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Finland’s ONKALO project is designed to isolate spent nuclear fuel deep underground at Olkiluoto—not to store every kind of radioactive waste. Fuel will cool in interim storage, be sealed in disposal canisters at a surface plant, then placed in the bedrock about 430 metres underground. The plan relies on several engineered barriers and the surrounding rock. In Posiva’s latest cited update, dated 4 August 2026, disposal had not begun: commissioning and testing remained underway, and the Government’s operating-licence decision was still a separate step.
What Finland is doing with spent nuclear fuel
ONKALO is Posiva’s name for the combined facility at Olkiluoto: a surface encapsulation plant and an underground repository. Posiva, established by Finnish nuclear power companies TVO and Fortum to manage their spent fuel, plans to dispose of fuel from the Olkiluoto and Loviisa power plants there. It is not intended for imported foreign spent fuel; Posiva says Finnish law bars importing foreign spent fuel for final disposal. (Posiva FAQ)
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The planned repository capacity is approximately 6,500 tonnes of spent nuclear fuel, according to Posiva’s current FAQ. That estimate depends on current assumptions about plant operation and licensing. The planned disposal depth is approximately 430 metres. (Posiva FAQ)
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How the disposal process is meant to work
- Cool the fuel. After removal from a reactor, spent fuel remains in interim storage at the power plant for decades, allowing its heat and radiation levels to decline.
- Encapsulate it at the surface. The cooled fuel is moved to ONKALO’s encapsulation plant and placed in disposal canisters.
- Move it underground. The canisters are transported into the repository tunnels in the Olkiluoto bedrock.
- Close the disposal locations. Bentonite buffer, tunnel backfill and plugs form parts of the engineered containment system around the canisters and tunnels.
Posiva describes the method as final disposal: once emplacement and closure proceed, the fuel is intended to remain isolated underground rather than be routinely retrieved for reuse. The facility’s design and safety case are described on Posiva’s final-disposal page.
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Why the safety design uses multiple barriers
No single component is meant to carry the entire safety burden. Posiva’s concept combines engineered barriers with the natural barrier provided by the Olkiluoto bedrock. The system’s components are:
- Disposal canister: encloses the spent fuel.
- Bentonite buffer: surrounds the canister in the deposition hole.
- Tunnel backfill and plugs: fill and seal underground openings.
- Bedrock: provides the geological setting around the engineered system.
Posiva’s safety case draws on site information, measurements, analyses and modelling. It considers bedrock and groundwater, the performance of engineered barriers, rare events such as earthquakes, and long-term environmental conditions including glacial periods. Posiva says its analyses consider conditions over a horizon of up to one million years; that is an assessment horizon, not an exact prediction of future events or a guarantee of zero risk. (Posiva FAQ; Posiva, Final disposal)
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The safety case is Posiva’s evidence for its proposal. STUK, Finland’s Radiation and Nuclear Safety Authority, assesses the proposal and safety documentation against regulatory requirements. Posiva says the safety case will be updated as the facility advances and, under the Finnish Nuclear Energy Act, at least every ten years. (Posiva FAQ)
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No disposal start is established by the latest cited update. On 4 August 2026, STUK published its safety assessment and gave the ministry a favourable safety statement on Posiva’s operating-licence application. That was a major regulatory milestone, but it was not the Government’s operating licence. Posiva said commissioning and tests—including work at the underground disposal facilities—still remained before disposal could start. Its stated end-of-2026 operational-readiness goal was a company target, not a confirmed start date. (Posiva, 4 August 2026)
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| Milestone | What it means | Status in the cited sources |
|---|---|---|
| Construction licence | Permission for construction, separate from authorisation to operate. | The Government granted it in November 2015. (OECD Nuclear Energy Agency, Finland 2026) |
| Operating-licence application | Posiva’s application for permission to operate the facility. | Submitted on 30 December 2021. (OECD Nuclear Energy Agency, Finland 2026) |
| STUK safety assessment | Regulatory assessment and safety statement informing the licensing process. | Favourable statement issued on 4 August 2026; not the Government’s operating licence. (Posiva, 4 August 2026) |
| Government operating decision | The separate decision on the operating licence. | Still a later step in Posiva’s 4 August 2026 account. |
| Commissioning and start of disposal | Testing and readiness work precede actual emplacement of spent fuel. | Commissioning and tests remained in progress in Posiva’s 4 August 2026 update; end-of-2026 was its operational-readiness target, not a confirmed disposal date. |
Posiva said the project had undergone more than 40 years of development and assessment by August 2026. It expects disposal operations to continue for several decades and, under current assumptions, well into the 22nd century. The total duration depends on how much spent fuel is generated and on power-plant lifetimes and licences. (Posiva, 4 August 2026; Posiva FAQ)
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.ONKALO is not a repository for every kind of radioactive waste
Finland uses different disposal routes for different waste categories. STUK’s guidance distinguishes deep geological disposal for spent fuel from bedrock repositories for low- and intermediate-level waste and near-surface facilities as an option for very-low-level waste. These are not interchangeable routes for the same material; operating waste already has separate disposal arrangements. (STUK, YVL D.5: Disposal of nuclear waste; Posiva FAQ)
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| Waste category | Disposal route described in Finnish guidance | Purpose |
|---|---|---|
| Spent nuclear fuel | Deep geological disposal at ONKALO, in Olkiluoto bedrock. | Final disposal of fuel from Finland’s TVO and Fortum plants. |
| Low- and intermediate-level waste | Bedrock repositories. | Separate disposal arrangements for these waste categories. |
| Very-low-level waste | Near-surface disposal may be used. | A separate route for a different waste category. |
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