Code Change Enables "Fitness for Service" Standard for Structural Welds

The design and fabrication of nuclear pressure vessels and piping components are governed by the rules of Section III of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. This Code, which aims to to ensure high levels of structural integrity for safe nuclear plant operation, requires radiographic examination of Class 1 and 2 pressure boundary butt welds to detect structural flaws introduced during welding.

Current ASME Section III weld acceptance criteria are based on a “workmanship standard.” This standard requires repair of cracks, lack-of-fusion defects, or incomplete penetration defects regardless of size, and requires repair of any porosity and slag defects above a certain specified size as measured from the radiographs. The technical basis for this standard is not clear, and field experience has shown that repaired areas are more susceptible to stress corrosion cracking. Fracture mechanics evaluations, supported by nondestructive evaluation (NDE) measurements of defect size and position, indicate that many of these defects would not lead to weld failure during the life of the power plant. At the time the Code was written, however, NDE and fracture mechanics methods were not accurate enough to permit this approach.

EPRI provided technical input to Section III to enact a “fitness for service” standard to replace the workmanship standard. The code changes were supported by several technical elements:

• Today’s NDE technologies provide reliable detection and accurate measurement of weld flaws.
• Today’s fracture mechanics approaches can accurately determine which flaws can safely remain without being repaired.
• A fitness for service standard can reduce fabrication costs and in many cases produce a final component with improved life and lower susceptibility to in-service failure.
• Analysis of worldwide field experience indicates increased susceptibility to stress corrosion cracking and increased probability of failure in welds that have been repaired.

The EPRI team worked with the Section III Code committees over a four-year period to develop the technical basis for the change, draft langauge for the Code Case, and address technical issues raised by the Section III Code committee. The new Code Case, N-818, was approved in December 2011.

The approved fitness for service standard will provide significant construction savings for new nuclear plants and help ensure that these plants can cost-effectively reach their designed life spans. Additionally, this Code change will benefit the existing fleet for repair, replacement, and modification activities that require Class 1 and 2 butt welds.