Model volumes and model areas

The site descriptive modelling is performed using two different scales of model volume, the regional and the local model volume. Generally, the local model is required to cover the volume within which the repository is expected to be placed, including accesses and the immediate environs. Hence, the description of the local model volume should be detailed enough for the needs of repository engineering and safety assessment. In addition to the description on the local scale, a description is also devised for a much larger volume, the regional model volume, in order to place the local model in a larger context and to allow for sensitivity analyses of, mainly, hydrogeological boundary conditions. The difference between the regional and local model volumes is the resolution of the modelled objects, which has been chosen to balance modelling efforts against the needs of downstream users of the model. This is a consequence of ensuring a homogeneous resolution throughout all parts of the model volumes.

In selecting the model volumes for version 1.2 /SKB 2005a/, and also for the geological model in modelling stage 2.1 /SKB 2006a/, some rules of thumb, taken from the strategy document for integrated evaluation /Andersson 2003/ were applied. Since the model volumes have not changed, the arguments for the selected volumes still remain and are repeated below. It also needs to be understood that the distinct model sizes primarily concern the development of the geological model in the SKB rock visualisation system (RVS). This is also the reason why the model areas and volumes have rectangular shape.

It should be noted that the RT 90 and RHB 70 coordinate systems (see section 2.5) are always used in the site descriptive model presented in the following chapters, even if this point is not always stated in the text. In addition, depth is used in the text as a synonym for elevation, i.e. referring to depth below sea level if not otherwise stated. However, the difference between depth below sea level and depth below ground surface at Forsmark is only a few metres due to the low altitude and flat topography of the Forsmark area.

2.9.1 Regional model area and volume

The regional model area is shown in Figure 2-5. This model area and the corresponding volume are the same as the regional model area/volume in version 1.2 /SKB 2005a/ and the arguments for selecting this area/volume remain.

• It includes the candidate area and it is not prohibitively large as it has a surface area of 165 km².

• It captures relevant portions of the steeply dipping regional deformation zones, which strike in a north-westerly direction and surround the candidate area. Any expansions of the regional model area to the north-west or south-east would not provide any significant changes in the regional geological picture. With these considerations in mind, the size of the regional model area is sufficient for the needs of geological modelling. It should be noted that the geological evolution (chapter 3) is assessed in the context of a much larger area than the regional model area.

• It adequately covers the variations in rock type in the candidate area and its immediate surroundings.

• It captures the main hydrogeological features of the region, as the boundaries perpendicular to the shoreline are judged to be sufficiently far away from each other that they do not influence the groundwater flow in the candidate area. The boundary to the south-west lies on the south-western side of a local topographic divide, and the boundary to the north-east lies north-east of a major bathymetric break in Öregrundsgrepen. Sensitivity analyses undertaken in version 1.2 of the site descriptive modelling addressed the proper locations of the boundaries in the regional hydro-geological model and the results confirm that the selected regional volume is also appropriate from a hydrogeological point of view.

• A depth of 2,100 m below sea-level is considered to provide a reasonable vertical extent for description and is the maximum depth down to which any meaningful extrapolations of deforma-tion zones could be made. To represent this depth in RVS, the size of the vertical dimension is

Figure 2‑5. Regional (black) and local (purple) model areas during and after model stage 2.1. The regional model area is the same as in model versions 0, 1.1 and 1.2. The local model area is smaller than in version 1.2 (blue line) and covers the north-western part of the candidate area selected as the target area for a potential repository.

2.9.2 Local model area and volume

The local model area (and corresponding volume) is the same as that selected for the geological modelling in stage 2.1 /SKB 2006a/. It is smaller in size compared with the version 1.2 local model area, but is contained within the boundaries of the version 1.2 local model area, as shown in Figure 2-5. It includes the target area selected for a potential repository, where site investigations during the complete site investigation phase were largely focused. The arguments for selecting this local volume were given in /SKB 2006a/ and are repeated below.

• Both to the north-east and south-west, it includes the boundaries to more inhomogeneous and banded bedrock outside the candidate area.

• It includes key rock boundaries within and immediately adjacent to the candidate area which help to define the structural framework within the tectonic lens.

• The north-western and south-eastern model boundaries are located well outside the outer borders of the repository area according to the repository layout from step D1 /Brantberger et al. 2006/.

Furthermore, the north-western boundary is positioned so that the parts of the tectonic lens below the current reactor site as well as potential access ramps from the SFR peninsula are included in the model.

• The surface area is c. 12 km², i.e. comparable with the size of 5 to 10 km² recommended in the general execution programme /SKB 2001/.

• A depth of 1,100 m beneath sea level permits inclusion of all information from the existing deep boreholes at the site. To represent this depth in RVS, the size of the vertical dimension is set to 1,200 m since the upper boundary is set to +100 m above mean sea level.

The coordinates defining the local model volume are (in metres):

RT 90 (RAK) system; (Easting, Northing): (1629171, 6700562), (1631434, 6702824), (1634099, 6700159), (1631841, 6697892).

RHB 70; elevation: +100, –1,100.