05000298/LER-2003-006
| Cooper Nuclear Station | |
| Event date: | 10-28-2003 |
|---|---|
| Report date: | 12-19-2003 |
| 2982003006R00 - NRC Website | |
2. DOCKET
PLANT STATUS
Cooper Nuclear Station (CNS) was in Mode 1 (Run) at 75% power at the lime of the manual reactor scram. Reactor power had been lowered from 100% due to the imminent loss of the main generator output line to the switchyard. Diesel Generator 2 [EllS:DG] was out of service for planned maintenance.
BACKGROUND
CNS is connected to the Mid-Continent Area Power Pool through 345 kV and 161 kV buses [EllS:BUI located in independent switchyards adjacent to the station. Connections to the 345 kV switchyard include the generator main transformer bank rEllS:XFMR), a 345/161 kV auto-transformer connecting CNS 345 kV and 161 kV substations and five 345 kV transmission lines. Circuit breakers (E1IS:521 are installed in the circuit to separate the connections to the 345 kV buses. Disconnect switches tEllS:DISC) are provided on each side of each circuit breaker. A line disconnect switch is provided on each of the five 345 kV lines and the station main transformer. The line disconnect switch between the generator main transformer bank and the switchyard was provided for flexibility during required maintenance activities.
EVENT DESCRIPTION
On October 28, 2003, at 0130 hours0.0015 days <br />0.0361 hours <br />2.149471e-4 weeks <br />4.9465e-5 months <br />, the Control Room was notified of a fire on a transmission structure located between the main generator output and the 345 kV switchyard. Due to the imminent loss of the main generator output line to the switchyard, a rapid power reduction was initiated, Station loads were transferred to the Startup transformer and a manual scram was performed at 0200 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br />. All control rods inserted and Primary Containment Isolation System (PCIS)[EllS:JM] Group Isolations 2 (Primary Containment), 3 (Reactor Water Cleanup) and 6 (Secondary Containment) initiated as expected due to vessel level shrink. The Emergency Core Cooling Systems did not initiate. Mode 3 (Hot Shutdown) was entered at 0201 hours0.00233 days <br />0.0558 hours <br />3.323413e-4 weeks <br />7.64805e-5 months <br />.
After the normal vessel level shrink, water level recovery resulted in an overfeed condition which tripped the operating Reactor Feedwater pump. During restoration of the feedwater pump to service, the vessel water level lowered and a second manual scram was inserted in anticipation of a low level scram signal.
Normal shutdown procedures were entered at 0300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br /> and a vessel cool-down was initiated.
The transmission structure fire was reported out at 0647 hours0.00749 days <br />0.18 hours <br />0.00107 weeks <br />2.461835e-4 months <br />.
BASIS FOR REPORT
This event is reportable in accordance with 10 CFR 50.73(aX2XivXA) as any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (aX2)(iv)(B) of this section." The following systems from paragraph (aX2XivXB) actuated during this event: Reactor Protection System and PCIS Groups 2, 3 and 6.
CAUSE
The transmission structure fire occurred when dust accumulation, attributed to nearby crop harvesting activities, on the insulators and structure cross member became wetted during high fog or light rain/mist conditions. This condition created a path for stray electrical currents from phase to phase, or phase to ground. The natural resistance in the cross member material generated sufficient heat to cause the fire.
NRCPORM3MA0.2021) The root cause of the event is the failure to properly ground the insulator strings on the wooden structure cross member.
SAFETY SIGNIFICANCE
There is no impact on Probabilistic Risk Assessment (PRA) assumptions due to this event. All equipment responded as expected and DG 2 unavailability is captured by the average test and maintenance terms utilized in the PRA model. The PRA model assumes a frequency of once per year for transients of this type. This event alone has a negligible impact on this assumed frequency of occurrence. The Conditional Core Damage Probability (CCDP) for this event with OG 2 unavailable was 7.19E-07/year. The risk incurred during the manual scram and subsequent reactor shutdown was less than 1 E-6 CCDP and is not risk significant.
This event is not considered a Safety System Functional Failure as defined in NEI 99-02, Revision 2, Regulatory Assessment Performance Indicator Guideline.
CORRECTIVE ACTIONS
Immediate Action A manual scram was inserted to de-energize the generator main transformer output line and the fire was extinguished by 0647 hours0.00749 days <br />0.18 hours <br />0.00107 weeks <br />2.461835e-4 months <br /> on October 28, 2003.
A visual inspection of a similar 345 kV structure identified that the insulator strings on that structure were not grounded. The inspection was performed on October 28, 2003.
Insulator strings on additional 345 kV transmission structures were verified by design to be grounded.
The damaged wooden structure and associated line disconnect switch on the main generator output line were evaluated and determined to not be required. The structure and line disconnect switch were removed. This action was completed on October 30, 2003.
Long Term Actions Grounding of the insulator strings on the identified 345 kV structure was completed on November 24, 2003.
PREVIOUS EVENTS
of the Reactor Protection System.
NRC FORM 3GeR 11-2001)
COMMITTED DATE
OR OUTAGE COMMITMENT
None N/A ATTACHMENT 3 LIST OF REGULATORY COMMITMENTS0 Correspondence Number: NLS2003127 The following table identifies those actions committed to by Nebraska Public Power District (NPPD) in this document. Any other actions discussed in the submittal represent intended or planned actions by NPPD. They are described for information only and are not regulatory commitments. Please notify the Licensing & Regulatory Affairs Manager at Cooper Nuclear Station of any questions regarding this document or any associated regulatory commitments.