[en] AECL is preparing an Environmental Impact Statement (EIS) of a concept for disposing of Canada's nuclear fuel waste. The disposal concept is that of a sealed vault constructed at a depth of 500 to 1 000 m in plutonic rock of the Canadian Shield. This report is one of nine primary references for the EIS. A probabilistic system variability analysis code (SYVAC3) has been used to perform a case study assessment of the long-term safety and environmental impacts for the EIS. This report describes the methodology for developing the SYVAC3-CC3 Geosphere Model (GEONET) which simulates the transport of contaminants from the vault through the geosphere to the biosphere. It also discusses the data used to construct the model, as well as assumptions and justifications for the data and model. The geosphere consists of the rock mass surrounding the vault, including the groundwater in the pores and cracks in the rock, the materials used to seal the shafts and exploratory boreholes at the site, and a domestic water well that is assumed to intersect the pathway of most rapid transport from the vault to the biosphere. GEONET simulates the movement of groundwater from the vault through the geosphere to discharge locations at the biosphere; the movement of contaminants in the groundwater by advection, hydrodynamic dispersion, and molecular diffusion; chemical sorption of contaminants onto minerals in the rock during transport; radioactive decay; and the rate of discharge of vault contaminants to the biosphere. Development of the Geosphere Model involves several steps. The initial step is to construct a conceptual model of the subsurface geological structure and ground water flow conditions using data from site investigations and laboratory tests. Once a conceptual model has been constructed, the coupled equations describing 3-D groundwater flow and heat transport are solved using the MOTIF finite-element code to calculate hydraulic head and groundwater velocity distributions. Next, the groundwater flow paths from the vault to discharge areas in the biosphere are determined by means of a particle-tracking code, TRACK3D. The flow paths and diffusion paths are used to construct a simplified 3-D pathways network composed of 1-D transport segments for GEONET. Sensitivity analyses are performed with MOTIF to determine the appropriate form of the pathways used in the GEONET approximation. GEONET modifies the head distribution predicted by MOTIF to account for the effects of pumping from a domestic well and calculates the mean groundwater velocities in the network by Darcy's law. It then solves the 1-D advection-dispersion-retardation equations for a radionuclide decay chain by using analytical response functions and numerical convolutions to determine the rate of movement of vault contaminants through the network of geosphere pathways. AECL has done a postclosure assessment case study, in which the assessment methodology was applied to a hypothetical reference disposal system. The geosphere characteristics of this system were assumed to be similar to conditions at the Whiteshell Research Area in southeastern Manitoba, one of AECL's geologic research areas. The hypothetical disposal vault, approximately 2 km x 2 km in area, was located at 500-m depth, near an assumed large low-dipping fracture zone. The fracture zone provides a relatively rapid groundwater pathway to the surface. Although the geosphere modelling approach is generic and can be applied to any disposal site in the Canadian Shield, we have used a site specific geosphere model for the assessment. This was done to show how the particular hydrogeologic and geochemical conditions of a site affect the transport of vault contaminants through the geosphere, to illustrate how the design and layout of the vault interact with these conditions. The Canadian Nuclear Fuel Waste Management Program is funded jointly by AECL and Ontario Hydro under the auspices of the CANDU Owners Group. (author)