05000529/LER-2004-003
| Palo Verde Nuclear Generating Station (Pvngs) Unit 2 | |
| 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 |
| Initial Reporting | |
| ENS 41304 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 5292004003R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
Pursuant to 10 CFR 50.73(a)(2)(iv)(A), this LER reports an event that resulted in the valid automatic actuation of one of the Unit 2 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 December 31, 2004 (reference ENS # 41304).
2. DESCRIPTION OF EVENT RELATED 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). (See section 10. OTHER INFORMATION, for a simplified diagram of the Palo Verde switchyard.) 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 lE power system and the Class lE 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 [ENS Code: EK] The standby power supply for each safety-related load group consists of one emergency diesel generator (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 December 31, 2004, at approximately 1042 Mountain Standard Time (MST), Units 2 and 3 were in Mode 1 (POWER OPERATION), operating at approximately 100 percent power. Palo Verde 525 kilovolt (kV) switchyard breaker PL-928 was open for scheduled annual maintenance or relay testing.
There were no failures of components with multiple functions involved in this event.
4. CHRONOLOGY OF RELEVANT EVENTS:
Prior to the event, on December 28, 2004,'Salt River Project (SRP) personnel (other, non- APS utility personnel) opened Palo Verde 525 kV switchyard breaker PL-928 for scheduled annual maintenance. Opening of the PL-928 breaker disconnects startup transformer NAN-X01 from the Palo Verde West switchyard bus and results in NAN-X01 being reliant on the East switchyard bus or the Westwing #2 transmission line for supply power.
On December 31, 2004, at 1042 MST, while performing Westwing #1 relay testing, APS T&D personnel inadvertently tested a relay for the Westwing #2 transmission line. The error resulted in the Westwing #2 line opening ("relaying-out") 525kV breakers PL-922 and PL-925. With the opening of breakers PL-922 and PL-925, and with breaker PL-928 already open, startup transformer NAN-X01 was isolated from the PVNGS switchyard and its supply power.
Subsequently, the downstream loads supplied by NAN-X01, i.e., Unit 2 NAN-S05 and Unit 3 NAN-S06 13.8 kV busses, lost power. Upon the loss of power to the Unit 2 NAN-505 and Unit 3 NAN-S06 busses, downstream power was lost to their respective engineered safety feature (ESF) transformers which supply power to the 4160 kV class lE busses.
This in turn caused the auto-start and loading of Unit 2 "A" EDG to supply power to 4160 kV class 1 E bus PBA-S03 and Unit 3 "B" EDG to supply power to 4160 kV class 1E bus PBB-SO4 The ESF auto actions for the loss of power functioned as designed and within their proper response times.
At 1043 MST, Units 2 and 3 entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1, AC SOURCES - OPERATING, Condition A, due to one offsite circuit being inoperable.
By 10:47 MST, SRP had closed PL-922 restoring power to the Westwing #2 transmission line and at 11:07 MST, PL-925 was closed restoring power to the startup transformer NAN-X01.
At 1217 MST, Unit 2 exited LCO 3.8.1, when NAN-S05 was declared operable.
At 1225 MST, Unit 2 entered LCO 3.8.1 Condition B, when an Auxiliary Operator (utility non-licensed) reported that there was an jacket water leak on the "A" EDG and it was declared inoperable. Because the jacket water leak was well within the makeup capacity of the jacket water subsystem, and the leak was in a location that did not challenge any of the electrical components, the DG was considered functional.
At 1236 MST, Unit 3 exited LCO 3.8.1, when NAN-S06 was declared operable.
On January 1, 2005, at 0600 MST, LCO 3.8.1, Condition B was exited when the Unit 2 "A" EDG was declared operable.
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 condition would not have prevented the fulfillment of any safety function and did not result in a safety system functional failure as defined by 10CFR50.73(a)(2)(v).
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, personnel safety impact, or appreciable economic significance.
6. CAUSE OF THE EVENT:
The cause of the event was human error. An APS T&D Protection/Control Technician, performing relay testing on the Westwing #1 line at the Westwing substation (approximately 44 miles from PVNGS), inadvertently tested a Westwing #2 relay resulting in the opening of 525 kV breakers PL922 and PL925 which isolated the Westwing #2 line from the Palo Verde switchyard.
Unusual characteristics of the work location include:
a. Identical equipment in close proximity b. Equipment labeling that was not clear and distinct to assist workers in differentiating identical equipment c. Lack of communication to identify sensitive equipment to worker d. Reliance on a single operator to perform repetitive work on sensitive equipment in high risk area.
7. TRANSPORTABILITY:
The loss of a single transmission line to"the switchyard, for whatever reason, coupled with ongoing work on switchyard components, is transportable. This type of scenario could possibly result in either the loss of a unit output (main generator trip and possibly a reactor trip), or the loss of a start-up transformer with subsequent loss of power to the associated 4160 kV class busses. The mitigation barriers already in place to lessen the probability and consequences of such scenarios include: switchyard administrative controls procedure; communication via a Palo Verde single-point-of-contact to the APS energy control center (ECC) and the Salt River Project for scheduled and emergent work; risk evaluation process for scheduled and emergent work including evaluation for inclement weather; and redundancy in the switchyard bus design.
8. CORRECTIVE ACTIONS:
Since this event was caused by personnel and circumstances outside of direct control of PVNGS management, "correctable opportunities" were identified in lieu of corrective actions. The correctable opportunities are:
1. Improve communication between Palo Verde, ECC, and Substation personnel to identify abnormal conditions that could negatively impact Palo Verde during certain work activities.
2. Identify work on extra high voltage (EHV) rated equipment that could negatively impact Palo Verde as "sensitive" to alert workers of a need for heightened awareness.
3. Implement a requirement for "flagging" to be used when working in areas of similar or identical equipment to assist the worker in properly identifying equipment.
4. Implement a requirement for two-party "concurrent verification" for work on EHV rated equipment that could negatively impact Palo Verde
9. PREVIOUS SIMILAR EVENTS:
Switchyard and EHV grid events that have affected PVNGS have been reported in LERs 50-528/2004-006-00 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.