caesium A soft silvery-white and highly reactive metal belonging to the alkali group of metals. It is a radiation hazard, because it can occur in two radioactive forms. Caesium-134 is produced in nuclear reactors, not directly by fission, but by the reaction. It emits beta- and gamma-radiation and has a half-life of 2.06 years. Caesium-137 is a fission product of uranium and occurs in the fallout from nuclear weapons. It emits beta- and gamma-rays and has a half-life of 30 years. Caesium-137 was the principal product released into the atmosphere, and hence the food chain, from atmospheric testing of nuclear weapons and from the Windscale fire and Chernobyl nuclear accidents. After the Chernobyl accident, which spread a radiation cloud across Europe, the European Commission proposed new and more restrictive limits on levels of caesium in food and drinking water['si:ziəm]n
