ML19329B223

Final Deficiency Rept Re Discontinuity Indications in Completed Reactor Coolant Pipe.Caused by Combination of Factors,Including Contamination from Airborne Particles & Differential Thermal Contraction Stresses

ML19329B223
Person / Time
Site:	Davis Besse
Issue date:	04/07/1975
From:	Roe L TOLEDO EDISON CO.
To:
References
NUDOCS 8001310571
Download: ML19329B223 (6)

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% E!aisam Docket No. 50-346 LOWELL E. ROE Mce President Facditaes Dessionment (4191 259-5242 April 7, 1975 Dr. Donald F. Knuth, Director Office of Inspection & Enforcement U. S. Nuclear Regulatory Commission Washington, D. C. 20555

Dear Dr. Knuth:

This letter is submitted in accordance with 10 CFR Part 50.55 as a consummation of the Interim Report dated May 2, 1974 regarding = ctor coolant pump suctiortfield_ weld _ linenrly disp _osed_indicat_iion 3 at the Davis-Besse Nuclear Power Station Unit No. 1.

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Radiographic examination of the four completed reactor coolant pipe to pump field welds revealed discontinuity indications. These welds are identified as WJ-2-1, -2, -3, and -4, and are located at the suction end of the pumps. The sketch in Attachment I shows the base and welding materials in th': joint. The original weld procedure and filler metals were based upon previous good experience in the B&W shops.

An evaluation of the cause of the indications, which were determined to be fissures, was conducted.

The conclusions reached determined that the indications (fissures) could have been caused by a combination of factors including contamination i frem airborne particles and differential thermal contraction stresses between the stainless steel base metal (c.asting) and the inconel weld metal, in combination with weld joint orientation.

Corrective Action Taken As a result of these conclusions, the following course of action was planned and carried out:

a. Protective covers were built around the weld to assure the s cleanliness of the welding environment.

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b. The pump casing was buttered with Type 316 stainless stcel weld metal, as shown in Attachment 2. With this practice, the pump casting would not be subject to differential thermal contraction stresses.

c. The repairs proceeded on a planned basis. That is, one of the four welds was selected to begin repairs and after its success, work on the other three welds would commence. Also, this weld was divided into 6 circumferential segments. Two of these segments (180 degrees apart) were " buttered" and welded to the half thickness level and examined by the radiographic method.

The results of the radiographic examination at the half thickness level revealed the presence of some small linear indications located on the pump side of the ineonel weld (stainless steel /inconel interface).

It was determined that these indications were located in the crater areas of the inconel weld metal. This was confirmed by liquid penetrant examinations and by a review of the welding techniques employed, such as location of starts and stops.

i Modified Repair Method As a result of these developments, the initial repair method was modified as described below:

a. Each end of each weld bead was to be thoroughly ground to assure removal of all small crater defects.

b. A layer of inconel buttering van t9 be deposited on t he stain-less steel buttering, as shown in Attachment 3. This, will decrease the dilution of the inconel weld metal with the stainless steel buttering and thereby decrease the passibility of minor crater defects in the diluted inconel weld.

This modified repair method was applied to two 1/6 segments of the weld.

The weld joint was completed to the half thickness level and examined by the radiographic method. The results showed the weld to be of acceptable quality with no indications.

Work was then initiated on two additional segments of the welF. Similarly, these segments were examined by the radiographic method. The results showed the weld to be of acceptably quality.

Work was then initiated on the remaining two segments of the first weld.

Similarly, these segments were examined by the radiographic method. The results revealed the presence of some small linear indications located in the first four and the last two segments. These indications were removed by grinding, the resultant cavities were welded, and the weld 3 e "'s.,

f radiographically examined. The results showed the weld to be of acceptable

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Work was then initiated on the remainder of the first weld. This weld has been completed and accepted by the radi-graphic method. It was initially planned to complete the remaining three welds by dividing each into fcar equal segments. The present plans have changed this approach to permit welding of alignment lugs on the weld and removal of the initial weld metal completely. This will allow the weld to be deposited uniformly around the circumference. Otherwise, the repair method for these welds will be identical with that used for the first weld.

i Safety Implications The micro-fissures in the field welds were disclosed by routine radio-graphic examination and liquid penetrant testing of the welds by the B&W Construction Company. Repair of the welds to acceptable quality criteria ensures the integrity of the primary coolant pressure boundary at this area.

Yours very truly,

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J. G. Keppler, Regional Director Office of Inspection and Enforcement Region III d

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. 'Offl G I N A L WELD JOINT-

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