Finland’s high-level nuclear waste disposal programme remains on schedule some 25 years after it began. Posiva is preparing a construction licence application for a final disposal facility at Olkiluoto and, if all goes well, the site should become operational in 2020.
The target for submitting an application for the facility is 2012, and the application will include a PSAR (preliminary safety assessment report), technical designs, system descriptions, and a large number of other reports. The safety case will form the central part of the material, all of which will be scrutinised by the Finnish Radiation and Nuclear Safety Authority (STUK).
The plan for final disposal is based on the KBS-3 system, which consists of engineered barriers and the natural barrier provided by the crystalline bedrock of the Fennoscandian Shield, where bedrock is between 2.5 and 3.4 billion years old. The main rock type at Olkiluoto is gneiss, a metamorphic rock with a granite-like composition.
The engineered barriers take the form of copper-iron canisters, which will contain lowsolubility spent fuel in the form of assemblies containing 150–200 fuel rods. The copper will act as a barrier against corrosion, while a cast nodular graphite iron insert will provide the required mechanical strength. The buffer between the canisters and the bedrock will consist of highly compacted bentonite clay, and backfill in the facility’s tunnels will be based on clay material with good swelling capabilities.
Bentonite contains high levels of smectite minerals and expands when it comes into contact with water, which will limit the movement of groundwater near the canisters. The bentonite barrier will also protect the canisters against mechanical wear from possible movements in the bedrock. Bentonite buckles under heavy loads but recovers its shape, and this characteristic will quickly seal any cracks that occur if the rock moves.
Containment for at least 100,000 years
Two variants of the KBS-3 concept are being considered. The reference concept is KBS-3V, in which canisters are placed vertically in individual boreholes. The alternative concept is KBS-3H, in which canisters are placed horizontally in long deposition drifts.
|Posiva began construction of its ONKALO underground characterisation facility at Olkiluoto in 2004. The project will reach the repository’s planned depth in 2010.
The canister has been perfected to endure very extreme conditions, such as earthquakes and the pressure imposed by a continental glacier. With a standard width of 1.05 m, the length of canisters varies to accommodate the three different types of fuel assembly used in Finnish nuclear power plants, at Loviisa 1 and 2, Olkiluoto 1 and 2, and Olkiluoto 3.
The main safety function of the canister is to ensure a prolonged period of complete containment for the spent fuel extending at least 100,000 years. This safety function rests primarily on the mechanical strength of the canister insert and the corrosion resistance of the copper surrounding it.
The safety functions of the buffer include protecting the canisters from external processes that could compromise containment and limiting and slowing down radionuclide release in the event of canister failure. The role of the host rock is to isolate the repository from the biosphere and normal human habitat, and to provide favourable and predictable mechanical, geochemical, and hydrogeological conditions for the engineered barriers, protecting them from potentially detrimental processes taking place above and near the surface. The rock will also limit and slow down the inflow and release of harmful substances from the repository.
As part of work to confirm the proposed site of the repository, Posiva is building an underground characterisation and research facility down to the planned disposal depth. The purpose of this facility, known as ONKALO, is to produce detailed information for the design and safety case that cannot be obtained from investigation on the surface. ONKALO also provides an opportunity to test and demonstrate processes for assessing rock suitability, the design of repository spaces, and excavation. Design work is ongoing and the layout is developing as excavation proceeds deeper and more knowledge is gained from the rock.
Construction was started in 2004 and is due to reach the target depth early in 2010. The remaining auxiliary rooms and test galleries will be excavated in 2010 and 2011. STUK is supervising the construction of ONKALO in the same way it would a nuclear facility.
After receiving its construction licence, Posiva will construct the encapsulation facility and the first disposal tunnels. The final step will be to apply for an operation licence, which Posiva plans to submit in 2018. Disposal activities are scheduled to start in 2020, and disposal of the current inventory of 5,500 tU (approx. 2 tU per canister) will take more than 100 years. This is because the thermal output of each canister is limited and spent fuel bundles must be cooled for 50 years, and the reactors supplying the spent fuel are expected to be operational for at least 60 years.
Graph: Layout of the final repository planned for Olkiluoto.