ML18054B370
| ML18054B370 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 01/12/1990 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML18054B368 | List: |
| References | |
| GL-83-28, NUDOCS 9001190197 | |
| Download: ML18054B370 (3) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFETY EVALUATION REPORT DOCKET NO. 50-255 PALISADES PLANT EtlCLOSURE 2 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 scrarn circuit breakers at Unit 1 of the Salem Nuclear Power Plant failed to open upon an automatic reactor trip signal from the reactor protection system (RPS). This incident was terminated rr.anually b)1 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 signal was generated based en 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 Cptrations (EDO), directed the staff to investigate and report on the generic iwp1ications 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 ar~ reported in NUREG-1000, "Generic Implications of the AWTS Events at the Salem Nuclear Power Plant".
As a result of this investigation, the Commission (NRC) requested (by Generic Letter 83-28 dated July 8, 1983) all licensees of operating r~actors, applicants for an operating license, and holders of construction perrr.its to respond to generic issues raised by the analyses of these two ATWS events.
The licensees were required by Generic Letter 63-28, Item 4.5.3 to confirm that or.-line functional of the reactor trip system (RTS), including independent testirig of the diverse trip features, was being perfonned.
Existing inte_rvals for on-line functional testing required by Technical Specificatior1s were to be reviewed to determine if the test intervals were adequate for achieving high RTS availability when accounting for considerations such as:
(1) uncertainties in component failure rates, (2) uncertainties in common rr.ode failure rates, (3) reduced redundancy during testing, ( 4) operator error during testing, and (5) component "wear-out" caused by the testing.
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- 2.0 DISCUSSION The NRC's contractor, Idaho National Engineering Laboratory (INEL), reviewed the licensee Owners Group availability analyses and evaluated the adequacy of the existing test intervals, with a consideration of the above five items, for all plants. The results of this review are reported in detail in EGG-NTA-8341 1 "A Review of Reactor Trip System Availability Analyses for Generic Letter 83-28, Item 4.5.3 Resolution," dated March 1989 and summarized 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 requestin9 Technical Specification changes to extend RTS surveillance test intervals (STIJ.
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 1 by quantitatively estimating the unavailability of the RTS.
These ar:alyses found that the RTS was very reliable and that the unavailability was dorr.inated by common cause failure and human error.
The ability to accurately estimate unavailability for very reliable syster.'s was considered extensively in NUREG-0460 1 "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, very little experienc.~ exists to support the estimates, and common case failure probabilities are difficult to estimate. Therefore, we believe that the RTS unavaila~ility estimates in these studies, while useful fer evaluating test intervals, must be used with caution.
NUREG-0460 also states that for systems with low failure probability, such as the*RTS, common mode failures tend to predominate, and, for a number of reasons,*
acditional testing will not appreciably lower RTS unavailability. First, testing more frequently than weekly is generally impractical, and even sc the increased testing could at best lower the failure probability by less than a factor of four compared to rr.onthly testing. Secondly, increased testing could possibly increase the probability of a common mode failure through increased stress on the system.-
Finally, not all potential failures are detectable by testing. In summary, NUREG-0460 provides additional justification to demonstrate ttiat the current monthly intervals are adequate to maintain high RTS availability.
3.0 CONCLUSION
All four vendors' topical reports have shown the currently cor.fioured RTS to be highly reliable with the current monthly test fotervals*.
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 in NUREG-0460 have shown that more testing more frequently than monthly wi 11 not lower appreciably the estimates of failure probability.
- Based on our review of the Owners Group topical reports, our contractor's indepEndent 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.