In the bowels of Finnish rock, heavy construction machines circulate through the deep tunnels of what will soon become a high-risk cemetery.
"Onkalo will be the first spent nuclear fuel storage facility in the world," says geologist Johanna Hansen, research and development coordinator at Onkalo.
On the island of Olkilloto in the Gulf of Bothnia, off the west coast of Finland (where Europe's largest nuclear reactor is already located), the project to solve the thorny problem of nuclear waste is nearing completion.
At a depth of more than 400 meters, the Onkalo reservoir is designed to hold a total of 6,500 tons of uranium, a capacity for all the fuel used by the five Finnish nuclear reactors throughout their lifetime.
The catacombs are built more than 400 meters deep (AFP).
Nuclear energy, which emits little carbon, represents a clean energy source, useful to contain global warming.
Some 400,000 tonnes of spent fuel have been extracted from the reactors so far, according to the World Nuclear Association, most of it stored in temporary storage.
The long-term storage of this waste is a problem that holds back nuclear projects in the world.
Copper capsules buried in the rock
According to the solution adopted by Posiva, the company that manages Onkalo, together with the Swedish authorities, the used uranium will be contained in thick copper capsules and buried in the rock, and then the tunnel will be sealed with an immense cuneiform plug in reinforced steel.
Work on Onkalo ("hollow" in Finnish) began in 2004 and the last tests will take place in early 2024.
The Onkalo deposit can hold a total of 6,500 tons of uranium (AFP).
When it is ready, it will move into the operational phase in the mid-2020s, Hansen says.
The time required (about 250,000 years for the most toxic waste) for radiation to be reduced to safe levels is one of the most important challenges in this type of device.
The appearance of the island of Olkiluoto could change before the radiation levels of uranium in its natural state are achieved.
A ticking time bomb?
In the coming millennia, the island could become part of the continent, or also become submerged due to rising sea levels due to climate change, according to Posiva.
But the company insists the capsules are designed to withstand noticeable changes.
The cassilas with the radioactive material can withstand about 250,000 years (AFP).
Some experts, however, do not think so.
Researchers at the Swedish Polytechnic School (KTH) question the safety of the device, citing the risks of copper corrosion, which could lead to leaks.
"It's not a solution, it's a risk reduction," says Jan Haverkamp, a nuclear specialist at the environmental NGO Greenpeace. In his opinion, Posiva does not pay "real attention" to the issue of copper oxidation.
The uranium will be contained in thick copper capsules and buried in the rock (AFP).
For the Finnish Nuclear Safety Authority, Onkalo is in line with the requirements.
Allison Macfarlane, a professor at the University of British Columbia in Canada, believes that there is no project "100% safe" but that Onkalo is "surely the solution that has been the subject of more research."
The alternative, he says, would be to "leave the debris on the surface indefinitely," a much riskier hypothesis.
One of the deposits is 8 meters deep and 1.75 meters in diameter (AFP).
In strictly selected locations, waste "will stay safe for thousands of years and tens of thousands of years," he says. "I don't think it makes much sense to project beyond," he adds.
Other possible burial sites have already been identified in Sweden, France and Switzerland. In Canada there is also another case.
"It's the world's unanimous solution to the nuclear waste problem," Macfarlane insists.
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