Modeling of the Diffusion of Hydrogen in Porous Graphite

Abstract

The graphite used in fusion devices as first wall material is porous and consists of granules and voids. The 1–10 µm granules are further composed of graphitic micro-crystallites (5–10 nm), which are separated by micro-voids. Understanding the hydrogen transport and trapping in such granules is an important aspect of understanding the effect of a realistic graphite structure on hydrogen recycling and hydrocarbon formation in graphite. We use Kinetic Monte Carlo (KMC) to study the diffusion of hydrogen in a typical granule of graphite. We use molecular dynamics (MD) to obtain the jump attempt frequency ωo and the migration energy Em of interstitial graphite which are inputs to the KMC. A consistent parameterization of MD within KMC is presented. The diffusion shows a non-Arrhenius behavior which can be explained with two types of different jump processes within the graphite crystal. The porous granule structure is constructed using statistical distributions for the crystallite dimensions and for crystallite orientations at a given porosity. The hydrogen trapping at inter-crystallite micro-voids are modeled by assuming that a fraction of the hydrogen atom flux transiting through the micro-void is trapped. We present a parametric study of the diffusion and trapping of hydrogen within the granule for various trapping fractions at the inter-crystallite micro-voids.