[en] The present invention evaluates radioactivity of activated ¹⁶N or ¹⁵C in a secondary main steam system when primary coolants are leaked in a steam generator, to rapidly detect the leakage of primary coolants. That is, the concentration of radioactivity of activated primary coolants is at first determined based on the data of measuring instruments which are controlled by a nuclear reactor primary coolant system data measuring and controlling device. A concentration at a certain position is determined based on the concentration at a position of a heat transfer tube and dose rate measuring value for radioactivity of leaked activated primary system measured by a γ^- ray detector which is controlled by a reactor secondary system data measuring and controlling device. Then, the concentration at a position of the steam generator is determined. Then, the amount of leakage at the heat transfer tube is evaluated by reversely calculating the concentration at a position of the steam generator. When the result of the evaluation is abnormal, this is informed by an abnormality alarm device. (I.S.)