Finland’s Posiva Oyj, the operator responsible for the final disposal of used nuclear fuel, said on 7 May that it had started the excavation of the final disposal tunnels at Olkiluoto, in the Eurajoki municipality. This is seen as a significant milestone for Posiva, coming after years of development activities on research and methodology for rock construction. The Radiation and Nuclear Safety Authority of Finland (STUK) verified before the excavation started that the pre-conditions specified for starting the work had been fulfilled.
“This is the culmination of years of research and development of rock construction that have produced procedures for construction of a nuclear facility suitable for the Finnish bedrock,” said Posiva's Construction Manager Juha Riihimäki. “The development of the methodology started with the construction of the Onkalo® facility in 2004.”
The first five tunnels to be excavated during the next 18 months mark the beginning of an extensive building effort. It is estimated that one hundred deposition tunnels will be excavated during the 100-year operational period of the final disposal facility, with a total length of about 35 kilometres. The maximum length of one tunnel is 350 metres. The tunnels are about 4.5 metres high and about 3.5 metres wide.
Riihimäki explained that about 30 canisters will be placed in one tunnel. Their number depends on how many deposition holes there are in the tunnel and that is determined by the volume of suitable rock based on the rock fractures. The 30 canisters placed in one tunnel can accommodate about 65 tons of used nuclear fuel.
The excavation of the first five tunnels is part of Posiva's approximately €500 million ($608m) EKA project that covers all the final disposal facilities needed, including both their construction and the equipment, as well as the start of the final disposal operation in the first deposition tunnel. The EKA project means that Posiva is the first operator in the world to start geological final disposal of used nuclear fuel. A similar repository is planned at Forsmark in Sweden.
Encapsulation of used fuel and the emplacement of the canisters in the deposition holes will start once the Finnish Government grants the operating licence for the final disposal facility. “The current estimate is that the final disposal operation will start in the mid-2020s, said Posiva's Programme Manager Kimmo Kemppainen. “The excavation and reinforcement of the tunnels, which have now been started according to tight regulatory requirements, signify for Posiva a transfer to the execution of the most complex and exacting rock structure system in Onkalo, he added. Posiva said dozens of subcontractors and 300 persons are currently working in the Onkalo area.
Posiva said in February that work to excavate a final disposal tunnel had started at the Onkalo underground characterisation facility. The tunnel is for the 'joint functional test' (YTK), which involves small-scale final disposal under actual conditions in preparation for commissioning 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. 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.
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.