[en] This report presents details of the modelling that was done to support the development of the simplified geosphere model (GEONET), which was used in the assessment that was presented in the Environmental Impact Statement on the proposed concept for the disposal of Canada's nuclear fuel waste. Detailed modelling of groundwater flow, heat transport and contaminant transport through the geosphere was performed using the MOTIF finite-element computer code and the particle-tracking code TRACK3D. The GEONET model was developed using data from the Whiteshell Research Area, with a hypothetical disposal vault located at a depth of 500 m. This report first briefly describes the conceptual model and summarises the two-dimensional (2-D) simulations that were used initially to define an adequate 3-D representation of the system. The analysis showed that the configuration of major fracture zones could have a large influence on the groundwater flow patterns. These major fracture zones can have high velocities and large flows. The proximity of the radionuclide source to a major fracture zone may strongly influence the time for a radionuclide to be transported from the disposal vault to the surface. Groundwater flow was then simulated and advective/convective particle tracking was conducted in the selected 3-D representation of the system, to aid in selecting a suitable form for the simplified model to be used in the overall systems assessment with the SYVAC3-CC3 computer code. Sensitivity analyses were performed on the effects of (a) different natural geometries of part of the model domain, (b) different hydraulic properties, (c) construction, operation and closure of the vault, (d) the presence of a water supply well and (e) the presence of an open borehole. These analyses indicated that the shape of the topography and the presence of a major low-dipping fracture zone focuses groundwater passing through the vault into a discharge area that is much smaller than the area of the vault. A domestic water supply well drawing water continuously from fracture zone LDl will reduce the area in the biosphere to which contaminated water from the vault is discharged, with the degree of focusing depending on the well depth and pumping rate. Minimum convective travel times to the surface could be reduced by 30 to 50% by the well. Incorporating a waste exclusion distance between the vault rooms and a major fracture zone can be an effective way to retard convective transport of contaminants from the vault. Excavation-induced damage caused by vault construction may have no detrimental effects on the long-term performance of the disposal system. Heat generated by the vault operation can reduce the earliest arrival time of contaminants from the vault to the surface. An open site-evaluation borehole penetrating the vault area may create a significant pathway for contaminants from the vault to the biosphere. Convective, dispersive and diffusive contaminant transport from the vault was also analysed with the MOTIF code. Results from this analysis were compared with those obtained from an equivalent simulation using a version of the simplified geosphere model, GEONET, in the SYVAC3-CC3 code. The good agreement indicates that the approximations made in describing contaminant transport in GEONET are reasonable. (author)