[en] The vented and pressure-equalized design of gas-cooled fast breeder reactor (GCFR) fuel rods and subassemblies has been studied for several years. The vented design imposes on the reactor designer the requirement of being able to predict the behavior and transport of all important gaseous and volatile radionuclides so that their expected distribution in the plant may be used for component design and safety analysis. The irradiation experiment data base is described from which information on the transporting species and the transported fractions of volatile and gaseous species is obtained. The data are analyzed to define the predominant transporting mechanism for the nuclides in each mass chain