How can we get a warning to the distant future?
Can we design warnings that last as long as the mortal dangers we create?
In a subterranean vault a quarter mile below the Satakunta region of Finland lies Onkalo, a repository for radioactive waste from the country's nuclear plants.
More than 20 years after the start of construction, Onkalo reached a milestone in 2024 as the first test casks were interred on-site. It was a successful test of the burial procedure that they'll follow for the real thing:
At the repository, used fuel will be placed in the bedrock, at a depth of about 430 metres. The disposal system consists of a tightly sealed iron-copper canister, a bentonite buffer enclosing the canister, a tunnel backfilling material made of swellable clay, the seal structures of the tunnels and premises, and the enclosing rock.
If Finland's nuclear regulator gives final approval, Onkalo could begin accepting radioactive waste within a year. If all goes according to plan, it will store all the nuclear waste Finland produces until the year 2100, when it will be permanently sealed.
Or so we hope.
Nuclear power: the benefits and the dangers
Nuclear power, as its backers never tire of pointing out, is reliable, carbon-neutral, and produces huge amounts of energy. But it poses some serious dangers of its own.
At the heart of a nuclear plant are fuel rods of enriched uranium. The atoms from the uranium are induced to split in a controlled chain reaction, liberating energy that's used to generate electricity.
When the uranium has been used up to the point that the fuel rods are no longer useful for generating power, they are still highly radioactive. In fact, spent fuel rods are more dangerous than new fuel rods. New fuel rods can be handled with relatively few precautions, while unshielded exposure to spent fuel rods would kill a person in minutes.
Conceptually, the reason for this is that uranium decays slowly, with a half-life on the order of billions of years, so it emits less radiation in a given amount of time. The fission products of uranium are themselves radioactive, and they decay faster—geologically speaking, that is, on the order of thousands of years, rather than billions—so they give off energy more quickly, emitting more radiation. (Another part of the answer is that uranium decays by emission of alpha particles, which can be blocked by clothing, while some of its fission products emit dangerously penetrating gamma rays.)
Spent fuel rods can be reprocessed to collect the residual uranium, which can be recycled into fresh fuel rods. Some countries do this. But nuclear reprocessing is expensive and hazardous. An accident at a Soviet reprocessing plant, the 1957 Kyshtym disaster, was the worst nuclear disaster in history until Chernobyl. It's also a way to obtain plutonium usable for nuclear weapons, so the technology is tightly restricted.
For all these reasons, many countries, including Finland, have decided against reprocessing spent fuel rods. The next-best option is to bury them deep underground, where they can be safely isolated until their radioactivity has decreased to harmless levels.
Is there a universal symbol for danger?
Spent fuel rods will be radioactive for tens of thousands of years, so the engineering demands of storage are formidable. The ideal site for a nuclear waste repository is deep in geologically stable bedrock, without inconvenient earthquakes or groundwater that might leak into the vault and corrode the storage containers. Ideally, it should be far from population centers and lacking any valuable ore veins that might tempt people to mine there. The designers even have to account for glaciation in far-future ice ages.
Onkalo and other planned repositories like it pose an enormous risk to future generations of humans. We've got to warn them. But how?
We can't depend on uninterrupted transmission of information over time. Memories fade, digital storage degrades, libraries burn, and stone carvings weather away to nothing. Civilizations collapse in war or disaster, and their knowledge can be lost with them. Over timescales of millennia, these are all very real possibilities.
The civilizations that come after us may not remember where the repository is or the reason it was constructed. If they dig into it, how can we warn them so they don't accidentally irradiate themselves?
A language like Linear A, which is "only" thirty-five centuries old, is indecipherable. It's likely that no written warning would still be comprehensible in ten thousand years—much less a hundred thousand. Even our basic symbology, like the red circle-and-slash or the yellow-and-black radiation trefoil, might mutate over time to the point of unrecognizability or be forgotten entirely.
Is there any way to communicate with the distant future? It's a fascinating philosophical problem to ponder.
Are there any truly universal symbols that would be understood by all humans from every culture throughout time? Or flip the question around: Do the ancient Egyptians, Sumerians, Chinese and Mesoamericans have any symbols that we would understand intuitively?
Nuclear semiotics
This field of inquiry, sometimes called "nuclear semiotics", has spawned a wild array of proposals for how to convey danger in a way that transcends language. People have suggested forbidding landscapes of jagged stone spikes, or structures that create a dissonant and mournful whistling when the wind blows through them; images of frightened and agonized faces, like The Scream; even genetically engineered animals that change color in the presence of radiation.
Another point of view—call it the security-through-obscurity view—is that marking the site in any way will only encourage curiosity. After all, modern-day archaeologists are drawn to excavate ancient tombs and pyramids because they're distinctive structures, and they feel no concern for the curses carved on their walls. This view asserts that the best way to protect Onkalo and other places like it is to leave no warnings, and hope that no one ever stumbles across it.
Since Onkalo will operate for decades to come, Finnish politicians don't yet have to decide what warnings, if any, should be posted when it's sealed up for good. But whatever they ultimately decide, the mere existence of the choice is a lesson on thinking in deep time. We're reaping the benefits today, but creating nuclear waste that will pose a danger for tens of thousands of years. It forces us to confront the question of what ethical responsibility we have to the distant future.
