Zircaloy-clad uranium metal fuel tubes containing dilute (<1000 ppm) Fe, Si, and Al additions that were irradiated under 1200 psi reactor pressure to about 5000 Megawatt days per metric ton of uranium (MWd/tU) swelled less than 3%; in contrast, U-1.5wt%Mo and unalloyed uranium tubes swelled 6 to 10%. External restraints such as provided by thick cladding and reactor pressurization contributed substantially to the volume stability of the tubes by limiting the volume increase that would normally be encountered at exposures beyond the characteristic threshold for swelling.