05000528/LER-2010-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 03-07-2010 |
|---|---|
| Report date: | 05-06-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 45748 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(a)(1)(i), Emergency Class Declaration |
| 5282010001R00 - NRC Website | |
�YEAR NUMBER NUMBERPalo Verde Nuclear Generating Station 05000528 2 OF -5Unit 1 2010 -- 001� --� 00 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) as an event or condition that resulted in an automatic actuation of the reactor protection system (RPS), a manual system actuation of an Auxiliary Feedwater Pump (AFB-P01), and manual system actuation of multiple Main Steam Isolation Valves (MSIVs). This event was reported to the NRC via the Emergency Notification System (ENS) on March 7, 2010, at 1223 (ENS 45748). The eight hour reporting requirement for the AFB-P01 and MSIV actuations specified in 10 CFR 50.72(b)(3)(iv)(A) was not satisfied.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The reactor protection system (RPS) (EllS:JC) consists of sensors, calculators, logic, and other equipment necessary to monitor selected parameters to effect reliable and rapid reactor shutdown (reactor trip). The system's functions are to protect the core specified acceptable fuel design limits and reactor coolant system (RCS) (EllS:AB) pressure boundary for incidents of moderate frequency, and to provide assistance in limiting conditions for certain infrequent events and limiting faults.
The non-class 1E AC system distributes power at 13.8 kV (NA) (EllS:EA), 4.16 kV (NB) (El IS:EA), and 480V (NG, NH) (EllS:EC, ED) for non-safety-related loads. During normal plant operation, power for the on-site non-class 1 E AC system is supplied through the unit auxiliary transformer connected to the generator isolated phase bus. Off-site power is provided through the 13.8 kV 1 E-NAN-503B breaker through 13.8 kV (Calvert) bus 1E-NAN-A03 to 13.8 kV bus 1E-NAN-S01. The 1E-NAN-S01 non-class lE 13.8 kV bus distributes power to:
- 2 - Reactor Coolant Pumps (RCPs) (RC) (EllS:AB)
- 2 - Circulating Water Pumps (CW) (El IS:KE)
- 13 - 480V Load Centers
- 1 - 4.16 kV bus (1E-NBN-S01) �
3. INITIAL PLANT CONDITIONS:
Palo Verde Unit 1 was in Operating Mode 1 (Power Operations) at approximately 100 percent thermal power at the initiation of this event. There were no major structures, systems, or components that were inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
On March 7, 2010, Unit 1 was operating at 100 percent power and local weather conditions included heavy rains and wind gusts up to 40 mph. Unit 1 experienced a plant trip at 1115 when a ground fault occurred on the non-class Calvert bus 1E-NAN-A03 connecting the non-class 1E 13.8 kV bus, 1E-NAN-S01, to breaker 1E-NAN-S03B. The fault propagated from phase A to phase B of the Calvert bus. Protective relays on 1E-NAN-S01 and 1E-NBN-S01 de-energized the buses. The loss of 1E-NAN-S01 de-energized RCPs 1A and 2A resulting in a reactor trip. The loss of 1E-NBN-S01 de-energized Condensate Pumps PO1A and PO1B.
The loss of two of the three normally operating Condensate Pumps caused a low Condensate System discharge pressure resulting in low suction pressure for both Main Feedwater Pumps (MFWPs). This condition resulted in a low discharge pressure for the Main Feedwater System and the temporary interruption of feedwater flow to the steam generators. Plant stabilization required manually tripping one MFWP and starting AFB-P01 at 1122.
Due to the loss of power on the affected non-class 1 E buses, some main turbine and main steam system valves failed to close resulting in higher than normal post shutdown steam flow. This higher than normal post shutdown steam flow resulted in an undesired RCS cooldown as plant conditions necessary for repair of the damaged bus had not been determined. After Auxiliary Operators manually closed the affected valves, further cooldown of the RCS occurred due to seat leakage of some secondary system valves. Therefore, the MSIVs were manually closed at 1456 to mitigate the undesired RCS cooldown. After evaluation, it was determined prudent to cool the plant down to Mode 5 for the bus repair.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The non-class 1 E buses are not required to achieve safe shutdown of the plant or mitigate the consequences of an accident.
� The Main Feedwater System and the Main Steam Isolation System were affected by the event. The Control Room staff initiated Auxiliary Feedwater flow to both steam generators and the steam generators were isolated by closing the MSIVs. The heat removal function was maintained through the Atmospheric Dump Valves until the unit was in Mode 5.
The event did not result in a transient more severe than those previously analyzed in the PVNGS UFSAR, Chapter 15, Accident Analysis. The specified acceptable fuel design limits and RCS pressure boundary limit were not exceeded. No automatic engineered safety feature (ESF) actuations occurred during the event and none were required. All safety-related buses remained energized from normal off-site power during and following the reactor trip.
There were no actual safety consequences as a result of this condition. The condition would not have prevented the fulfillment of the safety function; and, the condition did not result in a safety system functional failure as described by 10 CFR 50.73 (a)(2)(v).
6. CAUSE OF THE EVENT:
The control and maintenance of the installed configuration for 1E-NAN-A03 was not adequate to ensure a weather tight bus duct and adequate Noryl insulation resistance. The bus utilized Noryl sleeve insulation over the conductors, which has been identified through industry operating experience to fail.
Additionally, there was evidence of water intrusion in the outdoor vertical section of 1E-NAN-A03. A combination of the water intrusion and the likely degradation of the Noryl insulation resulted in a ground fault on phase A, which then ionized the air inside 1E-NAN-A03, resulting in a phase A to B fault that was then interrupted by bus protective relays.
7. CORRECTIVE ACTIONS:
Immediately following the event, the damaged section of 1E-NAN-A03 was removed and sent to an off-site vendor for repair. An initial extent of condition review was conducted on 13.8 kV Calvert buses that use Noryl insulation and are in an outdoor environment. Each of the three units has two applicable buses, (x)E-NAN-A03 and (x)E-NAN-A04. Two of the six buses (Unit 1, 1E-NAN-A04 and Unit 2, 2E-NAN-A04) were determined not to have had the Noryl insulation repaired or replaced. Visual examinations were performed for these two bus ducts to confirm the ducts were weather tight.
� The following additional corrective actions will be implemented for all three units:
- Improve the inspection and cleaning instructions in the preventative maintenance tasks for the Calvert buses and their associated ducts.
- Replace 13.8 kV outdoor Calvert bus Noryl insulation with new 3M heat shrink insulation or equivalent for the two buses that have not yet had the insulation replaced.
8. PREVIOUS SIMILAR EVENTS:
In the past three years, Palo Verde Nuclear Generating Station has not reported a similar event where the loss of a non-class power bus due to a bus failure resulted in a reactor trip.
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