What happens to nuclear waste, and what is Finland's Onkalo?
The hard part is a small share of hot used fuel that takes about 100,000 years to become less radioactive than natural uranium ore.
Much nuclear waste is surprisingly ordinary: paper, rags, tools and clothing with small amounts of mostly short-lived radioactivity. The hard part is a small share of very hot, very radioactive material, mainly from reactors. That has to be cooled first and then either or buried. Finland is building what is set to be the world's first long-term disposal facility for it, called Onkalo, which means small cave.
in a typical light-water reactor is about 95 percent uranium, 4 percent and about 1 percent heavier elements such as . The fission products cause most of the short-term radioactivity, and the heavy elements most of the long-term radioactivity. The fuel spends six to ten years in cooling pools, then moves to dry casks. Radioactivity falls by 99.5 percent after 100 years, though it takes about 100,000 years before spent fuel is less radioactive than natural uranium ore.
Fission products
- About 4% of
- Most of the short-term radioactivity
Plutonium and heavier elements
- About 1% of
- Most of the long-term radioactivity
That long clock is why waste is a political problem. Burial deep underground in a geological repository is a favoured solution for long-term storage. Onkalo is being dug into granite near the Olkiluoto power plant on Finland's west coast, deep underground. Each fuel canister will be sealed in copper and packed in clay.
A film makes the human puzzle plain: Into Eternity by Michael Madsen, whose focus is on how to mark such a place as dangerous for people in the distant future. Testing with empty canisters began in August 2024, and after a safety assessment by Finland's Radiation and Nuclear Safety Authority, drilling of the deposit holes began in August 2026. In 2012 researchers in Stockholm questioned whether the copper capsules resist corrosion, though follow-up studies by SKB, one of the companies planning such repositories, found that the suspected process does not exist.
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Recap
Spent fuel cools in pools for years, then is stored in casks or, in the long term, buried deep in rock.
💡 A trick to remember it · Cool it in a pool, keep it in a cask, then send it down in copper and clay.
Surprising fact · After about 100,000 years spent fuel is less radioactive than natural uranium ore, which is why a repository has to plan for deep time.
Sources (3)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.