05000353/LER-2011-002
| Limerick Generating Station, Unit 2 | |
| Event date: | 02-25-2011 |
|---|---|
| Report date: | 04-22-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 46641 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3532011002R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power in end-of-cycle operation with all control rods withdrawn. The stator cooling water (SCW) system (EIIS:TJ) temperature was being controlled manually due to a degraded automatic temperature control valve (EIIS:TCV).
Description of the Event
On Friday, February 25, 2011, Limerick Unit 2 was operating at 100% power. At 0910 hours0.0105 days <br />0.253 hours <br />0.0015 weeks <br />3.46255e-4 months <br />, a main generator stator coolant trouble alarm was received and a main turbine electro-hydraulic control (EHC) load set runback actuated. This was followed by a trip of the 2A and 2B Reactor Recirculation Pumps (EIIS:AD) as designed. The operators manually initiated the reactor protection system (RPS) (EIIS:JC) as directed by the "Recirculation Pump Trip" procedure (OT-112). The operators also entered the "Loss of Stator Water Cooling Runback" procedure (ON-114) and verified the expected automatic actions had occurred. All control rods inserted and all safety significant systems functioned as expected.
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 45 inches following the trip of the reactor recirculation pumps and the lowest level observed was plus 3.5 inches due to the post-scram void contraction. The digital feed water level control system (DFWLC) responded as designed. There were no emergency core cooling system (ECCS) actuations or challenges. The only automatic isolation actuated was the expected plus 12.5 inch isolation signal for Group IIB. All main turbine supervisory functions and main generator protective relaying functioned as designed.
An investigation determined that SCW high temperature actuation was due to a degraded SCW temperature control valve that required manual control as well as three SCW high temperature switches (EIIS:TS) that were improperly calibrated during the previous refueling outage.
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 (#46641) was completed on Friday, February 25, 2011 at 1148 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. A loss of both reactor 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 SCW system is equipped with three stator water outlet temperature switches with trip setpoints of 81 degrees Centigrade. The switches are arranged in 2 out of 3 trip logic. When the trip logic is actuated the system actuates a main turbine EHC load set runback and trips both reactor recirculation pumps following time delays of 9 seconds for 2A reactor recirculation pump and 18 seconds for 2B reactor recirculation pump. The reactor recirculation pump trips are bypassed when feedwater flow is less than 44 percent.
The SCW high temperature actuation was due to the following conditions that reduced the margin to the actuation:
1) improper calibration of the SCW high temperature switches resulted in low setpoints 2) the computer point used to monitor SCW outlet temperature indicated lower than actual conditions 3) the temperature band established for use during manual control was exceeded 4) the Mercoid high temperature switches have a large dead band from trip to reset 5) the Mercoid high temperature switch setpoints are susceptible to early actuation due to vibration
Cause of the Event
The root cause of the event was a failure to properly control a degraded SCW temperature control valve that required manual operator action in lieu of an automatic control function.
Corrective Action Completed The Unit 1 and Unit 2 SCW outlet temperature switches were re- calibrated.
The Unit 2 SCW temperature control valve was repaired and placed in automatic control.
Corrective Action Planned The Adverse Condition Monitoring and Contingency Planning (ACMP) procedure (OP-AA-108-111) and Operational and Technical Decision Making Process (OTDM) procedure (OP-AA-106-101-1006) will be revised to add a requirement to initiate an OTDM for any ACMP that substitutes a manual operator action for an automatic control function.
Previous Similar Occurrences There was one similar event in the last 3 years. Unit 1 LER 2010-001 reported a manual scram following an automatic trip of both reactor recirculation pumps. The event was caused by a 13 kV cable failure.
The 1B reactor recirculation pump tripped on a SCW runback.
Component data:
Equipment:
Component Number:
Manufacturer:
Model Number:
Generator Stator Winding Cooling Water Discharge TSH-M2-2T72A(B,C) M235 Mercoid Corp DA-38-103-6