Failed fuel detection device in FBR type reactor

In the present invention, damages of a reactor core fuel assembly in an LMFBR type reactor are detected. Failed fuels can be detected by disposing a delayed neutron detector in a support cylinder, which is supported verticallt movably in the cylinder and forms a coolants flow channel. The device is attached passing through the reactor upper mechanisms in the reactor container and is extended vertically toward the reactor core downwardly. The coolants uprising from the reactor core are intaken from a receiving member of an interface mechanism, entered to a lower failed fuel detector and then uprise. Further, the coolants flown from the handling head of the lower failed fuel detector pass through the interface-ratching mechanism and are flown in an upper failed fuel detector. In this way, the delayed neutrons from nuclear fission products are detected by means of the delayed neutron detector for conducting quantitative estimation to monitor the abnormality. (I.S.)