The evolution of molecular hydrogen from UO2(100) surfaces exposed to water vapor has been studied using temperature programmed desorption. No hydrogen evolution is observed from well-annealed, pristine surfaces. Significant amounts of hydrogen, however, desorb from highly defected, ion-sputtered surfaces at similar to400 K. Annealing studies show that the most of the defects are healed after heating to 750 K. Successive water exposures also heal these defects. These results indicate that the defect is likely an oxygen vacancy (reduced uranium site). On annealing, oxygen diffuses from the near-surface region to fill the surface vacancy. The dissociative adsorption of water also fills the oxygen vacancy. As the damage resulting from ion sputtering is similar to that caused by the recoil atom formed during radioactive decay, the thermal chemistry at defects may be an indirect, but significant, factor in the radiation chemistry of adsorbed species on alpha-emitting materials.