Olkiluoto Island, which is located in SW Finland, has been selected as the site for a deep geological repository for the high-level nuclear waste generated by the nuclear power plants operated by the Finnish power companies TVO and Fortum, and the repository is expected to become operational in the early 2020s. Posiva is an expert organisation responsible for the management and implementation of the nuclear waste repository at Olkiluoto and is owned by TVO and Fortum. Posiva submitted a construction licence application to the Finnish government in 2012 and an integrated description of the Olkiluoto site was part of that submission. In 2016, the Finnish Government granted a licence to Posiva for the construction of a final disposal facility for spent nuclear fuel. The repository design used in the present safety case is KBS-3V, in which the spent nuclear fuel (SNF) canisters are emplaced in vertical deposition holes surrounded by bentonite buffer.

Posiva has continued updating the multidisciplinary site description, toward the last licencing stage of the repository – the application of the operating licence. A primary focus of the site description is on the properties of natural fractures and brittle fault zones in the low permeability crystalline rocks at Olkiluoto; an understanding of the flow and transport processes in these features are an essential part of assessing long-term safety of the repository. A discrete fracture network (DFN) modelling methodology has been developed to provide an integrated description of geological, hydrogeological, geomechanical and transport properties of natural fractures and brittle fault zones at the site. Inputs to conceptualisation and parameterisation of the DFN model comes from 58 deep cored drillholes, a wide variety of geophysical investigations, many outcrops, kilometres of underground mapping and testing in the ONKALO research facility, and groundwater pressure monitoring and sampling in both deep and shallow holes.

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