There’s a timeless quality to the tunnels at Onkalo. And that’s the point. Carved into the rock more than 400 metres below ground, these passageways are part of a facility built for the long term. The extreme long term.
In fact, facilities such as Onkalo “should exist intact for at least 100,000, if not a million years”, says Allison Macfarlane at the University of British Columbia in Canada, who previously served as chair of the Nuclear Regulatory Commission in the US. “Those are geologic timescales.”
Onkalo is Finland’s answer to one of the more tricky questions posed by nuclear power: what do you do with the waste? We are going to have to get much better at answering it, given nuclear power is expanding again, driven partly by the hunt for low-carbon energy.
There’s broad consensus that burying the waste is the best option. But there is a catch. Radioactive elements can remain dangerous on timescales that make normal engineering look absurdly short-term. Egypt’s Great Pyramid of Giza is around 4500 years old. Some nuclear repositories will have to contain their contents for more than 20 times longer. Before the waste becomes harmless, civilisations may rise and fall, and ice sheets may come and go.
An earlier attempt to store waste on geological timescales failed spectacularly after just a few decades. The engineers at Onkalo, and at a handful of similar sites now being planned or built around the world, think they can do much better. So, as Onkalo prepares to accept its first waste shipments in the coming months, how exactly have the engineers future-proofed their facility?
The awkward thing about nuclear waste is that some of the most dangerous substances refuse to go away. Much of the material produced by the nuclear industry contains relatively low levels of radioactivity and can be handled with little drama.
But spent fuel and the materials used to build and shield the main nuclear reactor are another matter. Some of the radioactive elements they contain hang around for astonishing lengths of time. Iodine-129, for instance, has a half-life of almost 16 million years.
So, what do you do with it? Over the decades, researchers have entertained some spectacular ideas. One of the most eye-catching: blasting it into space.
“I have a one-word response to that, which is ‘Challenger’,” says Macfarlane. “We often send rockets up into space, and they blow up on the way. And if it’s carrying nuclear waste, that means that you get to breathe it and then greatly increase your risk of cancer.”
Given that possibility, in the end, the favoured solution is the same one we have been employing for decades: dig a very deep hole, put the waste inside and cover it up.
But even that’s easier said than done.
In 1967, authorities in what was then West Germany began storing radioactive waste in Asse II, a former salt mine. Over the following decades, more than 125,000 drums, mostly containing low- and intermediate-level waste, were stacked in chambers hundreds of metres underground. In the minds of the engineers, rock salt seemed like an ideal host. It is exceptionally impermeable to water and slowly deforms under pressure, meaning that if any cracks began to open up through which groundwater could flow, the rock could self-seal to keep the mine and its contents secure. But reality is rarely so simple.
By 1988, the operators had discovered that salty water was, in fact, flowing into the mine. The problem, as outlined in a 2011 report by the International Panel on Fissile Materials, was that the miners had excavated too close to the edge of the salt deposits and not backfilled enough of the caverns after they had finished with them. This weakened the underground structure and raised the risk of collapses, fracturing the salt faster than it could self-seal, and allowing water to seep in. The report suggested that the possibility of such collapses was flagged in 1979, but the warning was ignored.
The public became aware of the problem only in 2008. German authorities now have the unenviable task of retrieving the buried material: some 47,000 cubic metres of radioactive waste.
Asse II is about as stark a lesson in the difficulties of deep-time engineering as you could ask for. Getting radioactive waste underground is easy, but the devil is in securing it for time periods far beyond any normal engineering horizon. A barrier that looks reassuringly solid for a century may corrode, crack or shift over 1000 years. Stretch that timeline to 10,000 or 100,000 years, and engineering confidence drops.
One approach to this lack of confidence is to assume that extreme long-term storage might not, in fact, be required. Radioactive waste isn’t necessarily useless. Spent nuclear fuel still contains potentially valuable material. For instance, approximately 97 per cent of the material in used fuel can be recycled and mixed into fresh fuel, and future technologies may be able to extract even more use from it. Even if they can’t, people centuries from now may devise a better way to deal with the stuff.
Engineers in France are keeping that idea in mind. At an underground repository they plan to build in north-eastern France, known as Cigéo, a funicular railway will be on site so that nuclear waste can, in principle, be transported back to the surface in future.
Cigéo, which is scheduled to be operational by the 2050s, will be carved into sedimentary mudrock, an impermeable material that limits the flow of groundwater and should keep the buried waste dry. But retaining the ability to remove nuclear waste would also act as an insurance policy, just in case problems similar to those at the Asse II repository emerge.
“People [in France] want to be assured that if something goes wrong with the site that they can remove all the waste,” says Macfarlane.
There is another concern about truly long-term storage: people living millennia from now may have no knowledge that the nuclear repositories exist, and they might accidentally drill into one of them, causing an environmental disaster.
People have come up with some wonderfully odd ways to avoid this outcome. Some researchers, aware that languages change or even fall out of use over long periods of time, think it is important to signal danger without written words. At the Waste Isolation Pilot Plant in New Mexico – the first and only operational deep geological nuclear repository in the US, which opened in 1999 – researchers considered building vast earthworks and ominous monuments, including a landscape of giant thorns intended to warn future generations not to dig there. But that forbidding landscape was never built. One obvious snag is that mysterious ruins have a habit of attracting curious humans on the hunt for buried treasure.
Macfarlane thinks the effort is better spent elsewhere. “Just give up. It’s a lot of hubris to think that, ‘Oh, I can design a sign that people in the future will understand’,” she says.
