Experimental metallic uranium fuel elements were irradiated under power reactor conditions to >18 000 MWd/T with no indication that an exposure limit was being approached. This excellent performance is attributed to providing an axial hole in the fuel to accommodate fuel swelling internally without increasing the external dimensions of the fuel element. The irradiation test was conducted in the Engineering Test Reactor (ETR) in 260°C, 2000 psi water with coextruded, Zircaloy-2 clad, uranium rods. An axial hole representing 5,10, or 20% of the fuel volume was provided in the center of the fuel and was sealed from the coolant by a brazed and welded Zircaloy-2 end cap. Other variables in the test included cladding thickness and composition. The length, diameter, warp, and volume of each of 24 test elements were measured each reactor cycle in the ETR canal, and periodic neutron radiographs were obtained at Battelle-Columbus. Based on examinations of the neutron radiographs, it is concluded that the axial hole is acting as originally intended and has permitted an increase in the allowable exposures of a uranium rod by at least a factor of 5 and potentially much more. If 2% strain is allowed in the Zircaloy-2 clad, the rods with 5,10, and 20% holes would be expected to survive exposures to a maximum of ∼25 000, 40 000, and 70 000 MWd/T, respectively. The high exposures already achieved with this concept represent a real “breakthrough” in the technology of metallic uranium fuel, and the excellent behavior of the rods suggests still greater potential. It is anticipated that full advantage can now be taken of the tremendous economic incentives that exist for using metallic uranium in present day power reactors.