Modeling and Experimental Investigation on the Influence of Radiation Defects on Helium Behavior in BCC Iron

Fe-based alloys are important structural materials for both fission and fusion energy. For fusion applications, the challenges of radiation-induced changes in microstructure, properties and performance is further challenged by the concomitant production of helium from (n, alpha) nuclear reactions and fusion reactions. Due to the lack of a volumetric, high flux 14-MeV neutron source, studying these phenomena require the use of computational materials modeling and novel experimental methods. In this thesis, molecular dynamics (MD) simulations was used to investigate the synergistic interactions of helium with prismatic dislocation loops characteristic of those observed in neutron irradiated iron to determine how the presence of these loops modify the helium clustering and gas bubble nucleation process. In particular, multiple MD simulations have been conducted to investigate the role of a/2or aprismatic interstitial type dislocation loops on the helium clustering dynamics upon inserting interstitial helium into the body-centered cubic iron lattice at temperatures from 773 °K to 1173 °K. The simulations indicate a strong elastic interaction, which significantly influences the early stages of helium clustering that can aid bubble nucleation.

In addition, helium interactions with defect clusters in iron are probed experimentally, which has specifically focused on performing low energy helium ion implantation on neutron irradiated iron specimens followed by thermal desorption spectroscopy (TDS). The results show a limited increase in the dislocation line density, along with a relatively low volume fraction of cavities is observed after neutron irradiation of both single crystal and poly-crystalline iron samples. The TDS results indicate that the major helium desorption peaks shift to higher temperature due to the existence of the radiation induced defects, which are believed to act as strong trapping sites for helium.

Overall, the research performed in this thesis has confirmed that synergistic interactions occur between helium and point defect clusters, as well as the potential to combine computational modeling with experimental measurements using thermal desorption spectroscopy. The presented simulation and experimental results along with the proposed future work are expected to enhance the current understanding of the fundamentals of helium-defect interactions in neutron-irradiated iron.