ML20135E534

Forwards Util Response to NRC RAI Concerning GL 95-03, Circumferential Cracking of Steam Tubes. Util Response Clarifies Insp Performed During Outage

ML20135E534
Person / Time
Site:	Maine Yankee
Issue date:	12/03/1996
From:	Hebert J Maine Yankee
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
JRH-269, MN-96-172, NUDOCS 9612110333
Download: ML20135E534 (4)

Text

~ -

MaineVankee RELIABLE ELECTRICITY SINCE 1972 329 BATH ROAD = BRUNSWICK, MAINE 04011 + (207) 798-4100 i

' December 3,1996

' MN-96-172 JRH-269 UNITED STATES NUCLEAR REGULATORY COMMISSION Attention: Document Control Desk Washington, D.C. 20555

References:

~

a) License no. DPR-36 (Docket No. 50-309) '

b) MYAPCo Letter to USNRC dated June 27,1995 (MN-95-73) .

. c) USNRC Letter to MYAPCo dated October 21,1996

. d) MYAPCo Letter to USNRC dated November 29,1995 (MN-95-132)

Subject:

Response to Generic L'etter 95-03 "Circumferential Cracking of Steam Tubes" Request for i AdditionalInformation 1 Gentlemen:

l This letter provides additional information as requested in Reference (c) and supplements Maine i Yankee's response to Generic Letter 95-03, Reference (b). I NRC_REQUESTEDINEORMATION140ml The following areas have exhibited circumferential cracking:

a) Expansion transition circumferential cracking b) U-bend circumferential cracking c) Dented location (including dented TSP) circumferential cracking d) Sleeve joint circumfeVential cracking-m t- . . z. ;wq ., , ,, , .,

~

In your response to Generic Letter (GL) 95-03, the susceptibility of the U-bends and dented locations in your steam generators to circumferential cracking was not addressed. If these areas are susceptible to circumferential cracking, please submit the information requested in GL 95-03 per the guidance contained in the GL for these areas and any other area susceptible to circumferential cracking. The above response should clarify the inspections performed at these locations during the prior tube inspection outage and should clarify the inspections to be performed during the next scheduled steam generator tube inspection. For dented locations, if applicable, the criteria for determining which dents are/were examined should be provided. If a dent voltage threshold is used for such a determination, the calibration procedure used (i.e.,4.0 volts on 4-20 percent through-wall l ASME holes at 550/130 mix) should be provided. i O

)

9612110333 961203 f\

V PDR ADOCK 05000309 )

p PDR a

. . MaineYankee  !

l UNITED STATES NUCLEAR REGULATORY COMMISSION MN-96-172 Attention: Document Control Desk Page Two Maine. Yankee Response i

In its latest (1995) steam generator inspections, Maine Yankee inspected 100% of the tightest radius U-bends (tube rows 1 through 12 inclusive) in each ofits three steam generators with a Motorized Rotating Pancake Coil (MRPC). No circumferential cracks were detected. j l

The majority of the Maine Yankee horizontal tube suppons are of steel lattice bar (egg crate) design and steam generator tubes are not prone to circumferential cracking at their intersections. Two i partial drilled plate horizontal supports do exist near the top of the Maine Yankee S/G's. In each of Maine Yankee's 1993 and 1995 steam generator inspections,25 randomly selected dented drilled support plate intersections were examined with an MRPC. No circumferential cracks were detected. l The region of the steam generators containing the tight U-bends and the partial drilled support plates operates at a lower temperature than the hot leg top of tubesheet expansion transitions where circumferential cracking has previously been observed at Maine Yankee and are not thought to be j as susceptible to circumferential cracking. No circumferential cracks have ever been detected m these regions in the Maine Yankee steam generators. i 1

Maine Yankee is presently observing minimum to no primary to secondary leakage in its steam generators. Maine Yankee's low administrative and proposed Technical Specification (Reference l d) steam generator leakage limits of 50 and 150 GPD respectively, should ensure Maine Yankee i shuts down and makes s epairs before the stresr. in any tube exceeds its design criteria. l Because of the lower temperature, no previously detected circumferential cracks, and minimum to l I

no observed primary to secondary leakage presently being observed, Maine Yankee does not believe circumferential cracks of significance are present in the U-bends or at the partial drilled suppon plates of the Maine Yankee steam generators. I In the next scheduled steam generator inspection (Fall 1997), Maine Yankee will repeat the 100%

MRPC examination of the U-bends in rows 1 through 12.

