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Gas Diffusion in Metals: Fundamental Study of Helium-Point Defect Interactions in Iron and Kinetics of Hydrogen Desorption from Zirconium Hydride

  • Author(s): Hu, Xunxiang
  • Advisor(s): Wirth, Brian D
  • et al.

The behavior of gaseous foreign species (e.g., helium and hydrogen), which are either generated, adsorbed or implanted within the structural materials (e.g., iron and zirconium) exposed to irradiation environments, is an important and largely unsolved topic, as they intensively interact with the irradiation-induced defects, or bond with the lattice atoms to form new compounds, and impose significant effects on their microstructural and mechanical properties in fission and fusion reactors. This research investigates two cases of gas diffusion in metals (i.e., the helium-point defect interactions in iron and kinetics of hydrogen desorption from zirconium hydride) through extensive experimental and modeling studies, with the objective of improving the understanding of helium effects on the microstructures of iron under irradiation and demonstrating the kinetics of hydrogen diffusion and precipitation behavior in zirconium that are crucial to predict cladding failures and hydride fuel performance.

The study of helium effects in structural materials aims to develop a self-consistent, experimentally validated model of helium - point defect, defect cluster and intrinsic defects through detailed inter-comparisons between experimental measurements on helium ion implanted iron single crystals and computational models. The combination of thermal helium desorption spectrometry (THDS) experiment with the cluster dynamic model helps to reveal the influence of impurities on the energetics and kinetics of the He-defect interactions and to realize the identification of possible mechanisms governing helium desorption peaks. Positron annihilation spectroscopy is employed to acquire additional information on He-vacancy cluster evolution, which provides an opportunity to validate the model qualitatively. The inclusion of He - self-interstitial clusters extends the cluster dynamic model while MD simulations explore the effects of dislocation loops on helium clustering. In addition, the influence of pre-existing defects on helium behavior in iron is studied by applying a hybrid model, which includes the defect evolution during neutron irradiation and the subsequent He ion implantation and THDS. These modeling predictions will be assessed in future experiments.

The hydrogen desorption process from zirconium hydride and zirconium in vacuum is also studied by coordinated experimental and modeling methods. The production and verification of the desired δ-zirconium hydride is discussed while thermal desorption spectroscopy (TDS) is employed to obtain the hydrogen desorption spectra directly. In addition, a one-dimensional two-phase moving boundary model coupled with a kinetic description of hydrogen desorption from a two-phase region of δ-ZrH1.6±n and α-Zr is developed to compare with the TDS experimental results.

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