PURPOSE:To enable early specifying of a cooling system wherein breakdown of a heat transfer pipe occurs, by measuring a change in a dose rate by providing a radiation detector at a position whereat a secondary system main steam piping of each system is made to have a shielding effect. CONSTITUTION:The title apparatus comprises a dose rate measuring system 17 having radiation detectors 16 provided at positions which are located in the vicinity of secondary system main steam pipings 6 connecting steam generators 4 of a plurality of systems with a turbine 7, respectively, and whereat the pipings have a shielding effect for a radiation from pipings other than ones in the vicinity thereof, and an arithmetic device 18 which calculates a change in a dose rate by comparison with a normal state on the basis of data obtained from the measuring system 17 and can judge and specify the system wherein a leakage occurs, from the change in the dose detected by each radiation detector 16.