onkalo

Onkalo is the deep geological repository for spent nuclear fuel being developed at Olkiluoto in Eurajoki, western Finland. The name Onkalo means a hollow or cavern in Finnish. The project is designed to isolate high-level radioactive waste by placing spent fuel canisters in tunnels carved into stable granite bedrock beneath the surface, roughly several hundred meters underground.

The repository uses a multi-barrier concept to prevent radionuclide release. Spent fuel is sealed in corrosion-resistant copper canisters, which are surrounded by a bentonite clay buffer and sealed in deposition tunnels within the host rock. The design aims to provide long-term containment for up to 100,000 years, addressing potential scenarios through engineered barriers and the geological environment.

Onkalo is operated and developed by Posiva Oy, a joint venture owned by Teollisuuden Voima (TVO) and Fortum. The facility is intended to accommodate spent fuel generated by Finland’s nuclear power plants, notably those at Olkiluoto and Loviisa, and to transition Finland toward a closed fuel cycle with permanent disposal of waste.

Construction and development have progressed through extensive engineering, safety analyses, and regulatory review. Finland’s nuclear safety authority (STUK) and national authorities have overseen licensing and permitting as part of the process to enable disposal operations. As a project, Onkalo is widely regarded as the world’s first long-term deep geological repository designed for permanent disposal of civilian spent nuclear fuel, representing a key step in managing nuclear waste responsibly over extended timescales.