05000331/LER-2007-009
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 10-05-2007 |
|---|---|
| Report date: | 0-4-2007 |
| 3312007009R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Duane Arnold Energy Center 05000331 YEAR SEQUENTIAL NUMBER REVISION NUMBER
I.Description of Event:
Operation (LCO) for the 'A' Emergency Diesel Generator (EDG) (IEEE Code — EK), a loss of 480 Volt AC essential bus 1B42 (IEEE Code — ED) occurred during a circuit breaker 1B4234A clearance application. This condition resulted in a loss of both on-site emergency AC power sources from 0408 till approximately 0715.
Specifically, while setting electrical isolation to support replacement of the 'B' H2/O2 Analyzer Vacuum Pump (IEEE Code — IK) a fault occurred on the Motor Control Center (MCC) dual breaker cubicle 1B4234A/B. The fault resulted in an electric arc which produced an arc-blast propagating through the MCC cubicle and exiting at the bottom of the cubicle, which was at floor level. At the time of this event, the 'A' EDG was out of service for pre-planned maintenance. The loss of 480 Volt MCC 1B42 resulted in a loss of 'B' Emergency Service Water (ESW) (IEEE Code — BI).
The loss of 'B' ESW resulted in the 'B' EDG being incapable of performing its safety function. The loss of 'B' EDG resulted in a loss of the onsite emergency power safety function. At the time of this event, the 'A' ESW pump was inoperable, but available following'pre-planned maintenance.
II.Assessment of Safety Consequences:
As a result of this event, the plant entered multiple Technical Specification (TS) Actions, including a loss of safety function condition due to both ESW pumps being TS inoperable. Note that at the time of this event, the 'A' ESW pump was inoperable, but available. TS Action 3.7.3 Condition B for both ESW subsystems inoperable required the plant to be in. Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and Mode 4 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. This condition also resulted in an 8-hour immediate notification event under 10CFR 50.72 for an event or condition that alone could have prevented fulfillment of a safety function to mitigate the consequences of an accident due to both EDG's being unavailable for a period of approximately 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 7 minutes.
In conclusion, it has been determined that from 0408 till approximately 0715 on 10/05/07, a loss of both onsite emergency AC power sources occurred.
Specifically, at the time of the event, the 'A' EDG was out of service for preplanned maintenance. The loss of MCC 1B42 resulted in a loss of 'B' ESW. The loss of 'B' ESW resulted in the 'B' EDG being incapable of performing its safety function, and therefore a loss of the onsite emergency power safety function.
This event resulted in no personnel injuries.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Duane Arnold Energy Center 05000331 YEAR SEQUENTIAL NUMBER REVISION NUMBER This event did result in a Safety System Functional Failure due to both EDG's being unavailable at the same time.
III. Cause of Event:
The 1B4234A/B MCC breaker cubicle is a dual breaker cubicle. On the morning of 10/05/07, two operators were dispatched to implement a clearance that would open 'A' breaker within the cubicle and leave the 'B' breaker energized. The actions of the operators assigned were in accordance with the site's accepted practices.
When the operators achieved the correct breaker configuration, the operators attempted to open the MCC cubicle door. The operators had not fully overridden the breaker position/cubicle door interlock for the 'B' breaker, and therefore, the MCC cubicle internals were still interlocked with the MCC cubicle door. The outward force exerted by the operators was sufficient in this instance to cause outward movement of the common breaker stabs in the rear of the MCC cubicle.
This movement resulted in sufficient disengagement between breaker cubicle stabs and bus stabs to create an arc on at least one phase. Neither operator present was injured.
Following an initial investigation and follow-up Root Cause Evaluation (RCE) into this event, it was determined that the primary cause of the event was an equipment failure related to an underlying equipment deficiency that was exposed by Operator action. Specifically, the most probable cause of the arc flash was due to the 'C' phase stab for MCC breaker cubicle 1B4234A/B becoming separated from the right 'C' phase bus work. Due to the extent of the damage to the bus, it was not possible to determine the exact cause of the equipment failure.
Contributing to the event was the station management team's. failure to effectively identify and resolve previously identified issues with MCC breaker cubicle 1B4234A/B in the Corrective Action Program (CAP). Specifically, the CAP Screening process missed the underlying equipment deficiency of inadequate breaker stab to bus engagement as indicated by the intermittent effects of loads supplied by 1B4234B during previous breaker manipulations. Station managers focused corrective actions on the long-term issue of operating the dual configuration breakers during clearance activities. In addition, the RCE determined the consequences of the event could have been mitigated if the station had maintained the protected equipment program in a manner more consistent with industry best practices.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Duane Arnold Energy Center 05000331 YEAR SEQUENTIAL NUMBER REVISION NUMBER
IV. Corrective Actions:
Interim Actions:
An Operations Shift Order was issued documenting the new expectation that effective immediately dual breaker 480vac MCC cubicles would not be opened unless both MCC breaker handles are in the open position.
An Operations Shift Order was issued documenting the new expectation that effective immediately on-line risk management was being upgraded. Specifically, guidance has been generated to protect divisionalized safety equipment.
A review of the work schedule for the next 13 weeks was conducted to identify and remove any Protected Train conflicts.
Corrective Action to Repair Cause of Event:
Essential bus 1B42 was repaired and returned to service at 0715 on 10/05/07 after completion of maintenance.
V. Additional Information:
Previous Similar Occurrences:
A review of LERs over the previous 5 years revealed no similar occurrences.
El IS System and Component Codes:
1K — Containment Environmental Monitoring System EK — Emergency Onsite Power System ED — Low Voltage Power System — Class 1 E Bl— Essential Service Water Reporting Requirements:
This event is reportable under 10 CFR 50.72(b)(3)(v), 'Any event or condition that alone at the time of discovery could have prevented the fulfillment of the safety function of structures or systems that are needed to (D) Mitigate the consequences of an accident.