Nuclear Science User Facilities 68 This work examined friction stir welds that had been subjected to He or heavy-ion irradiation. Nanostructured ferritic alloys (NFAs) are strong candidates for Generation IV fission systems, but welding of NFAs is an immature technology.We used atom probe to study the effect of irradiation upon the nanostructures of the weld region. Project Description Our goal was to determine by irradiation whether friction stir welding (FSW)-processed NFA was grossly similar or different from irradiation of as-fabricated NFAs. This is important because, if the weld process should reduce the overall radiation tolerance of the material, then NFAs might not be usable in advanced Gen IV and fast reactor systems. NFAs show strong potential for application to the core structures in fast reactors or other advanced nuclear energy systems. However, any structure will require welding and joining, and welding of NFAs is not well developed.Traditional fusion (liquid state) welding is unusable because the oxide nanostructures in the NFA simply coarsen in the weld pool and the material ceases to be an NFA. As has been previously demonstrated, both by our group and other groups, FSW of NFAs can produce a structure that retains the nanostructured features, specifically the nano-oxide clusters. However, it is not presently known whether these nano-oxides retain the same radiation tolerance that makes as-fabricated NFAs attractive from an engineering point of view.Therefore, we chose to investigate the question of the radiation response of the nano- oxides in an NFA FSW .We performed heavy ion and helium ion irradiations on an NFA FSW zone at fission-relevant temperatures.This project involved using atom probe at the Center for Advanced Energy Studies (CAES) to investigate the irradiated zones. Accomplishments This work first accomplished the production of the FSW structure and irradiation using heavy ions and helium ions, both at Oak Ridge National Laboratory (ORNL).The co-principal investigator, Phil Edmondson, then carried the ion-irradiated samples to CAES and used the focused ion beam and atom probe to examine the different ion irradiated conditions. We then transferred the data to ORNL, where data analysis and manuscript preparation is underway. Future Activities Future activities involve finalizing the data analysis, preparing a manuscript, and submitting the manuscript to a journal for peer reviewed publication. An Atom Probe Tomography Investigation of the Response of Oxide-dispersion Nanoclusters to Non-similar Friction Stir Welds Chad Parish – Oak Ridge National Laboratory – This work provides basic science information on the ability to weld high performance radiation tolerant alloys for advanced nuclear systems.