Failed fuel rod detection method for nuclear fuel assembly

Presence or absence of water intrusion in a cladding tube is detected so as to judge as to whether a cladding tube of a failed fuel rod of a nuclear fuel assembly is raptured or not. That is, a pair of probe bases are inserted between fuel rods of a fuel assembly placed in a pool. A fuel rod is put between a micro wave oscillating portion and a temperature detecting portion of the probe base. If there is water intrusion in the cladding tube, the water is heated by micro waves oscillated in this state. The presence or absence of the water intrusion can be judged by detecting the temperature by way of the cladding tube. According to the method of the present invention, heating phenomenon is caused even by intrusion of a trace amount of water, and the detection is free from the effect due to the state of a close contact portion between the pellet and the cladding tube or the surface of the cladding tube. Further, since the temperature on the wide range of the cladding tube is elevated by the heat conduction, even if the intrusion of water occurs partially, it can easily be detected. (I.S.)