Onkalo - The Largest Underground Nuclear Waste Repository

Ceremonies will be held around the world on Tuesday to mark the 25th anniversary of the Chernobyl disaster but, in truth, Chernobyl is one event we're in no danger of forgetting. For one thing, the earthquake in Japan has given the world a second Level Seven incident on the International Nuclear Event Scale, refreshing public fears with almost cosmic timing. For another, the legacy of Chernobyl will be remembered for much, much longer than anyone would wish. According to estimates, this area of northern Ukraine will be uninhabitable for decades, if not centuries.

We like to think of our architectural treasures as milestones of human progress. The Egyptian pyramids, say, or the Eiffel Tower. Perhaps we imagine a Planet of the Apes-like scenario where our ruined monuments will stand as testament to our civilisation long after we're gone. But what will most probably outlive anything else we have ever built will be our nuclear legacy. Whatever its pros and cons as an energy source, one thing that's non-negotiable about nuclear power is the construction it necessitates. Less than a century after we first split the atom, we're now coming to appreciate the vast technological, engineering, financial and political resources nuclear technology demands. In terms of scale, complexity and longevity, much of this stuff makes Dubai's Burj Khalifa look like a sandcastle.

It is too early to know what will be done about Fukushima. A 20km exclusion zone has been imposed and radiation levels will not be brought down to safe levels for at least another six months. Even at Chernobyl, the 1986 accident is by no means dealt with. Immediately afterwards, the Soviets hastily cobbled together the most effective structure they could to contain further radioactive contamination. Unromantically named the Object Shelter, it was a concrete and steel sarcophagus resting on the remains of the ruined reactor. Owing to the high levels of radioactivity, it had been impossible to bolt or weld the Object Shelter together, so within a decade it was on the verge of collapse. 

Given that 95% of reactor four's nuclear materials are still inside, another nuclear disaster remains a possibility. Hence the current longer-term plan, called the New Safe Confinement. This €1.6bn (£1.4bn) project calls for the erection of an arch-shaped hangar, bigger than a football pitch and high enough to fit the Statue of Liberty inside. Because of the radiation levels, it must be built 500 metres away then slid over the top of the reactor and the Object Shelter. At 32,000 tonnes, it is just about the heaviest object ever moved.

"In some ways, this is how the engineers of the pyramids must have felt," says Eric Schmieman, chief technical adviser on the New Safe Confinement. "The steel structure has a design life of 100 years, so there are very rigorous requirements to demonstrate all the materials will last that long. The Eiffel Tower has been around that long but it's been protected from corrosion by painting. You can't repaint this because of the radiation."

The structure of the New Safe Confinement is carbon steel, protected by inner and outer layers of stainless steel cladding. Its purpose is not to shield radioactive emissions but to prevent the release of radioactive dust and other materials, and to keep out rainwater, which could carry contaminants into the water table. 

Work is currently proceeding on the foundations, and the arch will be assembled and slid into place by 2015. Then huge, remote-controlled cranes inside will dismantle the Object Shelter and begin retrieving the hazardous materials inside.

The structure will be visible from space, a hulking shell of steel in the midst of a landscape of industrial devastation. By the time it reaches the end of its 100-year life span, it is hoped that all the radioactive material will have been removed, but then comes the problem of where to put it. At the beginning of the nuclear era, the emphasis was very much on the power stations, including Basil Spence's heroic 1950s design for Trawsfynydd, in Snowdonia. But very little consideration was given to what came after. Those early power stations became obsolete: Trawsfynydd was decommissioned in 1991. What's more, the industry has so far generated nearly 300,000 tonnes of high-level nuclear waste, and counting. To be safe, it must be isolated from all living organisms for at least 100,000 years.

Current opinion is that the best thing to do with nuclear waste is put it underground in what is known as a "deep geological repository". At present, there are no such repositories in operation anywhere. In Britain, all the nuclear waste produced since the 1940s is stored above ground in Sellafield. Preliminary moves have been made towards finding a site in Cumbria but there's a powerful local resistance to such schemes, and no long-term solution is expected before 2040. In the US, a site was earmarked decades ago at Nevada's Yucca Mountain, 100 miles from Las Vegas, but the Obama administration finally abandoned the scheme last year.

Some countries are further ahead, though. Sweden's nuclear operation presents itself as a model for the rest of the world, and shows how much effort a fully joined-up operation requires. 

After cooling on site for a year, spent fuel from Sweden's three coastal nuclear sites is transported in purpose-built casks, on a specially designed ship, to a central interim storage facility. There, robotic arms transfer the fuel into storage cassettes underwater. These cassettes are then sent to another storage pool 25 metres beneath the facility to cool for at least another 30 years. Then the waste is moved to another plant to seal in copper canisters before it arrives at its final resting place in the geological repository.

Sweden has numerous other nuclear facilities, including the Äspö hard rock laboratory, an underground research laboratory open to visitors. Bizarrely, Äspö's surface buildings could be mistaken for a traditional farmstead: a collection of buildings in red and white timber. 

The folksy tweeness only points up how alien the rest of the nuclear landscape is. This is the heaviest of heavy industries, and it is often the least visible: a hidden parallel realm of anonymous industrial facilities, restricted zones, clinical chambers and subterranean vaults

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Sweden has identified a site for its deep geological repository, in Forsmark, but the Finns have been building theirs since 2004. Situated on the northwest coast, a few miles from its Olkiluoto nuclear power stations, it consists of a 5km-long tunnel spiralling 400m down to the bedrock, where a honeycomb of storage vaults fans out. Named Onkalo, whose literal translation is "cavity", it was the subject of a documentary last year, Into Eternity. Retitled Nuclear Eternity and broadcast on More4 tomorrow, the film fully appreciates the Kubrickian visual aspects of the nuclear landscape and the staggering challenges the project presents to our notions of permanence, history – even time itself.  

Onkalo will be ready to take waste in 2020, and then will be finally sealed in 2120, after which it will not be opened for 100,000 years. By that time, Finland will probably have been through another ice age. Little trace of our current civilisation will remain. The prospect of designing anything to last even 200 years is unlikely for most architects; the Egyptian pyramids are "only" about 5,000 years old.

Plan like an Egyptian 

This longevity poses Onkalo's custodians, and others in their position, with another unprecedented design issue: what sign should you put on the door? As one expert says in Into Eternity, the message is simple: "This is not an important place; it is a place of danger. Stay away from the site. Do not disturb the site." But how to communicate with people so far in the future? Put up a sign in a language they don't understand and they are sure to open it just to see what's inside. Ancient Egyptians on the pyramid planning committee probably grappled with the same issues.

One of the Finns suggests using an image of Munch's The Scream; another suggests a series of monoliths with pictographs and an underground library explaining the tunnel; another wonders if it is better not to tell anyone Onkalo is there at all. When a team pondered the same issue in the US in the 1990s, they came up with proposals for environments that communicated threat and hostility. They imagined landscapes of giant, spiky, black thorns or menacing, jagged earthworks, or vast concrete blocks creating narrow streets that lead nowhere.

If architecture is about designing spaces for human habitation, this is practically its opposite. These subterranean cities are places no human will ever inhabit or see, places designed to repel life and light. They are a mirror image to our towering achievements above ground and, like the pyramids, they are both monument and tomb. Every nuclear nation is compelled to build them, at great effort and expense, and to continue building them until we find a better way to deal with nuclear waste or a better alternative to nuclear power. Until then, we must live with the thought that in some unimaginable future aeons hence, this could be all that remains to prove our species was ever here.