Finnish radioactive waste management company Posiva has completed the procedure qualification test on the backfill installation at the Onkalo used nuclear fuel repository in Olkiluoto. During the test, almost 12 metres of tunnel were filled with backfill clay using the GBIS backfill installation system. The minimum goal for the test that took about three weeks was to fill a length of at least 10 metres of the tunnel.
At the repository, used fuel will be placed in the bedrock, at a depth of about 430 metres. The disposal system consists of 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.
The procedure qualification test on backfill installation was designed to demonstrate the operability of the systems and equipment in tunnel conditions. Another requirement was to achieve the specified result by deploying the operating methods and procedures that will be deployed during operation of the repository.
“The large, long combination machine almost fills the entire tunnel and compacts clay with large screws while pushing it into the tunnel,” said Posiva Development Manager Ari Maarni. “At the same time, it reverses slowly in order to achieve a uniform backfill degree. During the test, the transfer vehicle ran autonomously between the discharge station and the installation machine supplying more clay to the installation machine.”
The biggest challenges during the test were caused by the uneven floor of the tunnel, Maarni noted. “The docking of the transfer vehicle to the installation machine was difficult in points with large surface height differences. This slowed down the work. However, as the test proceeded a lot was learned and we were able to adjust the equipment to operate faster.”
The test was carried out in the 50-metre-long tunnel used for the Trial Run of Final Disposal. The remaining 40 metres of the tunnel will be filled in the Trial Run of Underground Operation, before the tunnel is finally sealed.
The next procedure qualification tests will focus on testing the buffer installation system as well as the canister transfer and installation vehicle. These installation procedure qualification tests will be carried in practice with each other and will be the last ones before the underground stage of the Trial Run of Final Disposal. This will test the operation of the complete final disposal facility using non-radioactive test elements instead of nuclear fuel.
Posiva, jointly owned by Finnish nuclear utilities Fortum and TVO, applied for a construction licence for a repository to the Ministry of Employment and the Economy in 2013 and the government granted the licence in November 2015. The site for a repository at Eurajoki, near the Olkiluoto NPP, was selected in 2000 and parliament approved the decision-in-principle on the repository project in 2021.
Once it receives the operating licence, Posiva can start the final disposal of the used fuel generated from the operation of TVO’s Olkiluoto and Fortum’s Loviisa NPPs. The facility will operate for about 100 years before it is closed.
