The diffusion constants of tritium and/or its isotopes for graphite so far reported scatter considerably in the Arrhenius plot. This, however, was not seriously considered, despite the fact that understanding of the scattering was indispensable to estimate the tritium inventory and permeation in/through the graphite first walls of D-T burning experimental devices. It was shown that in the present paper that there was 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 a- 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 could be interpreted as a consequence of the multi-diffusion channels.

Diffusion Constants of Tritium in Graphites and Compensation Effect. Ashida, K., Watanabe, K.: Journal of Nuclear Materials, 1991, 183[1-2], 89-95