Basis for ASME Section XI Code Case for Flaw Tolerance Evaluation of CASS Piping

Abstract

Thermal aging of cast austenitic stainless steel (CASS) piping is a concern for long term operation of nuclear power plants. The effects of aging in susceptible (i.e., high delta ferrite) CASS piping and components must be managed through the license renewal period. In the United States, utilities must follow the guidance for managing age-related degradation in the Generic Aging Lessons Learned (GALL) report, and this may include analyses or inspections to demonstrate that the piping systems remain flaw tolerant. The duplex structure and large grain size of the CASS materials poses challenges with non-destructive examinations (NDE) using ultrasonic inspection (UT) techniques of the CASS components which further complicates the ability to demonstrate that any existing flaws would not compromise piping integrity. Recent analytical studies of fully aged CF8M piping have resulted in a probabilistic fracture mechanics (PFM) model that has been used to develop allowable flaw sizes based on various levels of probability of failure. This model has been utilized extensively to generate a series of flaw acceptance limits that could be equated to the service levels in the ASME Code. A proposed ASME Section XI Code Case has been developed based on this PFM methodology for flaw tolerance evaluation of CASS piping components considering the effects of aging and uncertainties in material properties. The Code Case is currently under review by the ASME Working Group on Pipe Flaw Evaluation. This paper outlines the proposed Code Case and presents a technical basis for the flaw tolerance evaluation of CASS piping components. The ultimate objective of the flaw tolerance evaluation is to determine allowable flaw sizes in CASS components and determine target flaw sizes for NDE that will ensure safe operation with these components considering possible flaw growth. The flaw tolerance evaluation could be applied for a range of piping systems including pressurizer surge lines and primary system piping made from either statically or centrifugally cast stainless steel.