As the fuel is degraded or heated the more volatile fission products which are trapped within the uranium dioxide may become free. For example, see.
A report on the release of Kr, Ru and Cs from uranium when air is present has been written. It was found that uranium dioxide was converted to UO between about 300 and 500 °C in air. They report that this process requires some time to start, after the induction time the sample gains mass. The authors report that a layer of UO was present on the uranium dioxide surface during this induction time. They report that 3 to 8% of the krypton-85 was released, and that much less of the ruthenium (0.5%) and caesium (2.6 x 10%) occurred during the oxidation of the uranium dioxide.