ML20149H122

Forwards Maine Yankee 1994 S/G Insp Rept. Rept Provides Assessment of Operation W/Degraded SG Tubes Prior to Outage in Jul/Aug 1994.Rept Also Provides Assessment of Crack Growth Rate

ML20149H122
Person / Time
Site:	Maine Yankee
Issue date:	11/14/1994
From:	Whittier G Maine Yankee
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML20149H125	List: ML20149H122 ML20149H127
References
GDW-94-91, MN-94-104, NUDOCS 9411170295
Download: ML20149H122 (2)

Text

i MaineYankee RELIABLE ELECT RICITY SINCE 1972, 329 BATH ROAD

• BRUNSWICK, MAINE 04011 * (207) 798-4100 i

November 14, 1994 .

MN-94-104 GDW-94-91 I

. UNIT iTATES_ NUCLEAR REGULATORY COMMISSION f

'e Flint North One v ,

11555 ii.:kville Pike  !

Rockville, MD 20852 l

References:

(a) License No. DPR-36 (Docket No. 50-309) f (b) Maine Yankee Letter to the NRC, MN-94-82, dated August 8,1994,

Subject:

Steam Generator Tube Cracking ,

(c) NRC Letter to Maine Yankee dated August 19, 1994,

Subject:

Summary of August 9, 1994, Meeting on Maine Yankee Steam Generators

! (d) Maine Yankee Letter to the NRC, MN-94-90, dated August 23, 1994,  :

Subr ct: Maine Yankee Licensee Event Report 94-012, Steam l Generator Tube Inspection (e) Maine Yankee Letter to the NRC, MN-94-10?. dated October 14, .:

1994,

Subject:

Summary Report of Maine YanF e July / August,1994,  ;

, Steam Generator Inspection  !

Subject:

Summary Report of Maine Yankee July / August, 1994, Steam Generator L Inspection Gentlemen:

.This letter's enclosure provides a summary report of the Maine Yankee  !

July / August, 1994, Steam Generator (S/G) tube inspections in keeping with our  !

commitment to you, as stated in References (b), (c), (d), and (e).

The-enclosed report provides an assessment of operation with degraded steam generator tubes prior to the outage in July / August,1994. During the July / August outage, we inspected all steam generator tubes and found 303 tubes with circumferential crack defects. All but 24 of the tubes met the operating criteria, based on the average crack size limits, in Regulatory Guide 1.121.

We completed in-situ pressure testing on a sample of the 24 tubes that showed the most degradation based on eddy current test results. These tests demonstrated that even the most severely degraded tubes had substantial remaining structural integrity. Based on the test results, over 70% of the 24 tubes which exceeded Reg.

Guide 1.121 crack size criteria are expected to have met Rea Guide strength criteria. The remaining tubes would have been expected to remain intad with minor leakage during postulated accident conditions.

When assessing a hypothetical accident condition, the reactor coolant activity -

is a significant factor for consideration. Maine Yankee normally operates well below l allowable design limits for radioactivity and was operating in this condition prior j to the July / August outage.

• l 9411170293 941114 PDR ADOCK 05000309 ,

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L:\94mn\94104 g PDR

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yl3111C D 1RDNOC UNITED STATES NUCLEAR REGULATORY COMMISSION MN-94-104 PAGE TWO Because of the considerable tube integrity remaining in the most severely degraded tubes and the low coolant activity levels, we have concluded that the plant was operating safely prior to the July / August outage.

The enclosed report also provides an assessment of the crack growth rate. Maine Yankee's mean circumferential crack growth rate is 11.17% (averaged through wall) per effective full power year. Based on our assessment of the crack growth rate distribution, we believe the plant can be operated safely with a steam generator tube inspection each refueling outage. This conclusion will be reviewed following the inspection scheduled for our 1995 February refueling outage.

The enclosed report also summarizes the steam generator tube inspection program for the refueling outage scheduled to begin in late February,1995. The planned inspection includes = 100% bobbin exam and a 100% hot leg Motorized Rotating Pancake Coil (MRPC) exam neor the top of the tube sheet in all three steam generators. A 100% MRPC exam of the top of the tube sheet of the #3 steam generator cold leg is also planned. The MRPC exams will include the lessons learned during our steam i generator outage in July / August 1994 including review by an independent evaluator.

The report provides a detailed assessment of the inspection results from the July / August 1994 outage and, we believe, is responsive to questions received from the NRC staff. We would be pleased to meet with your staff to discuss this report and its conclusions further. Please call me if you would like to arrange such a meeting.

Very truly yours, khN c G. Douglas Whittier, Vice President Licensing & Engineering >

GDW/ jag Enclosures c: Document Control Desk Mr-. Thomas T. Martin Mr. J. T. Yerokun Mr. E. H. Trottier Mr. Patrick J. Dostie L:\94mn\94104

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