ML20206N400

Safety Evaluation Re Undefined Eddy Current Signals in Steam Generators.Steam Generators W/Undefined Signals Acceptable for Interim Period of Operation of Approx 3 Months,Pending Completion of Reevaluation of Eddy Current Data

ML20206N400
Person / Time
Site:	Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png
Issue date:	06/26/1986
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20206N381	List: ML20206N377 ML20206N400
References
NUDOCS 8607010452
Download: ML20206N400 (4)

Text

i aura,,,[$ UNITED STATES

[' q NUCLEAR REGULATORY COMMISSION 5 WASHINGTON, D. C. 20555 i[

\..../ SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION UNDEFINED EDDY CURRENT SIGNALS IN STEAM GENERATORS CONNECTICUT YANKEE ATOMIC POWER COMPANY HADDAM NECK PLANT DOCKET NO. 50-213

1.0 INTRODUCTION

By letter dated June 20, 1986, Connecticut Yankee Atomic Power Company, the licensee for the Haddam Neck Plant, informed the NRC that they had observed " undefined eddy current signals" in 575 tube ends of the steam generators (SGs) during the 1986 refueling outage. These signals, which are located within the 4.25 inch roll expansion region at the bottom of the tube sheet, are complex in nature and probably result from some combination of the following factors: skips in the roll expansion, .

roll expansion overlaps, tubesheet rippling, cold work of the tube, and flaws. Due to the unknown nature of these signals, characterization of the anomalies could not be performed. Therefore, no plugging or other corrective actions for these signals were taken during the outage.

A tube with an undefined signal was removed during this past outage from l Haddam Neck Plant SG Number 3 to assist in characterization of the undefined signals. Preliminary results of a destructive examination of the removed tube indicated that the undefined signal in that tube was due to inside diameter initiated cracks at the center of the roll expansion region. The cracks were 0.25 inches long, oriented 30 from l the longitudinal axis, and up to 82 percent through-wall. The cracks were caused by primary water stress corrosion.

8607010452 860626 PDR ADOCK 05000213 S PDR

The technical specifications currently state that corrective actions are to be taken for tubes with defects greater than or equal to 50 percent through-wall because the tubes may become unserviceable prior to the l next inspection. Unserviceable is defined as: the condition of a tube which leaks or contains a defect large enough to affect its structural integrity. Tube defects within the roll expansion region of the tube-sheet are not clearly distinguished by this technical specification requirement. As described later, the staff concludes that the structural integrity criteria for the SG tubes in this region are different than other tube regions; therefore, a license amendment to clarify these criteria is appropriate.

Programs are being initiated by the licensee with Westinghouse to further evaluate the safety significance of these defects in the roll expansion region. These programs are intended to further characterize the nature of these indications and perform appropriate analyses bases upon the limiting case type of defects. Based upon the results of these studies, the need for corrective action or a technical specification change will be assessed.

The staff has reviewed the results of the preliminary investigation completed by the licensee in order to determine the acceptability of plant operation for an interim period pending completion of the reevaluation of the undefined eddy current signals and for further detailed analytical studies.

2.0 EVALUATION The possibility exists that some of the undefined signals, which were identified during the last inspection, are defects of undefined magni-tude, sonb may have exceeded the plugging limit, and some may have been present as early as the end of Operation Cycle 4 (19h). Reanalysis of past eddy current data for a selected group of tubes indicates that S.

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f these signals were prc.sent in past coerating cycles and did not grow during the last operating cycle. Since there were no changes in these indications during this period, they are not expected to grow signifi-cantly during the proposed interi:n period of operation.

These undefined signals are all within the 4.25 inch roll expansion region at the bottom of the tubesheet, with the ma.iority of the signals concentrated at the center of the roll expansion. The restraint due to the tubesheet provides an additional margin of safety against the catastrophic failure of tubes. The expansion of the tube against the tdasheet will prevent the development of pressure differentials across the tube walls and thus pr. event tube burst or collapse in this region.

In addition, the tube cannot pull out of the tubasheet even if a circumferential 100 percent through-wall crack in the roll expansion region is postulated. The roll expansion is approximately 18 inches below the top of the tubesheet. Assurance that tube pull nut will not occur is provided by the restraint of non-defective roll expansion above the defect area, tube supports above the tubesheet, and the ,

strength and stiffness of the tube' itself.

1 l In the event that the postulated defects penetrate through the tube wall and develop a leak, the primary-to-secondary leakage limit specified in the technical specifications will prevent a large leak from developing.

l The technical specifications allow up to 150 gallons per day (gpd) per l SG or 0.107 gallons per minute (576 gpd total from all SGs) primary-to-secondary leakage before corrective actions must be taken. Based on past experience, leakage from any through-wall defect in the roll expansion region is expected to be small and within the allowable leakage limits.

Some primary-to-secondary leakage can be assumed if a through-wall defect is postulated. However, the maximum rate of any leakage would l

be controlled by the area of the annulus between the tube and the tubesheet and would be small. Further reduction of leakage would be provided by the non-defective roll expansion above the defect region.

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3.0 CONCLUSION

Based on the above considerations, the staff conc'ludes that the steam generators with the undefine.d signals in the roll expansion region of the tubeshee+ are acceptable for the interim period of operation of approximately three months, pending completion of the reevaluation of eddy current data and further detailed studies, and will not pose an undue risk to public health and safety.

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