05000298/LER-2004-003
| Docket Numbersequential Revmo Day Year Year Mo Day Yearnumber No 05000 | |
| Event date: | 06-01-2004 |
|---|---|
| Report date: | 07-27-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2982004003R00 - NRC Website | |
PLANT STATUS
Cooper Nuclear Station (CNS) was in Mode 1 (Run) at approximately 100 percent power when the identified condition occurred.
BACKGROUND
The High Pressure Coolant Injection (HPCI) System (EIIS:BJ ) is provided to assure that the reactor is adequately protected in the event of a small break in the nuclear system which does not result in rapid depressurization of the reactor vessel. The HPCI System permits the nuclear plant to be shutdown while maintaining sufficient reactor vessel water inventory until the reactor vessel is depressurized. The HPCI System continues to operate until reactor vessel pressure is below the pressure at which Low Pressure Coolant Injection (EIIS:BO) operation or Core Spray System (EIIS:BM) operation can be used to maintain core cooling. The HPCI System is also credited during a Station Blackout Event.
HPCI consists of a steam turbine assembly (EllS:TRB ) driving a multi-stage booster and main pump assembly and system piping, valves, controls and instrumentation. The HPCI turbine is driven by steam from the reactor which is generated by decay and residual heat. The steam is extracted from main steam line "C" (EIIS:SB) upstream of the main steam line isolation valves (EIIS:ISV). The HPCI lube oil system provides oil to operate the turbine stop and control valves. The HPCI auxiliary lube oil system is provided to supply oil requirements on initial HPCI turbine startup.
Exhaust steam from the HPCI turbine is discharged to the suppression pool. A drain pot at the low point in the exhaust line collects moisture present in the steam. Collected moisture is discharged through a trap to the suppression pool or bypassed to the gland seal condenser (EIIS:COND) if the trap fails. During standby conditions, condensate is drained to the gland seal condenser.
EVENT DESCRIPTION
On June 1, 2004 at 1915 Central Daylight Time (CDT), HPCI was declared inoperable. Prior to this declaration, the alarm from the HPCI exhaust drain pot high level sensor (EIIS:LS) had failed to clear. The alarm had been occurring hourly or less because the steam supply to the HPCI turbine inlet valve had been leaking. The HPCI auxiliary oil pump was placed in pull to lock as a precautionary measure to prevent the HPCI System from auto starting with excessive water in the exhaust piping. This resulted in HPC1 being declared inoperable and Limiting Condition for Operation (LCO) 3.5.1 Condition C was entered.
The alarm card directs that the pot be drained in this situation to clear the alarm. The alarm did not clear when other indicators verified that the pot had drained. The low side of the level switch was also drained with no results in clearing the alarm. The drain pot was then drained hourly. HPCI was available to be started manually if required.
An investigation indicated that the cause of the alarm not clearing was a problem with the level switch and not an accumulation of water in the HPCI exhaust line. The switch was found to be out of adjustment and was re-aligned.
The HPCI auxiliary lube oil pump control switch was removed from pull to lock. Hourly draining of the drain pot was discontinued. HPCI was declared operable and LCO Condition C was exited on June 2, 2004 at 1536 CDT.
BASIS FOR REPORT
This event is reportable in accordance with 10 CFR 50.73(a)(2)(v)(D) as "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident", since HPCI was declared inoperable.
CAUSE
The cause of the event was poor maintenance of the steam supply valve resulting in it leaking excessively. This in turn caused frequent cycling of the level switch which became misaligned and failed to operate. A contributing cause was that maintenance plans do not adequately address alignment and adjustment requirements of the HPCI exhaust drain pot level switch. Another contributing cause was placing the auxiliary lube oil pump in the pull to lock position.
SAFETY SIGNIFICANCE
The condition did not make HPCI unavailable. Although HPCI auto start was temporarily locked out, operator manual start would have been accomplished in response to a post-trip reactor pressure vessel low water level. In addition, the possibility of accumulated moisture in the HPCI exhaust line would not have adversely affected HPCI operation nor impacted primary containment integrity.
Therefore, this condition did not adversely affect, directly or indirectly, the CNS risk as described in the probabilistic risk assessment as established by the baseline reliability of components and equipment. The condition did not challenge a fuel, reactor coolant pressure, primary containment or secondary containment boundary. The condition did not impact the plant's ability to safely shutdown or maintain the reactor in a safe shutdown condition. The event is bounded by the baseline probabilistic safety assessment model and has negligible risk significance.
This event is considered a Safety System Functional Failure as defined in NEI 99-02, Revision 2, Regulatory Assessment Performance Indicator Guideline.
CORRECTIVE ACTIONS
Interim Action Before the HPCI steam supply valve is overhauled in the next refueling outage beginning in January 2005, a re- calibration and alignment of the HPCI exhaust line drain pot level switch will be performed. Since the level switch is still cycling at a high rate, this action will provide a higher probability of reliability of this component during the interim.
This action will be completed by October 29, 2004.
Long Term Actions The appropriate maintenance plan will be revised to ensure that it adequately addresses re-aligning the internal switch mechanism of the HPCI exhaust drain pot level switch. This action will be completed by August 30, 2004.
Refurbishment preventive maintenance will be established to prevent excessive seat leakage in the HPCI steam supply valve. This will prevent excessive leakage which will, in turn, obviate high cycling of the HPCI exhaust drain pot level switch. This action will be completed by October 21, 2004.
Determine if it is necessary to place the auxiliary lube oil pump in the pull to lock position when the HPCI drain pot high level alarm does not clear. This action will be completed by October 25, 2004.
Refurbish the HPCI steam supply valve. This will ensure that the root cause, excessive leakage, will be corrected which will prevent recurrence. This action will be completed during the next refueling.
PREVIOUS SIMILAR EVENTS
A number of instances of failures of the HPCI exhaust drain pot switch and previous leakage problems with the HPCI steam supply valve were found. Instances where the two events were linked were also noted. However, a specific plan to address reliability problems with the level switch by addressing the internal condition of the steam supply valve was not found. Because the safety function of the steam supply valve is to open and leakage through the valve did not affect this safety function, there was no impetus to link the two effects.