DIFFUSION CONSTANTS OF TRITIUM IN GRAPHITES AND COMPENSATION EFFECT

The diffusion constants of tritium and/or its isotopes for graphite so far reported scatter considerably in the Arrhenius plot diagram. This, however, has not been seriously considered, despite that the understanding of the scattering is indispensable to estimate the tritium inventory and permeation in/through the graphite first walls of D-T burning experimental devices. We show in the present paper that there is regularity in the scattering: namely, the presence of a linear relation between the logarithms of the pre-exponential factors and the activation energies (the compensation effect). The temperature dependence of the diffusion constants and the compensation effect were analyzed by assuming the presence of three distinct diffusion channels in graphite: channels along the alpha- and c-axis in a graphite grain and along the grain boundary. Computational simulations could reproduce fairly well the temperature dependence of the experimental data and the compensation effect by assuming proper effective pre-exponential factors for the respective channels, where the activation energies for the respective channels were chosen from the observed values. Namely, the scattering of the diffusion constants and the compensation effect can be interpreted as a consequence of the multi-diffusion channels.