Nigel Marks

  • Simulation of Radiation Damage in Graphite

    Nigel Marks1, Marc Robinson2, Irene Suarez-Martinez2, Helen Christie3, Daniel Roach3, Keith Ross3, Alice McKenna4, Thomas Trevethan4, Malcolm Heggie4

    1Discipline of Physics & Astronomy, Curtin University, Perth, Australia
    2Nanochemistry Research Institute, Curtin University, Perth, Australia
    3Department of Physics, University of Salford, Manchester, UK
    4Department of Chemistry, University of Surrey, Guildford, UK

    Despite being one of the original nuclear materials, surprisingly few molecular dynamics simulations have been performed to study radiation response in graphite. The large difference between the MD literature for graphite and that of metals and oxides can be traced to the challenges associated with the description of bonding in carbon, in particular the anisotropic interactions which are central to sp2 carbon. Aside from point defect energetics and estimates of threshold displacement energies, little is known from a computational perspective about radiation processes in graphite. In a modern context, understanding of damage in graphite is motivated by lifetime extensions associated with Advanced Gas Reactors in the UK and proposed Gen-IV technologies such as the high-temperature gas-cooled reactor where graphite is a moderator.

    We have performed what we consider to be the first systemic study of radiation response in graphite using molecular dynamics. Chemical bonding is described using the Environment Dependent Interaction Potential (EDIP) for carbon [1], while short-range interactions are modelled using the conventional ZBL approach. Cascade simulations reveal that graphite behaves a manner remarkably distinct from metals and oxides, with the cascade primarily generating point defects, in contrast to connected regions of transient damage as are familiar from metals and oxides. Other unique attributes include exceedingly short cascade lifetimes and fractal-like atomic trajectories which show a remarkable visual similarity to historical models from the literature [2,3]. Unusually for a solid, the binary collision approximation is useful across a wide energy range, and as a consequence atomic displacements and defect production are consistent with the Kinchin-Pease and Norgett-Robinson-Torrens models, respectively. Comparison of defect energetics computed with EDIP against values from density-functional-theory show that the underlying description of defect behaviour is sound. At the level of defect creation itself, the MD simulations quantify threshold displacement energies for which a broad range of values have been reported in the literature.

    [1] N.A. Marks, “Generalizing the environment-dependent potential for carbon,” Phys. Rev. B, 63, 035401 (2001).
    [2] R. E. Nightingale, “Nuclear Graphite”, United States Atomic Energy Commission, Academic Press, 1962
    [3] J. H. W. Simmons, “Radiation Damage in Graphite”, Pergamon Press, 1965.