After decades of construction, Onkalo — meaning “cave” in Finnish — is poised to become the first facility in the world for the permanent disposal of spent nuclear fuel, providing a secure resting place for tons of radioactive waste.
Located on the secluded island of Olkiluoto, along Finland’s west coast, the facility sits amid dense forests. The nearest town, Eurajoki, lies roughly 15 kilometers inland and is home to about 9,000 residents, many employed by the nuclear power plant or storage facility.
The €1 billion ($1.2 billion) project is expected to receive operational approval within months, marking a major milestone in nuclear waste management.
With the press of a button, an elevator plunges hundreds of meters into the darkness of Onkalo.
“We are now at about minus 430 meters (1,411 feet),” said geologist Tuomas Pere, navigating a vehicle through a complex network of man-made tunnels. “We are driving through 1.9-billion-year-old bedrock.”
The Associated Press toured the underground complex, which will soon be off-limits to humans.
Pere explained that the repository’s location — near three of Finland’s five nuclear reactors — was chosen for its stable migmatite-gneiss bedrock with a very low earthquake risk.
“It’s the isolation from civilization and mankind on the surface that’s important,” he said, standing in a dimly lit disposal tunnel scheduled to be permanently sealed. “We can dispose of the waste more safely than by storing it in facilities located on the ground.”
At a nearby encapsulation plant, unmanned machinery will seal radioactive rods in copper canisters before burying them over 400 meters underground. The canisters will be surrounded with bentonite clay, which absorbs water and provides an additional barrier.
Posiva, responsible for managing Finland’s spent nuclear fuel, says Onkalo can accommodate 6,500 tons of material. The canisters are engineered to remain sealed until the radioactivity decreases to safe environmental levels.
“The solution that we have, it’s the missing point for sustainable use of nuclear energy,” said Pasi Tuohimaa. Finnish nuclear power companies have been funding the project for decades, ensuring long-term waste management.
Posiva estimates it will take hundreds of thousands of years for the radioactivity to decline to background levels.
According to a 2022 report by the International Atomic Energy Agency, nearly 400,000 tons of spent nuclear fuel have been generated worldwide since the 1950s. Two-thirds remain in temporary storage, while about one-third is recycled.
Currently, global spent fuel is stored in pools at reactors or in above-ground dry cask facilities. No operational permanent underground repository exists for commercial nuclear waste. Sweden began construction of a facility in Forsmark last year, but it is not expected to open until the late 2030s. France’s Cigéo project has yet to begin construction amid opposition.
Onkalo is projected to operate until the 2120s, when it will be permanently sealed.
Still, experts warn that geological disposal carries uncertainties. Edwin Lyman, director at the Union of Concerned Scientists, said: “There’s no good option, but geologic disposal is the least bad among a range of bad options.” He noted that copper canisters will eventually corrode, though the process is expected to be slow enough that most radioactivity will decay safely.
Lyman emphasized that deep underground storage is preferable to keeping spent fuel above ground, where it is more vulnerable to sabotage. He noted that highly radioactive material is difficult to transport and reprocess immediately after discharge, but it becomes less risky over time.
Reprocessing separates uranium and plutonium for new fuel, but also poses proliferation risks, as recovered plutonium could theoretically be diverted to make weapons.
The long-term challenges of nuclear waste have inspired the development of nuclear semiotics, a field dedicated to creating warnings comprehensible thousands of years into the future.
Martin Kunze, leading a Nuclear Energy Agency group on long-term information preservation, devised a “nuclear message” printed on solid ceramic plates to alert future generations to buried waste. These plates are durable, inexpensive, and intended to be widely distributed around the repository.
Finnish officials say Onkalo embodies the nation’s long-term approach to nuclear energy. A 1994 law requires that radioactive waste generated in Finland must be handled, stored, and permanently disposed of domestically.
“Back then… some of the waste was still exported, but we wanted to take care of it ourselves,” said Sari Multala. “We also stick to the decisions, unlike many other countries.”
Multala did not dismiss the possibility of accepting limited foreign nuclear waste in the future, provided international regulations allow it.