The French National Agency for Radioactive Waste Management, which is overseeing the Cigéo project, has something less theatrical in mind. Its engineers are testing sapphire discs that can be etched with up to 40,000 miniaturised pages of text and images. These discs, each 20 centimetres across, would be tough enough to survive heat, scratching and exposure to chemicals in the environment. If stored somewhere near the Cigéo site, such discs might alert future generations to the repository’s existence.
In Sweden, meanwhile, researchers working for the country’s nuclear waste management company have drafted a 42-page digital document detailing a nuclear repository being built deep below ground at Forsmark, a site about 130 kilometres north of Stockholm. The plan is to update, translate and pass the document down through the generations.
There might be alternative ways to keep the knowledge alive. Sweden’s nuclear regulator has also suggested that weaving knowledge of repositories into education and culture may prove more durable than producing physical documents. There are potential precedents for this: a decade ago, anthropologists reported evidence that information about sea-level rise that occurred at least 7000 years ago has been preserved in stories told by Indigenous communities around Australia’s coast. The researchers speculate that the stories survived because the communities employed a complex system of crosschecks, in which younger and older generations were both aware of the stories and corrected one another if any errors crept in.
But in Finland, researchers have taken a different approach. Decades ago, several possible sites for Finland’s nuclear repository were scrutinised. One thing the researchers were looking for is a subsurface region so devoid of useful resources, such as oil, that no one is ever likely to consider digging there. The idea is that engineers don’t have to worry about alerting future generations to a nuclear repository deep below ground in such a resource-free location. Instead, the repository can be sealed up and then forgotten.
On the island of Olkiluoto in south-west Finland, they found what they were looking for. And since 2004, this is where they have been working on the Onkalo repository.
“Even now, when arriving here, you have pretty much a modern industrial site,” says Petteri Vuorio, the facility’s development director. But as you travel deeper and deeper, the site begins to feel different. “The surroundings become quieter and the rock becomes increasingly present as you go underground,” he says.
The bedrock at Onkalo is just about the best you could hope for in a nuclear store, the Finnish researchers think. It is composed primarily of gneiss, an incredibly dense and impermeable metamorphic rock. To add to the engineering security, the rock was carefully mapped out to avoid any areas with major fractures or tiny structural weaknesses that might one day allow groundwater to seep into human-made caves. It is in these fracture-free areas that Onkalo’s kilometres of tunnels now lie.
But that rock isn’t enough.
A system of multiple fail-safes known as KBS-3, developed by the Swedish nuclear waste company SKB, is being deployed, too. Spent fuel is first loaded into a cast-iron container for strength. Each container is then sealed inside a copper vessel with walls 5 centimetres thick. Studies have shown that elemental copper remains virtually unchanged by radioactive and high-pressure environments, making it a good candidate for such a protective shield.
Next, the finished canister is lowered into a hole drilled into the bedrock floor of one of Onkalo’s disposal tunnels. Bentonite, a type of clay, is then packed around the canister. This clay swells when wet to create a dense and nearly watertight barrier, so if any water ever does find its way into the tunnels, the clay should ensure the nuclear waste remains dry. The bentonite also cushions the waste against any movement in the rock.
This combination of protective measures should, the engineers argue, provide the closest thing to a fail-safe system that can survive for millennia.
“The engineered barrier should do the job itself, but if, for any reason, it doesn’t, then you’ve got really good geology,” says Clare Bond, a geologist at the University of Aberdeen, UK, who has no operational connection with the project.
In October 2026, the Finnish government granted a formal operating licence to run the Onkalo facility. In the months ahead, the first shipments of nuclear waste will arrive. Once Onkalo is completely full, all tunnels and shafts will be packed with more bentonite-rich material and sealed off. The facility will, quite literally, disappear back into the rock.
Years will pass, but the Onkalo site should remain sealed. Just 20,000 years ago, the site lay beneath a thick ice sheet, and over the nuclear repository’s vast lifetime, that may happen again – perhaps several times. “Weight from the ice disappears and the crust rebounds. And that can cause fractures,” says Bond. But she adds that significant research has gone into modelling the safety of nuclear repository sites like Onkalo under such climate shifts.
“There is still a lot to do, but we are happy so far,” says Vuorio. “Although there are still matters to solve, we are also quite close to the finish line.”
Onkalo may be the first bury-and-forget repository built with the deep future in mind, but it won’t be the last. There are plans for something similar in Canada, although construction is still some years away. Work has begun on the Forsmark repository in Sweden, and it will also employ the same multilayered protection system of copper, bentonite and bedrock. If all goes to plan, it should be operational in the 2030s.
The UK is further behind, still at the stage of selecting a site to deposit its nuclear waste – which includes the world’s largest stockpile of reactor-grade plutonium.
“The UK’s waste is varied and complex and is significantly different to the waste being disposed of in Finland, for example,” says Neil Hyatt, the chief scientific adviser to Nuclear Waste Services in the UK.
The country is currently evaluating two sites for a repository, according to Bond, both targeting impermeable mudrock in north-western England near the Sellafield nuclear site. Whichever is chosen, UK government documents suggest the plan is to build a permanent repository with a multilayered containment system broadly similar to the one that will be used at Onkalo.
At all these storage sites, engineers are being asked to think in terms of unfathomable timescales – an extraordinary challenge. But the truth is that only observers living in the distant future will know if these deep-time efforts prove to be successful. “Even after probably hundreds of visits underground, I feel humbled,” says Vuorio. “What we are doing here is something which has to be guaranteed to be like it is for thousands of years. This is something that is even hard to describe.”