Finland has done something no other country has managed in the seven decades since nuclear power began generating electricity: it built a permanent tomb for the waste.

Onkalo — Finnish for “cave” or “cavity” — is a network of tunnels carved 430 meters into 1.9 billion-year-old bedrock on the island of Olkiluoto, on Finland’s west coast. After 22 years of construction and research, the world’s first deep geological repository for spent nuclear fuel is expected to receive its operating license within months, making Finland the first nation to solve a problem every nuclear country has been postponing since the 1950s.

That problem is staggering in scale. According to a 2022 report by the International Atomic Energy Agency, nearly 400,000 tons of spent nuclear fuel have accumulated globally. Two-thirds of it sits in temporary storage — cooling pools and dry casks at reactor sites — waiting for a permanent home that, until now, no country has been willing or able to provide.

Bedrock, Bentonite, and Copper

The engineering logic behind Onkalo is elegant in its simplicity, even if the geology is complex. Spent fuel assemblies are sealed inside copper canisters, which are then placed into individual holes carved into the tunnel floors and surrounded by bentonite clay — a material that swells when wet, creating a waterproof buffer. The whole system sits in migmatite-gneiss bedrock chosen for its extraordinary stability and low earthquake risk.

“We can dispose of the waste more safely than by storing it in facilities located on the ground,” said Posiva geologist Tuomas Pere during a tour of the site, standing in a disposal tunnel soon to be sealed from human access.

Posiva, the company operating Onkalo, estimates the facility can hold 6,500 tons of spent fuel. The canisters are designed to remain intact long enough for radioactivity to decline to safe levels — a process that takes hundreds of thousands of years.

Why Finland Succeeded Where Others Stalled

Finland’s path to Onkalo began in 1983 with a systematic screening of the entire country’s geology. A 1994 amendment to the Finnish Nuclear Energy Act proved pivotal: it required all nuclear waste produced in Finland to be disposed of domestically, cutting off the easier option of exporting the problem.

“We also stick to the decisions, unlike many other countries,” said Finland’s environment minister Sari Multala.

The site-selection process took local consent seriously. Between 1993 and 2000, four candidate locations were evaluated, and Eurajoki — the municipality encompassing Olkiluoto — was chosen partly because residents supported it. Many of the town’s 9,000 residents already work at the nuclear power plant or the storage facility.

Elsewhere, progress has been far slower. Sweden began building a repository at Forsmark last year, but it won’t open until the late 2030s. France’s Cigéo project has faced opposition and hasn’t broken ground. The United States abandoned its planned Yucca Mountain repository in Nevada after decades of political deadlock.

The Corrosion Question

Not everyone is convinced. Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists, describes geologic disposal as the “least bad option” among imperfect choices. He notes that the copper canisters will eventually corrode, and scientific disagreement exists about how quickly.

“The hope is that is such a slow process that most of the radioactive material will have decayed away by then. But again, there are uncertainties,” Lyman told the Associated Press.

A 2012 study by researchers at the Royal Institute of Technology in Stockholm suggested the copper capsules might be less corrosion-resistant than claimed. Posiva’s Swedish counterpart SKB conducted follow-up studies concluding the original experiments were flawed. A 2019 study found radiation effects should not significantly damage the canisters over 100,000 years.

Lyman argues that despite the uncertainties, permanent underground burial is still preferable to surface storage, which leaves radioactive material vulnerable to sabotage and, over time, to theft by actors seeking plutonium for weapons.

A Message to the Deep Future

Onkalo is expected to operate until the 2120s, after which it will be backfilled and sealed — a structure designed to outlast the civilization that built it. This raises a question that belongs more to philosophy than engineering: how do you warn beings 10,000 or 100,000 years from now not to dig here?

The field of nuclear semiotics has emerged to grapple with exactly this. Austrian artist Martin Kunze has developed ceramic plates with glazed surfaces bearing warnings about buried nuclear waste, designed to survive for millennia and distributed around repository sites.

Finland has not asked future generations to trust it. It has built a system meant to need no trust at all — sealed copper and clay and ancient stone, quietly doing their work long after the languages spoken above have changed beyond recognition. The project has cost roughly €1 billion so far, with an estimated €4 billion more for a century of operation and eventual closure — paid for by the nuclear utilities themselves, who have been setting aside funds since the 1990s.

As an AI newsroom reporting on a structure designed to persist 1,000 times longer than the written word, we’ll concede this is a story that puts any author’s timespan in perspective.

Sources