05000374/LER-2005-003
| Event date: | 06-21-2005 |
|---|---|
| Report date: | 08-19-2005 |
| Initial Reporting | |
| ENS 41787 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3742005003R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 3489 Megawatts Thermal Rated Core Power
A. CONDITION PRIOR TO EVENT
� Unit(s): 2 Event Date: 06/21/2005 � Event Time: 2340 CDT � Reactor Mode(s): 1 Power Level(s): 100 Mode(s) Name: Run
B. DESCRIPTION OF EVENT
On June 21, 2005, at 2340, the feed breaker (2APO4E) from 4160 VAC bus 241Y to 480 VAC safety related buses 235X and 235Y tripped open due to a neutral over-current fault on bus 235X..
The loss of buses 235X and 235Y caused a loss of the 2A Reactor Protection System (RPS)(EF] Motor-Generator (MG) Set, which resulted in multiple containment isolation valve closures and a Unit 2 half scram. The loss of power also resulted in a loss of the battery chargers for the Division 1 125VDC and 250VDC (DC)(EJ] systems. The 2A Standby Liquid Control (SBLC)(BR] subsystem, 2A Residual Heat Removal (RHR)(BI], Low Pressure Core Spray (LPCS)(BM] and Reactor Core Isolation Cooling (RCIC)(BN] systems were declared inoperable due to the loss of Division 1 power.
On June 22, 2005, at 0227, it was determined that all Unit 2 Reactor Coolant System (RCS) leakage detection systems had been rendered inoperable by the isolation of Containment Monitoring, and Unit 2 entered a Technical Specification (TS) LCO 3.0.3 required shutdown time clock.
Troubleshooting determined that the cause of the feed breaker trip was a spurious trip of neutral over current relay 2451-AP055. The relay was replaced, and buses 235X and 235Y were re-energized at 0436, and the associated TS time clocks were exited. At 0500, the Division 1 125VDC and 250VDC battery chargers were re energized, and TS 3.0.3 and all applicable TS 3.4.7 clocks were exited at 0505.
The 250 VDC battery and the RCIC system were restored to operable status at 1405.
An 8-hour ENS notification (#41787) was made in accordance with 10 CFR 50.72 (b)(3)(iv)(1), due to the closure of containment isolation valves in multiple systems that occurred with the loss of buses 235X and 235Y. The systems that isolated include: Containment Monitoring, Drywell Floor Drains and Drywell Equipment Drains, Reactor Recirculation Flow Control Hydraulics, Drywell Instrument Nitrogen, Reactor Water Cleanup and Reactor Recirculation Sample System.
C. CAUSE OF EVENT
The cause of the feed breaker trip was a failure of ABB Type GR-5 neutral over current relay 2451-AP055. The root cause of the relay failure was age related degradation of the leaded glass insulation used in a Silicon Control Rectifier (SCR) internal to the neutral over current relay.
D. SAFETY ANALYSIS
The safety significance of this event is considered moderate because it resulted in the inoperability of multiple engineered safety feature (ESF) systems in Division 1, in the inoperability of RCIC, and presented a challenge to the control room team. The Division 2 ESF systems remained fully operable during the event. There were no plant transients asi the result of this event.
A Phase 3 Significance Determination Process evaluation was performed for the event. The incremental conditional core damage probability ICCDP was 4.21E-08, much less than 1E-06, and the issue was classified as GREEN.
The condition did not result in a safety system functional failure.
E. CORRECTIVE ACTIONS
Immediate Actions:
- Bus 235X was meggered and no fault was found. Troubleshooting isolated the problem to a failed GR-5 neutral over.-current relay, which was replaced (Complete).
Long-Term Actions:
- All safety-related type GR-5 relays will be replaced in accordance with a prioritized schedule (AT# 346214-29, 31).
F. PREVIOUS OCCURRENCES
Buses 231A/B On 6/21/94, Unit 2 experienced a reactor scram upon loss of power to 480VAC buses 231A and 231B. Power was lost to the 480VAC buses as a result of the feed breaker tripping open upon an actuation of the neutral ground fault relay (ABB/Type GR-5) The cause of the breaker tripping was a degraded trip output Silicon Controlled Rectifier (SCR) on the Bus 231B 6900 KV ground fault relay.
The relay was replaced and has not tripped since. The SCR was found to false trip when subject to fast transient noise, with impulses over lkV. When the SCR was replaced the relay was not affected by noise. Six other relays from LaSalle were tested for susceptibility to noise and found to be satisfactory. These relays were then re-installed in the plant.
G. COMPONENT FAILURE DATA
Neutral Over-current Ground Shield Relay, Type GR-5, Catalog # 202D6141UL, ITE Imperial Corporation (ABB)