2012:71 Identification of Brittle Deformation Zones and Weakness Zones

In preparation for the review of Swedish Nuclear Fuel and Waste Management Company’s (SKB) license application for disposal of spent nuclear fuel, Swedish Radiation Safety Authority  (SSM) is conducting studies to evaluate the performance of the multi-barrier principle on which the KBS-3 concept is based. Copper canisters containing the spent nuclear fuel are placed into granitic bedrock at about 500 m depth and embedded in bentonite clay. Thus, the rock, the clay and the copper canister are acting as barriers in order to retard the possibility of spent fuel to escape the repository and reach the biosphere.

The proposed area for the repository is surrounded and affected by deformation zones. Knowledge of the zones at depth is obtained from drill cores. No common methodology of identification of the brittle deformation zones exists. The concepts of “respect distances” and “full perimeter intersection criteria” developed by SKB are based on the understanding of fracture extension and the definition of deformation zones. A common methodology in identification of deformation zones is therefore desirable.

This study showed that an analyzing tool for characterization of brittle deformation zones is needed. It also showed that a methodology of defining different parts of the zones is needed.  One method in identifying brittle deformation zones is suggested in this study. It is based on the concept of clustering fractures due to their separation seen from the drill core.  The methodology is based on fixed steps and gives a good resolution. While comparing this methodology with others good consistency is shown.