High-energy irradiation of metastable austenitic 300-series steels by neutrons or ions appears to initiate formation of bcc-phases with different morphology and elemental composition compared to that of retained ferrite. Formation of specific Fe-rich ferrite was observed on the grain boundaries of annealed steel 12Cr18Ni10Ti, an analog of AISI 321, after irradiation in the center-core region of the BN-350 sodium-cooled fast reactor in Aktau, Kazakhstan, with doses up to 57 dpa at ~1×10-6dpa/s. The ferrite fraction determined from scanning electron microscopy closely matched the magnetic fraction determined using a ferritscope confirmed by x-ray diffraction analysis and selected area diffraction patterns. Chemical composition of the secondary phases was determined by energy-dispersive x-ray spectroscopy using a transmission electron microscope, showing Fe-rich ferrite domains. Fe-ion irradiation (2.3 MeV) was used to study the accumulation of ferrite and martensite phases at higher dpa rates. The very high etchability of these ferrite particles in alcohol-based solutions may signal very high etching in hot water, leading to accelerated intergranular cracking, especially upon long-term exposure during extended plant lifetimes