Irradiation effect on microstructure of modified SUS316 stainless steel cladding irradiated at elevated temperature to high dose

Yamashita, Shinichiro   ; Akasaka, Naoaki; Nishinoiri, Kenji; Takahashi, Heishichiro

A modified SUS316 stainless steel (PNC316) for high burn-up fast reactor core application has been successfully developed. The material has a typical composition of 16Cr-14Ni-2.5Mo-0.25P-0.004B-0.1Ti-0.1Nb and is used in the 20% cold-worked condition. To demonstrate irradiation performance of PNC316 cladding irradiation experiments has been successively carried out at the temperature ranging from 775 K to 905 K up to a maximum dose of 125 dpa. In this study, the microstructures of the irradiated specimens were carefully examined. Low-magnification image including some of needle-like precipitates indicates many helium bubbles attaching at the precipitate interface. On the other hand, a representative high-resolution image of the needle-like precipitate with the sizes of a few tens of nanometers long and a few nanometers wide shows a distinct interface structure between FCC matrix and the precipitate. As the results of diffraction pattern and image analyses this characteristic precipitate was identified as FeP phase. Based on the numerous results of TEM examination, it is found that a significant improvement in the swelling resistance of PNC316 was mainly derived from the formation of a stable phosphide that traps helium bubbles and retards the conversion of bubbles to voids.