Recent fast reactor driver fuel tests in the Experimental Breeder Reactor II (EBR-II) have demonstrated good performance of U-Pu-Zr fuel alloys to burnups >15 at. pct. Postirradiation examination of these tests has yielded a large amount of fuel-swelling data and metallographic information. These data show that prior to making contact with the cladding tube, metallic alloy fuel swells rapidly due to its high fission-enhanced creep rate and irradiation growth. Measurements of macroscopic fuel slug growth during the entire fuel pin lifetime can now be understood by properly taking into account the irradiation environment and microstructural changes for the various alloy compositions. It has been determined that fission rate, temperature, and plutonium concentrations influence the observed macroscopic swelling rate.