Defect formation in austenitic stainless steels during shutdown procedure of FFTF

Defect formation during a shutdown procedure of a fast reactor has been investigated. A Fe-16Cr-17Ni alloy and two phosphorus-containing alloys, Fe-16Cr-17Ni-0.024P and Fe-16Cr-17Ni-0.1P, were irradiated in the Fast Flux Test Facility (FFTF) using Materials Open Test Assembly (MOTA) during Cycle 10 and Cycle 11. In the case of Cycle 10, in which the reactor was shut down by a controlled procedure, interstitial-type dislocation loops were formed in addition to voids. In the case of Cycle 11, in which the reactor scrammed, stacking fault tetrahedra (SFT) were formed in addition to the voids. Standing on the rate theory of defect processes under irradiation, it was predicted that the interstitial loops were nucleated by the short neutron exposure during a shutdown process, but the SFT were formed as agglomerates of vacancies and their small clusters which were accumulated in the matrix as a result of balance of interstitials and vacancies under irradiation.