Welding Defects in Unit 3 Steam Generator

San Onofre’s Unit 3 reactor steam generator (currently leaking) had problems from the start.  According to this Nuclear Regulatory Commission Information Notice 2010-07: Welding Defects in Replacement Steam Generators:

The welding defects identified at the fabrication facility on the replacement steam generators (RSGs) for SONGS Unit 3 are unlike the weldability issues that are typically observed in the welding of nickel-based alloys. For the SONGS Unit 3 RSGs, surface preparation for the Alloy 152 butter was inadequate. Contamination from the air carbon-arc gouging (ACAG), used to remove the stainless steel cladding, resulted in elevated hardness of the material adjacent to the fusion line that remained on the surface after the grinding step. When the butter pass was applied, the metallurgical bond between the LAS and Alloy 152 butter weld pass was not as strong as it should have been.

The fabricator followed approved welding procedures for dissimilar metal welding of Alloy 690 to low-alloy steel (LAS) and had recently built two RSGs for SONGS Unit 2 according to these procedures without problems. However, for the SONGS Unit 3 RSGs, the fabricator requested and the licensee approved a deviation to allow using an alternative method (in this case ACAG) to prepare the LAS surface for butter application. According to the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code), Section IX, this deviation did not require the requalification of the welding procedure because this aspect of the weld joint preparation was not considered an essential variable.

ACAG could be used in the manufacturing or repair of any ASME Code Class 1, 2, or 3
component. The ACAG technique is an accepted procedure for removing metal. American Welding Society (AWS) C5.3:2000, “Recommended Practices for Air Carbon Arc Gouging and Cutting,” states that welding on a surface after ACAG “may generally be performed with a minimum of grinding or cleaning.” However, the standard also notes the limitation that the process “increases the surface hardness on cast iron and air hardenable metals. This may be objectionable.”

ACAG is not specifically covered in Section III of the ASME Code; however, ASME Code, Section XI, IWA-4461 covers the qualification and use of a thermal removal process like ACAG. In addition, 10 CFR 50.55a(b)(2)(xxiii) states:

The use of provisions to eliminate the mechanical processing of thermally cut surfaces in IWA-4461.4.2 of Section XI, 2001 Edition through the latest edition and addenda incorporated by reference in paragraph (b)(2) of 10 CFR 50.55a are prohibited.

Although all specific requirements or standards were met, this event illustrates that control over all aspects of welding ASME Code Class 1, 2, and 3 components can prevent welding defects like those found in the RSGs for SONGS Unit 3 from occurring.

About Donna Gilmore

California
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2 Responses to Welding Defects in Unit 3 Steam Generator

  1. Pingback: Edison says they have ample reserve power without San Onofre | San Onofre Safety

  2. Pingback: Inspectors find ‘unusual’ wear on San Onofre steam generator tubes | San Onofre Safety

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