A brief review is presented on the hydrogen diffusion in beryllium and graphite materials. It is shown that the hydrogen diffusion coefficients obtained at temperatures below 900 K should be used for evaluation of hydrogen inventory in graphite material. The effect of different parameters on hydrogen inventory is demonstrated by means of numerical calculations. The obtained value for hydrogen inventory in "virgin" RGT graphite is 0.1 g per metric ton material at a temperature of 850 K.