Abstract In Germany, the former potash and rock salt mine Morsleben is so far the only underground repository for radioactive waste operated on the basis of the Atomic Act. In 1986, the Morsleben repository was licensed by the authorities of the former German Democratic Republic for the disposal of low- and intermediate-level waste. In 1999, the federal government of Germany decided to cease disposal operations. At present, a license for the closure of the repository is being pursued by the Federal Office for Radiation Protection (BfS). BfS initiated a comprehensive scientific and technical program to keep Morsleben a safe repository. This program resulted in greater insight into the geological structure of the site, the rock mechanical deformation of the underground excavations, and the origin and age of brine seeps at a few locations in the mine. To reduce the radiological consequences of a brine intrusion scenario, the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), contracted by BfS, investigated several concepts for effective backfilling and sealing. Material selection and—in view of the specific geological situation—adequate positioning of sealing systems underground contribute substantially to the long-term safety of the repository.