The deposition tunnels are located at a depth of about 400-450 meters inside the Olkiluoto bedrock. Tunnels will be excavated inside the rock, and the final disposal canisters will then be placed in holes drilled in those tunnels.
The underground repository is divided into three parts:
- deposition tunnels where the canisters containing spent nuclear fuel will be placed
- central tunnels connecting the deposition tunnels and access tunnel and shafts
- underground technical auxiliary facilities.
The size of the final disposal facilities depends on the amount of the spent nuclear fuel. The basis is the final disposal of the amount of fuel equivalent to 5500 tU, which means a total of 2800 final disposal canisters. In this case, the volume of rock to be excavated for the facility, excluding the canister holes, is approximately 1.3 million cubic metres. According to current plans, the number of final disposal tunnels required is 137. The total length of the tunnels has been calculated to be 42 kilometres, and they are located within an area extending over 2 to 3 square kilometres.
An access tunnel and four vertical shafts lead from the surface down to the repository. The vertical shafts include a personnel shaft and a canister shaft as well as two ventilation shafts. Of these, the access tunnel, the personnel shaft and two ventilation shafts will be constructed while building the research tunnel ONKALO.
In Posiva's reference solution, the canisters will be placed in holes 6 to 8 metres deep that will be bored in the floor of the deposition tunnels. The holes will be sealed with precompressed bentonite clay. Alternatively, the canisters can be placed in horizontal tunnels, lined with a bentonite structure.
During final disposal operation, deposition tunnels are sealed as canisters are placed in tunnels. After placing the canisters in tunnels, the tunnels will be filled up as soon as possible. Compressed clay blocks will be used as filling material.
Phases of Excavating the Repository
When the final disposal operations begin, all deposition tunnels are not constructed at one time. About one half, and a maximum of 58 per cent, of the total volume of all repository facilities is unsealed at any one time.
There is a total of eight construction phases for repository. In addition to the phase when is built and the phase of preparation for the final disposal, there are six excavation phases.
When the construction takes place in phases, the investments can be broken down over a longer period of time. This also helps to minimize the amount of leakage water and the need for ventilation. Another advantage with several construction phases is that new data can be gathered and the decisions adjusted according to current state-of-art knowledge.