05000528/LER-2006-007
| Palo Verde Nuclear Generating Station (Pvngs) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5282006007R00 - NRC Website | |
NOTE: All times listed in this event report are approximate and Mountain Standard Time (MST) unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This LER (50-528/2006-007-00) is being submitted pursuant to 10 CFR 50.73(a)(2)(iv)(A), to report an event that resulted in the valid automatic actuation of one of the Unit 1 and one of the Unit 3 emergency diesel generators (EDGs) [EIIS Code: EB].
Pursuant to 10 CFR 50.72(b)(3)(iv)(A), Arizona Public Service Company (APS) made notification of this event to the NRC Headquarters Operations Officer on October 26, 2006.
Specifically, on October 26, 2006 at 11:53 PM a valid actuation of the Palo Verde Nuclear Generating Station (PVNGS) Units 1 and 3 Train B EDGs occurred as a result of under voltage on their respective safety buses. Both EDGs started and loaded as designed. No Emergency Plan declaration was made and none was required.
(Reference: ENS call # 42938)
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
Utility Grid System [EIIS Code: FK] The Palo Verde Nuclear Generating Station (PVNGS), is connected through the PVNGS switchyard (transmission system) to the Arizona-New Mexico-California-Southern Nevada extra high voltage (EHV) grid which is interconnected to other EHV systems within the Western System Coordinating Council (WSCC).
Onsite Power System [EIIS Codes: EA, EB, EC & PB] Offsite sources of power provide preferred power to the three units through the secondary windings of three startup transformers. The onsite power system of each unit is divided into two separate systems: the non-Class 1 E power system and the Class 1 E power system which is divided into two separate load groups. Power is supplied to the auxiliaries at 13.8 kV, 4.16 kV, and 480V levels. The onsite power system includes the Class 1E power system which provides auxiliary ac and dc power for equipment used to shut down the reactor safely following a design basis event. The Class lE busses of each unit must be energized in order to provide preferred or standby power to the safety related loads of each unit.
Standby Power Supply [El IS Code: EK] The standby power supply for each safety-related load group consists of one EDG, complete with its accessories and fuel storage and transfer systems. The standby power supply functions as a source of alternating current (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 October 26, 2006 Palo Verde Units 1 and 3 were in Mode 1 (Power Operations), operating at approximately 100 percent power. Startup transformer NAN-X01 "Y" winding had been removed from service and was in the process of being returned to service. No other major structures, systems, or components were inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
In the process of returning startup transformer NAN-X01 "Y" winding to service, maintenance workers had just completed installing potential transformer (PT) fuses in Unit 1 switchgear 1ENANS06, cubicle "G." After this task was completed, this cubicle door was in the process of being closed when a loss of power to Unit 1 and Unit 3 class 4.160 kV safety buses occurred.
The loss of power to the two safety buses was the result of an apparent spurious protective relay actuation of Startup Transformer X01 output breakers to Unit 1 NAN- S06 and Unit 3 NAN-506 busses. Unit 1 NAN-506 bus was being supplied by its Alternate breaker (1ENANSO6F) and Unit 3 NAN-S06 was being supplied by its Normal breaker (3ENANSO6C).
Preliminary information is that the two simultaneous Loss of Power (LOP) conditions occurred when the cubicle door was closed on 1ENANS06, Cubicle G. This condition apparently actuated an auxiliary relay located on 1ENANS06, Cubicle "G" door and caused the output breakers (feeder breakers 1 ENANSO6F (Unit 1 NAN-S06 bus supply) and 3ENANSO6C (Unit 3 NAN-S06 bus supply)) of Startup Transformer X01 to open.
There was no electrical fault or damage to any electrical components.
Both Units 1 and 3 entered Technical Specification LCO 3.8.1 Condition A for one required offsite circuit inoperable.
Both Unit 1 and Unit 3 were at approximately 100% power, at normal operating temperature and pressure prior to and following the EDG actuations. Unit 2 was de-fueled and was not impacted by the electrical disturbance. No other ESF actuations occurred and none were required.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this event and the event did not adversely affect the safe operation of the plant or health and safety of the public.
The event did not result in a transient more severe than those analyzed in the updated Final Safety Evaluation Report Chapters 6 and 15. The event did not have any nuclear safety consequences or personnel safety impact.
The condition would not have prevented the fulfillment of any safety function and did not result in a safety system functional failure as defined by 1. 0 CFR 50.73(a)(2)(v).
6. CAUSE OF THE EVENT:
An interim Significant Equipment Root Cause of Failure Analysis report has been completed. The report included the following preliminary causes.
Direct Cause: The loss of power (LOP) event that occurred was caused by the inadvertent tripping of the feeder breakers to switchgear buses 1ENANS06 and 3ENANS06 due to the actuation of relay 586S, resulting in an under-voltage condition at switchgear busses 1 EPBBSO4 (Unit 1) and 3EPBBSO4 (Unit 3). Feeder breakers 1ENANSO6F and 3ENANSO6C opened as a result of auxiliary relay 552X, located on the top of the 1ENANSO6G cubicle door, actuating; in turn energizing lockout relay 586S. Auxiliary relay 552X actuated due to induced vibration when the 1ENANSO6G cubicle door was closed.
Probable Root Cause: Auxiliary relay 552X is susceptible to vibration due to its location on switchgear cubicle door.
Contributing Causes: The switchgear doors are difficult to open and close due to aging which contributed to the actuation of relay 552X.
7.�CORRECTIVE ACTIONS:
Corrective action to replace the 552X relay was completed.
Interim corrective actions:
- CAUTION signs were installed on doors for cubicles 1ENANSO6G, 2ENANSO6D and 3ENANSO6D "Contact U-1 SM Prior to Opening" (complete).
- An Operations briefing/night order was issued on potential for offsite power loss when opening doors of switchgear cubicles 1 ENANSO6G, 2ENANSO6D and 3ENANSO6D (complete).
- Maintenance briefings are being conducted on the potential for offsite power loss when opening doors of switchgear cubicles 1 ENANSO6G, 2ENANSO6D and 3ENANSO6D.
- Permanent "CAUTION" signs will be installed on doors of switchgear cubicles 1ENANSO6G, 2ENANSO6D & 3ENANSO6D to warn of LOP hazards when opening or closing doors.
Any additional corrective actions taken as a result of this event will be implemented in accordance with the PVNGS corrective action program. If information is subsequently developed that would significantly affect a reader's understanding or perception of this event, a supplement to this LER will be submitted.
8.P PREVIOUS SIMILAR EVENTS:
In the past three years, Palo Verde reported loss of power to PVNGS emergency buses and EDG actuations in LERs 50-530/2006-003, 50-530/2006-004, 50-528/2006-003, 50-528/2005-001, 50-528/2004-006-00, 50-529/2004-003-00 and 50-530/2003-004-00.
However, these previous events or conditions did not involve the same underlying concern or reason as this event, such as the same root cause, failure, or sequence of events. There have been no previous similar events in the past three years that had a similar failure mechanism or that should have prevented this event from previously implemented corrective actions.