[en] Fractured rocks are composed of porous but almost impermeable rock matrix and water conducting fractures. The main characteristic of the fractured rock is the great heterogeneity in different scales that leads to preferential flow paths and channelling of the flow. Three distinct flow environments can be identified: channeling that causes variable flow in the individual fracture planes, transmissivity differences between fractures that leads to preferential flow paths and extensive fracture zones that provide highly transmissive connections over long distances. Large and transmissive fractures have an important role to the flow and transport properties of the fractured rock. Flow paths tend to accumulate on the large features that carry the majority of the flow. Modelling exercises have indicated persistence of the flow properties along the flow paths. This means that once a particle has entered a major flow path it tends to follow the high flow rate channel. The main challenge in spatial up-scaling of the retention properties is connected to the description of the flow characteristics in the fractured rock. The importance of individual fractures to the overall retention is proportional to the flow rate along the fracture. This means that simulations need to consider individual fractures. Fracture network modelling offers a suitable approach that is able to take into account the multiscale structure of the fractured rock and to determine retention properties of the flow paths. It also provides a straightforward way to up-scale transport properties along the preferential flow paths through the fracture network. However, the computational feasibility of the site scale applications in the performance assessment limits the range of different size fractures that can be taken into account in the fracture network simulations. Heterogeneity in the immobile zone properties may influence effective retention properties if the heterogeneity is coupled with a limited capacity of the immobile zone. Simulations show that importance of the limitation in the thickness of the immobile zone can be estimated by simply comparing penetration depth of the pore diffusion in the immobile zone to the actual thickness of the immobile zone. Applying typical properties of the limited rim zones around the water conducting fractures indicates that under PA flow conditions especially non-sorbing nuclides will saturate most of the immobile zones in the rim zones. In the PA conditions it is usually more important to take into account heterogeneity of the flow field than heterogeneity in the immobile zone properties when the solute retention is assessed. Heterogeneous flow properties lead to channelling of the flow that can take place between different flow routes and inside individual flow paths. Diffusional mixing inside an individual flow path will average out heterogeneity of the flow field, whereas mixing between different flow paths is possible only in the intersections of the flow paths and the solute flux is dominated by the flow routes of the highest flow rate. Modelling indicates that mixing along an individual flow path is quite efficient in the PA conditions. Therefore, channelling in the fracture planes is unlikely to be a major concern for the PA flow paths, but variability between different flow paths needs to be taken into account. (orig.)