Bare and zirconium-clad uranium-10 wt% molybdenum specimens were irradiated in NaK-filled capsules in the MTR. Irradiation conditions varied to include central-core temperatures ranging from 300 to over 1200 deg F, fuel burnups ranging from 0.36 to over 3.0 total at.% and fission rates in the range of 0.35 to 1.9 x 10/sup 14/ fissions/(sec)(cm/sup 3/) of alloy. Other parameters studied included the effects of heat treatment, changes in composition, different fabrication techniques, and changes in cladding thickness on the behavior of the fuel alloy. The objective of the irradiations was to determine the behavior of the fuel alloy under conditions approaching as closely as possible those to be encountered in the Enrico Fermi Atomic Power Plant as they were known at the time. The results indicated that the volume of the fuel alloy would increase conservatively at a rate of about 3.0% per at.% burnup as long as the critical temperature of 1000 to 1100 deg F was not exceeded and the gamma phase of the alloy did not transform during irradiation. If the critical temperature was exceeded, the alloy swelled until rupture or complete disintegration occurred. The occurrence of transformation during irradiation was noted at burnups in the range …