05000352/LER-2015-001
| Limerick Generating Station, Unit 1 | |
| Event date: | 02-23-2015 |
|---|---|
| Report date: | 04-24-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 50847 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3522015001R00 - NRC Website | |
On Monday, February 23, 2015, Limerick Unit 1 was operating at approximately 100% power.
At 2139 hours0.0248 days <br />0.594 hours <br />0.00354 weeks <br />8.138895e-4 months <br />, a valid automatic actuation of RPS (EIIS:JC) was initiated due to a valid reactor high pressure condition. The high pressure condition was caused by an unexpected closure of the 1C I/B MSIV. The valve (EIIS:ISV) failed closed when a fitting installed on the instrument gas supply tubing to the valve failed. The valve is designed to fail closed on a loss of the instrument gas supply.
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 (#50847) was completed on Tuesday, February 24, 2015, at 0102 EST. This event involved an automatic actuation of RPS.
Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(iv)(A).
Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no other structures, systems or components out of service that contributed to this event.
Reactor water level initially decreased to a minimum of -25.5 inches and increased to a maximum of +44.5 inches on wide range level instrumentation. The +54 inch high-level turbine trip setpoint was not exceeded. The reactor water level of less than +12.5 inches resulted in an isolation signal to the closed Group IIA and Group IIB residual heat removal (RHR) system valves as expected.
The post-scram investigation identified a failure of the 3/8 inch to 1/4 inch reducing port connector on the 1C I/B MSIV. The failed reducing port connection was replaced with a 3/8 inch port connector which is a more robust design. The three unaffected Unit 1 I/B MSIVs do not use the 3/8 to 1/4 inch reducing port connector. All of the I/B MSIVs are now using the same port connector design. The O/B MSIVs do not use a port connector.
The operators entered the procedure for reactor pressure vessel (RPV) control (T-101) and stabilized reactor parameters. The operators verified that all control rods were fully inserted.
The highest reactor pressure observed during the event was 1113 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 reactor pressure.
Unit Conditions Prior to the Event
Description of the Event
2. DOCKET
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), O fice of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
CONTINUATION SHEET
2. DOCKET
Analysis of the Event
There was no actual safety consequence associated with this event. The potential safety consequences of this event were minimal. The plant equipment performed as designed during the transient. The operators effectively stabilized reactor parameters and verified all control rods were fully inserted.
Exelon PowerLabs failure analysis determined that the fitting failed due to high cycle fatigue.
Review of the design configuration, recent modifications, and maintenance found no issues identifying inadequate tubing support or excessive system vibration.
Cause of the Event
The reactor scram was the result of a high reactor pressure condition due to the unexpected closure of the 1C I/B MSIV following the failure of a fitting on the instrument gas supply tubing to the valve. The cause of the event was that an undersized fitting was installed on the air supply tubing to the 1C I/B MSIV, which was not capable of withstanding existing cyclic stresses.
Corrective Actions Completed The 1C I/B MSIV instrument gas supply reducing port connector was replaced with a 3/8 inch port connector.
An inspection of Unit 1 I/B and O/B MSIV PCIG/Instrument Air tubing was performed to verify the correct fittings are installed as specified by the design drawings or subsequent evaluation.
The inspection found the correct fittings on the remaining Unit 1 I/B and O/B MSIVs.
Corrective Action Planned An inspection of Unit 2 I/B and O/B MSIV PCIG/Instrument Air tubing will be performed during the current refueling outage to verify the correct fittings are installed as specified by the design drawings or subsequent evaluation.
Previous Similar Occurrences There was no previous similar occurrence in the last five years of an automatic actuation of RPS due to an unexpected MSIV closure.
Component data:
System: SB Main/Reheat Steam System Component: ISV Valve, Isolation Component number: HV-041-1F022C-OP Manufacturer: A585 Weir Valves & Controls USA INC.
Model number: 21150-H DWG