Four prototype irradiation tests were conducted in the Fast Flux Test Facility to investigate the performance of a 2-yr mixed-oxide fuel system using titaniumstabilized stainless steel cladding and duct material for application in a commercial-scale liquid-metal reactor plant. Three of the tests were irradiated to the point of cladding breach to establish the lifetime capability of this fuel design. Details of the fuel element design, irradiation conditions and exposures, and postirradiation measurements are presented. Comparisons between measured and calculated behavior showed basically good agreement. A conservative failure analysis of the 676-fuel-pin data set from the four test assemblies indicated a 99.9% reliability for a peak burnup capability of 90 MW .d/kg metal.