Zircaloy-clad fuel elements containing UO2 pellets separated by thin graphite disks have been irradiated to maximum burnups of 800 MWh/kg U (33 000 MWd/tonne U) at a linear power range of 30 to 70 kW/m. Fission gas release and sheath strains were lower than experienced for conventional fuel under comparable conditions because of the lower bulk average fuel temperatures in disk elements. The irradiated disk elements also showed good internal stability, tolerance to power ramping, and acceptable defect behavior.