PURPOSE: To exactly detect the presence or absence of a fuel failure by computing the gross radioactivity concn. of the noble gas over the entire part of nuclides except the radioactivity concn. of the disturbing nuclide and computing a radioactivity emissivity from the radioactivity concn. of the noble gas of every nuclide and the flow rate of the exhaust gas of an exhaust gas system.

CONSTITUTION: The steam from a nuclear reactor 1 is discharged to a condenser 5 after working in a steam turbine 4. The exhaust gas is diverted as a sampling gas to a radiation measuring instrument α for detecting the fuel failure through an exhaust gas dehumidifying cooler 10 and a sampling pipe 12 and most of the exhaust gas is decreased in the radioactivity down to a safe level in an active carbon adsorption column 11. The sampling gas is diluted to a prescribed dilution rate by a dilution vessel. The radiation level of the noble gas is detected by a Ge semiconductor detector of the radiation measuring instrument. The result of the detection is spectrally analyzed by an analyzer of a signal processor by which the count rate of every nuclide is determined. The gross radioactivity concn. of the noble gas of the entire nuclide excluding the disturbing nuclide is computed from this count rate. The fluctuation in the radioactivity concn. of the noble gas in the case of the fuel failure in a reactor core 2 is thus exactly detected.