ML20056C460

Safety Evaluation Accepting Proposed Repair of Weld in Recirculation Piping Sys for One Cycle of Operation

ML20056C460
Person / Time
Site:	Quad Cities
Issue date:	06/17/1993
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20056C459	List: ML20056C458 ML20056C460
References
NUDOCS 9306240167
Download: ML20056C460 (2)

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UNITED STATES NUCLEAR REGULATORY COMMISSIO,N

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Qg~(f WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF-NUCLEAR REACTOR REGULATION WELD OVERLAY REPAIR OF A RECIRCULATION PIPING WELD COMMONWEALTH EDIS0N COMPANY OVAD CITIES. UNIT 2 DOCKET N0. 50-265 I. INTRODUCTION The purpose of this Safety. Evaluation (SE) is to determine whether the licensee's proposed overlay repair will permit Quad Cities, Unit 2, to return to safe operation. ,

II. BACKGROUND During the Q2R12 Unit 2 outage, Commonwealth Edison Company (CECO), the licensee identified a flaw in a weld (02-F28) of a 2-inch line on the recirculation system. CECO requests NRC approval to implement a temporary non-code repair for one cycle of operation. Ceco proposes to use the guidance of Generic Letter (GL) 88-01 "NRC Position on Intergrannular. Stress Corrosion Cracking (IGSCC) in BWR Austenitic Stainless Steel Piping."

III. DISCUSSION GL 88-01 does not exactly apply in this case because the line is only 2 inches in diameter, and the letter applies to lines 4 inches and larger. However, working within the guidelines of the letter would be conservative since the consequences of a small line break would not be as severe as for a larger-l i r.e . Furthermore, the licensee is requesting. approval for a temporary repair for only one cycle; the NRC approves' temporary repairs on a case-by-case' basis using standards when possible; and the piping is of IGSCC' susceptible material. Based on the above, the_ guidance in GL 88-01 would be most appropriate.

The weld, 02-F28, is a socket weld joining a recirculation line to a-valve body. This line provides bypass circulation.around the valve. The.line and valve are of ASTM A-312, Type 304 and A-351, Grade CF8M stainless steels, respectively. During the Unit 21993 outage, health physics-surveys _ performed near this weld found high levels of beta radiation usually associated with primary system leakage. A liquid penetrant examination of the weld showed a 3/32-inch to 5/32-inch long through-wall flaw. The flaw was difficult to see but was slowly weeping water under hydrostatic conditions. The licensee.

believed that a weld defect initiated the flaw and service stresses propagated it, i

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. The licensee proposed to apply a weld overlay in accordance with the guidance of GL 88-01, NUREG-0313, and ASME Section XI. The overlay design assumes a  ;

360 degree through-wall flaw and a safety factor of 2.77. Only layers with a ~

minimum ferrite content of 7.5 FN will be counted as part of the structural thickness of the final overlay. Low carbon filler metals ER308L or ER309L will be used. Controls on heat input and waterbacking will prevent i sensitization of the base metal and create favorable residual stresses.

The licensee has determined that because of the configuration of the line, I axial shrinkage is not a concern relative to increases in system sustained stress. However, there is a concern about the effect of axial shrinkage on an adjacent pipe support guide, so afte, welding, the position of the support guide will be reviewed and adjusted if necessary.

Because of the small pipe size, ultrasonic examination of this weld overlay is considered unreliable. Therefore, in lieu of the ultrasonic examination, each overlay layer will be checked by liquid penetrant.

Responding to NRC concerns raised in a teleconference, the licensee calculated the ef fects of overlay shrinkage on a potential failure path along the original weld-to-socket shoulder interface. The licensee performed crack growth calculations considering both IGSCC and fatigue. For IGSCC calculations, the licensee assumed an initial flaw size of 10 percent through-wall. After an 18-month operating cycle the flaw was predicted to grow to about 30 percent through-wall. At the sustained stress levels, present IGSCC growth is predicted to be well within ASME Section requirements after one cycle of operation. Fatigue calculations showed a 10 percent through-wall flaw could withstand thousands of application cycles (more than would be expected to occur in one cycle of operation) before reaching ASME Section XI maximum allowable flaw depths of 60 percent.

III. CONCLUSION The staff concludes that the licensee's proposed weld overlay repair at Quad Cities, Unit 2, on weld 02-F2B, done in accordance with the guidance in Generic Letter _88-01 is acceptable for operation for one fuel cycle. The licensee should either replace the subject weld or perform a Code repair during the next refueling outage.

Principal Contributor: L. Banic Date: June 17, 1993

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