The full-scale in situ system test (FISST) at the Onkalo underground characterisation facility near Olkiluoto has proceeded to the concreting phase in the construction of the end plug for the deposition tunnel, Finnish waste management company Posiva has announced.
The test ensures the functionality of storing nuclear fuel assemblies packed in the copper-steel canisters in tunnels drilled into rocks, as well as backfilling the tunnels with bentonite clay and sealing them with reinforced concrete plugs. The test involves the placement – within a 50-meter tunnel some 420m underground – of two test canisters containing heating elements to simulate the residual heat of used fuel. Conditions within the tunnel will be monitored using about 500 sensors. The test monitoring phase will continue for several years.
The canisters and the buffers were installed in the deposition test holes in the summer of 2018 and the backfilling of the tunnel with blocks and pellets started at the end of the summer. Tunnel backfilling was completed in early December.
Posiva said that the concreting phase in the construction of the end plug for the deposition tunnel was carried out on 3 May. A total of 130 cubic metres of concrete was poured to form the plug.
“We are talking about the largest continuous casting operation ever in Onkalo,” said R&D Coordinator Johanna Hansen. “Concreting work started at seven in the morning and was completed at seven in the evening on the same day. Once the formwork built for the concreting of the plug is removed, the surface of the plug will be the only visible part remaining from the FISST test.”
Once the FISST test is completed, Posiva will conduct an integrated systems test, which will see final disposal operations tested without actual used nuclear fuel. The test will provide more information concerning the functionality of the various phases of the final disposal process.
The site for Posiva’s repository at Eurajoki near Olkiluoto was selected in 2000. The Finnish parliament approved the decision-in-principle on the repository project the following year. Posiva – jointly owned by Finnish nuclear utilities Fortum and Teollisuuden Voima Oyj – submitted its construction licence application to the Ministry of Employment and the Economy in December 2013. Posiva studied the rock at Olkiluoto and prepared its licence application using results from the Onkalo underground laboratory, which would be expanded to form the basis of the repository. The government granted a construction licence for the project in November 2015. Construction work on the repository started a year later. Posiva is still required to obtain a separate operating licence for the facility. Operation of the repository is expected to begin in 2023.