Site description of Laxemar at completion of the site investigation phase. SDM-Site Laxemar

The Swedish Nuclear Fuel and Waste Management Company (SKB) has undertaken site characterisation in two different areas, Forsmark and Laxemar-Simpevarp, in order to identify a suitable location for a geological repository of spent nuclear fuel according to the KBS-3 method. The site investigations have been conducted in campaigns, punctuated by data freezes. After each data freeze, the site data have been analysed and modelling has been carried out with the overall purpose to develop a site descriptive model (SDM). The site descriptive model is used by repository engineering to design the underground facility and to develop a repository layout adapted to the site. It is also essential for safety assessment, since the SDM is the only source for site-specific input. Another important use of the site descriptive model is in the environmental impact assessment. An SDM is an integrated model of geology, thermal properties, rock mechanics, hydrogeology, hydrogeochemistry, bedrock transport properties and a description of the surface system. The site descriptive model compiled in the current report, SDM-Site Laxemar, presents an integrated understanding of the Laxemar-Simpevarp area (with special emphasis on the Laxemar subarea) at the completion of the surface-based investigations, which were conducted during the period 2002 to 2007. A summary is also provided of the abundant underlying data and the discipline specific models that support the site understanding. The description relies heavily on background reports that address, in particular, details of the data analyses and modelling of the different disciplines. The Laxemar-Simpevarp area is located in the province of Smaaland within the municipality of Oskarshamn, about 230 km south of Stockholm. The candidate area for site investigation is located along the shoreline of the strait of Kalmarsund, within a 1.8 billion year old suite of well preserved bedrock belonging to the Transscandinavian Igneous Belt formed during the waning stages of the Svecokarelian orogeny. The candidate area is circular-shaped with an area of 54 km2, and hosts the Laxemar subarea in its eastern part. The south-western part of the Laxemar subarea was selected as the focused area for the complete site investigation work, following the initial investigations at the site. Prior to the presentation of the SDM-Site report, three versions of a site descriptive model have been completed for the Laxemar-Simpevarp area and subjected to peer review. The last version, referred to as version Laxemar 1.2, constituted a preliminary site description that concluded the initial site investigation work and was presented in 2006. This preliminary site description formed the basis for a preliminary safety evaluation (PSE) of the Laxemar subarea, a preliminary repository layout, and the first evaluation of the long-term safety of this layout for a KBS-3 repository in the context of the SR-Can project. The final site descriptive model, SDM-Site Laxemar, builds on a coordinated geological model in 3D, into which other discipline-specific models have been integrated without any major conflicting interpretations. In particular, the bedrock thermal and strength properties of the bedrock at the site have been coupled to lithology and identified rock domains in the geological model and an integrated description has been devised that links the hydrogeology and the chemistry of the groundwater to hydraulic conductor domains (deformation zones) and hydraulic rock domains (and underlying fracture domains) and their division in depth zones. These mutually consistent results demonstrate that a fundamental understanding of the current state, properties and processes in Laxemar, from the surface down to potential repository depth, has been achieved. In addition, the properties of the area can be explained in the context of an understanding of its past evolution, throughout a long period of geological history. This integrated understanding of the area is presented in Chapter 11 of this report and this chapter effectively serves a s an executive summary of the modelling work. A systematic assessment of the confidence in the developed models, including treatment of uncertainties and evaluation of alternative interpretations, has been carried out. This assessment has taken into account the feedback obtained from the work on the preliminary repository layout step D1 and from the safety assessment SR-Can, as well as the feedback obtained on earlier versions of the site description. The overall outcome of this assessment is that most properties of importance for both repository constructability and long-term safety are bounded sufficiently, and that data collected underground now bears the prime potential to further reduce uncertainties in the focused volume. However, complementary studies to reduce some of these uncertainties are also currently in progress. Uncertainties outside the focused volume are higher, but are judged to be of less importance for the constructability and long-term safety of a repository.