[en] A considerable number of materials have been either proposed or investigated as packing materials for nuclear waste package systems. Almost always the expandable clays, such as the smectites contained in commercial bentonites, have received the most attention when their primary function is to retard groundwater flow. Other materials including zeolites, metals, and dessicants are considered as special-purpose additives. Materials that tend to hydrolyze and lead to porosity reduction, such as silicates, oxides, and sulfates, have also been suggested as packing materials. All these types of materials are also considered as components of tailored mixtures to achieve a broad range of packing material performance. Some of these materials are reviewed, along with proposed candidate materials, with respect to the properties required to function in a salt repository. The investigation of packing materials is composed of five studies which are discussed below. Initial candidates will consist of calcium hydroxide, a sodium silicate, and a cement-gypsum mixture in addition to the reference crushed salt. Consequently these tests will be necessary to determine properties of individual components and to optimize properties of mixtures. 13 refs., 7 figs., 1 tab