A sample of the partial drilled support plant intersections will be inspected with an MRPC in the 1997 steam generator inspection. The MRPC inspection sample (s) will be defined by the magnitude of denting observed at the intersections in the 1995 bobbin probe exams. Dent data from 1995 is available for all of the partial drilled support intersections. It is desirable to identify the MRPC samples before the 1997 outage to have the flexibility to conduct MRPC inspections with equipment set up in the hot leg channel heads while simultaneously conducting bobbin exams with equipment set up in the cold leg channel heads. Because the magnitude of dents has not been observed to change significantly between recent inspections, the difference in the inspection samples based on either 1995 or 1997 bobbin dent data is expected to be negligible. The 1995 dent calibration was made by setting the 400 Khz absolute voltage at 10.0 volts on a 0.010" radius dent in the ASME calibration standard. One volt was the dent reporting threshold.

- . . MaineYankee l UNITED STATES NUCLEAR REGULATORY COMMISSION MN-96-172 Attention: Document Control Desk Page Three The initial partial drilled support plate MRPC sample will be of the most dented intersections.

Expansion samples (if required) will examine successfully less dented intersections. The hot leg partial drilled support plate MRPC inspection program is as follows:

Initial Sample 20% of dented intersections of 5 volts or larger I

First Expansion Sample (If a Single Circumferential Crack is Observed in the Initial Sample) 100% of dented intersections 5 volts or larger &

20% of dented intersections between 1 and 5 volts Second Expansion Sample (If a Single Circumferential Crack is Observed in the Previous 1 to  ;

5 Volt Sample) 100% of dented intersections between 1 and 5 volts &

20% of nondented intersections l

Third Expansion Sample (If a Single Circumferential Crack is Observed in the previous Nondented intersections) 100% of allintersections &

Initiate and expand inspections of the cold leg partial drilled support plates as was done for the hot leg support plates.

NRCREQUESTED.INEORMATION NO.2 In your GL 95-03 response, it was indicated that a 20 percent sample of the sleeves installed during the present outage (i.e.,1995 outage) would be examined in the next outage and that 20 percent of-the cold leg expansion transition regionjust above the top of the tubesheet would be examined with techniques suitable to detect circumferential cracking (e.g., plus point and Cecco 5). In addition, it was indicated that a submittal would be made which addresses the expansion criteria. Please specify the expansion criteria to be used during the next scheduled steam generator tube inspection for these regions (i.e., cold leg expansion transitions and all installed sleeves).

MaineYankee Response If circumferential cracking is detected at the cold leg expansion transition, Maine Yankee will expand the test program to include all cold leg tubesheet transitions in the affected steam generator.

In the event that circumferential cracking is detected in installed sleeves, the expansion criteria will be based on the failure mechanism. If the presence of the crack is associated with the installation process, e.g. sleeves that have been rewelded, reworked, etc., the expansion would be limited to sleeves with this condition. If, however, the presence of the crack is not associated with a unique installation condition, all sleeved tubes will be examined in the affected steam generator.

W v

. . MaineYankee UNITED STATES NUCLEAR REGULATORY COMMISSION MN-96-172 Attention: Document Control Desk' Page Four NRC_ Requested.InformationNo.3 Discuss whether terrain plots will be used to analyze the RPC eddy current data at locations susceptible to circumferential cracking during your next steam generator tube inspection (i.e., the next inspection per your Generic Letter 95-03 response).

MainelankeeResponse During the 1995 MRPC examination, terrain maps were used for all recorded data. This practice will be continued during the next examination (Fall 1997).

We trust that the above information adequately addresses your questions. Should you have additional questions or require clarification on any of the above, please contact us at your convemence.

Very truly yours, e

James R. Hebert, Manager Licensing & Engineering Support Department CHE/JRH/mwf c: Mr. Hubert Miller Mr. J. T. Yerokun Mr. Daniel H. Dorman Mr. Patrick J. Dostie Mr. Uldis Vanags )

1 l

l