In the definition given in the Atomic Energy Act (AtG), those substances are termed "radioactive substances" whose content of radionuclides cannot be disregarded. Values occurring below legally specified exemption levels can be disregarded as defined by the AtG. One differentiates between nuclear fuels and other radioactive substances.

The scale for the classification "radioactive" is the exceeding of the exemption levels according to the provisions of the Radiation Protection Ordinance. All substances containing radionuclides must be classified as "radioactive" if their activity (that is, the number of nuclear transformations of a radionuclide or radionuclide mixture occurring per time unit) exceeds the exemption levels mentioned.