Finnish waste management company Posiva, jointly owned by Finnish nuclear utilities Fortum and Teollisuuden Voima Oyj, said on 3 May that the full-scale in situ system test (FISST) at the Onkalo underground characterisation facility near Olkiluoto had progressed to the concreting of the demonstration tunnel plug.  “For the time being, it is the biggest one in Onkalo. Concreting began in the morning at  seven and ended the same day at seven o'clock”, said Research and Development Coordinator Johanna Hansen.

The repository site near Olkiluoto was selected in 2000. The Finnish parliament approved the decision-in-principle for the project in 2001. Posiva submitted its construction licence application to the Ministry of Employment and the Economy in 2013. Posiva had prepared its licence application using results from the Onkalo underground laboratory, which is now being expanded to form the basis of the repository after the government granted a construction licence in 2015. Construction work began in 2016.

The FISST is intended to test  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 involved placing two test canisters, containing heating elements to simulate the residual heat of used fuel, within a 50-metre tunnel some 420m underground. Conditions within the tunnel are being monitored using about 500 sensors and this will continue for several years. Data from the sensors has been accumulated since August 2018, and nothing unexpected has emerged, said programme manager Pasi Rantamäki.

Posiva said that the concreting phase in the construction of the end plug for the deposition tunnel was carried out on 3 May 2019, and involved pouring some 130 cubic metres of concrete to form the plug. Operation of the repository is expected to begin in 2023 and Posiva still needs to obtain a separate operating licence for the facility.

Photo: Concreting of the plug for the full-scale in situ system test began in May (Credit: Posiva)