International Geologiical Congress - Oslo 2008

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GHZ-04 Earthquake hazard assessment and geotechnics

 

Respect distances: Rationale and means of computation

 

Raymond Munier, Svensk Kärnbränslehantering AB (Sweden)
Harald Hökmark, Clay Technology AB (Sweden)
Billy Fälth, Clay Technology AB (Sweden)
 

 

Though Sweden is currently regarded seismically dormant, huge earthquakes (M ≥ 8) have been shown to have occurred some 9000 years ago as the weichselian glacier retreated. Similar earthquakes are anticipated during the advance and/or retreat of future continental ice sheets.

The safe disposal of radioactive wastes requires that the canisters are placed sufficiently far from deformation zones as to not jeopardise the canister integrity. The mechanical impact on the canisters, as an effect of earthquakes, is by far the dominating factor of those studied. As the wastes are hazardous for thousands of years, the large earthquakes anticipated in the future have potential to harm the repository, thereby threatening the biosphere.

Though earthquake-generating structures can be safely avoided during the construction of the repository, the threat to the canisters lies mainly in the reactivation of fractures in the rock mass between deformation zones. The amount of slip that such a target fracture can host is a function of the fractures geometry and properties (e.g. size, orientation, surface friction) in relation to the distance to the hypocenter and the size (stress drop, slip velocity) of the earthquake. We propose a method to numerically simulate earthquakes of various magnitudes and the slip they induce on target fractures of varying geometry and distances to the earthquake source.

Using rock properties from site investigations, analogue shearing experiments and modelling of shearing across canisters, our 3D simulations (using the distinct element code 3DEC) suggest that canisters can be safely emplaced at a distance of 100 m or larger from the boundary of a deformation zone, if the canister is not intersected by fractures with radii up to 75 m. Should fractures with radii up to 150 m be accepted in a deposition hole, the respect distance must be increased to 200 m.

 

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