ML20058L639
| ML20058L639 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 10/19/1976 |
| From: | Wachter L NORTHERN STATES POWER CO. |
| To: | James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III) |
| References | |
| IEB-76-02, IEB-76-2, NUDOCS 9105300371 | |
| Download: ML20058L639 (2) | |
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. j-NORTHERN STATES POWER COMPANY Minneapolis, Minnesota 55401 October 19, 1976 Mr. James G. Keppler Director - Region III Office of Inspection and Enforcement United States Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellyn, Illinois 60137
Dear Mr. Keppler:
Monticello Nuclear Generating Plant l
Docket No. 50-263 License No. DPR-22 This letter is an update of our reply to I.E.Bulletin 76-02, dated April 15, 1976.
A destructive microscopic examination of three nylon bobbin HFA relay coils has been completed.
These coils were taken from normally de-energized DC relays which were located in an area which is not humidity controlled.
They had all been in service since initial plant construction (about 6 years).
The relay bobbins are constructed in such a manner as to house two individual, series connected, coils of wire side by side, with a nylon partition separating the two.
Proceeding on the assumption that corrosion would be most likely found in areas nearest the nylon material, examination of the wire was centered primarily on turns adjacent to the core of the bobbin (i.e.,
the inner-most turns of the coil).
One coil of each bobbin was unwound to within about 1/8" of the core and the remaining wire examined at about 50 foot intervals. The small diameter of the wire (7 mils) required that it be examined microscopically to obtain an adequate inspection. At each point of inspection, roughly one foot of vire was thoroughly examined at a magnification of 160 diameters for signs of discoloration, deterioration of the varnish coating, and corrosion of the wire.
The second coil on each bobbin received essentially the same treatment as the first except that in addition to examining the wire adjacent to the core, the rest of the wire was also examined microscopically every few hundred feet.
9105300371 761019
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' Examination of the wire by the unaided eye during unwinding and by microscopy at selec'.ed points failed to show any signs of deteriora-tion of either the copper wire or the varnish insulation. Examination of the bobbin under a magnification of ten diameters following removal of the wire showed no app ~arent deterioration of the nylon itself. A black residue from electrical tape used in splicing the coils at the core and wrapping the outside of each coil was noted on the bobbin where contact had been made with the tape.
This residue was believed to be adhesive from the tape and was not felt to be of any consequence.
We have been informed by General Electric Company personnel that one of the significant factors contributing to the corrosion failure mechanism is constant exposure to relative humidity in excess of 60%.
Based on the absence of this factor and the absence of any sign of corrosion in the three relays which were examined, we conclude that the probability of this type of failure occurring at the Monticello Nuclear Generating Plant is negligible. Therefore, relay or coil bobbin replacement is not planned.
Yours very truly,
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bd L. J /.achter Vice President - Power Production and System Operation LJW/kik c:
Mr. Victor Stello Mr. G. Charnoff Minnesota Pollution Control Agency Attn: Mr. J.W. Ferman KRC Office of Inspection and Enforcement Washington, D.C.
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