ML20245G521
| ML20245G521 | |
| Person / Time | |
|---|---|
| Site: | Big Rock Point File:Consumers Energy icon.png |
| Issue date: | 08/10/1989 |
| From: | Pulsifer R NRC |
| To: | |
| Shared Package | |
| ML20245G518 | List: |
| References | |
| GL-83-28, NUDOCS 8908160058 | |
| Download: ML20245G521 (3) | |
Text
_
hRdCg -
['g UNITED STATES -
&3 NUCLEAR REGtlLATORY COMMISSION o
O 5'
- E WASHINGTON, D. C. 20555 k.
- .+
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 Power 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 signal 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 (EDO), 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 implications of the Salem Unit 1 incidents are reported in NUREG-1000,
" Generic Implications of the ATWS 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 reactors, applicants for an operating license, and holders of construction permits to respond to generic issues raised by the analysis of these two ATWS events.
The licensees were required by Generic Letter 83-28, Item 4.5.3 to confirm 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.
1 Existing intervals for on-line functional testing required by Technical l
Specifications 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 mode failure rates; (3) reduced redundancy during testing; (4) operator error during testing; and (5) component " wear-out" caused by the testing.
- NOTE:
The RTS is called the Reactor Protection System (RTS) at 8ig Rock Point.
I 8908160058 890810 PDR ADOCK 05000155 P
PDC 1
i
2.0 DISCUSSION The staff has reviewed Censumers Power Company responses dated September 6, 1983, November 7, 1983 and June 1, 1984 regarding Generic Letter 83-28.
Big Rock Point on a weekly basis simulates a High Reactor Building Pressure by using the test circuit breakers.
This system test functionally actuates the Reactor Protection System (RPS)* one channel at a time to check the operability and reliability of the system.
On a monthly basis each RPS. channel is independently tested to verify proper RPS output for each sensor test circuit breaker.
During each refueling outage the RPS circuitry is checked by actuating the sensor or opening
" Links" to simulate an actuation so that it would perform its intended function.
Each of the pressure and level switches used as sensors are also calibrated during the outage when the set points are' verified and the operational recorder response is verified.
In November 1982, the NRC replied to Consumers Power Company response to SEP TOPIC VI-?0.A, " Testing of Reactor Trip System and Engineered Safety Features, Including Response Time." This evaluation was in regard to the reliability of the RPS and the need for changes to the Big Rock Point Technical Specifications as documented in the draft NUREG-0828,
" Integrated Plant Safety Assessment Report for the Big Rock Point Plant."
In the SER provided in the November 1982 reply, it was noted that although the Big Rock Point Technical Specifications do not require calibration of the initiation channels for the RPS they are controlled by plant test procedures.
The only area where these procedures do not conform to current Technical Specification requirements is calibration frequency and the licensee had noted and the staff agreed the operating experience had demonstrated acceptable failure rates and little instrument drift.
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, very little experience exists to support the estimates, and common cause failure probabilities 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.
j 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, additional testing will not appreciably lower RTS unavailability.
First, testing more frequently than weekly is generally
)
impractical, and even so the increased testing could at best lower the failure probability by less than a factor of four compared to monthly testing.
Secondly, increased testing could possibly increase the i
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 that the current monthly test intervals are adequate to maintain high RTS availability.
l 1 _ _ _ _ _ _ _ ___ _ _ _ _ _ _ _ _ _ _ _ _ _
l
n_.
i....
.,? 1.
1
..t.
-3.0 ' CONCLUSION L
With review of SEP. TOPIC VI.10.A and the above referenced letters the staff _ feels that currently' configured RPS.is highly reliable based on past Big Rock Point experience,.the weekly and monthly functionality tests, the sensor calibrations and the RPS Scram Sensor Test performed'at every refueling outage.
In addition, the analysis in NUREG-0460 have shown that for a number of reasons, more frequent testing than monthly will not appreciably lower the estimates of failure probability.
Based'on our review and the findings noted'in NUREG-0460, we conclude that the existing intervals for on-line-functional testing are consistent with achieving high RPS availability at Big Rock Point.
_ _ - _ _ _ _.. _ _ _ _.. _ _ _ _ _ _. - - _