ML20079A870

Forwards NEDC-21120, Monticello Feedwater Nozzle Cladding Crack Repair Rept, Per 751022 LER

ML20079A870
Person / Time
Site:	Monticello
Issue date:	12/31/1975
From:	Mayer L NORTHERN STATES POWER CO.
To:	Stello V Office of Nuclear Reactor Regulation
Shared Package
ML20079A872	List: ML20079A870 ML20079C409
References
NUDOCS 9105220313
Download: ML20079A870 (3)

Text

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NORTHERN 5TATES POWER COMPANY

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Mr Victor Stello, Direc*:or s- Q(.'N *' A Division of Operating Reactors \ ,

U S Nucicar Regulatory Ccanission .g

! Washington, DC 20555 =. D.*.11LT,

Dear Mr Stello:

o 0,)Y Q,S MOVIICELID NUCII.AR CENERATING PLANT .

Docket No. 50-263 License No. DPR-22 g.. p .

$ ,,.f ' y L Supplemental Evaluation Report 'on '3> ,z v (

E Feedwater N,o,zzle CladdinA Cracks M/ , n.. . .

s.! 4 iq Attached is the supplemental evaluation report covering feedwater nozzle cla dib}g, v ,.

crack experience at Monticello and initially reported in a Licensee Event Report dated October 22. 1975 (A0 75-20).

There are a number of additional items bearing on the conduc2 and quality of the Brinding repairs and modified sparger replacement which should be mentioned that are not included in the attached report. The following items increase our confidence in the repair-replacement work which we believe should minimize the '

probability and extent of recurrence.

1. .\ significant effort was expended to optimize working conditions in the vessel to achieve optimum worker perfomance. This included ,

r the decontamination and shielding of the vessel wall; use of air handlers and vacuuming to maintain a clean working area; installa-tion of a stable work platform; providing full supplied-air; and use of attendants to assist workers in suitup and unsuiting upon entry and exit from the in-vessel wc.rk area, r

2. Ievel I penetrant inspection work was performed by a volunteer group of engineers and supervisors who are full time NSP employees.

'Ihis approach provided a technically competent and conscientious working group who understood the significance and importance of the inspection effort in this application.

3. The penetrant in.pection procedure for this application was amended .-

to require a minimum penetrant dwell time of 15 minutes and a minimum developer dwell time of 25 minutes. We believe this amended procedure r.ignificantly improved the sensitivity of the in-spection method to indicate any presence of small, tight cracks.

Further, the acceptance criteria permitted no linear indication and no spot indications for final acceptance.

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9105220313 751231 PDR ADOCK O$000263 S PDR

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NORTHERN OTATEO POWER COMPANY Mr Victor Stello Page 2 December 31, 1975

4. As-left grinding repairs were determined by use of etching, dye-marking and replication casting techniques, thus providing accurate dimensioned definition of all repair grinding which penetrated the carbon steel material f or use in the stress evaluation.

5. In addition to actual repair grinding, the outer 1/32 to 1/16 inch of clad surface materials was removed using a " flapper-wheel" as part of the surface preparation for penetrant inspections. We believe this process has removed " tired" material which may have had a larger portion of its usage f actor consumed by previous ser-vice exposure, and that this significantly reduces the probability of crack indication recurrence.

6. The thermal sleeves of the replacement spargers were machined to final interference fit dimension prior to installation rather than sizing by cold-work expansion methods. We believe this increases the probable service life integrity of the fit because the sleeve starts without " memory" of cold work expansion.

In our judgment, all of the above increase our confidence that the unit was re-turned to service free of detectable indications in the nozzle area and that all practicable measures were taken to minimize service related recurrence of such indicetions.

Yours very truly, h*

• L 0 Mayer, PE Manager of Nuclear Support Services LOM/ak Attach cc: J G Keppler G Charnoff Minnesota Pollution Control Agency Attn: J W Ferman t

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