Regional hydrogeological simulations for Forsmark - numerical modelling using CONNECTFLOW. Preliminary site description Forsmark area - version 1.2

The Swedish Nuclear Fuel and Waste Management Company (SKB) carries out site investigations in two different candidate areas in Sweden with the objective of describing the in-situ conditions for a bedrock repository for spent nuclear fuel. The site characterisation work is divided into two phases, an initial site investigation phase (IPLU) and a complete site investigation phase (KPLU). The results of IPLU are used as a basis for deciding on a subsequent KPLU phase. On the basis of the KPLU investigations a decision is made as to whether detailed characterisation will be performed (including sinking of a shaft). An integrated component in the site characterisation work is the development of site descriptive models. These comprise basic models in three dimensions with an accompanying text description. Central in the modelling work is the geological model, which provides the geometrical context in terms of a model of deformation zones and the rock mass between the zones. Using the geological and geometrical description models as a basis, descriptive models for other geo-disciplines (hydrogeology, hydro-geochemistry, rock mechanics, thermal properties and transport properties) will be developed. Great care is taken to arrive at a general consistency in the description of the various models and assessment of uncertainty and possible needs of alternative models. Here, a numerical model is developed on a regional-scale (hundreds of square kilometres) to understand the zone of influence for groundwater flow that affects the Forsmark area. Transport calculations are then performed by particle tracking from a local-scale release area (a few square kilometres) to identify potential discharge areas for the site and using greater grid resolution. The main objective of this study is to support the development of a preliminary Site Description of the Forsmark area on a regional-scale based on the available data of 30 June 2004 and the previous Site Description. A more specific objective of this study is to assess the role of known and unknown hydrogeological conditions for the present-day distribution of saline groundwater in the Forsmark area on a regional scale. An improved understanding of the palaeo-hydrogeology is necessary in order to gain credibility for the Site Description in general and the hydrogeological description in particular. The latter will serve as a basis for describing the present-day hydrogeological conditions on a local scale as well as predictions of future hydrogeological conditions. Finally, this study aims to identify some of the issues that may impact the SR-Can Performance Safety Assessment (PSA) project, and where possible provide a preliminary evaluation of sensitivities to such issues. In particular, recommendations are made as to which uncertainties need to be addressed as part of SR-Can. The numerical modelling was performed by two separate modelling teams. The work presented in this report was conducted by The CONNECTFLOW Team involving modelling experts from Serco Assurance, Kemakta Konsult and Golder Associates