Onkalo’s Safety Case Rests on Barriers, Not Guarantees

Finland’s Onkalo repository is designed to isolate spent nuclear fuel underground, but no one can guarantee how an engineering system will perform over the timescales involved. That is the tension at the center of the project: the waste already exists and must be managed, while the period over which it remains radioactive far exceeds the record of human construction.

The total grows by about 10,000 tonnes a year, according to Richard Milne, and most of the fuel remains in temporary storage, in cooling pools or steel containers above ground. Nuclear power generates around one-tenth of the world’s electricity, and more reactors are planned.

Onkalo lies 430 metres underground on an island off Finland’s west coast. Run by Posiva, a joint enterprise majority-owned by Finland’s leading nuclear operator TVO, the €1bn facility took decades to plan and build; excavation began in 2004. Milne says waste is expected to arrive within about a year, after which the tunnel will be sealed. Philippe Bordarier, who describes the project as a major responsibility, says its 40-year development is relatively short beside the period the repository must address.

Onkalo’s case for long-term containment rests on both the surrounding geology and a sequence of barriers around the fuel. Tuomas Pere says the rock at Olkiluoto is seismically stable, with no earthquakes, and is 1.9-billion-year-old metamorphic crystalline rock. He describes it as rock that crystallized long ago and has changed little since.

Spent fuel will be sealed in copper canisters and lowered into vertical deposition holes. Bentonite clay will surround each canister; on contact with water, it swells to form a near-impermeable seal. Once the canisters are installed, bentonite backfill will be used to fill the tunnel.

Pere says the test deposition hole is exactly like the holes planned for the disposal tunnels. The animations shown alongside his explanation depict a machine lowering a canister into a hole and another moving through a tunnel to place backfill. A glass display model shows the facility’s tunnel system against a depth scale marked from zero to 300 metres. Together, the demonstration and description make the method concrete: the fuel is placed in individual holes, sealed in clay, and enclosed within a backfilled tunnel in the bedrock.

The barriers do not remove the problem of proving performance over time. Milne points out that some of the oldest surviving human structures are at least 5,000 years old; no engineering system has been built or tested over the timescales relevant to nuclear waste.

Oras Tynkkynen, who chairs the Green Group in Finland’s parliament, does not claim that Onkalo can be guaranteed to work for 100,000 years. He says such a guarantee would be dishonest. His conclusion is more limited: based on the information available, Finland’s chosen solution is safe enough, and is the best option available to deal with waste from its reactors.

That judgment has political backing. Finland’s parliament approved the project in 2001, with only three of 200 MPs voting against. But broad support and a decision to proceed are not the same as certainty about deep time. Onkalo is presented as a response to an existing obligation, not proof that a repository’s behaviour can be guaranteed indefinitely.

Onkalo is not a ready-made template for other nuclear countries. Tynkkynen says countries cannot simply copy and paste Finland’s approach; some may have better solutions. The location matters: Milne describes the project as relying on a seismically stable, oxygen-free underground environment, conditions he says few places can match.

The waste stream matters too. Onkalo is designed for one type of spent fuel from Finland’s modern nuclear fleet. Other countries have accumulated a broader mix, from different reactor designs and sources that include weapons programmes. A repository suited to one country’s fuel and geology may not suit another’s.

Milne says more than 70 reactors are under construction around the world, making effective disposal more pressing. Sweden, France and Canada are also developing permanent disposal sites. Finland’s distinction is that it will be the first to seal nuclear waste underground. The project offers a specific answer shaped by its geology and fuel—not a universal design.