Analytical models of hydrogen transport in graphite

• Significant differences exist among models, with differences in hypotheses, approximations, applicability. • A comprehensive transport model is provided, identifying knowledge gaps and proposing alternative formulations. • Model covers inter-granular and intra-granular diffusion, rate equations at reactive sites, and simplifying assumptions • Main gaps are chemical form of hydrogen during intra-granular diffusion and rate limiting steps for trapping in Trap 1 and Trap 2 sites. • Compiled experimental data and illustrated energy diagrams supporting the modeling options are provided. The importance of graphite-hydrogen chemical reactions to fusion, fission, and hydrogen storage applications, combined with the rapidly evolving knowledge on the underlying mechanisms, has led to the development of multiple models to describe hydrogen transport in graphite. Significant differences exist among these models, resulting from discrepancies in the modeling assumptions, intended degree of fidelity, and conditions of applicability. This paper attempts at reconciling these apparent differences by providing a comprehensive description of the constitutive equations governing hydrogen transport in graphite at high-temperature, identifying outstanding gaps in knowledge, illustrating how these different models approach them, and proposing alternative analytical formulations grounded on experimental results from hydrogen-graphite studies. Governing equations, closing relations, and simplifying assumptions are discussed for hydrogen transport at the inter-granular and intra-granular level, accompanied by compiled experimental data and illustrated energy diagrams associated to the proposed transport mechanisms. Analytical formulations are provided to reproduce competing hypotheses on the mechanisms, supporting the development of a range of computational models that can enable resolution of outstanding knowledge gaps through comparative testing against experimental data.