05000336/LER-2001-004
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3362001004R00 - NRC Website | |
1. Event Description On May 7, 2001, with the plant at 96 percent power, preventive maintenance (PM) work was being performed in the Unit 2 "A" Circulating Water Bay [NN]. The "A" Circulating pump [P], "A" travelling screen [SCN], and travelling screen in the adjacent "B" bay were tagged off for diver protection which allowed an accumulation of seaweed on the "B" screen over a period of several hours. When the "B" screen differential pressure reached its setpoint of 30 inches, the "B" Circulating Water pump automatically tripped. The reactor [RCT] was then manually tripped per procedure at approximately 1433 hours0.0166 days <br />0.398 hours <br />0.00237 weeks <br />5.452565e-4 months <br /> due to the loss of two Circulating Water pumps in one condenser [COND] [SG]. There were no significant complications following the trip and all safety related systems functioned as designed.
Preventive Maintenance work was scheduled to be completed on all four of the Unit 2 intake bays in late December, 2000, and early January, 2001. Due to the scheduling and planning of activities for the Unit 3 refueling outage this PM work was deferred until April, 2001. The schedule for the work allowed for one week on each of the four bays with work being completed in the "C" bay, followed by the "D" bay, the "A" bay, and finally the "B" bay.
Once work commenced, it was determined that the "C" and "D" bays would need additional maintenance and the work scope increased for those two bays. This expanded work scope further delayed the start of the work on the "A" and "B" bays and extended the start dates into the high seaweed season of late April and through May. The work on the "A" bay was then scheduled to start on May 7, 2001.
The evening just prior to the start date for the work there was a full moon causing extremely high tides which stirred up more debris in the bay. This coupled with the high seaweed season resulted in May 7, 2001, being a very high seaweed day. Intake bay work on Unit No. 3 was scheduled for the same day but was postponed due to the high seaweed conditions and was reported in the morning meeting. Due to calm conditions in the bay, low winds, and the historical Unit 3 intake bay problems when Unit 2 has had no problems, the decision was made to continue with the Unit 2 work.
At approximately 0730 hours0.00845 days <br />0.203 hours <br />0.00121 weeks <br />2.77765e-4 months <br /> the Unit 2 Shift Manager was notified that Unit 3 was experiencing seaweed problems and observed the calm bay conditions. Based on the calm conditions and historical Unit 3 problems the determination was made that there were no immediate intake structure concerns at Unit 2.
At approximately 1130 hours0.0131 days <br />0.314 hours <br />0.00187 weeks <br />4.29965e-4 months <br /> the "A" Circulating Water bay was taken out of service for the PM work to commence. For diver's safety the "B" travelling screen was placed in the "OFF" position and tagged as opposed to inserting the thermal barriers/stop logs. At 1423 hours0.0165 days <br />0.395 hours <br />0.00235 weeks <br />5.414515e-4 months <br /> the control room received an alarm for a high differential pressure across the trash rack [RK] and an increasing differential pressure across the travelling screen. A plant equipment operator (PEO) was called to inspect the intake bay. The PEO, was aware of the tag but did not know the "B" screen was turned off. He reported that the "B" screen was in the "OFF" position and noted the tag to the control room. The owner of the tag was notified and worked with the PEO to clear the divers from the intake bay and start the "B" screen in slow. The differential pressure continued increasing and approximately three minutes later the "B" screen was started in the fast position. About 30 seconds later the screen differential pressure setpoint of 30 inches was reached and the "B" Circulating Water pump tripped at 1433:02 hours. The reactor was than manually tripped by the control room at 1433:06 hours without significant complications.
Had the operators not taken action an automatic signal would have tripped the reactor. With the "A" CWP out for PM work and the trip of the "B" CWP, condenser vacuum would have been lost in the 1 A condenser which would have then tripped the main turbine [TRB] [TA]. The turbine trip would automatically trip the reactor. The operators manually tripped the reactor in anticipation of an automatic reactor scram.
This event is being reported pursuant to 10 CFR 50.73(a)(2)(iv) as an event that resulted in the manual actuation of the reactor protection system (RPS) including a reactor scram or reactor trip.
2. Cause The root cause of this event was determined to be that the risks and consequences of non-safety related work management decisions are not completely identified or assessed. Had the "B" travelling screen been in operation this event would not have occurred, yet procedurally tagging the screen to the "OFF" position was acceptable.
Additionally, during the planning and scheduling of the work the decision was made to tag the screens out as opposed to inserting the thermal barriers/stop logs which would have allowed the screens to continue running.
3. Assessment of Safety Consequences The CWP's provide water flow through the steam condenser that allows steam exiting the turbine to be condensed into water. In the normal plant configuration the "A" and "B" CWP's provide water flow through the "A" and "B" waterboxes in parallel. In the event that one pump is not running, the remaining pump provides flow through both waterboxes through a cross-tie valve. Loss of the remaining pump results in a complete loss of water flow through both waterboxes.
This event was of low safety significance. The reactor trip was uncomplicated and all safety functions were accomplished per design without complications.
4. Corrective Action Immediate corrective actions following the plant trip included stopping all circulating water bay work until an event review team had convened and investigated the event. Once the review team authorized the resumption of work in the bay an additional operator was stationed to monitor the differential pressure across both the trash racks and the travelling screens as well as to monitor the fish return and ensure the proper operation of the screenwash and traveling screen system.
To prevent recurrence a risk assessment process is being developed that utilizes the collective knowledge of plant personnel to identify those activities having the potential to jeopardize the safe and continuous operation of the plant. Results of the evaluation may conclude that some work is not suitable to be performed while the unit is operating or may require changes in plant configuration or compensatory actions to successfully perform the work activity.
Additional corrective actions are being addressed via the Millstone Corrective Action Program.
5. Previous Occurrences On April 29, 2001, while at 97 percent power, an automatic reactor trip occurred due to a loss of condenser vacuum. Following a maintenance overhaul of the "D" circulation water pump motor, preparations were being made to perform an uncoupled run. In order to run the motor uncoupled, electrical jumpers were required to be installed to bypass circulation valve/ pump start interlocks. During installation of the jumpers in the "D" circulating water pump breaker cubicle, the "C" circulating water pump tripped. With both the "C" and "D" circulating water pumps not operating, condenser vacuum decreased resulting in a reactor trip.
Energy Industry Identification System (ENS) codes are identified in the text as [XX].