Near Field Characterization of Hard Rock Spent Nuclear Fuel Repository by Seismic Reflection

Posiva and SKB conduct RD&D activities for the safety assessment and to test disposal techniques in real deep-seated conditions. Brittle deformation zones and large fractures are considered to pose a potential risk for the mechanical integrity of the spent fuel disposal canisters. These are to be avoided in positioning of the deposition holes that will host the canisters, and they need to be identified during construction of the deposition tunnels and ultimately the deposition holes. Results from two high resolution seismic surveys carried out in ONKALO and Äspö HRL in 2013 are presented here, providing continuity information for several large fractures identified through geological mapping to cut the tunnels and/or boreholes. These were recognizable in transmission and reflection images produced from the seismic data sets.

We show that reflection seismic surveys are relevant to the detailed characterisation of crystalline bedrock. Relatively small-scale features, even single fractures, can demonstrably be detected. On the other hand, the detection of some distinctive features, even large-scale, can be uncertain if the survey layout is spatially constrained. Combinations of borehole and tunnel measurements using measuring arrays with diverse orientations helped reducing the location ambiguities and should be used in the future wherever possible.