[en] The device of the present invention rapidly detects leakage of primary coolants due to rupture of heat transfer pipes in a steam generator of a PWR type reactor. That is, a neutron detector is disposed, as a dose rate measuring system, to a secondary main steam system, a secondary main steam pipeline, or a turbine. A calculation processing system compares the data obtained therefrom and a normal state, to judge the presence of an abnormal symptom due to leakage and calculate radioactivity concentration in the main steams at a measuring point based on the dose rate. With such procedures, if a heat transfer pipe in the steam generator should be ruptured, radioactive materials in the primary coolants reach the position of the neutron detector in several seconds. Based on the result, progress of the leakage is forecast, to estimate the scale of the ruptured portion with lapse of time. Since neutrons are an object of the measurement in the device of the present invention, the device does not undergo influence of gamma rays released from a radiation source nuclide present in natural radiation rays and the primary system. (I.S.)