The Long Journey in the Development of ISI Rules for CASS Piping

Abstract

The factors that make cast austenitic stainless steel (CASS) a good candidate for use in primary piping systems have significantly hampered the ability to detect, locate, and size flaws within the material. The difficulties in effectively inspecting CASS piping has been mirrored in the challenges associated with the development of CASS inspection requirements. Since the early 1970’s the United States Federal Regulations have made the Section XI inservice inspection (ISI) rules of American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code a mandatory requirement for all operating nuclear power plants. Section XI of the ASME Boiler and Pressure Vessel Code requires that the pressure boundary system of all light water reactors be inspected using a volumetric technique. However, the inhomogeneous coarse-grain structure and elastic anisotropy of the CASS material can produce beam skewing and distortion, high background noise, and uncertainty in the propagation velocity of the ultrasound beam. The difficulties associated with reliability detecting and sizing flaws in CASS material have been well known since the late 1970’s. To date there are no qualified procedures, equipment, or personnel or performance demonstration requirements for the UT examination of CASS pressure-retaining welds. Until Section XI, Appendix VIII qualification requirements are developed licensees will continue to follow the rules of Appendix III for the inspection of CASS material. In recent years there have been significant advances in the development of Section XI requirements for the inspection of CASS. This paper reviews the long and difficult journey, the changing strategies and approaches, the driving forces, and the anticipated new Code requirements for the inspection of CASS.