Caesium-137

• 1. Introduction

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  Caesium-137 was the material inside an 8 mm capsule that went missing from a truck in Western Australia on the 25th of January 2023. Health authorities issued an emergency alert covering a stretch of road about 1,400 km long. Western Australia's chief health officer, Andrew Robertson, warned that an exposed person could expect to receive the equivalent of "about 10 X-rays an hour". Experts said anyone who found the capsule should stay at least 5 m away. It was found on the 1st of February 2023. Why would an object that small trigger an alert on that scale? Where does this isotope come from, and what work was it doing on a truck? How does it travel through air, soil and sea? And why do so many of its stories involve people who had no idea what they were holding?

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• 2. Fission Leftover

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  Uranium-235 splits, and among the debris is caesium-137, one of the more common fission products. Other fissionable isotopes yield it as well, whether in nuclear reactors or in nuclear weapons. Trace quantities also arise from the spontaneous fission of uranium-238. It is counted among the most problematic of the short-to-medium-lifetime fission products. Glenn T. Seaborg and Margaret Melhase are credited with discovering it. Because it is almost purely synthetic, it did not exist in the environment before 1945.

  A half-life of about 30.04 years governs how long it lingers. It decays by beta emission into stable barium-137. In about 94.6% of decays, the first product is barium-137m, a metastable nuclear isomer. That intermediate state has a half-life of about 153 seconds. Usually it drops to the ground state and emits a photon, which happens in 85.1% of all caesium-137 decays. That photon is the source of all the gamma rays measured from a sample. The same signature would later let researchers tell old wine from new.

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• 3. Working Isotope

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  Calibration work uses small amounts of caesium-137 to check radiation-detection equipment. In medicine, it serves in radiation therapy. Industry uses it in flow meters, thickness gauges and borehole logging devices. Moisture-density gauges rely on it for the density reading, while americium-241 mixed with beryllium supplies the moisture reading.

  Industrial radiography is the notable exception, because iridium-192 and cobalt-60 are preferred there. Caesium taken from used nuclear fuel arrives mixed with stable caesium-133 and long-lived caesium-135, and separating the isotopes costs too much. Sources with higher specific activity tend to be made from highly soluble caesium chloride, so a damaged source carries a high risk of contamination. Water-insoluble versions can be made with ferrocyanides, but their specific activity is lower. A large emitting volume also harms image quality. Iridium and cobalt are chemically non-reactive metals, and high-flux reactors can make them at much higher specific activities by activating stable iridium-191 and cobalt-59. Caesium-aluminosilicate glasses, akin to the mineral pollucite, are another inert form. They have been used to demonstrate water-insoluble waste suitable for deep geological repositories.

  Caesium-137 is a waste product that fission reactors make in great quantities. Iridium-192 and cobalt-60 are made on purpose in commercial and research reactors, and their life cycle destroys high-value elements. Residual radioactivity and legal hurdles mean the material is rarely recovered from spent sources. In essence, the whole mass is lost for non-radioactive uses.

  The Trinity test on the 16th of July 1945 spread some caesium-137 into the atmosphere and scattered trace amounts around the globe. Researchers can therefore measure its characteristic gamma rays to judge whether a sealed container's contents were made before or after the first atomic bomb explosion. They have used this to check the authenticity of certain rare wines, most notably the purported "Jefferson bottles". Surface soils and sediments are dated by measuring its activity as well. Geologists also use it as a tracer for soil erosion and deposition, because it clings to fine sediments. The sealed sources inside gauges and clinics are another matter, because the accident record for them is long.

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• 4. Inside the Body

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  In the body, caesium behaves much like potassium and rubidium. Once inside, it spreads more or less uniformly, with the highest concentrations in soft tissue. Unlike group 2 radionuclides such as radium and strontium-90, it does not bioaccumulate and is excreted relatively quickly. Its biological half-life is about 70 days. Pancreatic tissue has been shown to be a strong accumulator of radioactive caesium and to secrete it into the intestine.

