[en] The release of gaseous and volatile fission products from UO₂ irradiated at temperature below approximately 1500⁰C is discussed in terms of simple diffusion models taking account of grain boundary bubble formation and the re-solution of atoms from them. It is shown that the release of stable gas is moderated by re-solution hence the rate of release from the fuel increases with burn-up virtue of the increasing fraction of grain boundary porosity remaining permanently open to the free surface. Re-solution is unimportant for active species but their release rates are affected in other ways by the behaviour of grain edge bubbles, specifically the frequency with which bubbles vent to the free surface and their probability of remaining permanently open. The reliance of the rate of release of active species on the grain boundary microstructure ensures that the release fractions of both stable and active fission products are closely associated. (author)