The effects of pile irradiations on the physical properties and corrosion resistance of U-- Mo, U-- Nb; and U--Si alloys are reported. The dimensional stability under irradiation of the gamma phase U-- Mo and U-- Nb alloys is excellent; however, an isotropic volume increase of 4 to 6% per wt.% burnup may limit the ultimate fuel element life. Corrosion resistance of the gamma-phase alloys appesrs to be improved when subjected to s neutron field; this is attributed to an irrsdiation induced stabilization of the gamma phases. The U/ sub 3/Si alloy, on the other hand, suffered severe deterioration, particularly of corrosion resistance. Changes in electrical resistivity, hardness, mechanical properties, and crystal structure are presented and the mechanisms producing the observed changes discussed.