The objective of this study is to understand the synergism between nanocluster irradiation evolution and radiation-induced segregation (RIS) in oxide dispersion strengthened (ODS) alloys. Nanoclusters play a critical role in the structure-property-performance relationships of advanced nuclear structural and cladding materials such as ODS alloys. But irradiation can induce coarsening or dissolution of these nanoclusters, having profound consequences on the structural and mechanical integrity of ODS alloys. Theoretical and experimental studies on ODS alloys – including those carried out by the PI’s team through NSUF support (RTEs 18-1198, 16-710, 15-569, 15-540) – have shown that several mechanisms interact in a complex manner to control nanocluster evolution during irradiation. One such mechanism purports that oxide nanoclusters can nucleate then grow, after being dissolved and/or broken apart by irradiation. But there has not yet been direct experimental evidence of stochastic nucleation of oxide nanoclusters during irradiation. Moreover, since nucleation occurs via atomic diffusion, understanding mechanisms of nanocluster nucleation inherently requires understanding of irradiation-enabled diffusion phenomena. To this point, our recent atom probe tomography (APT) results from ODS alloy 14YWT reveal dramatic nanocluster nucleation and RIS on grain boundaries following 450 degC, 100 displacements per atom (dpa) Fe ion irradiation. Specifically, O and Y enrich at grain boundaries, where they interact with Ti to nucleate discrete Y-Ti-O nanoclusters on the boundary. We thus hypothesize that RIS and nanocluster nucleation are closely coupled phenomena that potentially enhance one another. There is a critical need to understand nanocluster irradiation evolution mechanisms, including the co-evolution of grain boundary nanoclusters and RIS. This project will resolve our hypothesis and achieve the research objective by using APT to analyze RIS and nanocluster chemistries and morphologies on grain boundaries as a function of irradiation dose (ranging 50-200 dpa) and temperature (ranging 300-500 degC). Dose functionality will reveal the nanocluster nucleation and growth process; temperature functionality will inform the kinetics of the nanocluster nucleation process. Work will focus on ODS alloy 14YWT from the same heat that has been studied extensively in previous DOE and NSUF programs.