05000352/LER-2012-002
| Limerick Generating Station, Unit 1 | |
| Event date: | 04-19-2012 |
|---|---|
| Report date: | 09-18-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3522012002R01 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On Thursday, April 19, 2012, Limerick Unit 1 was operating at approximately 100% power. At 0753 hours0.00872 days <br />0.209 hours <br />0.00125 weeks <br />2.865165e-4 months <br />, a valid manual actuation of the reactor protection system (EIIS:JC)(RPS) was initiated as directed by the Recirculation Pump Trip procedure (OT-112) due to an automatic trip of both reactor recirculation pumps (EIIS:AD)(RRP) following a loss of main generator stator cooling water. The event was caused by a failure of a 13 kV/480 VAC transformer (EIIS:XFMR) that supplies electrical power to the 144D non-safeguard 480 VAC load center. The 13 kV breaker that tripped supplies electrical power to the 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 -3 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.
The pre-transient reactor pressure was 1041 psig. The maximum pressure observed was 1041 psig and minimum pressure observed was 950 psig during the transient. 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.
A brief low voltage condition on 124A load center caused by the electrical transient resulted in the tripping of the operating stator cooling water pump. The standby stator cooling water pump did not automatically start due to a loss of power to the 114A load center. Both reactor recirculation pumps tripped following the stator cooling water runback as designed.
The post-scram troubleshooting identified a fault on the transformer supplying the 144D load center. The 114A and 144D load center 480 VAC loads were re-energized after closing the load center tie breakers.
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 (#47850) was completed on Thursday, April 19, 2012 at 1110 EDT. This event involved a manual actuation of RPS.
Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(iv)(A).
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. 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 124A load center remained energized. The 1B Generator Stator Cooling Pump is powered from 124A load center and was operating when the transformer failed. The subsequent low voltage condition caused the 1B Generator Stator Cooling Pump to trip. The 114A load center trip de-energized the standby 1A Generator Stator Cooling Pump.
This resulted in a loss of stator cooling water to the main generator.
The operators initiated a manual actuation of RPS which inserted all the control rods.
Both RRPs tripped on the loss of stator cooling water runback.� The transformer fault was caused by partial current discharge (corona effect) where the High Voltage (HV) rods pass through the support board. A manufacturing defect resulted in improper insulation where the HV rod passes through the polyester support board.
Cause of the Event
The initiating event was a failure of a 13 kV/480 VAC transformer power supply to a load center due to partial current discharge (corona effect). The partial current discharge was caused by a manufacturing defect resulting in improper insulation of the HV line rod where it passes through the polyester support board.
Corrective Action Completed The 144D transformer was replaced and returned to service.
The 114A transformer was returned to service.
Previous Similar Occurrences There were three recent previous similar occurrences of reactor recirculation pump trips that resulted in manual actuations of RPS.
1)Unit 1 LER 2010-001-00 reported a failure of a 13 kV cable that resulted in a loss of stator cooling water and a trip of both reactor recirculation pumps which resulted in a manual actuation of RPS.
2)Unit 2 LER 2011-002-00 reported a stator cooling water high temperature actuation that resulted in a loss of stator cooling water and a trip of both reactor recirculation pumps which resulted in a manual actuation of RPS.
3)Unit 2 LER 2011-005-00 reported a main turbine first stage pressure instrument failure that tripped both reactor recirculation pumps which resulted in a manual actuation of RPS.
Component data:
System: 13 KV System Component: 144D Transformer, Technical Support Center Manufacturer: 258A ASEA, Brown Boveri Serial number: 24-29417