The objective of this project is to determine the influence of grain boundary radiation-induced segregation (RIS) on dislocation mobility in nuclear-relevant steels. Understanding the effects of irradiation on the deformation of steel is key to predicting load-bearing capabilities of reactor components to ensure safe operation. Dislocation slip is the dominant deformation mechanism in steels at nuclear-relevant temperatures, but studies on the effect of irradiation on dislocation slip have primarily focused on dislocation bowing around or cutting through irradiation-induced defects such as loops, voids, or precipitates. Meanwhile, dislocation interactions with grain boundaries (i.e. pileup or transmission) have not considered irradiation effects, namely the effect of grain boundary RIS. RIS is a non-equilibrium phenomenon that produces chemical concentration gradients at sinks such as grain boundaries. Because dislocation mobility is composition-dependent, it is hypothesized that RIS will alter the mobility and interaction of dislocations with grain boundaries. Consequently, RIS-related changes in dislocation-grain boundary interactions can affect the material’s capacity to accommodate strain at grain boundaries and triple junctions, as well as its susceptibility to irradiation-assisted stress corrosion cracking (IASCC). Thus, there is a critical need to understand the interplay of grain boundary RIS and dislocation mobility. This project will address this critical need by directly measuring dislocation mobility (i.e. dislocation velocities and flow stresses) near RIS-affected grain boundaries using transmission electron microscopic (TEM) in situ tensile testing. Work will focus on three neutron irradiated steels: 304L stainless steel (SS) irradiated in EBR-II to 23 dpa at 415°C; and two FeCrAl variants C35M (Fe-13Cr-5Al) and C37M (Fe-13Cr-7Al) irradiated in HFIR to 3 dpa at 330°C. The PI has already collected RIS measurements on these alloys through prior DOE/NSUF projects. These prior projects have also enabled the PI to demonstrate the proposed TEM in situ tensile testing methodology on baseline unirradiated materials; these results will provide a comparison for the proposed RIS-affected measurements, while also providing high-confidence proof of concept for the proposed work. Scientifically, this work will fill a critical knowledge gap on the role of RIS on dislocation plasticity. More broadly, these results will validate dislocation dynamics models that are used to produce constitutive plasticity models of irradiated steels to be introduced into MOOSE structural modules. The outcome of this project will provide a first-of-its-kind understanding of the implications of RIS on dislocation plasticity, which will enable researchers to construct more physically rigorous deformation models.