What is it? Cesium is a soft, silvery white-gray metal that occurs in nature as cesium-133. The natural source yielding the greatest quantity of cesium is the rare mineral pollucite.
There are 11 major radioactive isotopes of cesium (Isotopes are different forms of an element that have the same number of protons in the nucleus, but a different number of neutrons.) Only three have half-lives long enough to warrant concern: cesium-134, cesium-135 and cesium-137. Each of these decays by emitting a beta particle, and their half-lives range from about 2 to 2 million years. Of these three, the isotope of most concern for the Department of Energy (DOE) environmental management sites such as Hanford and INEEL is cesium-137, which has a half-life of 30 years. Its decay product, barium-137m (the "m" means metastable) stabilizes itself by emitting an energetic gamma ray with a half-life of about 2.6 minutes. It is this decay product that makes cesium an external hazard (that is, a hazard without being taken into the body).
Where does Cesium-137 come from? The three radioactive isotopes identified above are produced by nuclear fission. When an atom of usranium-235 (or other fissile nuclide) fissions, it generally split asymmetrically into two large fragments -- fission products with mass numbers in the range of about 90 and 140 -- and two or three neutrons. (The mass number is the sum of the number of protons and neutrons in the nucleus of an atom.) Cesium radionuclides are such fission products, with cesium-135 and cesium-137 being produced with relatively high yields of about 7% and 6%, respectively. That is, about 7 atoms of cesium-135 and 6 atoms of cesium-137 are produced per 100 fissions. Cesium-137 is a major radionuclide in spent nuclear fuel, high-level radioactive wastes resulting from the processing of spent nuclear fuel, and radioactive wastes associated with the operation of nuclear reactors and fuel reprocessing plants.
What's in the Environment? Radioactive cesium is present in soil around the world largely as a result of fallout from past atmospheric nuclear weapons test. The concentration of cesium-137 in surface soil from fallout ranges from about 0.1 to 1 picocurie (pCi)/g, averaging less than 0.4 pCi/g (or 0.3 billionth of a milligram per kilogram of soil).
Cesium is generally one of the less mobile radioaactive metals in the environment. It preferentially adheres quite well to soil, and the concentration associated with sandy soil particles is estimated to be 280 times higher than in interstitial water (water in the pore space between soil particles); concentration ratios are much higher (about 2,000 to more than 4,000) in clay and loam soils. Thus, cesium is generally not a major contaminant in groundwater at DOE sites.
*Excerpts from Human Health Fact Sheet, Environmental Assessment Division, Argonne National Laboratory