05000353/LER-2011-005

LER-2011-005,

Limerick Generating Station, Unit 2
Event date:	05-30-2011
Report date:	07-27-2011
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 46906	10 CFR 50.72(b)(3)(iv)(A), System Actuation
3532011005R00 - NRC Website

Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 2 (Startup) with all control rods fully inserted. The reactor was at approximately 146 psig and 362 degrees Fahrenheit.

Description of the Event

On Monday, May 30, 2011, Limerick Unit 2 was in OPCON 2 (Startup) and performing a soft shutdown to support completion of corrective maintenance on main turbine control valve CV-3. At 1134 hours0.0131 days <br />0.315 hours <br />0.00187 weeks <br />4.31487e-4 months <br />, all control rods were inserted to the full in position. At 1145 hours0.0133 days <br />0.318 hours <br />0.00189 weeks <br />4.356725e-4 months <br /> an unexpected "A2" reactor protection system (RPS) (EIIS:JC) channel actuation occurred and was reset. At 1147 hours0.0133 days <br />0.319 hours <br />0.0019 weeks <br />4.364335e-4 months <br /> the "A2" RPS channel actuated again. In addition, the 2A and 2B reactor recirculation pumps (EIIS:AD) tripped and the operators entered the "Recirculation Pump Trip" procedure (OT-112). At 1149 hours0.0133 days <br />0.319 hours <br />0.0019 weeks <br />4.371945e-4 months <br />, a manual actuation of RPS was performed as directed by OT-112.

The operating crew stabilized the plant using the appropriate Transient Response and Operating Procedures. The highest transient reactor level observed on the plant monitoring system (PMS) was plus 60 inches following the reactor recirculation pump trips. The digital feedwater level control system (DFWLC) responded as designed. There were no emergency core cooling system (ECCS) actuations.

An investigation determined that a failure of the main turbine first stage pressure trip unit (PIS-001-2N652C) (EIIS:PIS) caused the actuations. These trip units bypass the RPS turbine stop valve closure and turbine control valve fast closure trip functions when power is less than 29.5 percent rated thermal power. The trip units also bypass the reactor recirculation pump trip function for turbine stop valve closure and turbine control valve fast closure when power is less than 29.5 percent rated thermal power.

An 8-hour NRC ENS notification was required by 10CFR50.72(b)(3)(iv)W for a valid actuation of RPS. The ENS notification (#46906) was completed on Monday, May 30, 2011 at 15:46 ET. 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 were no actual safety consequences 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.

The RPS trip functions for turbine stop valve closure and turbine control valve fast closure are automatically bypassed when power is less than 29.5 percent of rated thermal power. Thermal power is sensed at the high pressure turbine first stage by four pressure transmitters. These instruments also are used in the reactor recirculation pump trip logic to bypass the turbine stop valve closure and turbine control valve fast closure trip logic when power is less than 29.5 percent of rated thermal power.

Failure analysis was performed on the degraded trip unit. The stationary wiper terminal on the Rll potentiometer was found to be loose when tugged. The cause of the loose terminal is unknown. The trip unit had been in service since 1986. The trip unit potentiometer was recently adjusted during the measurement uncertainty recapture 0010 implementation. There is no evidence that this adjustment contributed to this event.

Cause of the Event

The apparent cause of the event was a loose wiper terminal on the Rll potentiometer on the degraded trip unit (PIS-001-2N652C).

Corrective Action Completed The degraded trip unit was replaced.

Corrective Action Planned The remaining 510 model turbine first stage pressure trip units will be replaced.

Previous Similar Occurrences There were two similar events in the last 3 years. On two occasions manual actuations of RPS were performed following a trip of both reactor recirculation pumps. One event was reported in Unit 1 LER 2010-001-00 and the other event was reported in Unit 2 LER 2011-002- 00. The causes of the pump trips were different than this event.

Therefore, the previous corrective actions did not prevent recurrence.

MIC FORM 366A (10-2010) LICENSEE EVENT REPORT (LER) U.S. NUCLEAR REGULATORY COMMISSION

2. DOCKET

Component data:

Equipment:

Component Number:

Manufacturer:

Model Number:

Main Turbine First Stage Pressure Trip Unit PIS-001-2N652C R369 Rosemount Nuclear Instruments Inc.

510DU2