2012:54 Technical Note, Initial Review Phase for SKB´s safety assessment SR-Site: Geological structures and deformation zones, from site investigation to safety assessment

The site selected by the Swedish Nuclear Fuel and Management Co (SKB) for location of a deep geological repository, i.e. a storage for spent nuclear fuel at about 500 m depth in Precambrian metamorphosed rocks. The location of the site is Forsmark at the northern coast of Uppland and 122 km north of Stockholm. In geological terms Forsmark is located in the central part of the Fennoscandian Shield. The flat ground surface in Uppland today mainly coincides with a vast surface formed by long-lasting subaerial erosion more than 540 million years ago. Actually, the bedrock surface is exposed again after being covered by sedimentary rocks of Palaeozoic age (younger than 540 million years). 

The target volume, considered large enough to host a repository, is located in the core of a large-scale fold where the rock is composed of granite-like rocks (granites and granodiorites). The fold is bordered on each side, i.e. to the northeast and southwest, by subvertical regional brittle deformation zones (DZ) trending WNW and NW, respectively. The regional zones form a wedge closing north-westwards. The separation between the WNW and NW trending regional DZ varies from about 2.5 to 3.5 km in the local site area. The bedrock at the planned repository level may also contain rock (amphibolites) that may affect the utilization of the available rock volume; due to lower thermal conductivity a larger separation of canister positions is needed in amphibolites than in granitoids.

The actual target area is crossed by a steeply dipping regional DZ (>3 km)  trending ENE dividing the planned repository volume into two parts and along such DZ no canister position closer than 100m is allowed. Along borders of the target area there are steeply dipping regional DZ mainly trending WNW, NW and ENE.