Radioactive substances

Certain forms of atoms, known as radionuclides, have unstable nuclei in their atomic structures and emit energy and sub-atomic particles spontaneously in the process of reaching a more stable state. These unstable atoms, or the compounds in which they occur, are described as being radioactive and this process of change to a more stable form is known as radioactive decay. The rate of decay is constant and so the term half-life is used to measure the length of time required for half of the radionuclides in a given sample to have lost their radioactivity. Unstable radioactive compounds may have very short half-lives, but others are highly stable with half-lives measured to over one billion years.

Some radioactive compounds occur naturally, but humans have also developed techniques to create radioactive isotopes and these have become relatively widespread, though in very low concentrations, in many areas. The principal sources of radionuclides released into the ocean environment include nuclear weapons testing; fallout from accidents such as the Chernobyl accident in 1986 or from foundering of nuclear submarines; from the dumping of nuclear waste into the deep ocean and from discharges from nuclear power plants and nuclear reprocessing plants.

Levels of radionuclides are near nuclear power-plants and nuclear processing plants, however such levels may still remain within limits which are considered safe. Similarly, fallout from the Chernobyl accident led to increased levels of caesium-134 and caesium-137, although these levels are gradually diminishing in all areas except those where terrestrial runoff is still adding new radioactive materials. Overall, levels of radioactive pollution are low and increasing concern and regulations are helping to reduce these.