05000277/LER-2001-004
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 2772001004R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 was in Mode 1 and operating at 100% rated thermal power when the event occurred. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On 10/23/01, at approximately 1102 hours0.0128 days <br />0.306 hours <br />0.00182 weeks <br />4.19311e-4 months <br />, an automatic reactor scram occurred as a result of a main turbine trip caused by a generator lockout. The main turbine trip resulted in the main turbine control and stop valves going closed.
As expected when the turbine tripped, reactor pressure increased resulting in five safety relief valves lifting initially to control reactor pressure.
Reactor pressure was subsequently controlled by the main turbine bypass valves.
Additionally, both Reactor Recirculation pumps received a trip signal when reactor pressure exceeded 1106 psig. An alternate rod injection signal was also generated due to the high pressure signal.
As expected for this type of event, reactor water level decreased resulting in PCIS (EIIS: JM) group II and III isolations and a standby gas treatment (EIIS: BH) start. All isolations performed as designed.
No Emergency Core Cooling actuations occurred due to this event. All other systems responded as expected for the given plant conditions. The operating crew stabilized the plant per applicable procedures.
The scram and primary containment isolation was reset on 10/23/01 at approximately 1120 hours0.013 days <br />0.311 hours <br />0.00185 weeks <br />4.2616e-4 months <br />.
This report is being submitted pursuant to 10CFR50.73(a)(2)(iv) due to the actuation of the Reactor Protection System (RPS) and Primary Containment / Standby Gas Treatment Systems. Pursuant to 10CFR50.72(b), prompt notification of this event was made to the NRC on 10/23/01 at approximately 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br />.
Analysis of the Event
There were no actual safety consequences as a result of this event. Plant equipment responded as designed to the event.
Five main steam relief valves lifted momentarily due to the reactor high pressure condition which is expected for a turbine trip / generator lockout initiating event. Engineering reviewed the relief valve (EIIS: RV) response to the high pressure condition and there were no significant concerns identified.
Initially, it appeared that relief valve 'A' may have lifted at a slightly lower pressure than what the relief valve is set for. However, more detailed investigation into post event pressure data as well as other hydrodynamic affects determined that the relief valve operated as designed.
Because of the main steam high pressure signal, Alternate Rod Injection (ARI) actuated. This is a redundant feature to other Reactor Protection System (RPS) circuitry to ensure that control rods are inserted for a scram condition. There were no concerns noted with the operation of this equipment.
The generator lockout functioned as designed. Both the generator neutral overcurrent and overvoltage protective devices for the A phase were tripped resulting in the generator lockout. These protective devices responded as designed.
The isophase ductwork dampers provide control of cooling to the main generator conductors to remove heat. The dampers are manually operated devices that are positioned to provide optimum cooling to the main generator conductors. The dampers are not safety related equipment. Had the damper failed and not come in contact with the live conductor (i.e. no generator lockout / scram event), the unit would have continued to operate. Because the damper would not significantly have impeded the ventilation flow, there would not have been any significant impact on the cooling of the conductors.
Cause of the Event
Meggaring of the generator (EIIS: TB) resulted in the identification of a ground fault on 10/27/01. It was discovered that the A phase isophase bus ventilation damper (EIIS: DMP) became detached in the bus ductwork and fell onto the A phase conductor (EIIS: CBL). One side of the damper remained attached to the bus ductwork, resulting in a ground fault on the neutral phase. Further examination identified that the damper mounting flange had broke and was no longer attached to the mounting angle on the bus ductwork.
Cause of the Event, cont.
Further investigation determined that the tapped holes in the bus ductwork were stripped where the mounting angle is secured to the ductwork. This was attributable to previous maintenance performed during a refueling outage. It appears that three screws were over tightened during reinstallation of the damper which caused the tapped holes to become stripped. This allowed the screws to eventually fall out which led to the damper failure.
It was also identified that the preventive maintenance task for the isophase cooling system did not specifically review the integrity of the dampers on a routine basis.
Corrective Action Completed The isophase bus ductwork damper was repaired, thereby eliminating the neutral ground. Other similar dampers on Unit 2 were inspected and no similar concerns were noted.
Corrective Actions Planned Similar dampers on Unit 3 will be inspected during an outage of sufficient duration to permit inspections.
Appropriate procedures that control isophase damper work will be improved.
An upgraded design of the bolting to resist over tightening will be pursued.
Appropriate procedures will be updated to regularly inspect these dampers in the future.
Previous Similar Occurrences There were no previous events identified that involved a scram due to a generator conductor ground.