Irradiation test subassembly bent onto in-vessel storage rack in the experimental fast reactor "Joyo"

Sekine, Takashi ; Nakano, Tomoyuki 

During the 15th periodical inspection of the experimental fast reactor "Joyo", in-vessel visual inspections were conducted because of the malfunction of a fuel handling machine. These inspection results confirmed that the test subassembly part of instrumented type irradiation test device "MARICO-2" has been bent onto the in-vessel storage rack, and the regulating grids and guide tubes on the bottom of Upper Core Structure (UCS) have been deformed. After finishing irradiation test, MARICO-2 was transferred from the irradiation position to an in-vessel storage rack. Then the disconnection work of MARICO-2 test subassembly part from the upper structure was carried out. However, disconnecting failure was occurred because of the design deficiency of the separation mechanism. When the rotating plug was returned to the original position, the upper structure of MARICO-2 which was installed in the UCS dragged the test subassembly, and the head of the irradiation test subassembly has been bent.