05000315/LER-2003-001
| Donald C. Cook Nuclear Plant Unit 1 | |
| Event date: | 01-15-2003 |
|---|---|
| Report date: | 3-17-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 3152003001R00 - NRC Website | |
2003 001 00 05000-315 17. TEXT Of more space Is required, use additional copies of NRC Form (366A) Conditions Prior to Event Unit 1 - Mode 1, 100 percent power Unit 2 - Mode 1, 100 percent power
Description of Event
On Wednesday, January 15, 2003, at approximately 2010 hours0.0233 days <br />0.558 hours <br />0.00332 weeks <br />7.64805e-4 months <br />, a fault occurred in the Unit 1 main transformer (Manufacturer: VA Tech Elin, Model Number: TDQ116A20D9K-99) [EL]. The fault caused the current differential relays to actuate, resulting in automatic trip of the main transformer. A sudden internal fault within the main transformer ruptured the transformer tank and resulted in a loss of oil and a fire.
The loss of the main transformer precipitated an automatic trip of the main generator [TB] and an immediate turbine [TA] and reactor trip [JD]. The main transformer fire was extinguished within 35 minutes of the event with one minor reflash, which was promptly controlled and extinguished by onsite fire brigade members. One minor personnel injury occurred requiring offsite' medical attention. An Unusual Event (UE) was declared, based on a fire of more than 15 minutes in duration within the protected area. The UE was terminated at 2220 hours0.0257 days <br />0.617 hours <br />0.00367 weeks <br />8.4471e-4 months <br />.
In accordance with 10 CFR 50.72(b)(2)(iv)(B), 10 CFR 50.72(b)(2)(xi), and 10 CFR 50.72(b)(3)(iv)(A), all applicable event (EN #39513) and emergency plan required notifications were completed in a timely manner. The notifications included:
- a one-hour report for emergency plan entry,
- a four-hour report for the automatic reactor protection system (RPS) actuation [JE],
- a four-hour report for offsite notification of an oil spill, and.
- an eight-hour notification for the automatic actuation of an emergency safety features system (auxiliary feedwater) [BA].
The one-hour report was completed at 2105 hours0.0244 days <br />0.585 hours <br />0.00348 weeks <br />8.009525e-4 months <br /> and the remaining three reports were completed by a single notification at 2339 hours0.0271 days <br />0.65 hours <br />0.00387 weeks <br />8.899895e-4 months <br />.
As such, this LER is being submitted in accordance with the requirements of 10 CFR 50.73(a)(2)(iv)(A) for a condition resulting in an automatic actuation of the RPS.
Written notification of the oil spill was transmitted to the NRC via letter AEP:NRC:2401-03, Non-PCB Transformer Oil Spill National Pollution Discharge Elimination System And Groundwater Discharge Authorization Notification.
All safety-related components functioned properly and no significant discrepancies in equipment time responses were noted. The auxiliary feedwater system actuated due to low steam generator levels as expected.
2003 � 001 � 00 17. TEXT (If more space is required, use additional copies of NRC Form (366A)
Cause of Event
The Unit 1 main transformer experienced a sudden internal fault, with no prior indication, that resulted in a phase-to-phase flashover. Donald C. Cook Nuclear Plant (GNP) is conducting a formal root cause investigation into the failure of the Unit 1 main transformer. This root cause investigation is ongoing and includes hardware failure analysis with support from the vendor.
In accordance with the guidance established in NUREG-1022, Revision 2, "Event Reporting Guidelines," Part 5.1.5, "Supplemental' Information and Revised LERs," GNP will issue a supplement to this LER when the final root cause determination has been completed.
Analysis of Event
On Wednesday, January 15, 2003, at approximately 2010 hours0.0233 days <br />0.558 hours <br />0.00332 weeks <br />7.64805e-4 months <br />, a fault occurred in the Unit 1 main transformer. The fault caused the current differential relays to actuate resulting in an automatic trip of the main transformer. A sudden internal fault within the main transformer ruptured the transformer tank and resulted in a loss of oil and a fire. The transformer yard fire protection system actuated as designed and extinguished the fire. The main transformer and main generator protection circuitry performed as designed and the appropriate reactor protection and safeguards equipment functioned correctly. The reactor was shut down and the unit stabilized in Mode 3 as designed with all normal and emergency cooling systems available. This event did not adversely impact the plant's ability to mitigate the consequences of an accident and, therefore, had minimal safety significance.
The change in risk with respect to core damage and large early release frequency as a result of the transformer failure and subsequent plant trip have been qualitatively assessed and judged no different than any other unplanned reactor trips with the main condenser available. This assessment is based on the following considerations:
- The automatic plant trip functioned properly. The automatic trip features also functioned dependably, resulting in a safe and stable plant configuration.
- Transformer fires are well understood and the plant design features include an installed fire suppression system and the dividing walls (concrete and steel) to mitigate the consequences of the fire. Additionally, the transformer is located outside the plant structures, away from any risk-significant equipment, and does not have an effect on any safety function. Therefore, the fire did not degrade any system used to prevent core damage, assure containment integrity, or maintain defense-in-depth and safety margins.
17. TEXT (If more space Is required, use additional copies of NRC Form (366A)
- The subsequent fire suppression water and oil run-offs did not challenge any accident mitigation function. In addition, the main transformer does not provide any accident mitigation capability, nor is it modeled in CUP's Probabilistic Risk Assessment.
- The transformer failure does not directly contribute to the increased likelihood of any initiators, other than transients that result in a reactor trip.
Additionally, this event did not have an effect on the function of any emergency diesel generator.
Corrective Actions
Corrective actions included replacement of the main transformer and repair and/or replacement of other non-safety related components damaged by the fire.
Previous Similar Events
In 1995 with Unit 1 shut down, the main transformer experienced a high side (345kv) phase to ground and phase to bushing fault while back energized from the switchyard.
The fault did not result in a unit trip, unit transient, or emergency safety features actuation.