A radioactive isotope of the metallic element cesium. Cesium-137 is one of the major products formed when uranium undergoes nuclear fission, as in a nuclear reactor or nuclear weapons. During atmospheric testing of nuclear weapons after World War II, cesium-137 was a source of major concern to health scientists. Researchers found that the isotope settled to the ground, where it was absorbed by plants and eaten by animals, ultimately affecting humans. Once inside the human body, the isotope releases beta particles, which are carcinogens, teratogens, and mutagens. Large quantities of cesium-137 were released when the Chernobyl nuclear reactor exploded in 1986.