Abstract: The present invention provides a method for identifying failed fuel elements and the scale of damage accurately, quickly and at low cost without disassembling a fuel assembly and without discharging radioactive waste. Radiation detector(s) for measurement of gamma radiation emitted from surface of the irradiated fuel assembly and the fuel assembly are rotated and translated relative to each other, the dectected data of the radiation emitted from the surface of the fuel assembly is acquired, processed, a tomographic image is constructed from the radiation intensity distribution and is prepared and displayed, and a failed fuel element is finally confirmed from the tomographic image of the fuel assembly cross-section.