05000352/LER-2010-001
| Limerick Generating Station, Unit 1 | |
| Event date: | 06-23-2010 |
|---|---|
| Report date: | 08-23-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 46042 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3522010001R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. D14 Emergency Diesel Generator was out of service for maintenance at the time of the event. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On Wednesday June 23, 2010 Limerick Unit 1 was operating at 100% power. At 2051 hours0.0237 days <br />0.57 hours <br />0.00339 weeks <br />7.804055e-4 months <br />, a valid manual actuation of the reactor protection system (RPS) (EIIS:JC) was initiated as directed by the Recirculation Pump Trip procedure (OT-112) due to an automatic trip of both reactor recirculation pumps (RRP) (EIIS:AD) following a loss of main generator stator cooling water (EIIS:TJ). The event was caused by a failure of a 13 kV cable (EIIS:CBL5) that supplies electrical power to the 144D non-safeguard 480 VAC load center transformer, a failure of the 114A/124A load center control power undervoltage auxiliary relay (EIIS:RLY) to automatically provide control power to 124A load center and a failure of a reactor recirculation pump M-G set standby lube oil pump relay. The 13 kV breaker (EIIS:BKR) that tripped was the supply to 114A and 144D load centers.
The operators entered the TRIP procedure for reactor pressure vessel (RPV) control (T-101) and stabilized reactor parameters. The operators verified that all control rods were fully inserted.
Reactor level initially decreased to a minimum of -5 inches and increased to a maximum of +42 inches. The +54 inch high-level turbine trip setpoint was not exceeded. The reactor level of less than +12.5 inches resulted in an isolation signal to the closed Group IIB valves as expected.
Reactor pressure initially decreased from 1044 psig to approximately 1039 psig, then increased and stabilized at approximately 1042 psig. Reactor pressure remained less than the lowest safety relief valve (SRV) setpoint of 1170 psig; therefore, no SRVs actuated. The main steam bypass valves opened as designed to control pressure.
The loss of 114A load center de-energized the power supply to the operating stator cooling water pump and the operating reactor recirculation M-G set lube oil pumps. The standby stator cooling water pump did not automatically start due to a loss of control power to the 124A load center caused by an undervoltage auxiliary relay failure. The 124A load center remained energized during the event without control power for the supplied loads. The 1A RRP M-G Set tripped on a loss of lube oil due to a failure of the standby lube oil pump to start. This failure was due to a relay failure which preceded the stator cooling water runback trip. The 1B RRP M-G Set tripped on the stator cooling water runback as designed.
The post-scram troubleshooting identified a fault on the 13 kV cable supplying the 144D load center. The faulted cable section was replaced but has not been energized. The 114A and 144D load center 480 VAC loads were re-energized.
The post-scram troubleshooting also identified that 114A/124A load centers control power undervoltage auxiliary relay (27X) was failed. The failed undervoltage auxiliary relay has been replaced.
The north stack ventilation exhaust wide range accident monitor (WRAM) was rendered inoperable by the electrical distribution failure at the time of the scram which resulted in a loss of sample flow and a loss of indication.
This WRAM failure was identified by the operators on Thursday June 24, 2010, at 0200 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br />. The licensed operators entered the Technical Specification (TS) 3.3.7.5, Accident Monitoring Instrumentation, action 81 in Table 3.3.7.5-1, which requires restoration within 7 days. The licensed operators also entered the TS 3.3.2, Isolation Actuation Instrumentation, action "c" to place one trip system in the tripped condition within one hour and take the action required by Table 3.3.2-1. With both North Stack Effluent Radiation High trip systems inoperable, in Operational Condition 3, Table 3.3.2-1, action 23, requires being in at least Cold Shutdown within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Cold Shutdown was achieved at 1350 hours0.0156 days <br />0.375 hours <br />0.00223 weeks <br />5.13675e-4 months <br /> which satisfied the TS action at that time. However, firm evidence was later identified that the WRAM was rendered inoperable on Wednesday June 23, 2010, at 2051 hours0.0237 days <br />0.57 hours <br />0.00339 weeks <br />7.804055e-4 months <br />, for a period with the plant in OPCON 3, which exceeded the 12 hour1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> TS action requirement. Therefore, the inoperable WRAM condition existed for a period of time longer than permitted by the TS.
