ML19350B524

LER 81-002/01T-0:on 810301,during Eddy Current Exam of Steam Generator Tubes,Tubes Found Degraded & Defective.Caused by Corrosion &/Or Fretting.Defective Tubes Plugged

ML19350B524
Person / Time
Site:	Prairie Island
Issue date:	03/13/1981
From:	Hunstad A NORTHERN STATES POWER CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML19350B522	List: ML19350B521 ML19350B524
References
LER-81-002-01T, LER-81-2-1T, NUDOCS 8103200736
Download: ML19350B524 (3)

Text

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69 14 I 8 60 61 DOCK ET NUMSER aa EVENT DATE 7$ REPORT D ATE EVENT DESCRIPTION AND PROS A8LE CONSEQUENCES ITTTIl Eddy current enmination of Unit 2 steam generators tubes during refueling l Io 131 l showed degraded and defective tubes. See attachment. I loidi i l i

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JJ 36 31 42 43 44 47 CAUSE DESCRIPTION AND CORRECTIVE ACTIONS h l i 10 l l Corrosion and/or frettine. Defective tubes were plueRed. I I

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b NORTHERN STATES POWER COMPANY REPORTABLE OCCURRENCE 81-02 PRAIRIE ISIAND NUCLEAR GENERATING PIANT SUPPLEMEhTAL INFORMATION EVENT DESCRIPTION While conducting eddy current examination of tubes in 21 and 22 Steam Generators, thinning of several steam generator tubes was discovered.

This thinning falls into two distinct groups: thinning at the first and second tube support sheets, and thinning at the antivibration bars.

The thinning at the first and second support plates occurs on the cold legs of both steam generators. This appears to be the same mechanisn as that observed during the last eddy current inspec tion. The thinning occurs at the periphery of the cold leg side of the steam generator.

Although both steam generators exhibit this mechanism, the problem is more apparent in 22 Steam Generator. Previous examination of a tube pulled from 22 Steam Generator showed that the thinning is caused by corrosion.

See R0-80-03. Further investigation into the causes of the corrosion have started. Investigations conducted by NSP and Westinghouse are cover-ing the following areas: thermal hydraulic analysis, review of previous eddy current data, and development of a procedure for flushing chemicals from the tube support sheet.

The thinning at the antivibration bars is a new problem and this occurs only in 22 Steam Generator. An inspection of the antivibration bar region will be done. The inspection will include fiber optic examination of some of the damaged tubes and the associated antibration bar. Also, the inspection will attempt to determine the amount of movement of the affected tubes.

All defective tubes and some additional tubes were plugged. The following criteria were used. For tube support plate thinning, tubes with thinning in excess of 45% were plugged. The 45% limit at the first and second support plates was based on a review of our results by Westinghouse and their conclusion that these results were consistent with their assumptions in arriving at their 50% plugging criterion. An additional 5% allowance was made to provide for possible errors in analysis of the eddy current readings. In the antivibration bar area, all tubes showing more than 40%

thinning were plugged.

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n NORTHERN STATES POWER COMPANY REPORTABIE OCCURRENCE 81-02 PRAIRIE ISLAND NUCLEAR GENERATING PIANT SUPPLEMENTAL INFORMATION

SUMMARY

EDDY CURRENT EXN!INATIONS NUMBER OF TUBE INSPECTIONS TYPE AND EXTENT STEAM GENERATOR OF INSPECTION 21 COLD LEG 22 COLD LEG MF KHZ THROUGH Full Length 332 277 MF KHZ THROUGH U-Bend 649 2996 MF KHZ THROUGH 7 Support 349 79 NUMBER OF TUBES WITH INDICATIONS 21 S.G. 22 S.G.

TSP TSP AVB 90-100% 0 0 0 80-89% 0 0 1 70-79% 0 0 3 60-69% 0 0 1 50-59% 2 4 11 40-49% 6 21 7 30-39% 18 29 1 (Between AVB's) 21-29% 19 27 1 SUBTOTAL 45 81 25

< 20% 35 36 TOTAL 80 137 25