Finland is on the verge of becoming the first nation to bury spent nuclear fuel rods deep underground for the long term. Erika Benke visits the Onkalo site to find out more. 

'Onkalo' is a Finnish word for a cave or a hollow. It implies something big and deep: you don't know where an onkalo ends or whether it ends at all.

World's first permanent storage site

It's a fitting name for a huge grave made in Finland over the last 20 years. Onkalo, which lies 450m deep inside the bedrock of Olkiluoto island in the southwest of the country, is the world's first permanent storage site for spent nuclear fuel.

The gently winding road to Olkiluoto is lined with pine trees stretching high up to the sky. Nature has come back to life here after five months of winter. The ground is covered by a carpet of small yellow flowers and the air is filled with birdsong. It's almost too beautiful a setting for a major industrial site.

Olkiluoto is home to three nuclear reactors, which stand side by side on the seaside. The third launched only this year, becoming the first new reactor to provide power in western Europe in 15 years. These reactors, along with two others in Loviisa on the south coast, produce 33% of Finland's electricity.

A few minutes' drive away from the Olkiluoto reactors, construction of the world's first Geological Disposal Facility (GDF) for spent nuclear fuel is nearing completion.

'It's our generation of scientists' and engineers' responsibility to undertake the challenge to dispose of the waste, instead of leaving it to future generations.' Lewis Blackburn


Onkalo has cost €1bn to build and is expected to begin operations in about two years. Its arrival has been hailed as a game changer by many, including the International Atomic Energy Agency (IAEA). "Everybody knew of the idea of a geological repository for high-level radioactive nuclear waste, but Finland did it," said Rafael Mariano Grossi, IAEA director general, on a visit to the site in 2020.

Other countries including the UK, US, Sweden, France and Canada are already looking at a similar solution, says Gareth Law, professor of radiochemistry at the University of Helsinki, who is not involved in the Finnish project. "Finland is at least a decade ahead of everybody else."

Onkalo has been designed to store high-level nuclear waste that can remain radioactive for timescales hard for humans to fathom. But it has already sparked debates as to whether something could go wrong. So has Finland really found the answer to nuclear waste – and can anybody guarantee that it will remain safe in the deep future? 

Finland is building the world's first permanent nuclear storage site – right next door to three of its nuclear power stations. Image: Posiva.

Nuclear energy currently provides about 10% of the world's electricity. Being low-carbon, it's seen by many as an important player in the fight against climate change.

But it has also remained controversial in many countries due to its high cost and long build times, as well as concerns over the safety of nuclear reactors and the unresolved issue of what to do with nuclear waste. 

Still, several countries including the US and UK are building new reactors or upgrading existing ones to increase capacity, while others such as India, China and Russia are planning huge expansions.

Debates remain open about the safety of these plants themselves, but a solution also needs to be found for the huge amounts of spent fuel and radioactive waste being accumulated that remains dangerous to the environment and human health for hundreds of thousands of years.

Almost incomprehensible time span

This waste needs to be kept away from people and isolated from the environment for a time span almost incomprehensible to the human mind. This problem has made the safe management of radioactive waste one of the biggest problems with nuclear energy.

The IAEA estimates that there's about 260,000 tonnes of spent nuclear fuel in interim storage across the world as of 2016, the majority at reactor sites. About 70% of the world's spent fuel is in storage ponds, with the rest in concrete-and-steel containers called dry casks. 

Few believe this situation can be maintained indefinitely.

"We've all benefitted from nuclear energy for more than 60 years," says Lewis Blackburn, a lecturer in nuclear materials at the University of Sheffield in the UK. "It's our generation of scientists' and engineers' responsibility to undertake the challenge to dispose of the waste, instead of leaving it to future generations."

He says Onkalo has public support and was created through a democratic process. "It's exceptional and a huge milestone. Finland has set an example to the world of what can be achieved with successful cooperation and transparent communication with the public." 

A test deposition tunnel at Onkalo repository similar to the tunnels where canisters of spent nuclear fuel will be deposited for the long run. Image: Erika Benke.

Scientists have been grappling with how to dispose of high-level nuclear waste for decades now. Spent fuel is one of the most difficult sorts of waste to manage as it is highly hazardous. It produces a radiation level that could provide a lethal dose to a nearby person, says Blackburn.

According to Law, the scientific consensus is that geological disposal for hundreds of thousands years in specifically designed repositories like Onkalo, with a stable rock formation, is "the most feasible approach" for spent nuclear fuel. "We have engineering solutions for digging down and tunnelling down, putting the waste down, and building the barriers that we surround it with." 

The Finnish Radiation and Nuclear Safety Authority has concluded that the characteristics of the Onkalo bedrock are favourable for ensuring the safety of spent nuclear fuel final disposal.

