The Role of Helium on Cavity Growth and Swelling at High Damage Levels in Ferritic-Martensitic Steels

Abstract

The desire for cleaner energy sources for baseload power generation drive an interest in new nuclear power plant designs. The development of radiation-tolerant materials for new nuclear power plants requires extensive research programs. Traditionally, radiation effects studies have been conducted using materials test reactors followed by expensive and time-consuming post-irradiation examination due to the neutron induced radioactivity. Additionally, the damage rate, temperature and helium generation rate within a reactor are all correlated with each other and dependent on the flux of neutrons and gamma rays in a reactor making the analysis for the underlying mechanisms of cavity growth and swelling difficult. Ion irradiation experiments allow for the separation of single variable dependencies to uncover the processes and understand the mechanisms underlying cavity growth and swelling with orders of magnitude higher damage rates compared to reactor irradiations and no induced radioactivity. The objective of this thesis is to understand the process by which cavity growth and swelling changes with helium content across a wide range of damage levels. Alloys HT9, heat 84425, and T91, heat 30176, were irradiated with 5.0 MeV Fe2+ ions with 2.03-2.85 MeV He2+ ions at helium co-injection rates of 0 to 4 appm He/dpa at 460°C (HT9) and 445°C (T91) at damage levels from 17 to 650 dpa in the Michigan Ion Beam Laboratory. The swelling and dislocation evolution for all irradiation conditions were characterized. Scanning transmission electron microscopy (STEM) was used to characterize the cavities greater than 5 nm in diameter and dislocations. Transmission electron microscopy (TEM) was used to characterize the cavities less than 5 nm in diameter. The peak in swelling was observed at the highest examined helium-to-dpa ratio at the lowest damage level. The location of the peak was demonstrated to be due to the enhanced nucleation of cavities at higher helium co-injection rates. As the damage level was increased, cavity growth started to dominate swelling, so the increased bubble density and resulting increase in sink strength caused a shift in the peak swelling toward lower helium-to-dpa ratios. The results of the shift were that at an intermediate damage level of 50 dpa, the maximum swelling was observed to occur at an intermediate helium co-injection rate of 0.2 appm He/dpa. At damage levels greater than 150 dpa, the peak swelling was observed to occur without the co-injection of helium. Examining helium effects in existing literature, the mode of helium injection was demonstrated to not have an influence on the effect of helium. Specifically, at low damage levels, swelling is driven by the nucleation of cavities. As the damage level increases, higher helium levels correspond to higher cavity densities and higher sink strengths which retard swelling and shift the maximum swelling location to lower helium concentrations. At high damage levels, the peak in swelling as a function of helium concentration is observed to occur at 0 appm He. This work provides substantial insight into the impact of helium on the evolution of cavities.