Caesium is one of the radioactive fission
products created within a nuclear reactor during its operation.
Caesium-137 is one of the major radioactive
isotopes of caesium. It is a major
radioisotope in the radioactive waste
resulting from nuclear reactors.
Caesium-137 is radioactive for a long period of time (half-life is about 30
years) and can contaminate property, entailing extensive cleanup. It can also be
absorbed into the food chain.
Element - Radiation - Radioactivity
To read about this term in context:
GreenFacts Summary on the Chernobyl Nuclear Accident: