Hot cell crack initiation testing of various heats of highly irradiated 316 stainless steel components obtained from three commercial PWRs

Crack initiation testing sponsored under the International IASCC Advisory Committee Phase 2 and Phase 3 programs has led to the development of a sizeable crack initiation database which now contains more than 100 test data points. The database was generated over a 6-year period with a majority of the testing performed in 2006. All tests used O-ring specimens machined from three different heats of 316 CW SS thimble tubes removed from three different commercial PWRs. The O-ring specimens tested represent a wide range of dpa values (from 0 to 76 dpa) and have been tested under a broad range of stress values. The considerable variety of material conditions and test parameters evaluated has allowed for statistically significant results which were analyzed using a number of different approaches. The data were first analyzed using a typical test stress versus life analysis utilizing two different regression fits. This approach showed that stresses of approximately 50 to 55% of the irradiated yield strength are required to cause IASCC failure at end of reactor operating lifetimes, however, the fit qualities using this analysis approach were low. By re-evaluating the data using a test stress versus dpa analysis, the presence of an apparent stress threshold is revealed, below which, IASCC failure does not occur. This threshold reaches an asymptotic value of approximately 62% of irradiated yield strength at approximately 26 dpa. (author)