The control rod guide tube self-stands on the core support plate. The control rod is inserted in the control rod guide tube, and the control rod hangs from the upper structure of the reactor. At scrum in case of an earthquake, the control rod is detached and it sits on the seating structure in the control rod guide tube. The control rod guide tube is raised more easily than the fuel assembly by the vertical differential pressure of the core during operation, because the control rod guide tube is lighter than the fuel assembly. Therefore, it is necessary to restrain the rising of the control rod guide tube. The sleeve dashpot structure, in which a sleeve is attached on the upper surface of the receptacle tube, is employed. Moreover, the control rod guide tube is equipped with the control rod dashpot in order to restrain the vertical movement of the control rod. In case of an earthquake, horizontal vibrational behavior also arises. The horizontal vibration generates friction force in the vertical direction and it restrains the rising. In this seismic evaluation of the control rod guide tube and the control rod, only the vertical motion is taken into consideration because it becomes conservative side against the rising displacement.
The seismic evaluation of the control rod guide tube is performed in operating condition (the coolant is flowing upward); that of the control rod assembly (the control rod guide tube and the control rod) is performed in shutdown condition (the coolant is not flowing).
Seismic experiments were performed using a full-scale model of the control rod guide tube and the control rod, vertical excitation and water instead of liquid sodium in the actual plant, to verify the analysis method and to demonstrate the rising restraining effect of the sleeve dashpot and the control rod dashpot. In this study, the rising behavior of the control rod guide tube and the control rod was investigated. The effect of restraining rising by the sleeve dashpot and by the control rod dashpot was confirmed.
