Incore structure detection device

In an incore structure detection device, a first multi-joint arm connected vertically turnably and a second multi-joint arm rotatable around a vertical axis are connected by a joint memeber. A detection arm is constituted by turnably joining a top end of the second multi-joint arm having a ultrasonic sensor attached and the other end thereof. The top end of the detection arm is flexibly inserted into a region between structural materials and equipments in the chamber above the reactor core and a reactor container wall, to find parts dropped on the pool partition wall. When the multi-joint arm is rotated in a horizontal plane, vertical driving force is not necessary. Further, by covering the portion between the horizontal arm and the multi-joint arm and the joints between each of the multi-joint arms by extensible covers, the joints comprising a high temperature resistant electric motor and gears can be protected from highly corrosive liquid sodium. (N.H.)