The results are summarized of 15 yr of German research on spent fuel with respect to its suitability as a waste form disposed of in a repository located in the Gorleben salt dome. Within the multibarrier system for long-term isolation of high-level waste (HLW), the innermost engineered barrier "canistered spent fuel" contributes essentially to isolating radionuclides from the biosphere if a salt brine were to come into contact with the waste form. A large fraction of the radionuclide contents of the reacted fuel mass would become reimmobilized within secondary alteration products and on container corrosion products, but inevitably a certain nuclide-specific fraction would be released into the aqueous geochemical environment. The corrosion resistance of the fuel and the radionuclide mobility are not inherent materials properties but also depend on geological disposal conditions, packing concepts, and radioactive decay. In particular, the availability of oxidants is critical, controlling spent-fuel alteration rates and alteration products as well as radionuclide solubilities. Spent fuel is at least as suitable for final disposal as is HLW glass.