GEOLOGY OF OLKILUOTO - THE DISPOSAL SITE FOR SPENT NUCLEAR FUEL IN FINLAND

Olkiluoto Island, which is located in SW Finland, has been selected as the 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 December 2012 and an integrated description of the Olkiluoto site, including its geology, was a part of the background material. After their review, in early 2015, the Radiation and Nuclear Safety Authority in Finland (STUK) gave a positive statement to the Ministry of Employment and the Economy: "the final disposal facility designed by Posiva can be built to be safe".

Posiva has continued updating the multidisciplinary site description, which is aiming at the last licencing stage of the repository - the application of the operating licence. Understanding the geology of Olkiluoto is a substantial part of the site description work. It is based on data from hundreds of outcrops, tens of kilometers of drill core, extensive tunnel mapping programme and a wide variety of geophysical investigations. The Palaeoproterozoic bedrock in Olkiluoto consists of variably migmatised supracrustal high-grade metamorphic rocks: migmatized meta-pelites, meta-arenites and intermediate, pyroclastic metavolcanites. They are intruded by Paleoproterozoic felsic, granitic–tonalitic plutonic rocks and granitic pegmatoids, and diabase dikes. The rocks were metamorphosed simultaneously with the different phases of ductile deformation. Hydrothermal alteration at Olkiluoto can be subdivided into two different modes on the basis how the fluids have proceeded in the rock mass: fracture- or veinlet-controlled alteration and pervasive or disseminated alteration. The brittle deformation history of Olkiluoto can be reconstructed through paleostress inversion of fault-slip data from outcrops and drill cores and from K-Ar ages if fault gouge illites: seven distinct paleostress states can be identified, spanning in time from ca. 1.7 Ga to 1.0 Ga.