05000389/LER-2008-002
| St.I 05000389 Of 3Lucie | |
| Event date: | 06-04-2008 |
|---|---|
| Report date: | 08-04-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3892008002R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
Description of the Event
On June 4, 2008, with St. Lucie Unit 2 in Mode 1 at 100% power, Plant instrument and control personnel were performing maintenance on the 5B Feedwater (FW) Heater High Level Limit Switch LS-11-26B [EII:SJ]when two taped leads being routed into the housing became lodged in the elbow of the conduit. During attempts to retract the leads the tape became dislodged and the wires came in contact with the conduit resulting in a 20 amp fuse failing on the 120V Vital AC 2B panel circuit 1. This de energized the solenoids associated with the alternate and normal drain valves to the 5B, 4B and 3B feedwater heaters [EII:SJ]. The normal valves failed closed and the- alternates failed open. The 2B Heater Drain Pump [EII:SJ] then tripped on low level as indicated by alarm F-44, 2B Heater Drain Pump Flow Low/Heater Level Lo-Lo. As the feedwater (FW) Heater 4B alternate opened the level decreased to the trip set point.
Preparations for a rapid downpower were initiated. The 2A main feedwater pump tripped on low suction pressure 50 seconds after the heater drain pump tripped. The reactor was manually tripped in anticipation of a low steam generator level auto-trip. All safe shutdown equipment operated as expected.
Cause of the Event
The root cause analysis of the event determined the cause to be failure to implement adequate process controls to minimize risk during level switch replacement and drifting of the pressure switch set point causing a premature actuation of the switch during a feed water transient. This ultimately resulted in a manual reactor trip.
Several contributing causes associate with this event included; lack of detailed planning and the absence of a formal risk review, lack of single isolation of energized circuits in plant configurations, and inadequate taping of the leads for the level switch prior to job execution.
Analysis of the Event
This event is reportable under 10 CFR 50.73(a)(2)(iv)(A), as any event or condition that resulted in a manual or automatic reactor trip.
Analysis of Safety Significance The condensate, feedwater and heater drain system draws water from the condenser hotwells and feedwater heaters and pumps it to the steam generators feed nozzles. The system is a composite of several subsystems that work in conjunction with one another to supply pre-heated and deaerated high pressure feedwater to the steam generators for steam production and are not safety related except for the condensate storage tank (CST) which is the source of water for the auxiliary feedwater system. Failure of the 2B Heater Drain pump would have ultimately led to a low steam generator level auto-trip of the Unit without operator intervention. Actions taken by the operators to manually trip the Unit precluded that action. The condensate and feedwater system have no credited safety function and the unit operator were able to achieve a safe shutdown without impacting the health and safety of the public.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
Corrective Actions
The corrective actions and supporting actions are entered into the site corrective action program. Any changes to the proposed actions will be managed under the commitment management change program.
Immediate Corrective Actions:
- Performed stand-down and Human Performance walkdown with crews.
- Implemented the requirement during FIN work activities to have a risk review performed prior to implementation.
Corrective Actions:
1. Develop specific procedural direction for controlling energized leads during all work evolutions and provide direction for the use of the risk management process.
2. Design and implement modifications to address vulnerability when performing maintenance on the level switches.
3. Evaluate and incorporate industry best practices for handling all leads, include training plan.
4. Revise the preventive maintenance strategy which mitigates setpoint drift or implement design change to replace the main feedwater pump suction pressure switches.
Similar Events A search of the corrective action database for St. Lucie was performed to identify events related to manipulation of energized wires. One condition report (CR 96-2772) with similar concerns was found in which during maintenance on a level switch, leads were shorted which de-energized a portion of feedwater heater level control system, resulting in lifting feedwater heater safety reliefs and resulting in a significant plant transient.
Failed Components
20 amp fuse on the 120V Vital AC 2B panel circuit Degraded Components Feedwater (FW) Heater High Level Limit Switch LS-11-26B