05000321/LER-2004-005
| Docket Number(Si | |
| Event date: | 4-15-2004 |
|---|---|
| Report date: | 6-9-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3212004005R00 - NRC Website | |
FACILITY NAME (1) DOCKET LER NUMBER (6 PAGE (3)
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor Energy Industry Identification System codes appear in the text as (EIIS Code XX).
DESCRIPTION OF EVENT
On 4/15/2004 at 0700 ET, Unit 1 was in the Run mode at a power level of 2762 CMWT (100 percent rated thermal power). At that time, the "B" Reactor Protection System (RPS, EIIS Code JE) Motor Generator (MG) Set (1C71S001B) tripped. The RPS is designed to "fail-safe" so that during a loss of power or control signal the systems powered by the RPS change to their emergency configuration state. Thus, the trip of the bus caused Group 2, Group 5, and outboard small-bore Group 1 Primary Containment Isolation System (PCIS, EIIS Code JM) valves to receive an automatic isolation signal.
Those valves open at the time of the event closed per design. The Main Control Room Environmental Control System (MCRECS, EIIS Code VI) entered the pressurization mode; both units' Standby Gas Treatment Systems (SGTS, EIIS Code BH) initiated; the Primary Containment Hydrogen and Oxygen Analyzers (EIIS Code IK) isolated. Also, the Reactor Water Cleanup (RWCU, EIIS Code CE) system, the Fission Product Monitoring (FPM, EIIS Code U) system, and the operating Steam Packing Exhauster all tripped. Loss of power to SGT system logic components resulted in isolation of the secondary containment and the initiation of the Unit 1 and Unit 2 SGT system trains. By 0715 ET, licensed personnel energized the RPS bus from its alternate supply and affected systems were subsequently returned to their normal lineups.
Investigation of the RPS MG set trip focused upon the two most likely causes. These were either a motor fault, or a spurious breaker trip. On-site troubleshooting was performed and no problems were identified during initial investigation. It was determined that the motor, generator, and flywheel rotated freely. The motor insulation and winding resistance were found to be acceptable with no indication of a short circuit type fault. The feeder cable continuity was verified and the insulation resistance was acceptable. Visual inspections and current trip tests were performed on the feeder breaker. The instantaneous trip values were found slightly lower than the acceptance criteria on 2 phases. Phase 1 tripped at 440 amps, phase 2 at 426 amps, and phase 3 at 497 amps. The acceptance criteria specified in the procedure was 441 amps to 539 amps. Although Phase 1 and 2 instantaneous trip values were slightly lower than expected, the as found condition would not have caused the breaker to trip while the RPS MG set was in operation. However, as a result of the lower values (apparent 'drift'), the RMS-9 trip unit was replaced. The new RMS-9 trip unit tested satisfactorily.
An analyzer was connected to the RPS MG set circuit to monitor motor current and voltage while operating. On 4/17/2004 at approximately 0920 ET, when attempting to re-start the RPS MG set motor, the feeder breaker tripped immediately. The new RMS-9 trip device fault trip also indicated a short circuit. The analyzer captured the inrush current for all three phases; 398 amps on phase 1, 297 amps on phase 2, and 392 amps on phase 3. The inrush currents on Phase 1 and 3 were much higher FACILITY NAME (1) II� DOCKET LER NUMBER (6 PAGE (3) than expected. However, they were not high enough to have caused an instantaneous trip of the breaker. Since the new RMS-9 tripped in the same manner as the previous device another problem was present.
A troubleshooting plan was developed and the investigation continued. Although the motor did not trip during trouble shooting it was considered to be the most likely source of the problem. This was based upon experience, noted eccentricity, impedance imbalance, burn type smell on start under load, and two breaker trips. As a result, the motor was replaced, on Work Order 1040847402, with a spare motor.
The new motor was installed and the old RPS MG set motor was sent to a qualified repair shop for disassembly and inspection. Based on analysis of tests and examination findings, repair shop personnel and SNC component engineers concluded that the potential cause of the breaker trips were motor phase-to-phase intermittent arcing due to overall insulation degradation. This motor had never been rewound, and the windings were over 30 years old. Additionally this motor had an abnormally high core loss. Although the core loss found did not directly cause the problem, it is believed that it contributed to the winding insulation degradation.
Review of both units RPS MG Set motor history and nameplate data determined that there are two other motors (1C71S001A and 2C71S001A) that are similar in design and age as the replaced 1C71S001B motor. However, the 1C71S001B failure is the first motor failure with this potential cause identified. All of the RPS MG set motors for both units will continue to be monitored, maintained, and replaced in accordance with the plant's Equipment Reliability Improvement Program.
CAUSE OF EVENT
The cause of the RPS MG Set trip was not definitively determined. However, the most likely cause was considered to be age related motor winding degradation that was accelerated due to a motor with an abnormally high core loss. Although the motor had not failed, an intermittent motor fault is considered to have caused the supply breaker RMS-9 device to trip.
REPORTABILITY ANALYSIS AND SAFETY ASSESSMENT
This event is reportable per 10 CFR 50.73 (a)(2)(iv)(A) because an automatic actuation of a system listed in paragraph (a)(2)(iv)(B) occurred. Specifically, several Engineered Safety Features actuated in response to a trip of RPS power supply bus "B.
The RPS power supply system is designed to supply stable 120-volt AC power to a variety of plant instrumentation systems including the Process Radiation Monitoring System, the Neutron Monitoring System, the Reactor Protection System, the Primary Containment Isolation System, and the Offgas Radiation Monitoring System. A high degree of power stability is achieved by using two motor- FACILITY NAME (1) DOCKET LER NUMBER (6 PAGE (3) generator sets to condition the power supplied by the RPS power supply system. The electrical output of each motor-generator set energizes one of two RPS busses.
In this event, the "B" Reactor Protection System bus (1C71) tripped because of the RPS MG Set trip.
Upon loss of power or control signal, systems powered by the RPS de-energize to their "safe" configuration (i.e., they initiate their emergency or accident functions). All systems affected by this event responded per design for a power interruption, and this was verified by licensed personnel immediately after the event occurred. No unexpected actuations occurred. Had a design basis accident occurred during this event, all affected plant systems would already have been in their emergency configurations and no further automatic actuations of these particular systems would have been required to mitigate the accident.
Based on this analysis, it is concluded that this event had no adverse impact on nuclear safety. This analysis is applicable to all power levels.
CORRECTIVE ACTIONS
The degraded motor was removed and replaced with a new motor.
ADDITIONAL INFORMATION
No systems other than those previously described in this report were affected by this event.
This LER does not contain any permanent licensing commitments.
There was a previous similar event reported in the past two years in which a broken bolt for the neutral RPS cable resulted in an RPS bus trip. This event was reported in LER 1-03-001 and the corrective actions associated with this LER would not have prevented this event.
Failed Component Information:
Master Parts List: 1C71S001B Manufacturer: GE Manufacturer Code: G080 Model Number: 440027 Type: RPS MG set motor EIIS System Code: JE EIIS Component Code: MO Root Cause Code: X Reportable to EPIX: Yes