Nuclear accidents can be defined as the undesired events in which adverse effects of the ionizing radiation to man and environment are produced, and which are related to nuclear (fissile) materials. It is important to distinguish between nuclear and radiological accidents, the later being related to non-fissile radioactive materials (e.g. radiation sources used in medicine or in industry).

Although nuclear accidents can also happen during nuclear materials processing, storage or transport, the biggest threat comes from the power reactor accidents. Because of the presence of radioactive materials in considerable amounts, the consequences of such accidents can be serious and the affected areas can be large.