Cast austenitic stainless steel(CASS) used in main coolant pipes of the primary loop in pressurized-water reactor(PWR) nuclear power plants faces the problem of thermal aging embrittlement after long-time operation in the reactor environment. In this study

the aging parameter extrapolation method is used to evaluate the degree of thermal aging embrittlement and predict the service life of the mainly used Z3CN20-09 M steel after long-time operation. The evaluation results indicate that

when the aging parameter P is 3.48

the spinodal decomposition takes place on the ferrite of Z3CN20-09 M steels

which results in a thermal aging embrittlement tendency. In the meantime

the relationship between the total impact energy and aging parameter P of the Z3CN20-09 M steel after service at 288 ℃ to 327 ℃ is obtained by fitting. It is predicted that the Z3CN20-09 M steels used for the cold and hot leg pipe is safe when its extending service life is close to 60 effective full power years.