A 4-hour NRC ENS notification was required by 10CFR50.72(b)(2)(iv)(B) for an actuation of RPS when the reactor was critical. An 8-hour NRC ENS notification was required by 10CFR50.72(b)(3)(iv)(A) for a valid actuation of RPS. The ENS notification (#46042) was completed on Wednesday June 23, 2010 at 2357 EDT. This event involved a manual actuation of RPS and a condition prohibited by TS. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(iv)(A) and 10CFR50.73(a)(2)(i)(B).
Analysis of the Event
There was no actual safety consequence associated with this event. The potential safety consequences of this event were minimal. A loss of both recirculation pumps transient is categorized as an incident of moderate frequency per the Updated Final Safety Analysis Report (UFSAR) section 15.3.1 Recirculation Pump Trip, subsection 15.3.1.1.2.2. The plant equipment performed as designed during the transient with the exception of the load center control power undervoltage auxiliary relay, the 1A RRP M-G set standby lube oil pump relay and the WRAM. The operators effectively stabilized reactor parameters.
The 13 kV breaker that tripped was the normal supply to both 114A Generator Area Load Center and 144D Technical Support Load Center. The control power for the 114A and 124A load centers was de-energized when the 114A transformer tripped. The control power undervoltage auxiliary relay failed to automatically transfer the control power loads to the energized 124A load center control power transformer. The 124A load center remained energized without control power. The 114A load center trip de-energized the operating 1A Generator Stator Cooling Pump. The 1B Generator Stator Cooling Pump is powered from 124A load center and did not automatically start due to the loss of control power. This resulted in a loss of stator cooling water to the main generator.
The 1A RRP M-G Set tripped on a loss of lube oil due to a relay failure on the standby lube oil pump. The loss of stator cooling water tripped the 1B RRP M-G Set resulting in a loss of forced reactor coolant circulation. The operators initiated a manual actuation of RPS which inserted all the control rods.
Cause of the Event
The initiating event was a failure of a 13 kV cable power supply to a 480 VAC load center with a subsequent failure of a load center control power undervoltage auxiliary relay and a failure of a reactor recirculation pump M-G set standby lube oil pump relay. This caused both RRP M-G sets to trip which required a manual scram. The undervoltage auxiliary relay failure was caused by improper adjustment of the relay latching mechanism. The degraded relay was not identified since there was no routine testing or preventive maintenance on the undervoltage auxiliary relay. The standby lube oil pump relay failure occurred when contacts failed to reset as designed.
Corrective Action Completed The faulted section of the 13 kV cable from the 13 kV breaker to the 144D load center transformer was replaced and tested. The energization of the repaired cable is pending.
The 114A and 144D load center 480 VAC loads were re-energized.
The failed load center control power undervoltage auxiliary relay and the failed standby lube oil pump relay were replaced.
Corrective Action Planned The other non-safeguard load centers with the same relay for control power undervoltage automatic transfer will be tested.
Preventive maintenance will be implemented on the non-safeguard load center control power undervoltage automatic transfer equipment.
Previous Similar Occurrences There were no previous similar occurrences in the prior five years due to an electrical distribution failure.
Component data:
(Design, Manufacturing, Construction/Installation) System:�EA� Cause: B (Medium Voltage Power System) Component: CBL5 (Cable, Medium-Voltage Power) Manufacturer: A385 (Anaconda Company) (Design, Manufacturing, Construction/Installation) System:�EA� Cause: B (Medium Voltage Power System) Component: 27 (Relay, Undervoltage) Manufacturer: W120 (Westinghouse) Part#: 289B363A13 (Design, Manufacturing, Construction/Installation) System:�AD� Cause: B (Reactor Recirculation System) Component: RLY (Relay) Manufacturer: T009 (Tyco) Part#: 1423165-2