GEOLOGICAL SITE DESCRIPTION AND GEOCHRONOLOGY OF SPENT NUCLEAR FUEL REPOSITORY SITE, OLKILUOTO, SW FINLAND

The Olkiluoto Island in SW Finland was selected in 1999 as the final disposal site for spent Finnish nuclear fuel. Posiva is the nuclear waste management organisation in Finland responsible for research into the final disposal of spent nuclear fuel and for the construction, operation and eventual backfilling and closure of the final disposal facility.

Olkiluoto Site Description report (OSD) has been produced by the OMTF (Olkiluoto Modelling Task Force). The report represents a major part of the input from the technical and scientific activities that support Posiva’s safety case, which is needed for the application for construction licence. It covers the subject areas of the surface system, geology, rock mechanics, hydrogeology, hydrogeochemistry and transport properties.

Geological history and model of Olkiluoto is presented in the OSD and more detailed in geological model reports. The purpose of the geological model is to evaluate the geological properties and conditions of the rock mass on the repository site. The model provides the geometry and the geoscientific descriptions necessary to site characterisation, design and construction. In addition to studies on local bedrock, it is correlated to regional geology, and depositional and tectonic history of Olkiluoto is connected to framework of SW Finland and the Fennoscandian Shield.

The Paleoproterozoic gneisses (1.9–1.8 Ga) of southern Satakunta Region surrounding Olkiluoto are subdivided into two domains: a pelitic migmatite belt in the SW and a psammitic migmatite belt in the NE. The area between these belts is occupied by Mesoproterozoic rapakivi granites (1.58–1.55 Ga), Jotnian sandstones (1.4–1.3 Ga) and olivine diabases (1.27–1.25 Ga). Plutonic rocks composed of varying granitoids and small bodies of gabbro and diorite intruded the migmatites. After the formation, the gneisses in Olkiluoto area were reworked in polyphase ductile and brittle deformation and affected by several stages of alteration processes.

A geochronological compilation for the Olkiluoto site is prepared and linked to the evolution and events of SW Finland and Fennoscandian Shield. The compilation covers the Proterozoic sediments, intrusions and gneisses, limited indicators of Paleozoic, Mesozoic and Tertiary events, and mainly glacial processes in Quaternary.