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The diffusion of Xe atoms in UC a t temperature from 800" to 1,400"C was studied on samples irradiated to various doses up to 2 . 9~ 10l8 nvt. The apparent activation energy of the diffusion was found to vary with fluence, which corresponded to the extent of neutron or fission fragment damage in the specimen.The diffusion was observed to be enhanced at exposures below 1017nvt. At the higher doses, on the other hand, trapping of the gas atoms by defect clusters became evident, which inhibited the gas release.When the irradiated samples were once heated up to 1,4OO0C, the activation energy of Xe diffusion through them showed a constant value of 83+5 kcal/mol, which was independent of the neutron fluence.The scattered values of activation energy reported in literature are explained from the present results as resulting from differences in the activation energy of Xe diffusion, which depends on the extent of radiation damage retained in the UC.