Technical Basis for Revisions to ASME Section XI Acceptance Standards for Surface Flaws in Piping in Stress Corrosion Cracking Susceptible Materials

Abstract

Acceptance Standards for flaws in piping are provided in Section XI of the ASME B&PV Code to permit acceptance of relatively small flaws without the need to perform an analytical evaluation. The Acceptance Standards are based on maintaining large margins against failure, and are based on the assumption that flaw growth will be insignificant. The assumption of a small amount of flaw growth is justified when fatigue crack growth is the only crack growth mechanism. However, when stress corrosion cracking is operative, flaw growth could be significant. This conclusion was illustrated by comparison of the crack growth results due to fatigue and stress corrosion cracking in Pressurized Water Reactor (PWR), and Boiling Water Reactor (BWR), coolant environments. For this reason, IWB-3514 of Section XI prohibits use of the Acceptance Standards for planar surface-connected flaws that are detected in piping materials that are susceptible to stress corrosion cracking and are in reactor coolant environments. As part of a recent Code revision to include new Acceptance Standards tables for flaws in piping, restrictions on use of the Acceptance Standards of IWB-3514 have been refined and clarified. The recent Code revision now specifies different restrictions and requirements for use of the Acceptance Standards for such planar surface-connected flaws detected by preservice and inservice examination. In addition, similar restrictions have been imposed on use of the new Acceptance Standards for such planar surface-connected flaws in Class 2 piping in IWC-3514 of Section XI. The technical basis for the restrictions and requirements for use of the Acceptance Standards for planar surface-connected flaws in piping materials that are susceptible to stress corrosion cracking is provided in this paper.