In a dry air storage facility, the oxidation of irradiated nuclear fuel is considered as the main risk induced by a leakage that would put the nuclear ceramics in contact with air. The oxidation of UO2 in U3O8 leads indeed to a relative volume swelling of more than 30% which could cause the ruin of the fuel rod. In order to fill the lack of data on high burn-up fuel, some oxidation experiments are currently conducted in CEA with UO2 and MOX fuel. Fragments as well as slices of fuel rod, with a high burn-up, have been oxidized in air at 200°C for more than 4000 hours. The associated weight gain curves are presented together with the characterization of some samples picked up at different oxidation times. It is shown that the weight gain curve of high burn-up UOX is not parabolic-shaped, as predicted in literature, but sigmoid-shaped. Moreover it is also shown that the grain boundary oxidation is not uniform inside a fragment. This leads to a new description of the oxidation mechanism of irradiated fuel.