ML20138N645
| ML20138N645 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 10/30/1985 |
| From: | Withers B PORTLAND GENERAL ELECTRIC CO. |
| To: | Butcher E Office of Nuclear Reactor Regulation |
| References | |
| GL-85-09, GL-85-9, TAC-55383, NUDOCS 8511050269 | |
| Download: ML20138N645 (5) | |
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M M M COligXXIY Bart D Wahers Vce Presdent October 30, 1985 Trojan Nuclear Plant Docket 50-344 License NPF-1 Director of Nuclear Reactor Regulation ATTW:
Mr. E. J. Butcher, Jr., Acting Chief Operating Reactors Branch No. 3 Division of Licensing U.S. Nuclear Regulatory Coumission Washington DC 20555
Dear Sir:
Reference:
PGE (B. D. Withers) to NRC (E. J. Butcher, Jr.) Letter, August 2, 1985 As discussed in the above reference, we indicated our intent to participate in the Westinghouse Owners Group (WOG) program to optimize reactor trip breaker maintenance and surveillance practices. At this time, the WOG is proposing to:
1.
Not include bypass breaker testing in the Technical Specifications as recommended by NRC Generic Letter 85-09, 2.
Adopt the remaining Generic Letter 85-09 surveillance test requirements and allowable outage times as interim test requirements, and 3.
Administrative 1y control the interim requirements in 2., above, without Technical Specification changes until the requirements can be optimally determined by the trip breaker reliability model on which the WOG is now working.
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W M MCOf14MN1y Mr. E. J. Butcher, Jr.
October 30, 1985 Page 2 The WOG reconunendations are based on the evaluation provided in the Attachment. We plan to follow these WOG recomunendations.
Sincerely, "Y
Bart D. Withers Vice President Nuclear 3
Attachment c:
Mr. Lynn Frank, Director State of Oregon Department of Energy Mr. Robert L. King Cnairman of County Commissioners i
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- n Trojan Nuclear Plant Mr. E. J. Butcher, Jr.
Docket 50-344 Attachment
- License WPF-1 October 30, 1985 Page 1 of 3 The Westinghouse Owners Group (WOG) has been reviewing Generic Letter-85-09, which identifies Technical Specification requirements for reactor trip breakers, including:
- 1.
Adding surveillance tests on the bypass breakers, 2.
Testing both undervoltage trip attachment (UVTA) and shunt trip
. attachment (STA) trip functions in the periodic trip breakers tests, 3.
Allowing a 48-hour allowable outage time when a UVTA or STA device is inoperable, and 4.
Testing the UVTA and STA circuits when actuated by the remote manual trip switches.
The WOG has calculated the impact of the bypass breaker failure probability on the reactor trip system failure probability and concludes that the bypass breaker contribution is insignificant. These calculations are based o.. the trip breaker fault tree model presented in Supplement 1 to WCAP-10271.
In WOG Letter DC-106, which transmitted the WOG response to NRC questions on WCAP-10271, a typical Westinghouse PWR reactor trip unavailability was estimated to be 1.5 E-5.
No credit was taken for operation of the bypass breaker in the evaluation from which these calculations were derived. The impact'on the reactor trip system unavailability, including the unavail-ability of the reactor trip bypass breakers was calculated with the following results:
1.
The bypass breakers are placed in service only when one train of the Reactor Protection System (RPS) is in test. The only circumstance in which the bypass breaker could affect RPS unavailability is the cutset when one train is in test, a signal is generated in the operable redundant train and the main breaker fails to open.
i 2.
The unavailability of the RPS attributable to failure of a main trip breaker with the opposite train in test is 3.7 E-7 or 2.5 percent of 1
the total RPS unavailability (ie, 1.5 E-5).
This cutset constitutes the only configuration in which the bypass breaker can affect RPS i
l unavailability.
3.
Taking credit for the bypass breaker would reduce the probability value of this cutset to:
(3.7 E-7)(3.5 E-4) = 1.3 E-10 where 3.5 E-4 is the unavailability of the bypass breaker assuming bimonthly testing
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Trojan Nuclear Plant Mr. E. J. Butcher, Jr.
Docket 50-344 Attachment License NPF-1 October 30, 1985 Page 2*of 3 or, (3.7 E-7)(3.5 E-3) = 1.3 E-9 where 3.5 E-3 is the unavailability of the bypass breaker assuming testing on an 18-month interval.
Based on the above, it is recommended that testing of bypass breakers not be included in the Trojan Technical Specifications (TTS) periodic test of the main trip breakers. As shown above, testing the bypass breakers on a 2-month or 18-month test interval will result in an E-9 or E-10 level contribution to the RPS unavailability of approximately E-5.
Alterna-tively, the RPS unavailability increase that occurs by increasing the bypass breaker failure probability from 0 percent to 100 percent is only 2.5 percent at the RPS level. ~ Given the minimal impact of bypass breaker-testing, it is recommended that Trojan administrative 1y control bypass breaker testing outcAde of the TTS.
Generic Letter 85-09 also requires that both the UVTA and the STA function be tested during the periodic trip breaker surveillance test. Again, using the reactor trip breaker fault tree model discussed above, the WOG recalculated the impact of UVTA and STA testing on breaker unavailability.
The results of this evaluation showed that trip breaker unavailability increased by a factor of 2 when the surveillance test interval on either of the two diverse trip functions (UVTA or STA) was increased from 2 to 18 months. The impact of this increase in breaker unavailability on the overall reactor trip system unavailability was also evaluated. The result of this evaluation showed that the increase in RPS unavailability result-ins from a doubling of trip breaker unavailability is approximately 10 percent. This increase in RPS unavailability will proportionately
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increase the ATWS core melt probability. Therefore, no relaxation in the survelliance test frequency of the UVTA or STA functions is proposed at this time. These survelliance test intervals will be re-examined by the WOG when the additional work on the more sophisticated trip breaker model is completed.
Generic Letter 85-09 further recommends a 48-hour allowed outage time if either trip function is declared inoperable. Using once again the same breaker fault tree model, the breaker availability sensitivity to the
'48-hour allowed outage time was calculated.
The results showed unequal sensitivities for the STA and UVTA. Because this result does not support a significant increase in the 48-hour allowed outage time in Generic Letter 85-09, no relaxation in this parameter is recommended at this time. As in the above case of the surveillance test interval, the allowed 4
outage time will be re-examined by the WOG when the additional work on the i
more sophisticated reactor trip breaker model is completed.
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c Trojan Nuclear Plant Mr. E. J. Butcher, Jr.
i Docket 50-344 Attachment License NPF-1 October 30, 1985 Page 3 of 3 The final recommendation in Generic Letter 85-09 involves testing the manual reactor trip switch UVTA and STA circuits. Although a clear RPS unavailability improvement has not been shown for testing both UVTA and STA circuits, no change to this test is proposed at this time. The basis for this position is the infrequent test interval (18 months), and the fact that procedures to do the test have already been developed.
In conclusion, based on the WOG's current calculations of the reactor trip system unavailability, there is an insignificant reliability improvement from including periodic surveillance tests of the bypass breakers in the Technical Specifications. Thus, the proposed requirement in Generic Letter 85-09 to test the bypass breakers prior to the main breaker peri-odic survelliance test should be deleted. The remaining surveillance test requirements and allowable outage times proposed in Generic Letter 85-09 should be adopted as interim test requirements. However, these interim requirements should be administrative 1y controlled without Technical Specification changes until the requirements can be optimally determined by the trip breaker reliability model on which the WOG is now working.
BLK/kal 14910.1085
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