05000335/LER-2002-002
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3352002002R00 - NRC Website | |
FACILITY NAME (1) Lxx.;Kt i NUMBER (2) � LER NUMBER (6)
Description of the Event
On October 24, 2002, St. Lucie Unit 1 was in Mode 1 at approximately 6 percent power.
The operating crew was in the process of "latching" the turbine prior to placing the unit on line following the SL1-18 refueling outage.
� The 1B main feedwater pump [EIIS:SJ:P] was running in the recirculation mode, the 15 percent main feedwater bypass valve controllers [EIIS:SJ:FIC) were in AUTO, and steam generator water levels were being controlled between 60-67 percent.
The operating crew was attempting to latch the turbine from the control room. Two attempts to latch the turbine were unsuccessful and the reheat and intercept valves failed to remain open following the attempts. An attempt to latch the turbine locally from the turbine front standard was also unsuccessful.
Following these turbine latch attempts, the reactor operator at the feedwater control station noted that both steam generator water levels were increasing rapidly. Manual control of the 15 percent main feedwater bypass control valves was attempted but the operators were unable to stop the rise in steam generator water level. At this point, the crew realized that the partial latching of the turbine must have driven the 15 percent main feedwater bypass control valves to their post-turbine trip position (approximately 5 percent of full feed flow). There is no positive indication that the controllers are in their designed post-trip position. The 15 percent main feedwater bypass control valve's "override pushbuttons" were depressed and control of the valves was restored to the controller at the feedwater station.
However, due to steam generator swell, levels continued to rise and the 1B main feedwater pump automatically tripped when the steam generator water levels exceeded the high-high steam generator level setpoint.
Subsequent efforts to restart the 1B and lA main feedwater pumps and restore feedwater flow were unsuccessful because the main feedwater pump discharge valves failed to open. During the efforts to restore main feedwater, steam generator water procedural manual trip criteria. At this point the operators manually tripped the reactor. Auxiliary feedwater flow was initiated and the plant was stabilized in Mode 3. All safe shutdown systems responded to the trip as required.
Following the event a critique of operating crew performance was conducted. Several deficiencies were noted. The initial indication of rising steam generator levels was not communicated immediately to the entire crew. The potential failure mechanism of the feedwater controllers during turbine latch operation was not anticipated and subsequent diagnosis of the failure was not timely. In general, crewmembers were slow to exchange complete and relevant information and a team approach to problem solving was not clearly evident.
Llut,KEI
Cause of the Event
The cause of this event was the failure of the turbine valves to latch combined with a latent procedural weakness in that the adverse consequences of a "partial latch" condition were not addressed. During the latch attempts, the increase in turbine control system oil pressure was sufficient to arm the turbine control logic that generates a signal to the feedwater control system on a turbine trip. On release of the turbine latch pushbutton, control oil pressure decreased below the setpoint of the turbine trip logic, and a turbine trip signal was sent to the feedwater control system. This turbine trip signal drove the 15 percent main feedwater bypass valves to the post-turbine trip position. This initiated the steam generator over-feed condition that started the chain of events that ultimately lead to the manual reactor trip.
Contributing factors include weak crew performance, the failure of "just-in-time" simulator training to cover this potential failure mode during turbine latching, and the lack of direct information to the control room operators regarding the status of the 15 percent main feedwater bypass valve controllers.
Analysis of the Event
This event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A) as any event or condition that resulted in manual or automatic actuation of the reactor protection system.
The turbine front standard local latch malfunction was caused by mechanical binding in the DEH [EIIS:TG:XC] oil interlock box and has been corrected by Maintenance. The inadequate turbine latch remote pushbutton response is still under investigation.
The main feedwater pump discharge motor operated valves (MOVs) stalled when opening because ongoing sample streams depressurized piping downstream of the MOVs, thereby causing a high differential pressure condition across the valves.
The 15 percent main feedwater bypass valve controller response to a turbine trip signal is by design. The automatic repositioning of the 15 percent feedwater bypass valves provides enough feedwater post-trip to remove decay heat and prevent an excessive cooldown event. However, recovering manual control of the 15 percent main feedwater bypass valves requires that the controller be reset by the override pushbuttons. The need to reset the controller is not indicated or annunciated to the control room operating crew. The 15 percent main feedwater bypass valve controller design is shared with St. Lucie Unit 2, and any potential corrective actions planned for St. Lucie Unit 1 controller are applicable to the other unit.
Analysis of Safety Significance Reactor trips are analyzed events, and safe shutdown equipment responded per design.
Therefore, this event had no adverse effect on the health and safety of the public.
FACILITY NAME (1) PAGE (3) LER NUMBER (6) NUMBER (2)
Corrective Actions
1. St. Lucie implemented procedure changes to place cautionary notes prior to turbine latching steps in Operations procedures.
2. The turbine front standard local latch malfunction, that was caused by mechanical binding in the DEH oil interlock box was corrected.
3. A work request was initiated for repair of the control room remote latch function.
In addition, St. Lucie is evaluating the current practice of performing turbine latching from the control room and will pursue procedure or hardware changes, as appropriate.
4. St. Lucie will include this event in scheduled 2003 human performance training.
5. St. Lucie is developing an outage evolution "just-in-time" training list that will be included as scheduled Operations outage activities. This event will be discussed during this training.
6. St. Lucie is evaluating potential plant modifications for both St. Lucie units to provide either direct indication or annunciation of the status of the 15 percent main feedwater bypass valve controller.
Additional Information
Failed Components Identified Westinghouse Turbine DEH Controller Similar Events Cooldown Transient.