05000353/LER-2005-004
| Docket Number | |
| Event date: | 10-12-2005 |
|---|---|
| Report date: | 12-8-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 42054 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3532005004R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On Wednesday October 12, 2005 Limerick Unit 2 was operating at 100% power. At 22:24 hours an automatic actuation of the reactor protection system (RPS) occurred as a result of a malfunction of the Electro-Hydraulic Control (EHC) system (EIIS:JJ) that closed the main turbine control valves (EIIS:V) at power. The reactor scrammed when the average power range monitor (APRM) flux exceeded the high flux setpoint of 118.3%. The maximum indicated flux was 121.3%. 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 increased and then decreased to a minimum of -37 inches and increased to a maximum of +58 inches (wide range) then stabilized at +20 inches during level recovery. The +54 inch high-level turbine trip setpoint was exceeded resulting in tripping of the reactor feed pumps. Reactor pressure increased to a maximum of approximately 1136 psig, which is less than the lowest safety relief valve (SRV) setpoint of 1170 psig; no SRVs actuated. The reactor high pressure scram setpoint of 1096 psig was exceeded but RPS had previously initiated due to high APRM flux. The redundant reactivity control system (RRCS) setpoint of 1149 psig was not exceeded. The main steam bypass valves opened as designed to control pressure post scram.
Both reactor recirculation (EIIS:AD) motor-generator (M-G) set drive motor breakers tripped as a result of the manual main turbine trip and resultant main generator lockout as expected.
Approximately 28 seconds prior to the main turbine trip the main turbine first stage pressure decreased to less than the RPS bypass setpoint due to the control valve closure, which was caused by the invalid EHC signal. As a result, the end-of cycle recirculation pump trip (EOC-RPT) on turbine control valve fast closure at greater than 30 percent power was bypassed prior to the main turbine trip. The reactor recirculation pump low limit speed runback (28% pump speed) actuated on low reactor level (+12.5 inches) prior to the reactor recirculation M-G set trip.
A 4-hour NRC ENS notification is required by 10CFR50.72(b)(2)(iv)(B) for an actuation of RPS when the reactor was critical. An 8-hour NRC ENS notification is required by 10CFR50.72(b)(3)(iv)(A) for a valid actuation of RPS. The ENS notification (#42054) was completed on Wednesday October 12, 2005 at 23:55 EDT. 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).
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.
The initial investigation determined that the control valves closed due to a spurious demand signal. The resultant increase in reactor pressure caused neutron flux to exceed the APRM scram setpoint. Further investigation was performed but no defect was found and the initiator of the EHC circuit card failure could not be identified or reproduced. Therefore, the four circuit cards that were most likely the cause of the transient were replaced and laboratory analysis was performed. The failure did not recur during the laboratory analysis. All the cards performed satisfactorily during two weeks of testing.
Cause of the Event
The event was caused by a malfunction of the EHC system. The most likely cause was a transient malfunction of a circuit card.
Corrective Action Completed The four suspect circuit cards that most likely caused the transient were replaced and tested.
Corrective Action Planned A modification is being evaluated which would upgrade both units to Digital EHC.
Previous Similar Occurrences Several previous similar occurrences were identified. LER 1-86 011 reported a scram that occurred following an unexpected closure of the main turbine control valves due to a ground in an EHC circuit. LER 2-93-001 reported a scram on high pressure due to an EHC malfunction that occurred coincident with a BOP battery ground alarm. LER 2-95-010 reported a scram due to high pressure due to an EHC malfunction. LER 1-96-016 reported a scram initiated by an EHC malfunction. LER 2-97-005 reported a violation of maximum thermal power limit due to a pressure transient caused by an EHC malfunction.
Component data:
System: JJ� (Turbine Supervisory Control System) Component: CBD (Board, Control) Manufacturer: G080 (General Electric) Part#: 125D5788G0003 125D5788G0001 118D1516G0003 114D6003G0007