Litvinenko had ingested polonium-210, a poisonous radioactive isotope. Tritium is an example of a radioactive isotope. Radiocarbon dating based on the carbon-14 isotope has been particularly criticized. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. A common source used is the hydrogen isotope, tritium. Th is the only isotope of thorium occurring in quantity in nature. Examples include lethal effects to individuals, radioactive isotope to the environment, or reactor core melt. Nitrogen-15 is a rare stable isotope of nitrogen. Plutonium-238 was the first isotope of plutonium to be discovered. The only stable caesium isotope is Cs, with 78 neutrons. Fluorine-19 is the only stable isotope of fluorine. Caesium-133 is the only stable isotope of caesium. The ordinary isotope of hydrogen, with no neutrons, is sometimes called protium. H is a highly unstable isotope of hydrogen. Some elements occur in nature with only one stable isotope, such as fluorine.