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This article deals with the geological safety aspects of nuclear waste disposal in Finland. It notes that the planned nuclear waste disposal concept in Finland is based on the protection provided by technical barriers, the isolating and transport-resistant characteristics of the low-porosity Precambrian crystalline rock and on the stability of the Fennoscandian Shield. It offers information about an international permafrost study in northern Canada coordinated by Geological Survey of Finland (GTK). It explains the multibarrier principle, one of the fundamental aspects of deep geological disposal. It stresses the importance of understanding the bedrock processes from bedrock stability to biogeochemistry in the geological disposal of nuclear waste.

The article focuses on the geological and geophysical investigations in the selection and characterization of the disposal site for high-level nuclear waste in Finland. In particular, it describes the process that has led to the selection of the Olkiluoto site. It explains the aim of the detailed site investigations conducted in 1993-2000. It mentions some geophysical applications used at Olkiluoto during the Site Conformation Phase including a supplementary airborne survey as well as ground and drillhole surveying. Identified as one important part of the site investigations is an underground rock characterisation facility called ONKALO. It details the geological mapping of the ONKALO tunnel which is divided into round mapping, systematic mapping and supplementary studies.