Evolution of Microstructure and Microchemistry in Cold-worked 316 Stainless Steels under PWR Irradiation

Abstract

The evolution of microstructures and microchemistry was examined by transmission electron microscopy in coldworked SUS 316 stainless steel components irradiated in a pressurized water reactor to 1-73 dpa at 565-596 K. Homogenous nucleation of dislocation loops, helium bubbles and 0 precipitates was detected. The dislocation loops consisted of a high density of Frank loops and black dots. The black dots are considered to be small Frank loops, some fraction of which could be vacancy-type. The size distribution showed a double peak, which remained unchanged up to 73 dpa, suggesting that the dislocation structure was saturated under a balance of nucleation and disappearance. The helium bubbles were extremely dense and fine, resulting in swelling of less than 0.1%. Such bubble structure was formed under irradiation with a high He/dpa ratio. The 0 precipitation was detected at doses higher than 4 dpa with an increasing density for higher doses. The measured radiation hardening was almost explained by these visible microstructural features. Radiation-induced segregation at grain boundaries was confirmed to continuously develop up to 73 dpa whereas no significant segregation could be detected at Frank loops.

Fracture behavior of austenitic stainless steels irradiated in PWR

“…The irradiation temperature and dose rate were 293°C, 6 Â 10 À9 dpa/s for 2 dpa, 323°C, 2.8 Â 10 À8 dpa/s for 11 dpa, 323°C, 5.4 Â 10 À8 dpa/s for 22 dpa, 298°C, 7.8 Â 10 À8 dpa/s for 38 dpa and 305°C, 1.5 Â 10 À7 dpa/s for 73 dpa. Microstructure, grain boundary segregation and hardness were already examined in the previous study [16]. The data in the previous study indicated that evolution of microstructure, segregation and hardness mainly depended on dose although both dose rate and temperature were expected to affect the evolution to some extent.…”

“…Typical grain size was about 20-30 lm. The details of the tube material were described elsewhere [16]. In this study, experiments were conducted on specimens cut from several positions of the tubes, where the neutron doses were 2, 11, 22, 38 and 73 dpa.…”

“…Impact tests at À196, 30 and 150°C, and conventional and slow tensile tests at 30 and 320°C were conducted. Based on detailed fractography of the fracture surface, combined with the previous data of the same material [9,11,[15][16][17] and literature data, conditions for non-ductile fracture and underlying mechanisms are discussed.…”

A Multicomponent Model of Radiation-induced Segregation for Commercial Stainless Steels

“…In the low-dose-rate region of 10 À8 to 10 À6 dpa/s relevant to reactor irradiation, LWR data were reported at temperatures of 560-623 K at a dose rate of 10 À8 -10 À7 dpa/s. 8,11,12,40,41) At doses of 10-20 dpa, Cr depletion and Ni enrichment were 4-7 wt% and 10-14 wt%, respectively. Fast breeder reactor (FBR) data were also reported at a temperature of 593-653 K at a dose rate of 10 À7 -10 À6 dpa/s.…”

“…In commercial stainless steels, Mo is often enriched up to 10 wt% in unirradiated steels at grain boundaries and that Si reaches several wt% in stainless steels irradiated to several dpa, corresponding to a level higher than 10 at%. 8,11,12) Such high concentration of Mo and/or Si is likely to strongly affect RIS of major elements.…”