ML19325E220

LER 89-025-00:on 890928,main Turbine Trip Occurred,Followed Immediately by Reactor Scram.Caused by Spurious Actuation of Level Switch Due to Equipment Vibration.Plant Stabilized & Temporary Instruction Re Pump Shifting issued.W/891027 Ltr

ML19325E220
Person / Time
Site:	Cooper
Issue date:	10/27/1989
From:	Horn G, Krause R NEBRASKA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
CNSS896010, LER-89-025, LER-89-25, NUDOCS 8911020271
Download: ML19325E220 (4)

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COOPER NUCLEAR ST ATtoN -

Nebraska Public; Power District " "

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CNSS896010

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• October 27, 1989-

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a i U.S.. Nuclear Regulatory Commission Document Control Desk

• Washington, D.C.- 20555

Dear!Stra  :

Cooper Nuclear Station Licensee Event Report 89-025 is being forwarded as an

attachment to this letter.

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. Horn' .r r . Division Manager'of

. Nuclear Operations .

i  : Cooper. Nuclear Station

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Attachment; cc: R. D. Martin ~r L. G. Kunc1 R.,E. Wilbur V. L. Wolstenholm E :G. A. Trevors l

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• APPROVE) OMS 10. 31008104 LICENSEE EVENT REPORT (LER) '"P'a8 8 8'8"

8ACILITY NAME HI OOCEET NuhetER (2) PaGE (33 Cooper Nuclear Station o is lo jo lol2l9 l8 1 loFl 0 l 3

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NAME TELEPHONE NUMSER Ralph W. Krause 4;O l2 8;2 e5 l i 3l8;1l1 COMPLETE ONE LINE POR EACH COMPONENT P AILURE DESCRitEO IN THIS REPORT (13p CAust system COMPONENT "fo$,T'$3n CAust system COMPONENT R TAS E "d*%l"O M ANyl AC- pq f s

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YEs II* vu. comPon Expecrto suenotss/0N DA Tth ko AmeT. ACT ru,.it a, ran sona. o . .oe m wr en r. au, evo.anr~. unni no, On September 28, 1989, at 11:36 AM,'a main turbine trip occurred, followed immediately by a reactor scram. The Reactor vessel water level transient that resulted from the scram caused containment isolation Groups 2 (Primary Containment),

3 (Reactor Water Cleanup) and 6 (Secondary Containment) to occur. The turbine trip signal that initiated the event was turbine hydraulic control oil reservoir low level, which occurred approximately two minutes after the control oil pumps were shifted. It was later verified that reservoir level had remained in the normal range throughout the event.

The exact cause for the turbine hydraulic control oil reservoir low level trip signal could not be positively identified. It was concluded the trip signal was caused by spurious actuation of the level switch due to equipment vibration as a result of shifting control oil pumps.

The immediate actions taken were to stabilize the plant following the scram.

To minimize the possibility of recurrence, a temporary instruction was issued to limit control oil pump shifting, and a control oil system flush was scheduled for the next outage. Additionally, several system reliability improvements will be evaluated.

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Taxi u m . % w anc e-m.mm on g A. EVENT DESCRIPTION On-September 28, 1989, at 11

35 AM, a main turbine trip occurred, followed immediately by a reactor scram. The reactor vessel water level transient that resulted from the scram caused containment isolation Groups 2 (Primary Containment), 3 (Reactor Water Cleanup) and 6 (Secondary Containment including the initiation of Stendby Gas Treatment) to occur. The turbine trip signal that initiated the event was turbine hydraulic control oil reservoir low level, which occurred approximately two minutes after the Control Room Operator e,hifted running turbine hydraulic control oil-pumps.. The reservoir low level signal also tripped the running control oil pump. The trip signal was manually reset immediately following the turbine trip, and both control oil pumps restarted. It was subsequently.

. verified that the actual reservoir level remained within the normal operating range-(approximately 75% full) throughout the event. The control oil system was supplied by Westinghouse, the turbine generator manufacturer.

B. PLANT STATUS At the time of the event, the plant was operating at 100% of rated power (791 MWe).

C. BASIS FOR REPORT The report is being submitted in accordance with 10CFR50.73(a)(2)(iv),

the unplanned actuation of an Engineered Safety Feature; specifically.

the actuation of the Reactor Protection System, Primary Containment Isolation System, and Standby Gas Treatment System.

D. CAUSE l

The exact cause for the turbine hydraulic control oil reservoir low level trip signal could not be positively identified. The reservoir level .

l switch (Magnetrol Model A-153) was tested in place, removed from the reservoir and inspected, and the logic circuit components and viring [

inspected and tested, all with satisfactory results. Control oil system performance tests were also attempted. However, system characteristics o are sufficiently different while shutdown, such that duplicating system performance was not possible. Through a rigorous root cause analysis, it was subsequently concluded that the trip signal could only have been caused by a spurious actuation of the level switch due to equipment vibration. It is believed the vibration was caused by abnormal unloader valve actuations as a result of shifting the control oil pumps.

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E. SAFETY SIGNIFICANCE

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None. The design basis transient for this condition is a turbine trip I without bypass valve operation. Although the control oil pump tripped I when the main turbine tripped, sufficient control oil pressure was j available-via the installed accumulators to immediately open the bypass j valves. Because of the prompt action of the Control Room Operators to reset the reservoir low level trip signal and restart the control oil .i pumps, continued bypass valve operation was assured, and the transient l was enveloped by the analyzed transient. .i I

F. SAFETY IMPLICATIONS l 1

Minimal. As previously stated, the design basis transient for this )

condition is a turbine trip without bypass valve operation. If the  !

Control Room Operators had not, or could not reset the reservoir low j level trip signal, the plant transient would have been within the design basis analysis. . The pressure relief system and both the High Pressure Coolant Injection and Reactor Core Isolation Cooling systems were available, as well as the Suppression Pool Cooling mode of the Residual

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Heat Removal system to mitigate the event, j G. CORRECTIVE ACTION 1

). The immediate corrective actions taken to respond to the reactor scram l'

and main turbine trip were to reset the turbine control oil reservoir low level trip signal, start the control oil pumps, reestablish pressure control via the bypass valves and to stabilize the plant.

Following a rigorous root cause analysis, it was determined that the turbine trip could only have been caused by spurious operation of the control oil reservoir level switch. To minimize the possibility of l

recurrence, a temporary Operations Department Special Order was issued to limit the shifting of control oil pumps while the main turbine is l

operating, and an extensive control oil system flush is scheduled for the next refueling outage. Furthermore, several items will be evaluated as potential changes to improve system reliability prior to the 1990 refueling outage: reevaluate the vendor preventive maintenance l recommendations for the control oil system unloader valves, and determine I

l the feasibility of modifying the control oil reservoir low level trip logic.

l H. PAST SIMILAR EVENTS None.

'U.S. CPor 1968 dJ M 89 00010 s - NRC FORM 366A

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