Monitoring of a canister deposition hole

Svensk Kärnbränslehantering AB (SKB), the Swedish nuclear waste management company operates the Äspö Hard Rock Laboratory (HRL), Sweden. This laboratory has been constructed to investigate engineering and instrumantation methods that will be used for the safe storage of high-level nuclear waste in a deep underground repository.

As part of SKB's on-going research program two deposition holes, of the type that might be excavated in a future repository to hold a canister of spent fuel, have been excavated from the floor of a tunnel at 420m depth. Mock canisters will eventually be installed in the deposition holes and used to investigate the retrievability of the canisters at a later date.


The 1.75m diameter, 8m long deposition hole was excavated using a Tunnel Boring Machine (TBM) converted for vertical drilling. Excavation was performed in eleven 0.8m drilling steps. After each drilling step ASC monitored the rock volume for stress-related fracturing (AEs) and ultrasonically measured the changed in rock properties. Locations for AEs have been computed using in-house software and have an estimated location accuracy of 10cm. The AEs deliniate regions of small scale fracturing with millimetre dimensions around the deposition hole perimeter. The AEs are distributed in regions of induced compressive stress. The AE activity is believed to be associated with pre-existing macroscopic fractures, or similar weaknesses in the rock mass, that the excavation intercepts.

Changes in the elastic properties of the rock mass have been measured using ultrasonic surveys along 128 potential ray paths. The array geometry has been designed so that some of these paths pass within a few centimetres of the deposition hole wall. An average P-wave velocity change of -15m/s has been observed as the deposition hole is excavated past the ray path. This is primarily due to opening microfractures in an excavation damaged zone around the deposition hole. AE locations show this extending only a few 10s of centimetres into the rock mass. As both P- and S-wave velocities have been measured, estimates in changes of elastic properties have then been obtained for the damaged rock.



We thank SKB for their kind permission to use this data on our website.


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