05000368/LER-2007-001
| Docket Numbermonth Day Year Year Sequential Rev Month Day Year | |
| Event date: | 1-25-2007 |
|---|---|
| Report date: | 3-26-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| Initial Reporting | |
| 3682007001R00 - NRC Website | |
A. Plant Status At the time Control Element Assembly (CEA) 35 dropped into the reactor core, Arkansas Nuclear One, Unit 2 (ANO-2) was operating in Mode 1 at approximately 100 percent power.
Troubleshooting activities were in progress investigating abnormal voltage indications on the upper gripper coil of the subject CEA. At the time plant shutdown was initiated, reactor power had been reduced to approximately 47 percent to comply with Technical Specifications (TS) actions associated with azimuthal power tilt.
B. Event Description
At 1109 CST, on January 25, 2007, during troubleshooting activities associated with abnormal upper gripper coil voltages on CEA 35, which was fully withdrawn, the circuit breakers that feed the subject CEA tripped unexpectedly and the CEA dropped into the reactor core.
The dropped CEA resulted in entry into several TS action statements, the most limiting of which was TS 3.1.3.1.d, which requires that a trippable CEA that is misaligned from its group by more than 7 inches be restored to group alignment within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> or place the plant in at least Hot Standby within the following 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Other TS action statements entered included; TS 3.2.3 (Azimuthal Power Tilt), TS 3.2.1.b (Linear Heat Rate), TS 3.2.4.c (DNBR Margin), and TS 3.1.3.5 (Shutdown CEAs Fully Withdrawn) At 1115, Reactor Coolant System [AB] boration was commenced to reduce reactor power, as required by TS. At 1406, reactor power was stabilized at 47 percent, as required by TS 3.2.3.b.2.
Subsequent troubleshooting and repair efforts did not resolve the problem, and at 1717 on January 25, a normal plant shutdown was commenced. The reactor was manually tripped at 1821, in accordance with the plant shutdown procedure.
Repairs were completed and the plant was returned to power operation at 0200 on February 2, 2007.
C. Root Cause
The Control Element Drive Mechanism Control System [AA] moves CEAs in steps by sequencing power to five coils in a coil stack. In the normal holding mode, only the upper gripper coil is energized.
On November 1, 2006, it was identified that CEA 35 had transferred to its lower gripper coil as a result of the presence of elevated voltage on the upper gripper coil. CEA 35 was left on its lower gripper coil pending troubleshooting and repair efforts.
Inspection of the failed gripper coil indicated that the preliminary root cause of the failure was degradation of the coil insulation over its 20 year service life that led to internal shorting of the coil. It is believed that periods of elevated voltage on the upper gripper coils of the CEAs in the sub-group containing CEA 35 in the 1992-1993 timeframe may have accelerated the degradation.
The simple nature of the checks performed during outages and limited analysis of coil traces taken while the plant was at power are considered contributing causes of this event since they did not detect the degradation of the CEA coil.
D. Corrective Actions
The upper gripper coil of CEA 35 was replaced while the plant was shut down.
The plant was returned to power operation at 0200 CST, on February 2, 2007.
The upper gripper coils of the remaining CEAs in the sub-group containing CEA 35, which were exposed to periods of elevated voltage in the past, will be replaced during the next refueling outage to reduce the likelihood of a similar failure.
To further reduce the likelihood of the occurrence of similar events, a system monitoring plan will be developed which will include specific acceptance and replacement criteria for CEA coil trace reviews.
Additional upper gripper coils will be removed during the next refueling outage to gather information to finalize the root cause of the CEA 35 coil failure.
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E. Safety Significance
CEAs misaligned from their group position, specifically dropped CEAs, affect the power distribution within the core. In the case where a CEA which is normally fully withdrawn is dropped, power in the vicinity of the dropped CEA is reduced to a very low value. If reactor power remains substantially unaffected, power density increases in the rest of the core.
A dropped CEA event has been previously analyzed and specific compensatory actions are prescribed in the licensing basis documents.
Since the compensatory actions contained in the Technical Specifications and Core Operating Limits Report were complied with during this event, it is considered to be of minimal safety significance.
F. Basis for Reportability 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.
All required reports were made, as required by regulation.
G. Additional Information
A similar previous event in which a failed upper gripper coil resulted in a dropped CEA was reported in Licensee Event Report 2-1984-26-00. There were no actions to prevent recurrence taken with respect to this event. However, because it was believed that high voltage events during the early years of operation may have degraded the coils, all of the CEA upper gripper coils were replaced with coils containing different potting material which had better heat transfer were replaced in 1988. The upper gripper coil failure discussed in this report is the first involving one of the replacement coils.
Energy Industry Identification System (El IS) codes are identified in the text as [XX].
Attachment 2CAN030703 List of Regulatory Commitments Attachment to 2CAN030703 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.
COMMITMENT TYPE SCHEDULED
(Check One) COMPLETION
DATE
(If Required)
ONE TIME CONTINUING
ACTION COMPLIANCE
Develop a system monitoring plan to include specific acceptance and X replacement criteria for CEA coil trace reviews.
Remove additional upper gripper coils during the next refueling outage (2R19) to gather information to X finalize the root cause of the CEA 35 coil failure.
The upper gripper coils of the remaining CEAs in the sub-group containing CEA 35 will be replaced X during the next refueling outage.