Think about the longest-lasting entity established by humans. The Catholic Church, select dynasties, and the rare ancient university. Practically none endure for tens of thousands of years.
Now consider a challenge that necessitates just such a lengthy timeframe: spent nuclear fuel poses hazards for extremely extended durations, and its radioactive components must be kept away from the public and the ecosystem.
Finland is strategizing over a span of about 100,000 years, employing a combination of meticulous engineering and ancient rock predating complex animal life.
The location is designated as Onkalo, situated close to the Olkiluoto nuclear power station in Eurajoki, southwest Finland. It aims to securely enclose spent nuclear fuel for approximately 100,000 years and is anticipated to be the globe’s inaugural functioning deep geological repository for spent nuclear fuel.
It has yet to commence receiving radioactive waste. As of June 30, 2026, Finland’s Radiation and Nuclear Safety Authority, STUK, was still completing its safety evaluation of Posiva’s application for an operating license. The Finnish government cannot approve the license without STUK’s endorsement.
What captivates me here is less about the efficacy of the concept and more about the peculiarity of the gamble: that commonplace copper, expanding clay, and ancient stone can sustain protection for humanity long after the civilization that established the repository has vanished.
## What Onkalo truly is
Onkalo consists of a series of tunnels carved into hard crystalline rock. This rock is around 1.9 billion years old, forming part of the Fennoscandian Shield, an archaic piece of continental crust beneath Finland and its adjacent regions.
The disposal tunnels are set to be positioned approximately hundreds of metres beneath the surface. On one visit, the elevator indicator displayed 433 metres. When fully operational, Posiva states, the subterranean complex will encompass over 60 kilometres of tunnels.
Excavation commenced in June 2004. Initially, Onkalo served mainly as an underground research facility, constructed to investigate the bedrock and verify the site’s suitability. The Finnish government issued Posiva a construction license for the encapsulation and final disposal facility in 2015, subsequently steering the project towards being a functional repository.
The projected capacity is substantial. According to Posiva, Onkalo has the potential to hold around 6,500 tonnes of uranium in spent fuel, encased in about 3,250 disposal canisters. These canisters will be lowered underground and positioned individually into holes drilled into the bases of the disposal tunnels.
As Lauri Parviainen, a chemist at Posiva, stated, “Essentially, it must be safe indefinitely.” This represents a goal rather than a commitment that anyone can unequivocally ensure, and the straightforwardness of the wording adds to its intrigue.
## Copper, clay, and ancient stone
The design hinges on the KBS-3 method devised by Sweden’s nuclear waste firm SKB. Its foundational principle asserts that no single element of the system should bear the entire burden.
Spent fuel will be encased in a robust insert crafted from spheroidal graphite cast iron. Surrounding that is a thick copper outer shell, which offers resistance to corrosion. The lid is affixed using friction-stir welding to create a tightly sealed unit.
Each canister will then be enveloped in bentonite clay. As the clay takes in groundwater, it expands, fills voids, and creates a dense barrier surrounding the copper. Eventually, the deposition tunnel will be filled and sealed. Beyond those engineered defenses lies the surrounding bedrock, which impedes the flow of groundwater and any radioactive substances that may eventually seep out.
Emily Stein, a researcher in geological waste management at Sandia National Laboratories, articulated the rationale clearly: “You’re never depending on a single barrier.” Should one layer underperform compared to expectations, the remaining layers are designed to continue delaying or halting the release of radionuclides.
The copper garners the most focus as it is anticipated to serve as the primary corrosion barrier for an almost unfathomable duration.
Groundwater is already present in fractures at the repository’s depth, so the system is not predicated on eliminating every drop of water from the canisters. Instead, the developers expect the deep groundwater to evolve into a chemically reducing environment with minimal dissolved oxygen. Oxygen captured during construction should gradually be consumed by reactions occurring in the rock, clay, and groundwater, reducing one of the main causes of copper corrosion.
## The copper corrosion debate
Not everyone agrees with this perspective. The most prominent dissenter is Peter Szakálos, a corrosion researcher affiliated with KTH Royal Institute of Technology in Stockholm.
In a 2007 study with Gunnar Hultquist and Gunnar Wikmark, Szakálos reported hydrogen generation in experiments involving copper and deoxygenated pure water. The researchers interpreted their finding as evidence that copper could react with water itself even without dissolved oxygen present.
Szakálos has contended that this mechanism, combined with various forms of degradation, could potentially compromise the canisters. Speaking to *