Assessment of Backfilling Materials and Methods for Deposition Tunnels

The purpose of the study was to evaluate which combinations of materials and methods provide a safe solution for backfilling a deposition tunnel in a KBS-3V type repository for spent nuclear fuel. The comprehensive safety of the repository is provided on the basis of the multiple barrier principle and the backfill serves as an important element in this concept. To perform such a function under expected repository conditions, a variety of requirements have been set concerning the physical and chemical behaviour of the backfill. Since the physical properties of a soil are dependent on its void ratio, dry density criteria were determined for backfill materials based on theoretical calculations and experimental measurements of swelling pressure, hydraulic conductivity and deformation properties in various density states and water salinities. Several potential backfill materials, consisting of different types of bentonites, smectiterich clays and bentonite-ballast mixtures, were investigated. Two different backfilling methods were considered: 1) in situ compaction of backfill materials in inclined layers and 2) installation of pre-compacted blocks and bentonite pellets. Average material densities for the two alternative backfilling methods were estimated based on experience from field tests and simple density calculations. These densities were compared with the density performance criteria to assess whether the selected material/method combinations fulfilled the requirement set for the backfill. It was concluded that the highest safety margins were reached with pre-compacted blocks consisting of swelling clay and the lowest with in situ compacted materials. Recommendations were made that future development and investigation should focus on the block backfill concept.