Characterizations are being conducted on light-water reactor (LWR) spent fuel with peak burnups from ∼31 to 46 MWd/kgM and rod-average fission gas releases of ∼0.3 to 11%. Measured concentrations of nuclides agreed within ∼10 to 15% of the predicted amounts. Radiochemistry of deposits on the cladding interior surface, fuel ceramography, beta/gamma autoradiog-raphy, and gamma scan data were consistent with fuel burnups and apparent operating temperatures. Microstructural observations also indicated increased porosity at the fuel center in high-release fuels and at the edge in high-burnup fuels. Cladding oxide thickness increased with exterior cladding temperatures as did the trend in hydriding.