Why this one and not the others
A reactor accident releases hundreds of nuclides. Within a year almost all of them have decayed, and the contamination that remains comes down to two: caesium 137 and strontium 90.
Iodine 131 does the most damage in the first weeks. It concentrates in the thyroid, it is why potassium iodide tablets are distributed, and its half-life is eight days — after three months it is effectively gone.
Caesium 137 has a half-life of 30.1 years. Strontium 90 is similar at 28.8. But caesium is the more troublesome of the pair because it moves.
⚠️ Strontium follows calcium and locks into bone, which is dangerous but static. Caesium follows potassium, and potassium is everywhere — in soil, in plants, in every cell of every animal. It circulates.
The potassium mistake
Caesium sits below potassium in group 1 and forms the same singly charged ion of similar size. Nothing biological distinguishes them reliably.
Root systems take caesium up as if it were a nutrient. Grazing animals concentrate it. It passes into milk and meat. In humans it spreads through soft tissue rather than settling in one organ.
The biological half-life in an adult is about 70 days, so a person who stops ingesting it clears it within a year. The problem is that in a contaminated area the intake does not stop.
This is why restrictions after Chernobyl fell on food rather than on land. Reindeer in Lapland, sheep on Welsh hills and wild boar in Bavarian forests all carried enough caesium to exceed limits, in some cases for decades — boar especially, because they root for underground fungi that concentrate it.
Goiânia
In September 1987 a private radiotherapy clinic in Goiânia, in central Brazil, was demolished. A caesium 137 teletherapy source was left in the ruins.
Two scrap dealers took it, broke the housing open, and found a powder inside that glowed blue in the dark. They thought it was valuable, or beautiful, or both. They sold pieces of it and gave pieces away.
⚠️ A six-year-old girl, Leide das Neves Ferreira, ate a sandwich with the powder on her hands. She died a month later. Three adults died with her. Two hundred and forty-nine people were contaminated, and 41 houses had to be demolished.
The source held about 93 grams of caesium chloride — a soluble salt, which is why it dispersed so readily once the capsule was open.
The IAEA classifies it as level 5 on the international scale, the same level as Three Mile Island, and still cites it as the worst radiological accident that did not involve a reactor.
Why it is so hard to clean up
Caesium binds tightly to clay minerals in soil. That is fortunate and unfortunate at once: it stays near the surface instead of reaching groundwater, and it stays near the surface instead of washing away.
The consequence is that decontamination means moving earth. After Fukushima, crews scraped the top few centimetres from farmland, roads and school yards across a wide area, bagged it, and stacked the bags. Millions of tonnes of soil now sit in interim storage because there is nowhere agreed to put it.
Chemical washing works in a laboratory and poorly in a field, because the caesium is held inside the clay lattice rather than on its surface.
⚠️ One approach that does work is agricultural rather than industrial. Adding potassium fertiliser to contaminated land gives plants an abundant supply of the element caesium imitates, and they take up proportionally less of it. It does not remove the caesium; it keeps it out of the food.
What is still measured
Thirty-nine years after Chernobyl, caesium 137 remains the isotope surveyors look for. Roughly a quarter of the original release has decayed away; the rest is in soil, bound to clay minerals that hold it near the surface.
The exclusion zone is defined largely by caesium contours. So is the Fukushima zone, where the same isotope drove the evacuation and the same soil chemistry governs the cleanup — removing the top few centimetres of earth, because that is where the caesium is.
⚠️ Its persistence has one useful consequence. Caesium 137 did not exist in nature before 1945, so its presence in a sediment layer, an ice core or a wine bottle dates the material to after the first atmospheric tests. Wine fraud investigators use it: a bottle claiming to predate the war should have none.