Since the stable valences of U in its oxides are IV, V, and VI, the formation of reduced species in UO2 is problematic. This is demonstrated in XAFS measurements of UO2 irradiated with protons and helium nuclei. The addition of O occurs by clustering that results in UO2-U4O9 phase separation, facilitated by the virtually identical U sublattices of these two species. Ion beam irradiation that displaces O ions could give the same behavior, but whereas disorder is observed the signatures of U4O9 are not. This is most likely because, whereas the O-enriched regions with mixed valence U(IV,V) would be stable, the U(III) formed in the O-depleted regions is not. A different mechanism is activated. This could also be important in the chemistry of the fuel and the O potential formed in response to the temperature gradient.