The use of the ferritic/martensitic HT-9 alloy as the cladding and duct material for the Core Demonstration Experiment (CDE) directly contributed to the attainment of the high fuel burnup levels critical to the viability of an economical liquid-metal reactor fuel system. The CDE, a partial core loading of fuel and blanket assemblies in the U.S. Department of Energy’s Fast Flux Test Facility, has successfully attained its irradiation exposure goal of 3 yr. Consisting of ten fuel and six blanket assemblies in a heterogeneous core configuration, the CDE has clearly demonstrated the capability of the advanced fuel and blanket designs to attain high burn ups and fast fluences. Each CDE fuel assembly consisted of 169 large-diameter fuel pins comprising mixed-oxide annular fuel pellets in sealed HT-9 cladding tubes. Each CDE blanket assembly consisted of 91 large-diameter pins comprising solid depleted uranium dioxide pellets in sealed HT-9 cladding tubes. The maximum-exposure CDE fuel assembly reached a peak pellet burnup of 163 900 MWd/tonne metal (M) and a peak fast fluence (E > 0.1 MeV) of 23.3 × 1022 n/cm2. The maximum-exposure CDE blanket assembly reached a peak pellet burnup of 43100 MWd/tonne M and a peak fast fluence (E > 0.1 MeV) of 22.8 × 1022 n/cm2. Lead test fuel assemblies built to CDE specifications continue their successful irradiation and have attained burnups of >238 000 MWd/tonne M with accumulated fast fluences (E > 0.1 MeV) of >38 x 1022 n/cm2. In-core measurements of HT-9 ducts and withdrawal loads of the assemblies indicate that duct distortion will not be a factor that limits the lifetime of the fuel or blanket assemblies. Comparison of the measured and predicted coolant outlet temperatures from the peak CDE fuel and blanket assemblies indicate the irradiation of the CDE has proceeded as planned. The CDE represents a tremendous success in demonstrating the lifetime capabilities of this advanced oxide system using the HT-9 ferritic alloy for structural materials.