05000287/LER-1978-006-03, /03L-0:on 780210,Control Rod 3,Group 4,dropped Into Core During Breaker Test,Causing Power Runback to 55%. Caused by Stator Short Due to Excessive Moisture Entering Through Nonleakproof O-ring Seal.Stator Replaced

/03L-0:on 780210,Control Rod 3,Group 4,dropped Into Core During Breaker Test,Causing Power Runback to 55%. Caused by Stator Short Due to Excessive Moisture Entering Through Nonleakproof O-ring Seal.Stator Replaced

ML19308B531
Person / Time
Site:	Oconee
Issue date:	03/13/1978
From:	Ken Wilson DUKE POWER CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML19308B526	List: 05000287/LER-1978-006, Forwards LER 78-006/03L-0 05000287/LER-1978-006-03, /03L-0:on 780210,Control Rod 3,Group 4,dropped Into Core During Breaker Test,Causing Power Runback to 55%. Caused by Stator Short Due to Excessive Moisture Entering Through Nonleakproof O-ring Seal.Stator Replaced
References
LER-78-006-03L-01, LER-78-6-3L-1, NUDOCS 8001090533
Download: ML19308B531 (2)

LER-1978-006, /03L-0:on 780210,Control Rod 3,Group 4,dropped Into Core During Breaker Test,Causing Power Runback to 55%. Caused by Stator Short Due to Excessive Moisture Entering Through Nonleakproof O-ring Seal.Stator Replaced

Event date:
Report date:
2871978006R03 - NRC Website

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g Io Isi i The unit was of f-line from 0850 on Febrt. ry 10 to 0520 on February 22.

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.1 DUKE POWER COMPANY OCONEE UNIT 3 Report Number:

R0-287/78-6

- Report Date:

March 13, 1978 Occurr'ence Date:

February 10, 1978 Facility: Oconee Unit 3, Seneca, South Carolina

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Identification of Occurrence:

Control Rod Stator Failure During RPS Breaker Test Conditions Prior to Occurrence:

100% Full Power Description of 0,currence.

On February 10, 1970 at 0842, during the performance of a RPS breaker test, Control Rod 3, Group 4 fell into the core causing a power runback to 55 per-cent. At 0850, it was determined that an A-phase-to-neutral short had occur-red and that stator replacement would be necessary.

During the above mentioned RPS breaker test, all CC phases for the Safety Groups are deenergized. Normally 2 phases, A and CC are energized to hold the Safety Groups.

However, the system design is such that only one phase need be energized to hold the groups in place.

The A phase had already started on this particular rod, so when the CC was deenergized, the rod fell.

Unit shutdown was 1.nitiated to replace the stator and when the replacement was -accomplished the unit was returned to power operations.

Apparent Cause of Occurrence:

The short was caused by corrosion and moistu're buildup within the stator cavity probably caused by. failure of the 0-ring seal to be leak-proof.

A similar incident was reported in R0-270/77-17 which was transmitted by my letter of Janucry 27, 1978.

Analysis of Occurrence:

The rod drop caused an immediate power runback to 55 percent, due to assym.

metric control rod configuration. A unit shutdown was eventually required to replace the stator.

Instrumentation and controls properly handled the runback and shutdown insuring that the reactor remained in a safe configura-tion at all times and that no applicable station limits were exceeded,

thus, the health and safety were not endangered by this incident.

- Corrective Action:

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The stator was replaced following unit shutdown.