The Surrounding Bedrock
Spent nuclear fuel will be disposed of in a depth of about 400-450 meters inside Olkiluoto bedrock. Deep inside the bedrock, the prevailing conditions are stable and predictable.
The main rock type in Olkiluoto bedrock is migmatic gneiss. The bedrock in the area is approximately 1,800 to 1,900 million years old. There are various types of structures in the rock, some of them water conductive. The final disposal tunnels and the disposal holes for the canisters will be positioned inside the bedrock in such a manner that any major water-conducting structures will be avoided.
Why Is Bedrock Important in the Final Disposal?
The bedrock protects the canisters against external impacts, creates mechanically and chemically stable conditions to the repository and limits the amount of groundwater coming into contact with the final disposal canisters. The bedrock in Finland is stable, and the possibility for any large-scale movements inside the bedrock is minimal.
Research results indicate that hundreds of metres down in the bedrock, groundwater is virtually void of oxygen and flows very slowly, which is why its corroding effect on the canisters and spent nuclear fuel is very small.
The bedrock also effectively stops direct radiation emanating from the canisters because rock two metres thick alone is sufficient to attenuate the radiation to the level of natural background radiation.
If spent fuel would, due to unforeseen circumstances, come into contact with groundwater, the substances dissolved from it would mainly remain in the bentonite and the bedrock surrounding the canisters.
Why the Depth of Several Hundred Meters?
With the disposal of the canisters in the depth of more than 400 meters, the effect of potential changes above ground and in the atmosphere to the area immediately surrounding the final disposal facility will be avoided. Furthermore, the canisters are outside the reach of normal human activity (such as drilled wells) and the facility is very hard to enter.
Deeper inside the bedrock, the stress conditions in the rock would make the construction of facilities more difficult. The salinity in the groundwater would also increase, which can have an adverse effect on the functioning of the bentonite.
Studies Help to Know the Bedrock Better
Olkiluoto bedrock has been studied ever since the 1980s. There have been a number of studies, some of which are still underway, using such methods as aboveground drilling and investigation trenches, as well as constructing the underground rock characterisation facility, ONKALO.