SKB’s conceptual model for flow through the fractured rock at Forsmark is based on a division between highly fractured deformation zones (described in terms of deterministic geometry), and the more sparsely fractured rock (described in terms of statistical geometry).
The model for the highly fractured deformation zones includes several assumptions that are not strongly supported by data, which may lead to underestimation of groundwater flux through the repository volume. However, the impact of these assumptions on flows to deposition holes is judged to be minor, compared with uncertainties in the model for the more sparsely fractured rock.
This assessment focuses mainly on the statistical model for the sparsely fractured rock in the vicinity of the deposition tunnels in the proposed repository. This model, referred to as the Hydro-DFN model, is described in terms of a statistical population of disc- or square-shaped, uniformly conductive plates, representing fractures, which are randomly located and oriented in a given volume of rock. The population of these plates is described by statistical distributions of geometric and hydraulic properties. Flow is assumed to be entirely through networks formed by chance though intersections among these plates (fractures).