05000461/LER-2006-001
| Docket Numbersequential Revmonth Day Year Year Monthnumber No. Day Year None 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4612006001R00 - NRC Website | |
PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
Unit: 1 � Event Date: 3/20/2006 � Event Time: 0450 Central Standard Time Mode: 1 (Power Operation) � Reactor Power: 96 percent
DESCRIPTION OF EVENT
On 3/20/06, at about 0450 hours0.00521 days <br />0.125 hours <br />7.440476e-4 weeks <br />1.71225e-4 months <br /> with the plant at about 96 percent power, operators observed Switchyard [FK] 345 kilovolts gas circuit breakers [BKR] for the Main Power Transformer [XFMR] trip open and received a Main Generator [EL] / Turbine [TA] trip, resulting in an automatic reactor scram. At the time of the event, the plant was at 1108 Megawatts Electric and 3329 Megawatts Thermal, with condenser vacuum stable at 27.8 inches of mercury. No work was being performed at the time of the event.
Operators took immediate actions in accordance with off-normal procedure CPS 4100.01, Reactor Scram, including immediately verifying that all control rods fully inserted.
About 4 seconds after the scram, as expected, reactor water level dropped below the Low, Level 3 trip setpoint, causing Primary Containment Isolation Valves [ISV] in Groups 2 (Residual Heat Removal (RHR) [BO]), 3 (RHR), and 20 (miscellaneous systems) to receive signals to shut. Operators entered Emergency Operating Procedure (EOP)-1, RPV Control. The Level 3 trip cleared 10 seconds later. (At about 0524 hours0.00606 days <br />0.146 hours <br />8.664021e-4 weeks <br />1.99382e-4 months <br />, operators verified that the valves responded to the Level 3 trip as required.) Approximately 32 seconds after the scram, the Motor [MO]-Driven Reactor Feed Pump [SJ] [P] (MDRFP) started unexpectedly, resulting in all three Reactor Feed Pumps running (including the two Turbine [TRB]-Driven Reactor Feed Pumps); the MDRFP did not inject into the vessel. Issue Report 468516 and Work Order 903843 have been initiated to investigate the unexpected start of the MDRFP.
About 10 seconds later, reactor water level swelled and reached the Level 8, high water level trip setpoint, causing all three Reactor Feed Pumps to trip off.
The MDRFP was restarted at about 0456 hours0.00528 days <br />0.127 hours <br />7.539683e-4 weeks <br />1.73508e-4 months <br /> to control reactor water level. Reactor pressure was controlled using turbine bypass valves.
At about 0605 hours0.007 days <br />0.168 hours <br />0.001 weeks <br />2.302025e-4 months <br />, operators exited EOP-1, all EOP entry conditions were cleared and the plant was in stable condition in Mode 3 (Hot Shutdown).
A troubleshooting team investigating the cause of this event identified that the C phase neutral Current Transformer (CT) [XCT] of the Main Generator output was found to have an open circuit. The open circuit was traced to a junction box [JBX] under the Main Generator. A loose terminal screw caused the open circuit on the CT lead wire by causing high resistance and overheating of the wire. The open circuit caused the Generator Differential Overcurrent 87-G1 [87] relay to trip due to a sensed current imbalance. Actuation of the 87-G1 relay resulted in a Generator Trip / Lockout and an automatic Turbine Electro Hydraulic Control (EHC) [TG] trip. The Turbine EHC trip caused a Turbine Control Valve [V] fast closure and a Reactor Protection System [JC] signal for the automatic scram.
Issue Report 468357 was initiated to perform a root cause evaluation of the reactor scram and to identify corrective action.
No automatic or manually initiated safety system responses were necessary to place the plant in a safe and stable condition. No other inoperable equipment or components directly affected this event.
CAUSE OF EVENT
The cause of this event was inadequate workmanship that occurred during Generator High- Potential testing, as a result of a process weakness. The inadequate workmanship most likely occurred during the eighth refueling outage. Terminal screws were most likely loosened to install test leads, and following removal of the test leads, the terminal screw was not adequately retightened, resulting in a failure of the wire termination.
SAFETY ANALYSIS
This event is reportable under the provisions of 10CFR50.73(a)(2)(iv)(A) as an event that resulted in an automatic reactor scram while the reactor was critical.
The response and behavior of the plant and operator performance during this event were compared to the Generator Load Rejection transient discussed in Chapter 15 of the Updated Safety Analysis Report and the General Electric Transient Safety Analysis Report and were determined to be within those analyses. This event posed no challenges to fission product barriers. All systems responded as required for an event without bypass valve failure.
No safety system functional failures occurred during this event.
CORRECTIVE ACTION
Process controls are being added to ensure that lifted and landed leads will be adequately tightened in the future. The quality control checklist of the contractor will be revised to include controls similar to those used in the Clinton Power Station procedure for Wire Removal/Jumper Installation, including a step to perform a pre-job brief on the requirements. (468357-19) The other Main Generator CT connections have been verified to be tight.
Technicians removed the CT from the neutral bus bar under a temporary modification due to concerns that the CT may have been stressed and could fail. A replacement CT was not available.
PREVIOUS OCCURRENCES
None
COMPONENT FAILURE DATA
Manufacturer� Nomenclature� Manufacturer Model Number Serial Number None The following table identifies commitments made in this document. (Any other actions discussed in the submittal represent intended or planned actions. They are described to the NRC for the NRC's information and are not regulatory commitments.)
COMMITMENT TYPE
COMMITMENT ONE-TIME ACTION Programmatic (Yes/No) (Yes/No) This document has no regulatory commitments.