Cast Austenitic Stainless Steel (CASS) Pipes, especially those with elevated delta ferrite content, are subjected to significant thermal aging effect during the operation of nuclear power plants. The thermal aging effect leading to a decrease in fracture toughness of CASS material causes a reduction in critical crack size that is insensitive to ultrasonic testing. Specifically, based on the elastic–plastic fracture mechanics (EPFM) theory, the J-integral acting as the driving force on cracks is compared with J-R curve representing the material fracture toughness and the judgement whether a crack has undergone rapid fracture can be determined. A collective data of saturated thermal aging materials shows a high degree of dispersion, based on which, a modified defect evaluation method for CASS pipes using probabilistic fracture mechanics will be proposed in this study to accommodate the current nondestructive evaluation limitations, providing a basis for the development of in-service inspection acceptance standards of this specific category.