Radioactive fission gas release from coated particles containing UO2 or (Th-U)O2 fuel kernels was studied by taking into account the following parameters: (a) porosities of kernels and materials surrounding them, (b) irradiation temperature, (c) burrnup, and (d) thermal neutron flux. The main results follow. First, the structure of the kernels is modified during irradiation and, consequently, the mechanism and rate of fission gas release vary. Second, for a dense fuel, released activity results from recoil species ejected by the external surface of the kernel and reemitted from the surrounding porous carbon. Finally, for an initially porous fuel or for a heavily irradiated dense fuel, recoil atoms reemitted from the internal open porosity of the kernel and atoms ejected by knockout give the most important contributions to the release.