05000528/LER-2011-001
| Palo Verde Nuclear Generating Station (Pvngs) Unit 1 | |
| Event date: | 02-21-2011 |
|---|---|
| Report date: | 08-03-2012 |
| Initial Reporting | |
| ENS 46633 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 5282011001R01 - NRC Website | |
All times are Mountain Standard Time and approximate unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This Licensee Event Report (LER) is being submitted pursuant to 10 CFR 50.73 (a)(2)(iv)(A) to report a valid automatic actuation of the circuitry that starts the emergency diesel generator (EDG) for Unit 1 and Unit 3 following an undervoltage condition on the affected safety bus for each unit on February 21, 2011. This event was reported to the NRC on February 21, 2011, via the event notification system (ENS 46633).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The safety related equipment for each Palo Verde Nuclear Generating Station (PVNGS) unit is divided into two load groups. Either one of the associated load groups is capable of providing power for safe plant shutdown. Each alternating current (AC) load group includes one 4.16 kV bus (EIIS: EB).
The preferred and alternate power sources for each load group is offsite AC power (EIIS: EK).
Offsite power is supplied from a startup transformer (EIIS: EA) through intermediate buses (EIIS: EA) to a class 4.16 kV bus. The standby power supply for each safety related load group consists of one EDG (EIIS: EK), complete with its auxiliaries and fuel storage and transfer systems. The standby power supply functions as a source of AC power for safe plant shutdown in the event of loss of preferred power and for post accident operation of engineered safety feature (ESF) loads.
3. INITIAL PLANT CONDITIONS:
On February 21, 2011, Palo Verde Unit 1 and Unit 3 were in Mode 1 (Power Operation) at 100 percent power, normal operating temperature and normal operating pressure. There were no inoperable structures, systems, or components at the time that contributed to this event.
4. EVENT DESCRIPTION:
On February 21, 2011, at 2001, a valid loss of power (LOP) actuation occurred due to an undervoltage condition on the Unit 1 train 'B' and Unit 3 train 'A' 4.16 kV safety buses which resulted in an automatic actuation of the circuitry that starts the Unit 1 train 'B' EDG and Unit 3 train 'A' EDG. Both EDGs started and loaded as designed.
The undervoltage condition on the Unit 1 and Unit 3 safety buses was the result of a protective relay actuation associated with the AE-NAN-X02 startup transformer which de- energized the transformer and the Unit 1 13.8 kV intermediate bus 1E-NAN-S06 and the Unit 3 13.8 kV intermediate bus 3E-NAN-S05. The affected intermediate buses provide offsite power to the Unit 1 safety bus 1E-PBB-SO4 and the Unit 3 safety bus 3E-PBA-S03 respectively.
Both Units 1 and 3 entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1, Condition A, for one required offsite circuit inoperable. On February 22, 2011, at 0106, Unit 3 restored offsite power to safety bus 3E-PBA-503 from the alternate supply and exited the LCO condition. At 0153, Unit 1 restored offsite power to safety bus 1E-PBB-SO4 from the alternate supply and exited the LCO condition.
Unit 2 safety buses and required offsite circuits were unaffected by the AE-NAN-X02 startup transformer trip. Unit 2 safety buses, Unit 1 safety bus 1E-PBA-S03 and Unit 3 safety bus 3E- PBB-504 were supplied by startup transformers AE-NAN-X01 and AE-NAN-X03.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
There were no inoperable structures, systems, or components at the time that contributed to this event. The EDGs responded as designed to the undervoltage condition on their respective safety buses. The conditional core damage probabilities for this event were calculated to be 7.56E-10 and 1.56E-9 for Unit 1 and Unit 3, respectively. This event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. There were no actual safety consequences as a result of this event.
This event did not prevent the fulfillment of a safety function nor did it result in a safety system functional failure as described by 10 CFR 50.73 (a)(2)(v).
6. CAUSE OF THE EVENT:
The cause of the undervoltage was a cable splice failure on the cable for the 'Y' winding of the AE-NAN-X02 startup transformer.
The failed splice was installed with an approved specification that allowed the use of an insulation taping method. The work order instructions lacked the necessary detail for the insulation taping method. The cable splice failed due to a marginal amount of tape applied on the crimp connector adjacent to the flared edge of the crimp connection. The tape was not able to withstand localized electrical stresses.
7. CORRECTIVE ACTIONS:
Immediate corrective actions were taken to restore offsite power to Unit 1 safety bus 1E-PBB-504 and Unit 3 safety bus 3E-PBA-503. Alternate supply power was provided from startup transformers AE-NAN-X01 and AE-NAN-X03 respectively, through their associated intermediate buses to the safety buses.
The specification for cable splicing and terminations has been revised to remove the allowance for taping of cable splices for 5kV to 15kV cables.
The preventive maintenance program for the four existing underground medium voltage taped cable splices at Palo Verde has been revised. This revision requires additional periodic testing of the affected cable splices using improved technology to maintain the reliability of the splices.
8. PREVIOUS SIMILAR EVENTS:
No similar events resulting from a cable splice failure have been reported by Palo Verde in the past three years.