Technical Basis for Revisions to ASME Section XI Code Case N-597-2 on Requirements for Analytical Evaluation of Pipe Wall Thinning

Code Case N-597-2 provides procedures and acceptance criteria for the evaluation of piping items subjected to wall thinning mechanisms such as flow-accelerated corrosion (FAC). The acceptance criteria ensure that margins equivalent to those of the ASME B&PV Code are maintained. Subsequent to the publication of Code Case N-597-2, the U.S. Nuclear Regulatory Commission (NRC) found the Code Case conditionally acceptable. A number of task items have been undertaken by the ASME Section XI Working Group on Pipe Flaw Evaluation (WGPFE) to address the NRC conditions. A 2006 ASME Pressure Vessels and Piping (PVP) Division conference paper was published to provide an expanded explanation of the technical basis for Code Case N-597-2. A 2009 PVP paper was published to provide results of validation of evaluation procedures and acceptance criteria in Code Case N-597-2 against experimental and historic wall thinning events. More recently, revisions to Code Case N-597-2 have been made and were proposed as N-597-3. Significant changes have been made in the proposed revised Code Case to clarify the technical requirements and address the NRC concerns over N-597-2. The technical basis for revising Code Case N-597-2 is provided in this paper.