DIMENSIONAL CHANGES AND MICROSTRUCTURE OF IN-PILE CREPT FERRITIC-MARTENSITIC STEEL

Pressurized tube specimens of Japanese Ferritic-Martensitic Steel (JFM S) were irradiated in Fast Flux Test Facility (FFTF) Materials Open Te st Assembly (MOTA) to 37.5 dpa at 680 and 793 K. Diametral creep strai n following irradiation at 680 K was 0.11% even at zero hoop stress an d slightly increased with increasing the hoop stress. The creep strain following irradiation at 793 K increased from zero to 0.23% with the increase in hoop stress from zero to 86 MPa. As for the void swelling, nothing was recognized. The martensitic phase in JFMS was stable afte r the irradiation at 680 K, but this was not the case at 793 K where a considerable recovery of dislocation structures was found. Following irradiation at 680 K, a high density of fine spherical G phase precipi tates which were accompanied by the strain fields around them were obs erved, while large Mo-rich Laves phase particles were observed followi ng irradiation at 793 K. Analysis of dislocation Burgers vectors revea led that dislocations having Burgers vector of type a[100] were observ ed only after the irradiation at 680 K and that a large anisotropy was indicated in a[100] type of Burgers vector populations for a stressed specimen, but no significant anisotropy was observed in the Burgers v ectors of a specimen irradiated without hoop stress. The average diame ter of small precipitates in the specimen irradiated at 680 K under st ress was significantly larger than that without stress. Finally, the o bserved dimensional changes were attributed to both the precipitation- induced volume expansion and the Stress-Induced Preferential Absorptio n (SIPA) irradiation creep for 680 K irradiation and Climb-Controlled Glide (CCG) irradiation creep at 793 K.