During a previous solute transport study (TRUE Block Scale) at the Äspö Hard Rock Laboratory in Oskarshamn, Sweden, an extensive characterization of a 100m scale rock volume was undertaken with an emphasis on fracture and deformation zone geometry and connectivity. The construction of a new experimental tunnel (TASS) adjacent to the TRUE Block allowed the opportunity for the detailed characterization of one of the key hydraulic conductors, a brittle-ductile shear zone named Structure #20. Utilizing high-resolution HDR and UV digital images alongside hydraulic interference analysis, borehole core and image log analysis, tunnel wall mapping and mineralogical analysis of fracture fillings, this study focused on understanding the geometric, mineralogical, structural and hydraulic heterogeneity of Structure #20 at a scale (centimeter to decimeter) important to radionuclide transport understanding. The result was a greater understanding of the in-plane variability of an important class of structural features in rock types typical for proposed high-level spent nuclear fuel repositories in the Nordic countries.
1. THE ÄSPÖ HARD ROCK LABORATORY
The Äspö Hard Rock Laboratory (HRL) is a key element in the research and design of geological spent nuclear fuel repositories in Sweden. Seated approximately 450 m beneath the island of Äspö off the eastern coast of Sweden near the city of Oskarshamn, construction of the Äspö HRL began in 1990 and ended in 1995, though the excavation of new experimental niches and tunnels continues . The geology of Äspö HRL is dominated by granitoids of the Trans-Scandinavian Igneous Belt, but also includes fine-grained granite dikes and sheets, mafic inclusions and xenoliths . The age (1.85 Ga), mineralogy and chemistry of the rocks at Äspö are similar to those at the candidate sites (Forsmark, Laxemar and Olkiluoto) for spent nuclear fuel repositories elsewhere in the Nordic countries.
Äspö is run by the Swedish Nuclear Fuel and Waste Management Company (SKB), an organization tasked with the disposal and safety of the legacy of Sweden’s nuclear power program. Äspö HRL is used for the development of systems and processes for safe long-term storage of spent nuclear fuel using the KBS-3 concept.