Continuous measuring method for radioactive iodine in off-gas

In the present invention, the amount of iodine in off-gases discharged from a nuclear fuel reprocessing facility, a nuclear reactor facility, a radioactive waste facility or a facility using radioactive iodine is measured and monitored while eliminating interference due to radioactive rare gases mixed together with iodine. Namely, in the present invention, a portion of the off gases discharged from each of the facilities is continuously circulated to an adsorbing material. Energy of the radioactive iodine adsorbed to the adsorbing material is measured by a semiconductor detector simultaneously with the measurement for the energy of mixed radioactive rare gases. The energy of K α X-rays or γ-rays of the radioactive iodine are discriminated based on the signals of the semiconductor detector by a multiple wave-height analyzer. Discrimination signals are flattened, and interference of radioactive rare gases is eliminated. The amount of radioactive iodine is thus measured continuously. As the semiconductor detector, Ge, CdTe, HgI2 or GaAs is used. According to the present invention, radioactive iodine in the off gases can be continuously measured while eliminating interference of mixed radioactive rare gases. (I.S.)