A new degradation mechanism has been observed in Monel 400 Steam Generator tubing material, a nickel-copper alloy (63Ni-28Cu-2½Fe) with the ASME material designation SB-163/N04400. The location is above the top preheater support plate of the two re-circulating steam generator in one of the units of the Pickering Nuclear Generating Station.

This paper provides a brief description of the regulatory environment, OPG’s steam generator life cycle management plans, the Canadian Industry’s fitness-for-service guidelines for steam generator tubes, and the afflicted steam generators. The paper then goes on to discuss the following activities that were conducted to support the technical basis to justify that the steam generators fit to be returned to service:

• Inspection scope expansion, methods, and results.

• Examination of removed tubes.

• Condition monitoring assessment.

• Operational assessment.

• Burst-pressure tests of removed tubes and of fabricated test specimens.

• Degradation specific flaw model and acceptance standards.

• Flaw growth rate predictions.

• Plugging limit adopted.

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