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Finland’s plan is to cool spent nuclear fuel for decades, seal it in disposal canisters at Olkiluoto, then place it deep in the bedrock at the ONKALO facility. Its safety design combines engineered barriers with the surrounding rock. But ONKALO was not yet accepting fuel for final disposal in Posiva’s 4 August 2026 update: Finland’s nuclear regulator had issued a favourable safety assessment, while the Government’s operating-licence decision and commissioning work remained separate steps.
What ONKALO is—and what it is for
ONKALO is Posiva’s name for the combined facility at Olkiluoto: a surface plant for encapsulating spent fuel and an underground repository where the fuel is intended to be placed for final disposal. The planned disposal depth is approximately 430 metres, and the estimated capacity is approximately 6,500 tonnes of spent nuclear fuel, based on current plant-operation and licensing assumptions, according to Posiva’s FAQ.
Posiva was established by Finnish nuclear power companies TVO and Fortum to manage their spent fuel; it is not a power generator. ONKALO is intended for fuel from TVO’s Olkiluoto plant and Fortum’s Loviisa plant, not for foreign spent fuel. Posiva says Finnish law prohibits importing foreign spent fuel for final disposal.
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- Interim cooling: Spent fuel remains in interim storage at the power plants for decades, allowing it to cool before final disposal.
- Encapsulation: The fuel is moved to ONKALO’s surface encapsulation plant and placed in disposal canisters.
- Underground disposal: The canisters are taken into the Olkiluoto bedrock and placed in the repository at the planned depth of approximately 430 metres.
Posiva describes the intended process and facility on its final-disposal page. The fuel’s long interim-cooling period is part of the planned sequence; ONKALO is not simply a storage building for newly removed fuel.
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How the safety concept uses multiple barriers
Posiva’s safety concept combines engineered barriers around the fuel with the Olkiluoto bedrock as a natural barrier. The barriers are intended to work together rather than make safety depend on a single component.
- Disposal canister: Holds the spent fuel for emplacement underground.
- Bentonite buffer: Surrounds the canister as part of the engineered barrier system.
- Tunnel backfill and plugs: Form additional engineered barriers in the repository tunnels.
- Olkiluoto bedrock: Provides the natural geological barrier around the underground repository.
Posiva says its safety case draws on site information, measurements, analyses and modelling. It considers the bedrock and groundwater, the engineered barriers, rare events such as earthquakes, and long-term environmental conditions including glacial periods. Its analyses consider time horizons of up to one million years; that is the horizon addressed in the safety assessment, not a precise forecast of the future or a guarantee that every event can be predicted. See Posiva’s explanation of its safety case.
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The applicant’s safety case and the regulator’s safety assessment are different things. Posiva prepares the evidence for its proposal; Finland’s Radiation and Nuclear Safety Authority, STUK, assesses whether the proposal and its safety documentation meet the requirements. Posiva says Finnish law requires the safety case to be updated at least every 10 years as the facility advances.
Is ONKALO already operating?
Not for final disposal, according to Posiva’s 4 August 2026 update. On that date, STUK had published its safety assessment and given the ministry a favourable safety statement on Posiva’s operating-licence application. That was a significant regulatory milestone, but it was not the Government’s operating licence and did not mean that disposal had begun.
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Posiva said commissioning and testing remained, including work at the underground disposal facilities. It described the end of 2026 as its operational-readiness goal. That was a company target in the cited update, not a confirmed date for the start of disposal. For the status as reported that day, see Posiva’s 4 August 2026 announcement.
The licensing milestones are distinct:
| Milestone | Date or status in the cited sources | What it means |
|---|---|---|
| Construction licence | Granted by the Government in November 2015 | Authorised construction; it was not the operating licence. |
| Operating-licence application | Submitted by Posiva on 30 December 2021 | Started the application process for operating the facility. |
| STUK safety assessment | Favourable safety statement issued on 4 August 2026 | A regulatory assessment provided to the ministry; not the Government’s operating decision. |
| Government operating decision and commissioning | Still pending or in progress in Posiva’s 4 August 2026 update | Separate steps remained before disposal could start. |
The construction-licence and application dates are reported by the OECD Nuclear Energy Agency’s March 2026 account of Finnish nuclear legislation. Because licensing and commissioning status can change, the dates and pending steps above describe the cited August 2026 update, not a claim about later decisions.
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ONKALO is not Finland’s repository for every kind of radioactive waste
Finland uses different disposal routes for different waste categories. STUK’s guidance distinguishes deep-bedrock disposal for spent fuel from repositories for other waste types:
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|---|---|---|
| Spent nuclear fuel | Deep geological disposal in bedrock | ONKALO is designed for this category. |
| Low- and intermediate-level waste | Repositories at intermediate depth in bedrock | These wastes have separate disposal arrangements in Finland. |
| Very-low-level waste | Near-surface disposal may be used | This is a different route from ONKALO’s deep repository. |
The distinctions are set out in STUK’s YVL D.5 guidance on nuclear-waste disposal. ONKALO should not be described as a general-purpose facility for all radioactive waste.
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How long the project is intended to run
Posiva says final-disposal operations are expected to last several decades and, under current assumptions, continue well into the 22nd century. The capacity estimate and operating duration depend on how much spent fuel is generated, as well as power-plant lifetimes and licences. Posiva also describes more than 40 years of project development and assessment in its 4 August 2026 announcement; that figure refers to preparation history, not time spent operating a repository.
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