05000352/LER-2008-002
| Limerick Generating Station, Unit 1 | |
| Event date: | 03-22-2008 |
|---|---|
| Report date: | 05-19-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3522008002R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 87% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On Saturday March 22, 2008, Limerick Unit 1 was operating at approximately 87% power with power ascension in progress. At approximately 19:39 hours, an automatic actuation of the reactor protection system (RPS) (EIIS:JC) occurred. The actuation of RPS was caused by a main turbine control valve fast closure signal from a main turbine trip. The operators entered the trip procedure for reactor pressure vessel (RPV) control (T-101) and stabilized reactor parameters. All control rods fully inserted.
Reactor wide range level indication decreased to -3.0 inches and then stabilized at +35 inches. Reactor pressure was approximately 1015 psig prior to the automatic scram, peaked at 1120 psig, and stabilized at approximately 955 psig after the scram. The reactor high-pressure scram setpoint of 1096 psig was exceeded but RPS had previously initiated due to the main turbine control valve closure. The redundant reactivity control system (RRCS) setpoint of 1149 psig was not exceeded.
The lowest main steam relief valve (MSRV) setpoint of 1170 psig was not exceeded; therefore, no MSRVs actuated. The main steam bypass valves opened as designed to control pressure post scram.
Primary containment isolation signals were automatically initiated at +12.5 inches reactor level. A Group 2 isolation occurred at +12.5 inches but the isolation valves were in the closed position prior to the event. The reactor recirculation pumps motor-generator set breakers tripped due to the main generator lockout.
An investigation determined that an invalid power/load unbalance (PLU) condition actuated the main generator protection relays (EIIS:RLY) resulting in a Main Turbine trip with a subsequent automatic actuation of RPS. The invalid PLU condition was caused by a relay internal failure, which affected the main generator load input signal to the PLU circuit.
This event resulted in an actuation of RPS when the reactor was critical and a valid actuation of RPS. The 4-hour ENS notification required by 10CFR50.72(b)(2)(iv)(B) and the 8 hour notification required by 10CFR50.72(b)(3)(iv)(A) were performed on Saturday March 22, 2008 at 2050 hours0.0237 days <br />0.569 hours <br />0.00339 weeks <br />7.80025e-4 months <br /> (#44088).
This event resulted in an automatic 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 turbine trip with bypass transient is categorized as an incident of moderate frequency per UFSAR section 15.2.3, "Turbine Trip". The plant equipment performed as designed during the transient and the operators effectively stabilized reactor parameters.
Failure analysis of the degraded main generator over current relay determined a relay tap screw was cross-threaded into a bus bar resulting in inadequate tap screw seating. The inadequately seated tap screw increased the resistance in the circuit, which caused an internal flashover between the A and B phases.
The electro hydraulic control (EHC) power/load unbalance circuit monitors main turbine power and main generator load.
If power exceeds load by greater than 40 percent the circuit actuates the main generator protection relays. Main turbine cross around steam pressure provides the power input and the main generator megawatt signal is the load input.
Cause of the Event
The automatic RPS actuation was caused by a Main Turbine trip, which resulted from a Main Generator lockout. The lockout was the result of an invalid actuation of the PLU circuit, which was caused by a relay internal failure.
The root cause of this event was the generator protection relay logic failed due to a defective relay.
Corrective Action Completed The degraded relay was removed from service and isolated from the PLU circuit inputs.
Corrective Action Planned The main generator relay testing procedures will be revised to include a circuit loop signal verification test to ensure reliability of newly installed equipment.
Previous Similar Occurrences There were no previous similar occurrences of a scram caused by a malfunction in the PLU circuit in the past three years.
Component data:
Component description: Main Generator Overcurrent Relay Component number: 350-G101 Manufacturer: G080 General Electric Company Model number: 12SBC31A1D