The development of a safety assessment model of the geosphere for a repository sited in the crystalline basement of northern Switzerland

Abstract A modelling study of groundwater flow and radionuclide transport in the geosphere surrounding a repository for vitrified high-level waste sited in the crystalline basement of northern Switzerland has been carried out as part of the recently completed Kristallin-I project. The study derives information from an analysis of seven deep boreholes, along with seismic surveys, regional surface geological mapping and hydrochemical and hydrogeological analysis. This paper describes the transport model itself and how, by means of a suite of supporting models, field data are translated into input parameters. Supporting models include: (i), a conceptual model of the proposed siting region; (ii) a set of hydrogeological models, the formulation of which is guided by the conceptual model and by hydrochemical and isotopic data. Sources of uncertainty in the conceptual model are described and a parameter sensitivity study is presented that illustrates the resultant uncertainty in radiological consequences. A particular source of uncertainty is the heterogeneous nature of the geosphere, giving rise to a complex and incomplete dataset from which ranges of input parameter values must be derived.