05000261/FIN-2010005-03
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Finding | |
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| Title | Emergency Diesel Generator Inoperable in Excess of Technical Specifications Completion Time Due to Output Breaker Failure |
| Description | A violation of TS 3.8.1.B was identified when the B EDG was inoperable in excess of the TS allowed outage time. Enforcement discretion was exercised for this violation. No performance deficiency was identified. On February 22, 2010, the B EDG was removed from service for planned maintenance. During post maintenance testing the output breaker for the B EDG failed to close. The breakers failure to close was unrelated to the maintenance activity. The licensee entered the issue into the corrective action program as AR 382604 and initiated a root cause and extent of condition review. A new output breaker was installed and tested on February 24, 2010. The licensee determined the cause of the breaker failure was due to a vendor workmanship error, which included a defective Shunt Trip Attachment (STA) movable core in the breaker control circuit. Based on the failure mechanism, the licensee, using engineering judgment, concluded the B EDG had been inoperable for greater than the 7 days allowed by TS 3.8.1.B.4 and Condition C. The last successful breaker closure was January 28, 2010. This corresponded to approximately a 27 day period of inoperability. As discussed in the licensees root cause report, an inspection of the STA movable core revealed that the leading edge of the core was not chamfered to 1/32 as required by design specifications. Additionally, the leading edge of the moveable core exceeded the maximum outside diameter (OD) design specification by 0.004 in one area. The moving core slides within a brass sleeve on the STA. The brass sleeve inside diameter (ID) has a tolerance of 0.008. The moveable core must be free to rotate within the brass sleeve during STA operation. The investigation revealed the internal binding of the movable core occurred when the maximum OD region of the moveable core aligned with the minimum ID of the brass sleeve. Because the STA is procured from the vendor as part of a complete breaker or replaced as a complete assembly, the cause was not reasonably within the licensees ability to foresee and correct. An assessment of the significance of the event was performed by the inspectors. This review resulted in the matter being assigned a risk assessment of low to moderate significance. In addition, the licensees risk evaluation determined that the increase in core damage probability was also low to moderate significance. The event was mitigated by the redundant A EDG and Dedicated Shutdown Diesel Generator being available to respond to an event. The licensee concluded that actions to recover the B EDG, such as the discovery that the STA had positioned the trip bar in such a way which would not allow the breaker to close or replacing the affected breaker with a spare, could be accomplished in an estimated time frame which ranged from one to four hours. The inspectors reviewed the licensees assessment and corrective actions for the event, and determined they were appropriate to the circumstances. All similar breakers at the Robinson Plant which are susceptible to this failure have been inspected with no deficiencies noted. Prior to implementation of these inspections, satisfactory compensatory actions were put in place which ensured successful operation of similar breakers. The inspectors determined a violation of TS 3.8.1.B occurred since the B EDG was inoperable in excess of the TS allowed outage time (7 days). The inspectors determined that this violation was more than minor because it affected the equipment performance attribute of the Mitigating System cornerstone and because it affects the cornerstone objective of ensuring mitigating system availability. The inspectors determined that the breaker failure was not a performance deficiency because the cause of the failure was not reasonably within the licensees ability to foresee and correct to prevent the failure. Because a performance deficiency was not associated with this issue, it was not subject to evaluation under the formal Significance Determination Process (SDP) using Inspection Manual Chapter 0609. |
| Site: | Robinson  |
|---|---|
| Report | IR 05000261/2010005 Section 4OA3 |
| Date counted | Dec 31, 2010 (2010Q4) |
| Type: | Violation: Severity level Enforcement Discretion |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71153 |
| Inspectors (proximate) | M Bates J Hickey C Welch R Musser D Bollock R Williams W Deschaine |
| INPO aspect | |
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Finding - Robinson - IR 05000261/2010005 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Robinson) @ 2010Q4
Self-Identified List (Robinson)
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