ML17202U777
| ML17202U777 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 08/06/1990 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML17202U776 | List: |
| References | |
| GL-83-28, NUDOCS 9008080257 | |
| Download: ML17202U777 (2) | |
Text
I UNITEDSTATES e
N LEAR REGULATORY COMMISSION ENCLOSURE 1 WASHINGTON, D. C. 20555 SAFETY EVALUATION REPORT GENERIC LETTER 83-28, ITEM 4.5.3 REACTOR TRIP SYSTEM RELIABILITY FOR ALL DOMESTIC OPERATING REACTORS
1.0 INTRODUCTION
On February 25, 1983, both of the scram circuit breakers at Unit 1 of the Salem Nuclear Po~er Plant failed to open upon an automatic reactor trip signal from the reactor protection system (RPS). This incident was terminated manually by the operator about 30 seconds after the initiation of the automatic trip signal. The failure.of the circuit breakers was determined to be related to the sticking of the undervoltage trip attachment. Prior to this incident, on February 22, 1983, at Unit 1 of the Salem Nuclear. Power Plant, an automatic trip signa1 was generated based on steam generator low-low level during plant startup.
In this case, the reactor was tripped*
manually by the operator almost coincidentally with the automatic trip.
Following these incidents, on February 28, 1983, the NRC Executive Director for Operations (EOO), directed the staff to investigate and report on the generic implications of these occurrences at Unit 1 of the Salem Nuclear Power Plant. The results of the staff's inquiry into the generic implications.
of the Salem Unit 1 incidents are reported in NUREG-1000, 11Gener*ic ImpHcations of the ATWS Events at the Salem Nuclear Power Plant.* As a result of this*
investigation, the Commission (tJRC) requested (by Generic Letter 83-28 dated July 8, 1983) all licensees of operating reactors, applicants for an operating license, and holders of construction permits to respond to generic issues raised by the analys*es of these ATWS eve'nts.
The licensees were required by Generic Letter 83-28, Item 4.5.3,to conffrm that on-line functional testing of the reactor trip system (RTS), including independent testing of the diverse trip features, was being performed at all plants.
Existing intervals for on-line functional test,ing required by Technical Specifications were to be reviewed to determine if the test intervals were adequate for achieving high RTS a\\*a1lab11ity when accounting for considerations such as:
(1) uncertainties in component failure rates; (2) uncertainties in*
com.on mode, failure rates; (3) reduced redundancy during testinp; (4) operator error during testing; and (5) component "wear-out" caused by the testing.
2.0 DISCUSSION The.NRC's contractor, Idaho National Engineering Laboratory (INEL), reviewed the licensee o~ners Group availability analyses and evaluated the adequacy of the existing test intervals, with a consideration of the above five 9008080257 900806 PDR ADOCK 05000237 P
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!' items. for all plants. The results of this review are reported in detail in EGG-NTA 8341. NA Review of Reactor Trip System Availability Analyses for Gene.ric Letter 83-28. ltem 4.5.3 Resolution.
0 dated March 1989 and*summadzed in this report. The results of our evaluation of Item 4.5.3,and*our review of EGG-NTA-8341, are presented below.
The Babcock & Wilcox (B&W), Combustion Engineering (CE), General Electric (GE), and Westinghouse (W) Owners Groups have submitted topical reports either in response to GL 83-28, Item 4.5.3 or to provide a basis for requesting Technical Specification changes to extend RTS surveillance test intervals (STI).
The owners groups' analyses addressed the adequacy of the existing intervals for on-line functional testing of the RTS, with the considerations required. by Item 4.5.3, by quantitatively estimating the unavailability of the RTS.
These analyses found that the RTS was very reliable and that the unavailability was dominated by common cause failure and human error.
The ability to accurately estimate unavailability for very reliable systems was considered extensively in NUREG-0460, *Anticipated Transients Without Scram for Light Water Reactors,~ and the ATWS rulemaking.
The uncertainties of such estimates are large because the systems are highly reliable. Since very little experince exists to support the estimates, the common cause failure pro~abilities are difficult to estimate. Therefore, we believe that the RTS unavailability estimates in these studies, while useful for evaluating test intervals, must be used with caution..
NUREG-0460 also states that for systems with low failure probability, such as the RTS, coT1111on mode failures tend to predominate, and, for a number of.
reasons, additional testing will not appreciably low:r RTS unavailability.
Fir1t 1 testin~ more frequently than weekly is generally impractical, and even
-so the increased testing could at best lower the failure probability by less than ~ factor of four_compared to monthly te~ting. Secondly, increased testing coul~,result in the possibility of a common mode failure through increas~d stress on the system.
Finally, not all potential failures are detectable by testing.
In summary, NUREG-0460 provides additional justification to demonstr~te that the**
tu~rent monthly test intervals are adequate to maintain high RTS availabiltty.
3.0 CONCLUSION
All four vendors' topical reports have shown the currently configured RTS to*
be highly reliable with.the current monthly test intervals. Our contractor has reviewed these analyses and performed independent estimates of their own which conclude that the current test intervals provide high reliability. In addition, the analyses 1n NUREG-0460 have shown that for a number of reasons, more frequent testing than monthly will not apprecia~ly lower the estimates of failure probability.
Based on our review of the Owners Group topical report~. our contractoi's 1r1dependent analysis, and the findings noted in NUREG-0460, we conclude that the existing intervals, as recommended in the topical reports, for on-line functional testing are consistent with achieving high RTS availability at all operating reactors.