Radioactive contamination, also called radioactive contamination, is the deposition or presence of radioactive materials on surfaces or within solids, liquids or gases (including the human body), where their presence is unintended or unwanted (as defined by the International Atomic Energy Agency) .

This pollution poses a risk due to the radioactive decay of pollutants, which produces harmful effects such as ionizing radiation (i.e. alpha, beta and gamma rays) and free neutrons. The degree of danger is determined by the concentration of pollutants, the energy of the radiation emitted, the type of radiation, and the proximity of the contamination to the body’s organs. It is important to be clear that contamination creates a radiation hazard, and that the terms “radiation” and “contamination” are not interchangeable.

Sources of radioactive contamination can be classified into two groups: natural and man-made. After nuclear containment is discharged into the atmosphere or a nuclear reactor’s containment is breached, the air, soil, people, plants and animals in the vicinity will all be contaminated with nuclear fuel and nuclear fission products. A spilled vial of radioactive material, such as uranyl nitrate, contaminates the ground and any pieces used to clean up the leak. Widespread cases of radioactive contamination include the Bikini Ring, the Rocky Flats plant in Colorado, the Fukushima Daiichi nuclear disaster, the Chernobyl disaster, and the area around the Mayak facility in Russia.

Sources of radioactive contamination can be natural or man-made.

Radioactive contamination is due to several reasons. It can be caused by the release of radioactive gases, liquids or particles. For example, if radionuclides used in nuclear medicine were spilled (by accident, or, as in the Goiânia radiation accident, out of ignorance), then people could spread the material as they walked around.

Radioactive contamination can be an inevitable consequence of certain processes, such as the release of radioactive xenon in the reprocessing of nuclear fuel. In cases where radioactive materials cannot be suppressed, they can be diluted to safe concentrations. For a discussion of environmental pollution on alpha emitters, please see Actinides in the Environment.

Nuclear fallout is the distribution of radioactive contamination from 520 nuclear explosions that occurred in the atmosphere from the 1950s to the 1980s.

In nuclear accidents, a source term is defined as a measure of the type and amount of radioactivity released, such as a reactor containment failure. The United States Nuclear Regulatory Commission defines it as “the types and quantities of radioactive or hazardous materials released into the environment after an accident.”

Contamination does not include residual radioactive material, which remains at a site after the nuclear weapon has been dismantled. Therefore, radioactive material in properly sealed and labeled containers is not referred to as contamination, even though the units of measurement can be the same.