Onkalo, like the rest of Finland, is very stable geologically and the risk of earthquakes is low. "The rock in Onkalo is migmatite-gneiss: a mixture of two different rock types in one rock," explains Antti Joutsen, principal geologist with Posiva. "It's almost two billion years old and it's very hard." 

Construction of deposition tunnels

This is important because the rock is one of the three safety barriers in the disposal concept. It also has to be stable enough to allow the construction of deposition tunnels and holes deep below the ground. 

Onkalo's proximity to an existing nuclear power station was also relevant to the decision to site it there, says Law. "With nuclear investments, one of the problems around the world is nimby[ism], or 'not in my back yard'," he adds.

"People in this part of Finland already had nuclear energy on their doorstep and accepted it. So putting the waste here was not that much of a leap of faith for them." (Read more about how the Finnish nuclear waste project embraced far-sightedness.)

Onkalo will take spent fuel rods without reprocessing them first. Some scientists say reprocessing – the process of separating plutonium and uranium from spent nuclear fuel that produces reusable nuclear material and high-level nuclear waste – would make them safer by reducing the overall volume of waste that is left.

"You can [also] reclaim uranium and plutonium which can be used to make fresh fuel," explains Marja-Siitaru Kauppi, a lecturer at department of chemistry at the University of Helsinki who has done research for Posiva. Around a third of the world's nuclear waste has been reprocessed, which can then be vitrified (turned into glass.)

But others say reprocessing increases the risk of nuclear terrorism. In the long term, vitrified waste could also dissolve in contact with groundwater. Posiva, the company set up to build and run Onkalo, itself says reprocessing is technically demanding and expensive. It also still produces some amount of high-level waste, points out Johanna Hansen, research and development coordinator at Posiva. "So a disposal facility would be needed in any case." 

An elevator shaft, currently behind a door marked with two red Xs, will take canisters containing nuclear fuel rods to a landing area 437m (1,430ft) deep. Image: Erika Benke.

Finland decided to build a facility at Onkalo more than two decades ago when the country's two nuclear producers agreed to dispose of their spent nuclear fuel in one location. The Finnish government approved plans to construct the facility in December 2000, saying that "of the studied disposal options, deep disposal in the bedrock offers the best and most realistic possibilities to isolate high-level nuclear waste from the biosphere and the human habitat".

The Onkalo repository is based on the KBS-3 concept developed by the Swedish Nuclear Fuel and Waste Management Company, in cooperation with Posiva. The idea is to create three barriers around nuclear waste: first, put it in copper canisters, then wrap the canisters in bentonite – a water-absorbing clay – and finally, bury them in tunnels deep in the bedrock.

Encapsulation plant

"The process starts in the encapsulation plant which is on the surface," says Hansen who has worked on the project for more than 20 years. "Here we'll put the spent fuel rods into a disposal canister that has two parts: there's an inner shell made of cast iron and an outer shell made of 5cm- thick copper."

A lid is then welded onto the canister, which is transported to a storage area in the disposal facility, she says. Once the site is ready, these will be transferred to a horizontal deposition tunnel 450m below the ground, where final disposal will happen.

'Onkalo will take all the high-level nuclear waste produced by Finland's five nuclear power plants in their entire life cycles.' Antti Joutsen


An elevator will initially lower the canisters down to a landing area some 437m deep. When the author visited in May 2023, this elevator shaft is behind a door marked with two large red Xs in a well-lit spacious service area. Safety is key here: it's still very much an active building site. From here, a robotic transfer vehicle will take the canisters to a deposition hole, says Hansen.

Five dead-end deposition tunnels have been built so far, all 350m long. Another 85 will be constructed over the years as the facility fills up. Each deposition tunnel has about 40 vertical circular holes in its floor, each 8m deep and 2m wide. In total, there's room for about 3,000 canisters, one for each vertical hole.

"They'll hold a total of 5,500 tonnes of waste," says Joutsen. "So Onkalo will take all the high-level nuclear waste produced by Finland's five nuclear power plants in their entire life cycles."

Filled with bentonite

After each canister has been buried, its hole will be filled with bentonite, says Hansen. "When we've put a canister in each hole, we'll backfill the tunnel with more bentonite and seal it with concrete."

Final disposal of spent nuclear fuel will start sometime in the next few years. Posiva estimates that it will take 100-120 years before the repository is full. At that point the entire facility will be sealed off, allowing the canisters to lay, hopefully undisturbed, for at least 100,000 years, with their lethal radioactive content isolated from the outside world.

Although the risk of earthquakes is low here, seismic activity has been taken into account, says Joutsen. "There will be several ice ages in the next million years and they will pose a risk of earthquakes. There will be a 2-3km thick ice sheet on top of Onkalo that will push the Earth's crust downwards by hundreds of metres. Onkalo's been built to withstand that."

When the ice age ends, the crust will start rising again – which is when earthquakes with the power to break up the canisters could happen, he adds. "To prevent that, we're putting them in the best possible locations: the disposal holes are in unfractured sections of the bedrock." 

