preventive l
maintenance during power operation.
Performance of RTB preventive maintenance at power unnecessarily increases susceptibility to spurious l
Trending of preventive maintenance parameters over the l
past eight years indicates no adverse trends in RTB performance.
It is not anticipated that the increase in preventive maintenance interval vill result in a reduction of RTB reliability.
Preventive maintenance vill otherwise continue to be performed in house and in accordance with General Electric service advisories.
The increase in preventive maintenance interval vill not affect the performance of monthly functional testing of RTB.
Technical
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Specification required monthly functional testing vill continue to j
assure the high RTB reliability.
The inclusion of a diverse trip mechanism in the DBNPS RPS design provides an independent backup for the RTBs which would tend to offset any unexpected effect on RTB reliability associated with increasing the RTB preventive maintenance interval.
In view of the increased potential for spurious trips during on line RTB protection and demonstrated RTB reliability, Toledo Edison has concluded that it is appropriate to increase the RTB preventive maintenance interval eighteen months with a maximum 25% extension to permit scheduling coincident with planned refueling outages.
If you have any questions regarding this change of commitment, please
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contact Mr. R. V. Schrauder, Manager - Nuclear Licensing at (419) 249-2366.
Verytrulzyours,
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PVS/ach P. M. Byron, NRC Region III, DB-1 Senior Resident Inspector cc:
A. B. Davis, Regional Administrator,.NRC Region III J. B. Hopkins, NRC/NRR DB-1 Senior Project Manager Utility Radiological Safety Board t
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