Following ratification by the Finnish government in 2001, construction of the world’s first deep geological repository for spent nuclear fuel began in 2004 at Olkiluoto, an island off Finland’s south-west coast. The repository is situated at a depth of 400-450 m in the high-grade metamorphic bedrocks of the Fennoscandian shield, primarily composed of low-permeability Palaeoproterozoic gneisses and granites (Paulama¨ki et al., 2002). Construction and operation of the repository are fulfilled by Posiva, a nuclear waste management organisation. A key component of Posiva’s upcoming operational licence application is an updated site descriptive model (SDM), characterising the geological, hydrogeological and hydrogeochemical features of the island (Hartley et al., 2018). The updated model builds on several previous iterations of the Olkiluoto SDM (Hartley et al., 2010; Fox et al., 2012; Hartley et al., 2012; Hartley et al., 2017).

A primary objective of the updated SDM is to produce a quantitative description of the properties of naturally-occurring fractures in the bedrock. Groundwater flow through the void space of such fractures represents the principal transport pathway for any arising radionuclides, and is therefore a fundamental consideration for the long-term safety assessment of the repository. Fracture properties are pertinent to a number of geoscience disciplines including geology, rock mechanics, hydrogeology, solute transport and hydrogeochemistry. This work demonstrates how the discrete fracture network (DFN) methodology (e.g. Dershowitz, 1985) provides a conceptual framework in which these various disciplines can be integrated to develop a combined ‘hydrostructural’ model of fracturing, flow and transport processes at Olkiluoto.

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