GDF is not the only proposed idea for the safe long-term management of radioactive waste. Norway, for example, is considering deep borehole disposal, where waste would be lowered in canisters to a much deeper 3,500m in thin boreholes without construction tunnels.

Other scarcely credible options have also been suggested, such as shooting nuclear waste into space in a rocket or putting it in subduction zones – regions in which the Earth's tectonic plate goes beneath another plate.

Still, it makes sense to continue exploring alternatives – not least because some scientists are concerned about Finland's approach.

'There might be wiser people after us who have new ideas about what to do with nuclear waste.' Marja-Siitaru Kauppi


Posiva says its theoretical and laboratory studies show that Onkalo will provide long-term protection from nuclear waste. But there are critics concerned about the safety of the KBS-3 disposal concept used at Onkalo.

Three protective barriers

KBS-3V is based on a multi-barrier principle: spent fuel is contained within three protective barriers: copper canister, bentonite clay and bedrock. The most important barrier is the copper canister, and Onkalo makes the assumption that copper does not corrode under anaerobic conditions – that is, in the absence of oxygen, such as when buried in the bedrock with bentonite clay around it.

But researchers at the Swedish Royal Institute of Technology (KTH) have raised doubts about the corrosion resistance of unalloyed copper canisters: they say their experiments suggest that copper could corrode even under anaerobic conditions.

"You can fill up the tunnels with bentonite clay and rock but nature will do its course," says Peter Szakálos, a chemist at the KTH Royal Institute of Technology in Stockholm. "Water will enter the tunnels and it will corrode copper much faster than the theoretical model predicts."

Corrosion cracks could break up a lot of canisters in Onkalo within a hundred years, argues Szakálos.

Posiva has rejected these claims after conducting its own research into corrosion. A paper by Posiva scientists and independent experts concluded that "stress corrosion of the copper shell is considered unlikely under repository conditions". It also argues that corrosion observed in experimental conditions in a laboratory is not relevant to conditions 450m below the ground in Onkalo.

'We're trying to construct something that should work longer than the homo sapiens has been alive. The time span is mind-boggling.' Antti Joutsen


This conflicting evidence has delayed plans to build a repository in Sweden based on the same KBS-3 concept as Onkalo. But in January 2022, the Swedish government gave the green light to building a GDF in Forsmark, about 130km north of Stockholm – a development viewed by Hansen of Posiva as one that should put the copper controversy to bed.

"We have a robust, multi-barrier concept and we're prepared for scenarios that might happen in the next million years," says Hansen. "For example, we looked at what happens if a canister starts leaking early on, what if the clay is wrongly installed, what if there's more water in the tunnels and the water chemistry is different from what we expected. The results of our risk assessments show that we're on the safe side."

For now, at least, Finland and Sweden are going ahead with the plans, though Szakalos says there will be more research on copper corrosion. 

Both Onkalo and the proposed Swedish facility aim to keep radioactive waste safe for at least 100,000 years. "We're trying to construct something that should work longer than the homo sapiens has been alive," says Joutsen. "The time span is mind boggling."

Tunnels sealed off

When the facility is full, the tunnels will be sealed off, the buildings on the surface demolished and the land restored. "There will be animals, trees and plants, and houses," says Joutsen. He says he would happily buy a plot of land in Onkalo if he were alive in 120 years. 

What comes later is more uncertain. It is entirely possible that all knowledge about Onkalo will be lost in a few thousand years.

Social scientists have been wrestling with the challenge of how to communicate with humans, or anybody else that might come after us in deep future, to warn them about the dangers we leave behind. (Read more about how to build a nuclear warning for 10,000 years' time.)

One discussion about marking Onkalo centred on trying to find a universally understood sign of danger, should language disappear in the future. The current thinking from Posiva, decision makers and researchers, however, is not to mark Onkalo at all.

"Future generations don't need to know about this," argues Joutsen. "It's so deep and so isolated. There will be a lot of copper and plutonium down here and a sign may encourage people to start digging to retrieve it. A bit like the Pyramids in Egypt that were looted by tomb robbers."

No final decision has been made though. There's no rush as the facility will remain open for 100 to 120 years.

Kauppi says it is important to put the debate about marking the site in a perspective that people can relate to. She often reminds her students at the University of Helsinki that radioactivity was discovered only about 120 years ago.

"We're now talking about things that could be happening in the next hundreds of thousands of years. We don't even know what could happen in the next hundred years."

Kauppi says she is "very confident" Finland is doing a good job with Onkalo, but believes the door should still be kept open to other solutions.

"There might be wiser people after us who have new ideas about what to do with nuclear waste. They may invent something better. We just have to do our best now, with our current knowledge based on honest research." 

Author: This article was written by Erika Benke and first appeared on BBC Future Planet.