The Olkiluoto island was chosen as the site of the repository from among many survey sites on the basis of location surveys and safety assessments carried out between 1987 and 2000. The underground research facility ONKALO, which Posiva started to construct in 2004, has enabled research at the actual disposal depth. The aim has been to preserve the good characteristics of the rock as far as possible in the construction phase of the underground research facility.
The research carried out during the construction has confirmed earlier assumptions about the rock properties. Based on the research, it is now known that there are few cracks in the bedrock at the disposal depth the bedrock is stable at a depth of hundreds of meters, the roundwater is oxygen-free the groundwater flow is minimal.
The safety of final disposal is supported with information obtained through research. The bedrock, the materials used in the disposal process, and the functionality of the technical solutions are subjected to safety studies.
An extensive body of assessment documentation, known as the safety case, is prepared to prove the safety of final disposal. The most central part of the safety case is scenario analysis whereby the behavior of the final disposal system installed deep in bedrock is assessed on the basis of postulated event sequences. The temporal scopes of the analyses stretch far into the future, and they consider factors such as climate change and the formation of a thick glacier as well as associated rock movement.
Further proof of the safety of final disposal for the safety case may be gathered by observing so-called natural analogs—in other words, by making comparisons with nature.
Natural analogs are examples, observed around the world, of how naturally occurring materials have behaved under different conditions over long periods of time. By studying nature, it is possible to establish how copper, clay, and radioactive substances behave under different conditions over long periods of time. This has led to a better understanding of the representativeness of the models used to describe the functionality of the disposal system.
It is now known, for example, that clay has effectively stopped the migration of uranium as well as other radioactive substances from a radioactive uranium deposit deep in the bedrock in Cigar Lake, northern Canada. No radiation levels differing from the normal natural background radiation levels can be observed above ground in the area.
Furthermore, natural copper formations found around the world have persisted for up to millions of years, which can be considered as proof of the long-term strength of copper canisters placed deep in the bedrock. In Hyrkkölä in southern Finland, for example, copper occurs in its original form in granite rocks despite having been exposed to groundwater containing sulfide under oxidizing conditions.