Finnish radioactive waste management company Posiva Oy said on 26 February that work to excavate a final disposal tunnel has started at the Onkalo underground characterisation facility at Olkiluoto in Eurajoki. The tunnel is for the 'joint functional test' (YTK), which involves small-scale final disposal under actual conditions in preparation for commissioning of the geological repository. The YTK will be used to demonstrate that the processes and procedures related to final disposal are acceptable as a condition for the facility being granted an operating licence.
At Onkalo, used fuel will be placed in the bedrock, at a depth of about 450 metres. The disposal system will comprise a tightly sealed iron-copper canister, a bentonite buffer enclosing the canister, a tunnel backfilling material made of swellable clay, the seal structures of the tunnels and premises, and the enclosing rock.
Posiva said a tunnel with an approximate length of 80 metres is being excavated and four deposition holes will be drilled into it as part of the YTK. The excavation has been preceded by careful preparation and planning in cooperation with the Radiation and Nuclear Safety Authority (STUK). A central tunnel also needed for the test was excavated earlier.
Then the new tunnel reaches 48 metres, excavation will be paused, the base of the tunnel will be levelled out and the required compaction work will be done, said Posiva worksite manager Kimmo Lehtola. He noted that the excavation is a method test that pays special attention to the area between 19m and 48m.
"Surveys and subsurface imaging measurements will be performed in the region of this area," he said. "They are used to confirm that the bedrock has no water flow paths or discontinuities caused by the excavation. The method test is used to demonstrate that we can perform the work according to the plans and requirements."
Excavation of the tunnel will then continue and is expected to reach 80m in April/May. The test tunnel is shorter than the actual final disposal tunnels, which will be about 350m in length.
The YTK will take place in 2023 and will test final disposal using the technology designed for it. However, the canisters placed in the bedrock will not contain used nuclear fuel. "Once the tunnel for the joint functional test is complete, excavation of the first five final disposal tunnels will begin," said Posiva construction manager Juha Riihimäki. "Actual final disposal activities will begin in one of the tunnels around 2025."
The site for Posiva's repository was selected in 2000. The government had granted a construction licence for the project in November 2015 and construction work on the repository in 2016. The Finnish parliament approved the decision-in-principle on the repository project in 2001.
Posiva is jointly owned by Finnish nuclear utilities Fortum and Teollisuuden Voima Oyj.