Geoscientific understanding of the Forsmark site

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Ahlbom, Kaj۱; Skagius Elert, Kristina۲
Holding Date 04 October 2008

Multidisciplinary investigations to characterize a potential repository for spent nuclear fuel have been completed in two areas in Sweden: Forsmark and Oskarshamn. The results from ongoing integrated modelling and repository engineering evaluation provide input to forthcoming safety and environmental impact analyses to be included in the application to construct a repository at one of the sites.
Forsmark is located 120 km north of Stockholm. The candidate area is approximately 10 km2 in size. It is located within a tectonic lens, composed mainly of c. 1870 Ma years old metagranite, that trends NW-SE. The lens is surrounded by highly deformed and heterogeneous bedrock, which is strongly anisotropic. The multidisciplinary investigations have included a wide range of surface and borehole surveys to evaluate geological, ecological, hydrogeological, hydrogeochemical, transport, rock mechanical and thermal conditions and properties. Rigorous measures have been taken to ensure data quality.
The integrated analysis of the results have been continuously presented in site descriptive reports that describe the current state of knowledge, as well as the natural processes that affect the long-term development of the area. The models have been updated successively as the database has developed with time. During the recurrent modelling procedure, data needs were identified and the site investigation programme was adjusted accordingly.
Key characteristics of the bedrock are:
•The bedrock inside the candidate area is geologically homogeneous and dominated by metagranite with high quartz content from surface down to at least 1,000 m depth. It has high thermal conductivity and high compressive strength.
•The candidate area is divided into two structural elements by a gently dipping fracture zone. The potential repository is located in the footwall to this zone and is transected solely by steeply dipping zones. Gently dipping zones are common in the hanging wall.
•The fracture frequency outside fracture zones in the repository bedrock is low below c. 200 m depth and even lower below 400 m depth. Only a few of these fractures carry water.
•Down to a depth of c 200 m, there is a high frequency of sub-horizontal fractures with apertures. These are hydraulically interconnected over long distances and, together with the gently dipping zones, are the main water conductors.
•Rock stresses are relatively high in the upper part of the bedrock but the gradient towards depth in the repository rock is low.
•Both the salinity and age of the groundwater increase with depth but the water composition at depth in the repository bedrock is different from that in the gently dipping zones to the south-east. It is inferred that water in the repository bedrock has been isolated from the surface for a long time (36Cl and 4He analyses indicate > 1.5 Ma), whereas the water in the gently dipping zones contains signatures from the Littorina Sea that covered the area 9,500 to 5,000 years ago.