Caesium (IUPAC spelling; cesium
in American English) is a chemical
element; it has symbol Cs and atomic
number 55. It is a soft, silvery-golden alkali
metal with a melting point of 28.5 °C (83.3 °F; 301.6 K),
which makes it one of only five elemental metals that are liquid at or near room
temperature. Caesium has physical and chemical properties similar to those
of rubidium
and potassium.
It is pyrophoric and reacts with water even at
−116 °C (−177 °F). It is the least electronegative
element, with a value of 0.79 on the Pauling
scale. It has only one stable isotope, caesium-133.
Caesium is mined mostly from pollucite.
Caesium-137 (13755Cs), cesium-137 (US), or radiocaesium, is a radioactive isotope of caesium that is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons. Trace quantities also originate from spontaneous fission of uranium-238. It is among the most problematic of the short-to-medium-lifetime fission products. Caesium-137 has a relatively low boiling point of 671 °C (1,240 °F) and easily becomes volatile when released suddenly at high temperature, as in the case of the Chernobyl nuclear accident and with atomic explosions, and can travel very long distances in the air. After being deposited onto the soil as radioactive fallout, it moves and spreads easily in the environment because of the high water solubility of caesium's most common chemical compounds, which are salts.
Caesium-137, along with other radioactive isotopes caesium-134, iodine-131, xenon-133, and strontium-90, were released into the environment during nearly all nuclear weapon tests and some nuclear accidents, most notably the Chernobyl disaster and the Fukushima Daiichi disaster.
Caesium-137 in the environment is substantially anthropogenic (human-made), these bellwether isotopes are produced solely from anthropogenic sources. Caesium-137 is produced from the nuclear fission of plutonium and uranium, and decays into barium-137.
Caesium-137 is a radioisotope commonly used as a gamma-emitter in industrial applications. Its advantages include a half-life of roughly 30 years, its availability from the nuclear fuel cycle, and having 137Ba as a stable end product. It has been used in agriculture, cancer treatment, and the sterilization of food, sewage sludge, and surgical equipment. Radioactive isotopes of caesium in radiation devices were used in the medical field to treat certain types of cancer, but emergence of better alternatives and the use of water-soluble caesium chloride in the sources, which could create wide-ranging contamination, gradually put some of these caesium sources out of use. Caesium-137 has been employed in a variety of industrial measurement gauges, including moisture, density, leveling, and thickness gauges. It has also been used in well-logging devices for measuring the electron density of the rock formations, which is analogous to the bulk density of the formations
The isotopes 134 and 137 are present in the biosphere in small amounts from human activities, differing by location. Radiocaesium does not accumulate in the body as readily as other fission products (such as radioiodine and radiostrontium). About 10% of absorbed radiocaesium washes out of the body relatively quickly in sweat and urine. The remaining 90% has a biological half-life between 50 and 150 days. Radiocaesium follows potassium and tends to accumulate in plant tissues, including fruits and vegetables. Plants vary widely in the absorption of caesium, sometimes displaying great resistance to it. It is also well-documented that mushrooms from contaminated forests accumulate radiocaesium (caesium-137) in the fungal sporocarps. Accumulation of caesium-137 in lakes has been a great concern after the Chernobyl disaster. Experiments with dogs showed that a single dose of 3.8 millicuries (140 MBq, 4.1 μg of caesium-137) per kilogram is lethal within three weeks; smaller amounts may cause infertility and cancer. The International Atomic Energy Agency and other sources have warned that radioactive materials, such as caesium-137, could be used in radiological dispersion devices, or "dirty bombs".
Fukushima Daiichi disaster
In April 2011, elevated levels of caesium-137 were also found in the environment after the Fukushima Daiichi nuclear disasters in Japan. In July 2011, meat from 11 cows shipped to Tokyo from Fukushima Prefecture was found to have 1,530 to 3,200 becquerels per kilogram of 137Cs, considerably exceeding the Japanese legal limit of 500 becquerels per kilogram at that time. In March 2013, a fish caught near the plant had a record 740,000 becquerels per kilogram of radioactive caesium, above the 100 becquerels per kilogram government limit. A 2013 paper in Scientific Reports found that for a forest site 50 km from the stricken plant, 137Cs concentrations were high in leaf litter, fungi, and detritivores, but low in herbivores. By the end of 2014, "Fukushima-derived radiocaesium had spread into the whole western North Pacific Ocean", transported by the North Pacific current from Japan to the Gulf of Alaska. It has been measured in the surface layer down to 200 meters and south of the current area down to 400 meters.
