Caesium is one of the radioactive fission products created within a nuclear reactor during its operation. More: 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.