05000368/LER-2006-001

LER-2006-001, Completion of a Plant Shutdown Required by Technical Specifications Due to Loss of Motive Power to Certain Containment Isolation Valves as a Result of a Phase to Ground Short Circuit in a Motor Control Cubicle

Docket Number
Event date:	10-30-2006
Report date:	12-21-2006
Reporting criterion:	10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown
3682006001R00 - NRC Website

A. Plant Status At the time of this event, Arkansas Nuclear One, Unit 2 (ANO-2) was operating in Mode 1 at approximately 67 percent power. Power ascension to full power was in progress after Refueling Outage 2R18.

B. Event Description

At 1238 CST, on October 30, 2006, the ANO-2 Control Room received several alarms concurrent with the attempted start of Containment Cooling Fan 2VSF-1A [BK]. Investigation revealed that the supply circuit breaker for Motor Control Cubicle (MCC) 2B-53 [EC] had tripped. This MCC supplies power to 2VSF-1A and several additional Technical Specifications (TS) related components (Red Train).

Shortly after the alarms were received, a fire alarm associated with the 2B-53 switchgear room was also received. At 1244, ANO-2 entered abnormal operating procedure OP-2203.034 (Fire or Explosion) due to a report of smoke emanating from the 2B-53 switchgear room. At 1248, operators on the scene reported that the source of smoke was a small fire in MCC 2B-53 and that the fire had been extinguished.

At 1304, ANO-2 declared an Alert Emergency Class based on a fire that affected one train of safety related equipment. Appropriate notifications to offsite organizations, including the NRC, were made, as required.

The loss of the 2B-53 MCC resulted in entry into several TS action statements, the most limiting of which was TS 3.6.3.1, Action D, which requires that an inoperable containment isolation valve be returned to operable status or the associated penetration be isolated within four hours or that the unit be placed in Hot shutdown in six hours and cold shutdown in the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />. Red train containment isolation valves affected by the loss of MCC 2B-53 were component cooling water supply and return to the reactor coolant pumps, main feedwater block valves, main steam [SJ] isolation valve bypass, and the main steam supply to emergency feedwater [BA] pump 2P-7A.

Other TS Action Statements entered included; TS 3.7.1.2 (red train of emergency feedwater inoperable), TS 3.6.2.3 (red train of containment coolers inoperable), TS 3.7.3.1 (red train of service water [BI] inoperable), and TS 3.8.1.1.b and TS 3.4.4 ( #1 emergency diesel generator [EK] inoperable due to loss of its fuel oil transfer pump). As a result of declaring the red train of service water inoperable, the action statements of TS 3.5.2.b (ECCS), TS 3.6.2.1 (Containment Spray [BE]), and TS 3.7.6.1 (Control Room Emergency Ventilation) were also entered.

Plant conditions remained stable at approximately 67 percent while the extent of the damage to the MCC was evaluated. At 1925, plant shutdown was initiated as required by TS 3.6.3.1, Action D, in order to facilitate repairs. The Alert was terminated at 2000. The reactor was manually tripped at 2134, in accordance with the plant shutdown procedure. Repairs were completed and the plant was returned to power operation at 0609 on November 1, 2006.

C.� Root Cause The cause of the fire in MCC 2B-53 was determined to be a misalignment of the circuit breaker stabs that connect the 2VSF-1A circuit breaker to the bus bars that resulted in both sides of the "C" phase stab and its associated spring clip ending up on one side of the bus bar, thereby creating a high resistance connection. Subsequent starts of 2VSF-1A likely resulted in arcing which increased the resistance at the connection due to oxidation at the contact points. When an attempt to start 2VSF-1A was made on October 30, the large inrush current of the motor at the high resistance connection caused the spring clip to melt. It cannot be determined if the molten metal from the spring clip caused a fire in the bottom of the MCC which created a conductive vapor in the cabinet or if the clip simply vaporized, creating the conductive environment. Nonetheless, the result was that the conductive vapor created an arc path to ground between the bus bars and the metallic dust pan in the bottom of the MCC, resulting in significant damage to the bus bars. The upstream circuit breaker tripped immediately, terminating the over-current event.

The root cause of this event was determined to be an inadequate design of the MCC (manufactured by ITE), which allows flexing of the cubicle while the circuit breaker is being connected to the bus since alignment guides are installed only on the bottom of the cubicle.

Additionally, the insulating barriers between the breaker cubicle and the bus bars are flimsy and the stab cutouts are too large. These factors make it possible to misalign the breaker stabs such that both stab fingers can be positioned on the same side of the bus bar.

A contributing cause of this event was inadequate corrective actions following a previous similar event. The inadequate design of the subject MCCs was identified in a root cause evaluation associated with a similar event in 1991. However, the corrective actions taken subsequent to that event did not, in this case, correct the design deficiency or implement effective procedural controls as a compensatory measure. For example, the procedure used to reinstall the subject circuit breaker in the MCC contains a statement that requires verification "to the best extent possible" that positive connection is made when the breaker is installed. This guidance is vague and was typically interpreted to mean that a voltage check should be performed after breaker installation. This practice would not necessarily identify a misaligned breaker stab.

D. Corrective Actions

Corrective actions completed to date include:

• Cleaned and repaired MCC 2B-53. New style insulating barriers of sturdier construction with smaller stab cutouts were installed.

• Tested the affected circuit breakers and associated loads.

• Inspected stab engagement on all ANO-2 MCCs with accessible panel backs. No additional stab engagement issues identified.

• Returned MCC 2B-53 to service.

• A memo was distributed to the electrical and relay shops of both ANO units stating that, until an acceptable stab alignment verification method is established, no circuit breakers will be inserted into energized MCCs. In addition, for those MCCs that are accessible from the rear, stab engagement will be visually verified after breaker installation.

Acceptable methods for verifying bus stab engagement for MCCs which cannot be accessed from the panel back will be developed and incorporated into applicable plant procedures and work instructions. This action is expected to be completed by March 15, 2007, for ANO-1 and June 15, 2007, for ANO-2.

In addition, an evaluation will be completed by March 13, 2007, to determine if the new style insulating barriers, which are of sturdier construction than the original barriers and have smaller stab cutouts, should be installed in all ANO-1 and 2 ITE MCCs to eliminate the potential for stab misalignment during breaker installation activities.

E.�Safety Significance 2B-53 is a Class lE 480 VAC MCC that supplies power to various red train safety related loads. Class lE electrical systems are designed such that a single failure will not prevent a safety system from performing its design function(s). Throughout this event, the redundant green train safety systems remained operable and capable of ensuring safe shutdown of the plant and maintaining safe shutdown conditions.

E. Safety Significance (Continued) ANO electrical systems are designed to provide protection from extended overload and fault current conditions that could result in significant damage to the plant. During this event, the supply circuit breaker for 2B-53 opened upon sensing the fault current condition, thereby removing the source of energy that was feeding the fire. The fire was completely contained within the MCC and was quickly extinguished.

Considering that redundant safety related systems remained operable and capable of mitigating any design basis event had the need arisen, and that the fire was contained within a single MCC and quickly extinguished, thereby minimizing damage to plant equipment, this event was of minimal safety significance.

F. Basis for Reportability 10CFR50.72(a) requires immediate NRC notification of the declaration of an emergency class.

In addition, 10CFR50.72(b)(2)(i) requires an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> notification regarding the initiation of any nuclear plant shutdown required by the plant's Technical Specifications whereas 10CFR 50.73(a)(2)(i)(A) requires a 60 day report regarding the completion of any nuclear [plant shutdown required by the plant's Technical Specifications.

As this event involved the declaration of an emergency class and the initiation and completion of a plant shutdown required by the plant's Technical Specifications, it is reportable pursuant to the above referenced criteria.

All required reports were made, as required by regulation.

G. Additional Information

A previous similar event was reported by ANO in Licensee Event Report 50-368/1991-007-00.

However, the corrective actions implemented in association with that event were not adequate in preventing the event discussed in this report.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

Attachment 2CAN120601 List of Regulatory Commitments Attachment to 2CAN120601 List of Regulatory Commitments The following table identifies those actions committed to by Entergy in this document. Any other statements in this submittal are provided for information purposes and are not considered to be regulatory commitments.

TYPE SCHEDULED

(Check One) COMPLETION

ONE TIME CONTINUING DATE

COMMITMENT ACTION COMPLIANCE (If Required) Develop an acceptable method to verify proper bus stab engagement ANO-1: 3/15/2007 for motor control cubicles which X cannot be accessed from the panel ANO-2: 6/15/2007 back and incorporate into appropriate procedures and work instructions.

Determine if installation of new style insulating barriers in all ANO-1 and 2 X 3/13/2007 ITE MCCs is appropriate.