Fe-Cr-Ni austenitic stainless steels are vital core internal structural materials in light water reactors (LWRs) and used as pressure vessel cladding due to especially their corrosion resistance. Even though these steels are widely used, they suffer from embrittlement, swelling, and stress corrosion cracking (SCC) during irradiation. In order to increase the sustainability of LWRs and advance the development of Gen IV reactors, advanced steels that can tolerate the radiation, temperature, and corrosion environment need to be developed. Increasing the volume fraction of grain boundaries can enhance irradiation tolerance in nanostructured materials as grain boundaries serve as effective sinks for irradiation induced defects. While UFG steels have been shown to possess reduced void swelling and embrittlement after irradiation, there have been limited studies on the radiation induced segregation (RIS) and precipitation in nanostructured materials. Because RIS is driven by vacancy flux towards defect sinks through the inverse Kirkendall effect, a decrease in the total number of defects due to nanostructuring is expected to lower the vacancy flux and therefore reduce RIS. This project will systematically study the effect of grain size on RIS by comparing segregation behavior after ion irradiation in CG, UFG and NC SS304. A correlative technique using procession electron diffraction via TEM-ASTAR on atom probe tips will allow for comparing grain boundaries with the same or similar character but different grain sizes, and this work will be the first to use a correlative technique consisting of APT and TEM-ASTAR to understand RIS separately as a function of grain size and as a function of grain boundary character. Additionally, this experiment will compare the effects of grain size on intragranular segregation and precipitation. It is anticipated that results of this project will lead to development of strategies to reduce irradiation induced segregation and precipitation through nanostructuring and accordingly improve the lifetime of structural materials and cladding.