The rare gases have not been shown to exhibit measurable equilibrium solubility in metals, nor do any common metals exhibit measurable permeability to the rare gases. By means of nuclear reactions, however, “solid solutions” of rare gases in metals may be produced which permit the rare gas diffusion process to be studied. Results of work on the system radiokrypton-uranium are presented. Diffusion of radiokrypton from small cylinders of irradiated normal uranium was found to be negligible at temperatures below 1000°C. The diffusion rate was found to be quite temperature-sensitive and was considerably enhanced by thermal cycling. Swelling of the metal specimen during the diffusion process and the fact that the theoretical time dependence was never observed, leads to the conclusion that gas escape is by way of grain boundaries or microcracks. A possible mechanism to explain the thermal cycling behavior is presented.