[en] United Kingdom Nirex Limited has studied in detail the geology and hydrogeology at a site near Sellafield in Cumbria, NW England to determine the suitability, or otherwise, of the site as the location for a deep repository for intermediate-level and certain low-level radioactive wastes. A key factor in determining site suitability is the nature of groundwater flow in the repository host rock; the Borrowdale Volcanic Group. In the host rock, flow takes place predominantly through a limited subset of discontinuities, mainly fractures, parts of which form networks of connected channels. Within this overall understanding of the nature of groundwater flow, there is a wide range of possible geometries for the flow channels. Determination of the applicable conceptualisation of the flow system must be soundly based on site characterisation data as a prerequisite for any numerical modelling study. In this presentation, a brief description of aspects of the core characterisation studies which were used to identify the location, orientation and mineralogical characteristics of the particular discontinuities referred to as Potential Flowing Features (PFF) is provided. These features have either demonstrable present day open porosity or display evidence of groundwater flow as part of the evolution of the current groundwater system. The PFF information has been integrated with data derived from hydraulic testing to provide the basis for robust conceptual models, which have been used in a performance assessment of the site