05000219/LER-2007-001
| Oyster Creek | |
| Event date: | 01-7-2007 |
|---|---|
| Report date: | 09-17-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2192007001R00 - NRC Website | |
Unit Conditions Prior to the Event.
The unit was in Power Operation at 99.8% reactor power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
An automatic reactor scram occurred at 05:21 on July 17, 2007, due to a low reactor water level condition, following a trip of the "C" RFP. The "C" RFP overload alarm was received in the main control room followed by the breaker trip alarm at 05:20. Reactor vessel water level decreased rapidly due to the loss of feed water flow. The Reactor Operator started manually decreasing Reactor Recirculation Pump speed to lower reactor power to 70% reactor power in order to meet the capacity of the two remaining RFPs. The Oyster Creek plant design does not have an automatic Reactor Recirculation Pump runback feature to reduce power in the event of the loss of an RFP. The reactor operator was not successful in reducing reactor power to within the capacity of two RFPs prior to reaching the low-level automatic scram set point.
Reactor water level continued to lower as expected due to normal shrink and reached the Lo-Lo Reactor Water Level setpoint. Lo-Lo level actuations include containment isolation and the initiation of the Standby Gas Treatment System, Low Pressure Core Spray, and the Isolation Condensers. Reversal of the initial void collapse and heating of the colder feed water resulted in a level increase to high in the control band and eventually above 180 inches above the top of active fuel (TAF) which initially precluded the use of the Isolation Condensers. Electromatic Relief Valves (EMRVs) were manually opened on three occasions during the recovery to control pressure and also resulted in lowering of reactor vessel water level to the point where the Isolation Condensers could be utilized to maintain reactor pressure. The isolation condensers were then used to cool down the plant to where Shut Down Cooling could be placed into service at 150 psig.
As a result of the heatup of the Torus, Containment Spray ESW was placed in Torus cooling to reduce Torus Temperature at 06:57. Shutdown cooling was placed into service at 10:05 and the unit reached cold shutdown at 14:00. All plant systems performed as designed.
Analysis of the Event
There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. The transient led to an automatic scram and subsequent operator actions to maintain the vessel level above the top of the active fuel terminated the event.
The "C" RFP trip is attributed to an internal motor ground fault and was not caused by human performance. The RFP motor was original equipment and had never been replaced. The motor was scheduled to be replaced in the next refueling outage (1R22) in 2008.
The Operations Director reviewed operator performance for this event and identified weaknesses in operator performance. A cause determination for these identified weaknesses revealed that training and repetition needs improvement to address some of the issues. Simulator sessions provided to all operating crews after the event determined that the training provided to Operations personnel to mitigate a scram due to a RFP trip was sufficient but improvement opportunities were identified for immediately verifying and communicating critical parameters at the onset of the event, improving communications within the control room staff to enhance decision-making, and ensuring sufficient turnover of critical parameters during turnover of tasks between control room staff.
Cause of the Event
The cause of the "C" RFP trip is attributed to an internal motor ground fault. This motor was original equipment having never been replaced. The motor was scheduled to be replaced during the next refueling outage (1R22) in 2008.
Corrective Action Completed.
The failed RFP motor was replaced and the plant returned to full power operation. Operator training was conducted to emphasize communication of critical parameters between control room staff during transients. Preventive maintenance activities had been created prior to this event for periodic RFP motor refurbishments.
Corrective Action Planned.
Operations will implement corrective actions to enhance the communication of critical parameters during transients. In addition, Dynamic Learning Activities will be developed for operator performance issues that can be addressed by repetition in a simulator training setting.
Previous Similar Occurrences On January 25, 2006, the "B" Reactor Recirculation Pump tripped. An internal fault was the cause of both the recirculation pump motor and RFP motor failures. The "C" RFP motor was scheduled for replacement during the next refueling outage (1R22) in 2008.
Component Data.
Component: 'C' Reactor Feed Water Pump Motor Cause: Internal ground fault System: Feedwater Component: Motor (P-2-2C) Manufacturer: General Electric Model number: 31E724