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Summary:
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This third version of the Olkiluoto Site Report, produced by the OMTF (Olkiluoto Modelling Task Force), updates the Olkiluoto Site Report 2006 (Andersson et al. 2007) with the data and knowledge obtained up to December 2007. The main product of the modelling has been to develop an updated version of the descriptive model of the site (the Site Descriptive Model), i.e. a model describing the geometry, properties of the bedrock and the water and the associated interacting processes and mechanisms. The Site Descriptive Model is divided into six parts: the surface system, geology, rock mechanics, hydrogeology, hydrogeochemistry and migration, which are presented in individual chapters. Five separated models are presented: the geological, rock mechanics, hydrogeological, hydrogeochemical and migration models.Chapter 1 presents an outline of the report, explains its objectives and scope and sets out how it has been updated, compared with the previous Site Report, to take into account the various suggestions and recommendations from STUK and other review groups. It is also introduces and explains the integrated modelling methodology, the nomenclature used in the descriptions of the models and the prediction/outcome studies. Chapter 2 provides a brief overview of the data used for producing the Site Description. Chapters 3 to 8 present the descriptive modelling, which involves interpreting data, interpolating or extrapolating between measurement points and calibrating the model against data, based on the various assumptions made about each conceptual model. Chapter 9 presents the results of the prediction/outcome studies, as carried out during the construction of the ONKALO and the impact of the construction itself. These prediction-outcome studies include the disciplines of geology, rock mechanics and hydrogeology; and, in addition, the investigation of the EDZ is presented. Chapter 10 presents the overall consistency and confidence assessment in the Site Model 2008 and focuses on discussing the main issues of importance that are judged to require further attention. Chapter 11 presents an integrated description of the understanding of the Olkiluoto site, based on the characterisation and observations carried out up to the end of 2007. Chapter 12 concludes Site Report 2008, by listing its overall achievements and discussing issues to be considered for further site characterisation and modelling. The main advances since Site Report 2006 are:
- The geological model has been revised according to new data and interpretations. This has improved the consistency between the locations of the deformation zones in the geological model and the hydraulic zones in the hydrogeological model.
- New 3D seismic data have been incorporated within the geological model and an initial model for the eastern part of the Island is presented. Site-scale brittle deformation zones are extrapolated to intersect the surrounding regional lineaments, unless prohibited by direct observations to the contrary. The alteration model has been revised, showing a clear correspondence between the illitisation and the site-scale fault zones.
- A first account of the development of the brittle deformation history of the site is provided.
- A new geological DFN model has been developed, that considers mapped fracture traces from both the surface and the ONKALO.
- A new stress state model and fracture and fracture zone properties are presented.
- A new hydrogeological DFN model has been developed.
- An updated site scale (EPM) flow model has been developed.
- There has been an improvement in the understanding of the groundwater composition under low transmissive conditions. The importance of organics in controlling the groundwater composition of recently-infiltrated groundwaters has been clarified, due to the availability of many more isotopic and microbial data.
- A migration chapter has been added, which covers both the migration properties of the rock mass (e.g. matrix porosity, diffusivity and sorption) and its flow-related properties, as derived from the hydrogeological DFN modelling.
In other aspects of the Site Report, the 2008 model version is more a refinement of the 2006 model. Generally, there have been few surprises in the data obtained from the ONKALO and the Olkiluoto characterisation programmes, and the revisions to the model are concerned with details.
Chapter 10 of the report concludes that there is a high level of confidence in key aspects of the current Olkiluoto site descriptive model. There is less confidence in some aspects of the model and twenty-six critical issues have been identified, however, this lack of confidence can be handled, to a considerable extent, by providing wide uncertainty ranges and bounding estimates, or by the development of alternative models. The lack of confidence in these specific areas does not necessarily, therefore, warrant further characterisation and modelling. As outlined in Chapter 12, some issues are, however, more significant, including:
- Characterising the eastern part of the island, such that the level of understanding becomes comparable with that of the central part of the island.
- Further advancing the rock stress and rock strength regime to allow reliable stability estimates to be made at relevant repository depths.
- Further advancing the hydrogeochemical understanding of processes of importance for long-term safety, e.g. processes affecting the sulphide content, the origin of methane, the distribution of groundwaters at the detailed scale and the relationship between groundwaters and pore waters.
- Further advancing the understanding of detailed-scale migration properties, including site-specific data on the matrix properties.
- Further focusing on the ability to forecast and describe conditions of importance for long-term safety at the tunnel scale, i.e. as support to the current RSC (Rock Suitability Criteria) programme.
These issues will be addressed in Posiva’s further characterisation and modelling work.
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