Radioactive materials were dispersed into the atmosphere immediately after the disaster and account for most of all such materials leaked into the environment. 80% of the initial atmospheric release eventually deposited over rivers and the Pacific Ocean, according to a UNSCEAR report in 2020. Specifically, "the total releases to the atmosphere of Iodine-131 and Caesium-137 ranged generally between about 100 to about 500 PBq [petabecquerel, 1015 Bq] and 6 to 20 PBq, respectively.
Once released into the atmosphere, those that remain in a gaseous phase will simply be diluted by the atmosphere, but some that precipitate will eventually settle on land or in the ocean. Thus, the majority (90~99%) of the radionuclides which are deposited are isotopes of iodine and caesium, with a small portion of tellurium, which is almost fully vaporized out of the core due to their low vapor pressure. The remaining fraction of deposited radionuclides are of less volatile elements such as barium, antimony, and niobium, of which less than a percent is evaporated from the fuel.
Approximately 40–80% of the atmospheric releases were deposited over the ocean.
In addition to atmospheric deposition, there was also a significant quantity of direct releases into groundwater (and eventually the ocean) through leaks of coolant that had been in direct contact with the fuel. Estimates for this release vary from 1 to 5.5 PBq. Although the majority had entered the ocean shortly following the accident, a significant fraction remains in the groundwater and continues to mix with coastal waters.
According to the French Institute for Radiological Protection and Nuclear Safety, the release from the accident represents the most important individual oceanic emissions of artificial radioactivity ever observed. The Fukushima coast has one of the world's strongest currents (Kuroshio Current). It transported the contaminated waters far into the Pacific Ocean, dispersing the radioactivity. As of late 2011 measurements of both the seawater and the coastal sediments suggested that the consequences for marine life would be minor.
The possible presence of other radioactive substances, such as strontium-90 or plutonium, has not been sufficiently studied. Recent measurements show persistent contamination of some marine species (mostly fish) caught along the Fukushima coast.
Initial discharge
A large amount of caesium entered the sea from the initial atmospheric release. By 2013, the concentrations of caesium-137 in the Fukushima coastal waters were around the level before the accident. However, concentrations in coastal sediments declined more slowly than in coastal waters, and the amount of caesium-137 stored in sediments most likely exceeded that in the water column by 2020. The sediments may provide a long-term source of caesium-137 in the seawater.
Data on marine foods indicates their radioactive concentrations are falling towards initial levels. 41% of samples caught off the Fukushima coast in 2011 had caesium-137 concentrations above the legal limit (100 becquerels per kilogram), and this had declined to 0.05% in 2015. United States Food and Drug Administration stated in 2021 that "FDA has no evidence that radionuclides from the Fukushima incident are present in the U.S. food supply at levels that are unsafe". Yet, presenting the science alone has not helped customers to regain their trust in eating Fukushima fishery products.
2023 discharge
The most prevalent radionuclide in the wastewater is tritium. A total of 780 terabecquerels (TBq) will be released into the ocean at a rate of 22 TBq per year. Tritium is routinely released into the ocean from operating nuclear power plants, sometimes in much greater quantities. For comparison, the La Hague nuclear processing site in France released 11,400 TBq of tritium in the year of 2018. In addition, about 60,000 TBq of tritium is produced naturally in the atmosphere each year by cosmic rays.
Other radionuclides present in the wastewater, like caesium-137, are not normally released by nuclear power plants. However, the concentrations in the treated water are minuscule relative to regulation limits.
"There is consensus among scientists that the impact on health is minuscule, still, it can't be said the risk is zero, which is what causes controversy", Michiaki Kai, a Japanese nuclear expert, told AFP. David Bailey, a physicist whose lab measures radioactivity, said that with tritium at diluted concentrations, "there is no issue with marine species unless we see a severe decline in fish population".
Ferenc Dalnoki-Veress, a scientist-in-residence at the Middlebury Institute of International Studies at Monterey, said regarding dilution that bringing in living creatures makes the situation more complex. Robert Richmond, a biologist from the University of Hawaiʻi, told the BBC that the inadequate radiological and ecological assessment raises the concern that Japan would be unable to detect what enters the environment and "get the genie back in the bottle". Dalnoki-Veress, Richmond, and three other panelists consulting for the Pacific Islands Forum wrote that dilution may fail to account for bioaccumulation and exposure pathways that involve organically bound tritium (OBT).
