[en] A mathematical method of iodine release limitation is presented which, in assessing the radiological effectiveness of ¹²⁹I, takes advantage of the fact that the chemical behaviour of ¹²⁹I resembles that of ¹³¹I and relies on the already extensive knowledge of the chemical and biological behaviour of ¹³¹I. If this method is used for calculating permissible ¹²⁹I emission rates it is stated that no unnecessary restrictions need be imposed on a fuel reprocessing plant and that the grazing season for the pasture-cow-milk pathway can be taken into account. The concept is currently in use at the Karlsruhe Nuclear Research Center and seems to be appropriate for licensing of nuclear fuel reprocessing plants. (U.K.)