Method for measuring the adsorbed radioactivity source term of fission nuclide 131I in lead–bismuth reactors

131I is a typical fission product in nuclear reactors. It has a strong affinity for the human thyroid gland and is an important nuclide of concern in radiation protection under both normal and accident conditions of reactors. Accurate measurement of its radioactivity is of great significance. The main measurement method involves collecting 131I in aerosols using an activated carbon adsorption device and then analyzing it with a γ spectrometer. This study proposes a bidirectional ratio method to obtain the γ detection efficiency through integrating the depth distribution coefficient of 131I in activated carbon. Compared with the traditional methods of preparing standard uniform volume sources or using destructive analysis, this experimental method is more reasonable and reliable. In addition, to verify the reliability of this method, experimental verification of the proportional coefficient of bidirectional measurement was conducted, and experimental work on the correlation analysis between sampling flow rate and distribution coefficient was provided. The experiments show that the detection efficiency of the iodine cartridge source with a real depth distribution coefficient obtained by the volume source integration method incorporating a probability function can quickly and conveniently obtain the γ detection efficiency of non-uniformly distributed iodine cartridge sources, providing a feasible approach for accurately measuring the radioactive activity of the fission product 131I in the environment.