  A 1961 experiment found that mice given a high dose had a 50% fatality rate within 30 days. A 1972 experiment exposed dogs to a heavy whole-body burden of caesium-137, and they died within 33 days. Animals with half of that burden all survived for a year. A 1960 mouse study found high levels of caesium-137 in the first day after exposure in the mucus glands of the colon, the pancreas, cartilage, tendons and skeletal muscle. After 24 hours, cartilage and skeletal muscle showed the highest activity.

  A 2003 study looked at children from a caesium-137-polluted area of Belarus. They suffered from chronic diseases rarely found in children elsewhere in the country. Autopsies on 52 children who died of various causes showed the highest concentrations in the thyroid, at 2054 becquerels per kilogram, give or take 288. The adrenals followed at 1576 give or take 290, and the pancreas at 1359 give or take 350. The brain, at 385 give or take 72, and the liver, at 347 give or take 61, held the least.

  Prussian blue, a compound of iron and cyanide, treats accidental ingestion by binding chemically to the caesium. It reduces the biological half-life to 30 days. That same compound reappears in plans for cleaning contaminated land.

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• 5. Travelling Fallout

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  Caesium has a relatively low boiling point of 671 degrees Celsius. When it is released suddenly at high temperature, as in a nuclear accident or explosion, it turns volatile and can travel very long distances in the air. After it lands as fallout, the high water solubility of its most common compounds, which are salts, lets it move and spread easily through the environment.

  Atmospheric nuclear weapon tests released caesium-137 alongside caesium-134, iodine-131, xenon-133 and strontium-90. Bombs detonated in the Arctic and bombs exploded in or near the stratosphere sent caesium-137 down onto upper Lapland, Finland. Measurements there in the 1960s were reportedly 45,000 becquerels. Figures from 2011 have a midrange of about 1,100 becquerels, and no increase in cancer cases has been identified.

  At Chernobyl, the 1986 disaster left caesium-137 and strontium-90 as the principal source of radiation in the zone of alienation around the plant. They pose the greatest health risk there, and will for the next few hundred years or so, owing to their roughly 30-year half-life and biological uptake. An estimated 12000 km² of Germany is contaminated with caesium-137 from the disaster. That corresponds to 1.1% of all the caesium-137 released in Europe. In Scandinavia, some reindeer and sheep still exceeded the Norwegian legal limit 26 years afterward. By then the caesium-137 had decayed by more than half, yet it could have been locally concentrated by much larger factors. A second accident, in Japan in 2011, would soon raise the same questions.

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• 6. Fukushima Aftermath

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  In April 2011, elevated levels of caesium-137 began turning up in the environment after the nuclear disasters in Japan. By July 2011, meat from 11 cows measured 1,530 becquerels per kilogram, considerably above the Japanese legal limit of 500 becquerels per kilogram at that time. In March 2013, a fish caught near the plant carried a record 740,000 becquerels per kilogram. The government limit was 100.

  A 2013 paper in Scientific Reports examined a forest site 50 km from the stricken plant. Concentrations there were high in leaf litter, fungi and detritivores, but low in herbivores. The ocean told a larger story. By the end of 2014, in the paper's words, "Fukushima-derived radiocaesium had spread into the whole western North Pacific Ocean". The North Pacific current carried it from Japan to the Gulf of Alaska. Researchers measured it in the ocean surface layer down to 200 meters, and down to 400 m south of the current area.

  Caesium-137 is reported to be the major health concern in the affected land. Several techniques under consideration would strip a large share of the caesium from contaminated soil without destroying its organic material. One of them is hydrothermal blasting. The caesium would be precipitated with ferric ferrocyanide, which is Prussian blue, and would be the only waste needing special burial sites. The aim is to bring annual exposure from the contaminated environment down to 1 mSv above background levels. The most contaminated area must remain off-limits, but decontaminating some less affected areas could allow 22,000 residents to return.

