The world’s first permanent underground repository for spent nuclear fuel is nearing operations in Finland, marking a major step in solving one of nuclear energy’s toughest problems.
Built deep inside 1.9 billion-year-old bedrock, the facility, known as the Onkalo nuclear waste repository, is designed to store highly radioactive waste for up to 100,000 years and is expected to receive final approval within months, according to a report by Associated Press.
Nuclear waste has long posed a challenge for the energy sector. While nuclear power produces low carbon emissions, the spent fuel remains dangerously radioactive for millennia, requiring isolation from humans and the environment.
At Onkalo, that solution lies hundreds of meters underground. Waste will be sealed in copper canisters and buried in tunnels carved into stable bedrock, where multiple layers of protection are designed to prevent leaks.
The repository sits more than 400 meters below the surface in ancient rock formations selected for their stability. Engineers chose the site for its low seismic activity and ability to contain radiation over extremely long periods.
“It’s the isolation from civilization and mankind on the surface that’s important,” geologist Tuomas Pere said while explaining why the remote location was selected, as quoted by AP.
“We can dispose of the waste more safely than by storing it in facilities located on the ground.”
The process involves sealing spent [nuclear fuel] inside corrosion-resistant copper canisters, which are then placed in underground tunnels.
These are packed with bentonite clay, a material that swells when wet, creating an additional barrier against water movement and radiation release.
The facility is being developed by Posiva, which estimates it can store up to 6,500 tons of spent nuclear fuel. Once filled, the tunnels will be sealed permanently, leaving the waste isolated for tens of thousands of years.
Despite its advanced design, the approach is not without concerns. Experts say uncertainties remain about how materials will behave over extremely long timescales.
Future risks still debated
“My view of nuclear waste disposal is that there’s no good option, but it’s important to find the least bad option,” Edwin Lyman of the Union of Concerned Scientists told the news agency.
He added that geologic disposal is likely “the least bad option among a range of, you know, bad options.”
One concern is the long-term durability of copper canisters [ USA is using stainless steel canisters], which could eventually corrode. While engineers expect this to take thousands of years, the exact timeline remains uncertain.
“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 said.
Still, keeping waste underground is widely seen as safer than leaving it above ground, where it could be exposed to environmental risks or security threats.
Beyond engineering, the project raises a unique challenge: how to warn future generations about the danger buried below.
Researchers are exploring long-term communication methods, including durable markers and symbols that could be understood thousands of years from now.
Globally, most nuclear waste is still stored in temporary facilities. Onkalo’s near-completion signals a shift toward permanent solutions, as countries look for safer ways to manage radioactive materials over the long term.
Jaak