05000352/LER-2012-005
| Limerick Generating Station, Unit 1 | |
| Event date: | 07-18-2012 |
|---|---|
| Report date: | 09-17-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 48117 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(a)(1)(i), Emergency Class Declaration |
| 3522012005R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. The 114A Load Center Transformer (LCT) (EIIS:XFMR) was de-energized and the 114A Load Center (LC) loads were being supplied through the 114A/124A LC tie breaker. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On Wednesday July 18, 2012, Limerick Unit 1 was operating at 100% power. At 0816 hours0.00944 days <br />0.227 hours <br />0.00135 weeks <br />3.10488e-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 (RRPs) (EIIS:AD) following a loss of main generator stator cooling water. The event was initiated by a fault on the 124A Generator Area LCT 13 kV cable connection (EIIS:CON). 124A LCT is a 13 kV/480 VAC non-safeguard LCT.
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 -11 inches and increased to a maximum of +41 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 was 1041 psig and decreased to approximately 966 psig, then stabilized. 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.
At 0832 hours0.00963 days <br />0.231 hours <br />0.00138 weeks <br />3.16576e-4 months <br />, an Operations supervisor identified flash-over damage on the 124A LCT and emergency action level (EAL) HU3 threshold #2 was met for an explosion within the protected area boundary affecting a Table H2 area. The affected Table H2 area was the control enclosure.
At 0839 hours0.00971 days <br />0.233 hours <br />0.00139 weeks <br />3.192395e-4 months <br />, an Unusual Event (UE) was declared. At 0947 hours0.011 days <br />0.263 hours <br />0.00157 weeks <br />3.603335e-4 months <br />, the UE was terminated.
The loss of 124A LC de-energized the power supply to the operating (1B) and standby (1A) stator cooling water pumps. Since 114A LCT was out-of-service the 114A/124A LC tie breaker was closed with both 114A LC and 124A LC supplied by 124A LCT. The 1A and 1B RRPs tripped on the stator cooling water runback as designed.
The post-scram troubleshooting identified a fault on the 124A LCT 13 kV cable. The faulted cable and 124A transformer were replaced. The 124A and 114A LCTs were returned to service.
During the investigation extent of condition (EOC) review, thermography was performed on 11 similar transformer air termination cabinets (ATC) and no hot connections were identified.
A one-hour NRC ENS notification was required by 10CFR50.72(a)(1)(i) for declaration of an Unusual Event. 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. The ENS notification (#48117) was completed on Wednesday July 18, 2012, at 0933 EDT. A follow-up ENS notification was made at 1022 hours0.0118 days <br />0.284 hours <br />0.00169 weeks <br />3.88871e-4 months <br /> due to the UE termination.
This event involved a manual actuation of RPS. Therefore, this LER is being submitted pursuant to the requirements of 10CFR 50.73(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.
Both RRPs tripped on the loss of stator cooling water runback. The operators initiated a manual actuation of RPS which inserted all control rods.
Cause of the Event
The cause of the failed transformer was a high voltage connection clamp that was larger than the 13 kV cable size and the cable was not installed properly. The cable in use was a solid conductor and the clamp used was designed for stranded cable. This resulted in overheating and subsequent failure of the connection which damaged the cable and the LCT. The investigation determined that this connection is not disturbed during routine maintenance. Therefore, this is believed to be a manufacturing issue.
Corrective Action Completed The 124A LCT supply cable was upgraded to a stranded cable with a crimped lug connection.
The faulted 124A LCT was replaced and returned to service.
Corrective Action Planned Similar LCTs will be upgraded to stranded cable with crimped lug connections.
Previous Similar Occurrences There were four 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 with all control rods inserted.
4)Unit 1 LER 2012-002-01 reported a failure of a transformer 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.
Component data:
System:�13 KV System Component: 124A Transformer, Generator Area Manufacturer: 258A ASEA, Brown Boveri Model number: DWG OLV-C-1874 Serial number: 24-35156-05