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• 7. Found by Strangers

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  In 1987, an abandoned clinic in Goiânia, Brazil, held a radiation therapy system that had been improperly disposed of. Someone removed it and cracked it open to sell in junkyards. The glowing caesium salt was then sold to curious buyers who did not know what it was. Four deaths were confirmed, along with several serious injuries from radiation contamination.

  In 1989, a capsule measuring 8 x 4 mm was found inside the concrete wall of an apartment building in Kramatorsk, in the Ukrainian SSR. It is believed to have been part of a measurement device, lost in the late 1970s and mixed into the gravel used to build the block in 1980. Two families lived in the apartment over 9 years. By the time the capsule was found, 6 residents of the building had died, 4 of them from leukemia, and 17 more had received varying doses of radiation.

  In 1994, three brothers scavenging for scrap metal broke into a shed at a nuclear waste storage facility in Tammiku, Estonia. They did not know what the facility was. A caesium-137 source had been released from a damaged container, and one brother received a whole-body dose of 40 Gy. He died of radiation poisoning 12 days later.

  In 1997, several Georgian soldiers suffered radiation poisoning and burns. Investigators traced the cause to training sources left abandoned, forgotten and unlabeled after the dissolution of the Soviet Union. One was a caesium-137 pellet in the pocket of a shared jacket. At a distance of 1 m it released about 130,000 times the level of background radiation.

  On the 5th of April 2024, an emergency regime was introduced in the Russian city of Khabarovsk. A local resident had accidentally discovered that radiation levels in an industrial area had jumped sharply. Volunteers from a dosimetric control group measured a reading of 1600 times the safe value. Employees of the Ministry of Emergency Situations fenced off an area of 30 by 30 meters and found a caesium capsule from a defectoscope. They placed it in a protective container and took it away for disposal. Novaya Gazeta was the first to report the incident.

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• 8. Lost in the System

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  In March 2015, the Norwegian university in Tromsø lost 8 radioactive samples, including caesium-137, americium-241 and strontium-90. They had been moved out of a secure location to be used for education. When it was time to return them, the university could not find them. As of November 2015, they were still missing.

  On the 3rd of March 2016, unusually high levels of caesium-137 were detected in the air in Helsinki, Finland. Finland's Radiation and Nuclear Safety Authority, known as STUK, found measurements about 1,000 times the usual background level. Its investigation traced the source to a building from which STUK itself and a radioactive waste treatment company operate.

  In May 2019, thirteen people were exposed to caesium-137 at the Research and Training building in the Harborview Medical Center complex in Seattle, Washington. A contract crew was moving the caesium from the lab to a truck when the powder spilled. Five people were decontaminated and released. Eight who were more directly exposed went to the hospital, and the research building was evacuated.

  Some sources ended up in furnaces. In 1998, at Los Barrios in Cádiz, Spain, a recycling company accidentally melted down a mass of caesium-137 that came from a gamma-ray generator. In 2009, a Chinese cement company in Tongchuan, Shaanxi, was demolishing an old, unused cement plant without following the standards for handling radioactive materials. Caesium-137 from a measuring instrument went out with eight truckloads of scrap metal to a steel mill, where it was melted into the steel.

  On the 23rd of February 2023, a caesium-137 capsule went missing from a steam power plant in Prachinburi province, Thailand. Officials from the Office of Atoms for Peace and the provincial administration began a search. The Thai public was not notified until the 14th of March. On the 20th of March, the office's Secretary-General and the provincial governor held a press conference. They said they had found caesium-137 contaminated furnace dust at a steel melting plant.

  On the 18th of August 2025, the United States Food and Drug Administration announced that shipments of shrimp from Indonesia had been contaminated. This was only the first of several major recalls of irradiated shrimp processed by the Indonesian company Bahari Makmur Sejati, also known as BMS Foods. Investigators traced the source of the material to a metal scrapyard in the Modern Cikande industrial estate near Jakarta. The company that supplied the shipping containers was also based there.

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